Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review

Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
energies
Review
Thermal Performance through Heat Retention in
Integrated Collector-Storage Solar Water Heaters:
A Review
Ruth M. Saint 1, *    ID
                           , Céline Garnier 1 , Francesco Pomponi 2   ID
                                                                           and John Currie 3
 1   School of Engineering & the Built Environment, Edinburgh Napier University, Edinburgh EH10 5DT, UK;
     c.garnier@napier.ac.uk
 2   Resource Efficient Built Environment Lab (REBEL), Edinburgh Napier University, Edinburgh EH10 5DT, UK;
     f.pomponi@napier.ac.uk
 3   Scottish Energy Centre, Edinburgh Napier University, Edinburgh EH10 5DT, UK; j.currie@napier.ac.uk
 *   Correspondence: r.saint@napier.ac.uk; Tel.: +44-(0)131-455-2837
                                                                                                        
 Received: 16 May 2018; Accepted: 19 June 2018; Published: 20 June 2018                                 


 Abstract: Solar thermal systems are a long-standing technology that is receiving increased attention,
 in terms of research and development, due to ambitious climate change targets and the need for
 renewable energy solutions. Integrated collector-storage solar water heaters (ICSSWHs) are a potential
 contributing solution and numerous studies have focussed on the optimisation of their thermal
 performance and efficiency. A major drawback of these systems is the heavy heat losses experienced
 during non-collection periods. To combat this, various heat retention strategies have been proposed
 and evaluated, including baffles plates, additional insulation, multiple glazing layers, selective
 coatings, and phase change materials. This paper aims to bring together these studies through
 a systematic review of the existing literature surrounding the performance of ICSSWH systems,
 focusing on heat retention. This review provides a comprehensive and up-to-date point of reference
 on relevant research and developments for researchers in this field.

 Keywords: integrated collector-storage solar water heater (ICSSWH); thermal performance;
 technological development; heat retention; solar energy




1. Introduction
     The current global situation sees ever increasing fossil fuel use with a subsequent increase in
climate impact. Given the commitments made to reduce greenhouse gas emissions and mitigate climate
change [1], the move towards renewable energy sources need to be accelerated. Solar energy has great
potential to contribute a near zero-carbon solution to the world’s energy demand [2]. In Scotland
the consumption of heat accounts for 53% of the energy consumed by homes and businesses and the
nature of the Scottish climate requires a large proportion of this demand to be dedicated to space
and water heating. Domestic hot water use is the second most significant energy use in the home at
approximately 13% of total Scottish heat energy demand [3]. On average, Scotland has a 10% greater
number of degree days than the UK as a whole, thus necessitating additional space heating. Therefore,
solar hot water systems present a promising alternative to fossil fuel powered heating. Given the high
specific heat capacity of water there is great potential for energy gain and storage in these systems,
often yielding efficiencies of up to 70% [4].
     There are several types of solar thermal systems; the most common, commercially available
ones being the flat-plate and evacuated tube collectors. A hybrid flat-plate system, the integrated
collector-storage solar water heater (ICSSWH) has certain advantages over its more prevalent
opponents. The absorber plate and storage tank of the ICSSWH are combined into one compact


Energies 2018, 11, 1615; doi:10.3390/en11061615                                    www.mdpi.com/journal/energies
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, 1615                                                                                                           2 of 26
Energies 2018, 11, x FOR PEER REVIEW                                                                                              2 of 26


space
unit    savinggreat
     offering   capabilities   unlikecapabilities
                     space saving      other flat-plate
                                                  unlikeand  evacuated
                                                          other flat-platetube
                                                                           andcollectors;
                                                                                evacuatedwhich     require awhich
                                                                                            tube collectors;   bulky
externala water
require    bulky storage
                  externaltank
                             waterand,  consequently,
                                     storage tank and,additional    piping,
                                                         consequently,        pumps,piping,
                                                                         additional    and valves   which
                                                                                               pumps,   andincrease
                                                                                                              valves
the  heat  loss and  failure   points  within  the system.   Heat  loss is  a major  drawback    for  solar
which increase the heat loss and failure points within the system. Heat loss is a major drawback for solar  thermal
systems,systems,
thermal     especially  duringduring
                   especially     non-collection   periods
                                         non-collection      and and
                                                         periods   overcast   days.
                                                                       overcast  days.ToTocombat
                                                                                           combatthis,    numerous
                                                                                                    this, numerous
recent studies
recent   studies have
                 have focused
                        focused on on improving
                                       improving heat
                                                   heat retention
                                                         retention and
                                                                    and system
                                                                          system efficiency.
                                                                                   efficiency. This
                                                                                               This paper
                                                                                                     paper aims
                                                                                                             aims to
                                                                                                                  to
critically  review  such   bodies   of  knowledge,   identify existing   trends,  challenges,  and
critically review such bodies of knowledge, identify existing trends, challenges, and opportunities, opportunities,
and single-out
and   single-out areas
                  areasfor
                         for further
                              furtherresearch.
                                       research.

2. Background
2. Background
       The evolution
      The     evolution of   of solar
                                 solar water
                                         water heaters
                                                  heaters (SWHs)
                                                             (SWHs) has  has been
                                                                               been an an impressive
                                                                                           impressive journey
                                                                                                            journey beginning
                                                                                                                        beginning in    in
 America with
America      with the
                    the first
                          first of
                                of its
                                    its kind,
                                        kind, ‘The
                                                ‘The Climax
                                                       Climax Solar-Water
                                                                  Solar-Water Heater’,
                                                                                   Heater’, patented
                                                                                              patented in 1891  1891 [5].
                                                                                                                       [5]. However,
 once extensive
once   extensive oil oil and
                         and gasgas reserves
                                     reserves were
                                                 were discovered
                                                       discovered research
                                                                        research into
                                                                                    into these
                                                                                          these systems
                                                                                                  systems halted
                                                                                                              halted until
                                                                                                                       until the
                                                                                                                               the early
                                                                                                                                    early
 1970s [6].
1970s   [6]. Presently,
              Presently, interest
                             interestin insolar
                                            solarwater
                                                   waterheating
                                                           heating has has been
                                                                            been revived
                                                                                   revived in in response
                                                                                                   response to to global
                                                                                                                   global initiatives
                                                                                                                            initiatives
 aimed at
aimed    at the
             the promotion
                  promotion of    of renewable
                                     renewable energy
                                                    energy technologies
                                                              technologies in  in order
                                                                                   order to mitigate greenhouse
                                                                                                          greenhouse gas (GHG)
 emissions [7,8].
emissions      [7,8]. There are numerous different models of SWH                SWH systems
                                                                                         systems but they they all
                                                                                                                 all have
                                                                                                                     have the the same
                                                                                                                                    same
 goal—todeliver
goal—to      deliverhothotwater
                             waterto tothe
                                         theend-user
                                              end-userwhenwhenrequired,
                                                                   required,efficiently,
                                                                                efficiently,and
                                                                                              and  atat a reasonable
                                                                                                      a reasonable        cost.
                                                                                                                        cost.     Figure
                                                                                                                                Figure   1
 1 shows     an  ICSSWH        and   illustrates   the  general     key  components        of  a  SWH:
shows an ICSSWH and illustrates the general key components of a SWH: an absorber surface to collect        an  absorber     surface     to
 collect incident
incident              solar radiation,
            solar radiation,      a storage  a tank
                                                storage   tankcombined
                                                      (either     (either combined
                                                                               with thewith       the collector
                                                                                           collector                or a distributed
                                                                                                        or a distributed       system),
 system),with
glazing      glazing
                  highwith      high transmission,
                         transmission,                    insulated
                                            insulated housing        forhousing     for the collector/storage
                                                                          the collector/storage        unit, and cold unit,   and inlet
                                                                                                                           water     cold
 water  inlet   and   hot
and hot water outlet pipes. water   outlet   pipes.
       Thethree
      The    threemost
                     mostcommon
                              commonSWH    SWH     designs
                                                designs    areare   concentrating
                                                                concentrating          collectors,
                                                                                  collectors,         evacuated
                                                                                                 evacuated     tubetube     collectors,
                                                                                                                      collectors,    and
 and flat-plate
flat-plate           collectors.
             collectors.              The of
                             The choice      choice
                                               whichof      which
                                                        design         design can
                                                                   to employ     to employ         can bebydetermined
                                                                                       be determined            the environmentalby the
 environmental
conditions           conditions
               and the     heating and      the heating
                                      requirements       of requirements        of the end-user.
                                                             the end-user. Flat-plate        collectorsFlat-plate    collectors
                                                                                                           have varying             have
                                                                                                                                designs
 varying designs
(serpentine,           (serpentine,
                parallel                 parallel tubes
                            tubes or ICSSWH)        and, as orthe
                                                                ICSSWH)
                                                                   cheapestand,     as three
                                                                               of the   the cheapest
                                                                                              options,of  arethe  three
                                                                                                               used       options, are
                                                                                                                     extensively      for
 used  extensively      for  domestic     water   heating     applications    due    to the  performance
domestic water heating applications due to the performance advantages relative to cost [9]. They collect       advantages        relative
 to cost
both       [9].and
       direct     They     collect
                     diffuse          both direct
                                radiation    which is and     diffuse radiation
                                                         advantageous                  which
                                                                             in overcast         is advantageous
                                                                                             conditions,     also solarin      overcast
                                                                                                                           tracking    is
 conditions,
not  requiredalso thussolar   tracking
                         lowering         is notcost
                                       system     required     thus lowering
                                                       and complexity.        Thesystem     costof
                                                                                     majority     and   complexity.
                                                                                                     studies             The majority
                                                                                                                surrounding        these
 of studies
systems    aimsurrounding
                 to evaluate and  these    systems
                                       optimise       aim
                                                   their     to evaluateincluding
                                                         performance,        and optimise opticaltheir   performance,
                                                                                                    efficiency,             including
                                                                                                                 thermal efficiency,
 optical
and      efficiency,
      heat  retention.  thermal
                           There efficiency,
                                    have been and      heat retention.
                                                   a number      of papers There
                                                                              (see have
                                                                                     Tablebeen    a number
                                                                                            1) over    the lastofdecade
                                                                                                                   papersaiming
                                                                                                                             (see Tableto
 1) over these
review    the last   decade
                  heat          aiming to review
                         loss reduction                these
                                              strategies   andheat    loss reduction
                                                                  to appraise    the work strategies
                                                                                              being and       to appraise
                                                                                                        conducted      in the the   work
                                                                                                                                 area  of
 being conducted
thermal    performance. in the area of thermal performance.




      Figure 1.1. Diagrammatic
      Figure      Diagrammatic representation of the key
                                                     key components
                                                         components of
                                                                    of aa SWH,
                                                                          SWH, shown
                                                                               shown as
                                                                                     as an
                                                                                        an integrated
                                                                                            integrated
      collector-storage(ICS)
      collector-storage (ICS)type.
                              type.
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, 1615                                                                                                                        3 of 26



      Table 1. Summary of papers reviewing the current research and outcomes surrounding design factors
      of solar thermal systems.

   Reference    Year                     Title                          System(s) Reviewed                      Design Aspects Reviewed
                                                                                                        Additional insulation; Baffles; Cavity
                          Integrated collector storage solar                                            evacuation; Collector material; Glazing;
      [6]       2006                                             ICSSWH
                          water heaters                                                                 phase change material (PCM); Reflectors;
                                                                                                        Selective absorber surfaces
                          A comprehensive review on solar                                               Additional insulation; Collector material;
      [4]       2011                                             Thermo-syphon
                          water heaters                                                                 Glazing; Selective absorber surfaces
                          Review of solar water heaters                                                 Additional insulation; Baffle Plates;
      [10]      2013      with integrated collector-storage      ICSSWH                                 Glazing; Inlet valve configuration; PCM;
                          units                                                                         Reflectors; Selective absorber surfaces
                          Solar water heating systems and        ICSSWH; Thermo-syphon;                 Additional insulation; Baffle plates; PCM;
      [11]      2013
                          their market trends                    Direct/indirect circulation; Air       Reflectors
                                                                 Flat-plate; Evacuated tube;            Additional insulation; Baffle plates;
                          Recent advances in the solar water
      [12]      2013                                             compound parabolic concentrator        Collector material; Glazing; Reflectors;
                          heating systems: A review
                                                                 (CPC)                                  Selective absorber surfaces
                          A review of solar collectors and
                                                                 Non-concentrating (inc. flat-plate);   General overview of current technologies;
      [13]      2013      thermal energy storage in solar
                                                                 concentrating collectors               Fins; PCM
                          thermal applications
                          Review of water-heating systems:
                          General selection approach based       ICSSWH; Thermo-syphon; Active;         Fins; PCM; Reflectors; Selective absorber
      [14]      2014
                          on energy and environmental            PV/T; PCM                              surfaces
                          aspects
                          Building integrated solar thermal
      [15]      2015                                             PV/T & solar thermal                   General overview of current technologies
                          collectors—A review
                          Integrated collector storage solar
                                                                                                        Additional insulation; Baffle plate/inner
      [16]      2015      water heaters: Survey and recent       ICSSWH
                                                                                                        sleeve; PCM; Thermal diodes
                          developments
                          Performance enhancement of             Flat-plate; PV/T; Evacuated tube;
      [17]      2015                                                                                    Nanofluids; Selective absorber surfaces
                          solar collectors–A review              Compound parabolic
                          Innovation in flat solar thermal                                              Additional insulation; Collector material;
      [18]      2016      collectors: A review of the last ten   Flat-plate; ICSSWH; PV/T               Fins; Nanofluids; Selective absorber
                          years experimental results                                                    surfaces
                          A review of water heating system       Flat-plate; Evacuated tube;
      [19]      2016                                                                                    Auxiliary immersion heating; Nanofluids
                          for solar energy applications          Concentrating; ICSSWH
                          Recent developments in
                                                                                                        Additional insulation; Baffle plate/inner
      [20]      2016      integrated collector storage (ICS)     ICSSWH
                                                                                                        sleeve; Thermal diodes
                          solar water heaters: A review



     Table 1 provides a summary of recent review papers in the field of solar thermal systems, detailing
the types of systems and the design aspects reviewed. Of the review papers listed, only a handful
evaluate the array of thermal optimisation methods available in one article. Smyth et al. [6] present
the most comprehensive review of the technical aspects of ICSSWH systems. Kumar and Rosen [10]
also review a number of optimisation strategies, collating key literature. However, the main problem
is the distributed nature of the information with regards to heat retention strategies and the need
for that information to be updated. Therefore, the aim of this paper is to present an up-to-date,
comprehensive review of the current research pertaining to the improvement in thermal performance
and heat retention with a particular focus on ICSSWH systems.

3. Heat Gain and Retention Strategies
     ICSSWH systems are the simplest of the SWH designs, combining the collector and storage
tank into one compact unit. This creates direct contact between the working fluid (water, in direct
systems, as it must be potable) and the absorber plate. As a result, ICSSWH are subject to heavy heat
losses, especially during overcast and non-collection periods [21]. Figure 2 illustrates the heat transfer
mechanisms in a typical ICSSWH system showing the number of ways heat can be lost. The collector
surface absorbs solar radiation and converts it to heat which then gets transferred directly to the water
through convection. Due to the buoyancy effect, thermal stratification builds up in the water body
with the hottest water at the top of the tank where the majority of heat is lost through convection,
conduction and radiation. Convective heat losses occur between the absorber plate and the glazing
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, 1615                                                                                                           4 of 26



and the air cavity needs to be optimised to act as an insulating gap, promoting convective heat transfer
           Energies 2018, 11, x FOR PEER REVIEW                                                                           4 of 26
to the collector     whilst also minimising losses when ambient temperatures drop. Conductive                                         heat
loss from the    back and
           the glazing    and theside   walls,needs
                                    air cavity   as well     as the absorber
                                                      to be optimised               surface,
                                                                         to act as an insulatingisgap,
                                                                                                   a major     problem
                                                                                                       promoting            for ICSSWH
                                                                                                                    convective
systems andheatthey      need
                  transfer        to be
                              to the       heavily
                                       collector      insulated.
                                                  whilst              Long-wave
                                                          also minimising                radiative
                                                                               losses when    ambient heat   losses have
                                                                                                         temperatures     drop.the largest
impact andConductive
              occur betweenheat lossthe
                                      from  the back and
                                          absorber         sideand
                                                       plate     walls,
                                                                     theasglazing,
                                                                           well as thesuppressed
                                                                                        absorber surface,  is aextent
                                                                                                       to an    major problem
                                                                                                                        by the air cavity.
           for ICSSWH systems and they need to be heavily insulated. Long-wave radiative heat losses have the
The glazing    needs to have a very high transmissivity as well as optical efficiency to ensure as much
           largest impact and occur between the absorber plate and the glazing, suppressed to an extent by the
incident solar   radiation
           air cavity.            can reach
                         The glazing    needs tothe collector
                                                  have a very highsurface     as possible
                                                                     transmissivity    as wellwhilst
                                                                                              as opticalagain   minimising
                                                                                                          efficiency  to ensure radiative
losses. Numerous strategies exist to overcome these losses and to promote heat gain. The literature
           as  much    incident   solar  radiation can  reach  the  collector surface   as possible whilst  again minimising
surroundingradiative
                theselosses.
                          methodsNumerous     strategies exist
                                        of optimising           to overcome
                                                            thermal            these losseswill
                                                                        performance          and be
                                                                                                  to promote
                                                                                                     reviewed   heatingain. The detail in
                                                                                                                        greater
           literature surrounding these methods of optimising thermal performance will be reviewed in greater
the following    segments.
           detail in the following segments.




                     Figure 2. Illustration of the heat transfer processes in an ICSSWH. Adapted from Smyth et al. [6].
         Figure 2. Illustration of the heat transfer processes in an ICSSWH. Adapted from Smyth et al. [6].
              3.1. Additional Insulation
3.1. Additional Insulation
                 Insulationis a simple and effective way of retaining heat, particularly at night. The two main
              types of insulation are opaque and transparent. Opaque insulation is primarily used around the back
     Insulation    is a simple and effective way of retaining heat, particularly at night. The two main
           and sides of the systems where ambient heat losses through conduction and convection are high but
types of insulation    are opaque
           there is no need            and transparent.
                             for transmittance                  Opaque
                                                 of solar radiation         insulation
                                                                     [22,23].             is primarily
                                                                              A large percentage of heat used    around the back
                                                                                                         is lost through
and sides of
           thethe   systems
               glazing  as this where
                                needs toambient
                                          be exposed heat    losses
                                                        to gain solarthrough
                                                                       irradiance.conduction    and convection
                                                                                   This can potentially be alleviated are
                                                                                                                      by high but
there is nopartially
            need for insulating  the air cavity,
                        transmittance            usingradiation
                                            of solar     transparent[22,23].
                                                                      insulatedA glazing
                                                                                   largematerials,  and/or
                                                                                           percentage    of by  applying
                                                                                                             heat  is lost through
           an insulated cover during the night and non-collection periods.
the glazing as this needs to be exposed to gain solar irradiance. This can potentially be alleviated by
partially insulating    the airInsulation
           3.1.1. Transparent     cavity, Materials
                                           using transparent
                                                       (TIMs)         insulated glazing materials, and/or by applying
an insulated cover     during     the night   and    non-collection
                TIMs represent a new class of thermal insulation used      periods.
                                                                              to reduce unwanted heat losses from air
              gaps and evacuated spaces. TIMs are transparent to solar irradiation yet they can provide good
3.1.1. Transparent   Insulation
           thermal insulation.    Materials
                               Early attempts to(TIMs)
                                                 use these materials, such as experimental work by McCracken
              [23], led to overall decreases in system efficiency, by ~20% during daytime collection, due to their
     TIMspoor
           represent     a new class
                 solar transmittance     of in
                                      (0.85  thermal    insulation
                                               this case).           used toand
                                                           However, research reduce   unwanted
                                                                                development          heat
                                                                                               of these    losses from air
                                                                                                        materials
gaps and evacuated      spaces.   TIMs   are  transparent    to solar irradiation yet they    can  provide   good thermal
           has led to greater collector performances and they could be greatly beneficial in increasing the solar
insulation. Early attempts to use these materials, such as experimental work by McCracken [23], led to
overall decreases in system efficiency, by ~20% during daytime collection, due to their poor solar
transmittance (0.85 in this case). However, research and development of these materials has led to
greater collector performances and they could be greatly beneficial in increasing the solar gain of
thermal energy systems. TIMs include organic-based transparent foams, honeycomb or capillary
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
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            gain of thermal energy systems. TIMs include organic-based transparent foams, honeycomb or
structures,capillary
            and inorganic    glass foams (e.g., silica aerogel). There are numerous configurations available
                     structures, and inorganic glass foams (e.g., silica aerogel). There are numerous
in terms ofconfigurations
             cellular geometry,     asterms
                          available in illustrated   ingeometry,
                                            of cellular Figure 3as[24].
                                                                    illustrated in Figure 3 [24].




                  Figure 3. Diagrammatic representation of different cellular geometries. (A) Absorber-parallel; (B)
      Figure 3. Diagrammatic representation of different cellular geometries. (A) Absorber-parallel;
                Absorber-perpendicular; (C) Mixed configuration; (D) Cavity structures; (E) Homogeneous. The
      (B) Absorber-perpendicular;       (C) Mixed
                arrows indicate the absorptance       configuration;
                                                and reflectance         (D) Cavity
                                                                of the different TIMs instructures;   (E) Homogeneous.
                                                                                          terms of incident solar
      The arrows   indicate
                irradiance.  the absorptance
                            Adapted from Kaushikaand
                                                  and reflectance
                                                      Sumathy [24].of the different TIMs in terms of incident solar
      irradiance. Adapted from Kaushika and Sumathy [24].
                Kaushika and Sumathy [24] conducted a review of solar TIMs, presented in Table 2. It was found
          that a honeycomb material had the highest efficiencies when compared with a single and double
      Kaushika
          glazedand   Sumathy
                  system          [24] conducted
                           and a transparent  slab ofa methyl
                                                       reviewmethacrylate
                                                              of solar TIMs,
                                                                          (MMA).presented    in Table
                                                                                    It can also  be seen2.that
                                                                                                            It was found
that      efficiency increases with TIM thickness.
      a honeycomb material had the highest efficiencies when compared with a single and double
glazed systemTable
               and 2.a Efficiency
                       transparent     slab of methyl methacrylate (MMA). It can also be seen that efficiency
                                  comparison of single glazing, double glazing, MMA slab, and honeycomb cover
increases withsystem.
               TIM thickness.
                      Data taken from Kaushika and Sumathy [24].

                                                                       Efficiency (%)
      Table 2. Efficiency comparison
                     TIM Thickness (m) of single glazing, double glazing, MMA slab, and honeycomb cover
                                        Single Glazing  Double Glazing MMA Slab Honeycomb
      system. Data taken from
                           0.025 Kaushika and   Sumathy [24].0.32
                                             0.106                        0.299      0.328
                              0.05                 0.11               0.329           0.374           0.41
                              0.075                0.119              0.346Efficiency 0.396
                                                                                      (%)             0.439
     TIM Thickness (m)         0.1                 0.128              0.357           0.404           0.45
                                      Single Glazing        Double Glazing              MMA Slab                Honeycomb
                  Schmidt and Goetzberger [25] determined that, for a single-tube absorber mounted in an
              0.025                      0.106                    0.32                   0.299                   0.328
            involute reflector, the ICS performance was almost constant for a TIM (polycarbonate honeycomb
              0.05                       0.11                    0.329                   0.374                    0.41
            structure) thickness ranging from 50–150 mm. Chaurasia and Twidell [26] compared two identical
              0.075                      0.119                   0.346                   0.396                   0.439
            ICS units, one with TIM and the other without. The system with TIM had a total heat loss factor of
               0.1                       0.128                   0.357                   0.404                    0.45
            1.03 W/m2K, with hot water at temperatures 8.5–9.5 °C higher the next morning, compared with 7.06
            W/m2K for the glass only system. The study found that, by using TIM, storage efficiency was 24.7%
     Schmidthigher;
                 and39.8%  compared to
                      Goetzberger        15.1%
                                      [25]      without thethat,
                                            determined       TIM. for
                                                                   Reddy  and Kaushikaabsorber
                                                                       a single-tube      [27] conducted a series
                                                                                                   mounted      inofan involute
            comparative studies on the effect of different TIM configurations for a rectangular ICSSWH. It was
reflector, the   ICS performance was almost constant for a TIM (polycarbonate honeycomb structure)
            reported, and later corroborated [28,29], that an absorber-perpendicular configuration (transparent
thickness ranging
            honeycomb from   50–150
                         structure     mm. Chaurasia
                                   immersed                 andexhibited
                                               in an air layer)  Twidellsuperior
                                                                           [26] compared
                                                                                   efficienciestwo
                                                                                                overidentical  ICS units, one
                                                                                                     corresponding
with TIM and      the other without.
            absorber-parallel            The system
                               configurations  (multiplewith
                                                          coversTIM  had a totalfilms).
                                                                 of glass/plastic  heat Reddy
                                                                                          loss factor of 1.03 W/m
                                                                                                 and Kaushika    [27] 2 K, with
hot waterconcluded      that the 8.5–9.5
             at temperatures                ◦ C higher
                                   double-wall   structuredthepolycarbonate
                                                                next morning, TIM compared
                                                                                   sheet was the withmost
                                                                                                       7.06effective
                                                                                                             W/m2 K for the
            configuration and a system with a thickness of 10 cm can achieve average solar collection and storage
glass onlyefficiencies
             system. in The   study found that, by using TIM, storage efficiency was 24.7% higher; 39.8%
                           the range of 20–40% and bulk water temperatures between 40 and 50 °C.
compared to 15.1% without the TIM. Reddy and Kaushika [27] conducted a series of comparative
studies on3.1.2.
            the Opaque    Insulation
                  effect of different TIM configurations for a rectangular ICSSWH. It was reported, and
later corroborated      [28,29],
                 A number         thathave
                            of studies   an absorber-perpendicular              configuration
                                              experimentally reviewed the collector               (transparent
                                                                                        performance                honeycomb
                                                                                                      of fully-versus
           partially-exposed
structure immersed      in andesigns.   Tripanagnostopoulos
                               air layer)   exhibited superior et al. efficiencies
                                                                      [30] evaluatedover
                                                                                     the performance  of two absorber-parallel
                                                                                          corresponding       double-
           vessel systems utilising asymmetric compound parabolic concentrator (CPC) reflectors where one
configurations (multiple covers of glass/plastic films). Reddy and Kaushika [27] concluded that the
           had both tanks fully exposed while the other was partially exposed. The latter showed improved heat
double-wall    structured
           retention  during polycarbonate        TIMbut
                              non-collection periods    sheet
                                                            the was     the most
                                                                fully exposed       effective
                                                                                 design        configuration
                                                                                         exhibited               and a system
                                                                                                   greater collection
with a thickness    of Smyth
           efficiency. 10 cmetcan al. achieve   average
                                      [31] investigated    solar
                                                         two       collection
                                                              ICSSWH      systemsand   storage
                                                                                   enclosed      efficiencies
                                                                                             within             in the range of
                                                                                                     a concentrating
20–40% and collector.
               bulk One   system
                      water       was a fully exposed,
                             temperatures       between 1.0 m
                                                            40long     50 ◦ C. tank and the other a 1.5 m long tank
                                                                andcylindrical

3.1.2. Opaque Insulation
      A number of studies have experimentally reviewed the collector performance of fully-versus
partially-exposed designs. Tripanagnostopoulos et al. [30] evaluated the performance of two
double-vessel systems utilising asymmetric compound parabolic concentrator (CPC) reflectors where
one had both tanks fully exposed while the other was partially exposed. The latter showed improved
heat retention during non-collection periods but the fully exposed design exhibited greater collection
efficiency. Smyth et al. [31] investigated two ICSSWH systems enclosed within a concentrating collector.
One system was a fully exposed, 1.0 m long cylindrical tank and the other a 1.5 m long tank with the
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, 1615                                                                                                                   6 of 26



top third heavily insulated. Unlike Tripanagnostopoulos et al. [30], the insulated system exhibited
a 13% increase in collection efficiency and increased thermal retention of up to 37% in the upper
insulated section due to stratification. However, these results are perhaps reflecting the greater storage
capacity and lower average water storage temperatures of the 1.5 m tank. Chaabane et al. [32] also
found, through a numerical CFD study, that a partially insulated tank effectively retained heat during
non-collecting periods. For the studied CPC mounted ICS system the authors found that the additional
insulation did affect the optical efficiency, with slightly lower water temperatures observed–up to
7.5 ◦ C difference       between
               Energies 2018,           the REVIEW
                              11, x FOR PEER  insulated and non-insulated systems. However, this                                trade-off was
                                                                                                                           6 of 26
worthwhile with higher temperatures (up to 7 ◦ C) at the start of the collection period and a ~25%
               with the top third heavily insulated. Unlike Tripanagnostopoulos et al. [30], the insulated system
reduction in exhibited
               thermala losses.13% increase in collection efficiency and increased thermal retention of up to 37% in the
      Using an    experimental
               upper    insulated section set-up,    Souza et al.
                                             due to stratification.    [33] and
                                                                    However,   theseSwiatek
                                                                                     results are et  al. [34]
                                                                                                 perhaps       studied
                                                                                                          reflecting       a parallelepiped
                                                                                                                     the greater
ICSSWH system  storage   capacity
                     which        wasandpartially
                                          lower average   water storage
                                                     heated.     Souzatemperatures      of the 1.5 m
                                                                           et al. [33] studied        tank.
                                                                                                    the     Chaabane
                                                                                                         effect        et al. [32] stratification
                                                                                                                 on thermal
               also found, through a numerical CFD study, that a partially insulated tank effectively retained heat
within a cavity    with a heat flux applied only to bottom 0.2 m while Swiatek et al. [34] built on this and
               during non-collecting periods. For the studied CPC mounted ICS system the authors found that the
applied the heat      flux to
               additional          a 0.2 m
                              insulation      section
                                            did          in the
                                                 affect the        middle
                                                              optical         of the
                                                                      efficiency,  withcavity.
                                                                                         slightly Beyond
                                                                                                   lower waterthetemperatures
                                                                                                                   heated zone, the front
               observed–up
plate of the cavity      in both to 7.5  °C difference
                                      studies     was between
                                                        entirelytheinsulated
                                                                       insulated and
                                                                                   withnon-insulated
                                                                                           extrudedsystems.      However, Souza
                                                                                                         polystyrene.         this     et al. [33]
               trade-off was worthwhile with higher temperatures (up to 7 °C) at the start of the collection period
found that by      only heating a short section at the bottom of the cavity the evolution of thermal
               and a ~25% reduction in thermal losses.
stratification within      the                                                                                                                 ◦
                     Using    an tank     was unsatisfactory
                                   experimental   set-up, Souza et al.with
                                                                        [33] a maximum
                                                                             and              temperature
                                                                                  Swiatek et al. [34] studied a difference
                                                                                                                 parallelepipedof only 4.2 C
being reported.ICSSWH       system which was
                    The adaptations            made  partially
                                                        in theheated.     Souza et
                                                                  later study       al. [33] enhanced
                                                                                  yielded     studied the levels
                                                                                                             effect on
                                                                                                                     of thermal
                                                                                                                         stratification, with
               stratification within a cavity with a heat     ◦ flux applied only to bottom 0.2 m while Swiatek et al. [34]
a maximum temperature                different of 20.1 C [34].
               built on this and applied the heat flux to a 0.2 m section in the middle of the cavity. Beyond the heated
                  zone, the front plate of the cavity in both studies was entirely insulated with extruded polystyrene.
3.1.3. Night Cover
             Souza et al. [33] found that by only heating a short section at the bottom of the cavity the evolution
                  of thermal stratification within the tank was unsatisfactory with a maximum temperature difference
     As well of
              asonly
                 insulating
                       4.2 °C being thereported.
                                         air cavity,    an insulating
                                                   The adaptations    madenight       cover
                                                                              in the later     canyielded
                                                                                            study    be utilised
                                                                                                            enhanced  to levels
                                                                                                                          reduceof the issue of
night-time heat   losses. Kumar
             stratification,               and Rosen
                              with a maximum               [35] undertook
                                                   temperature    different of 20.1a °C
                                                                                     comparative
                                                                                         [34].           performance investigation of
rectangular ICSSWHs,          assessing
             3.1.3. Night Cover
                                               various    heat   loss   reduction       strategies.     The    following five cases were
assessed: (1) single glass cover without night insulation; (2) single glass cover with night insulation
                   As well as insulating the air cavity, an insulating night cover can be utilised to reduce the issue
cover; (3) double     glassheat
             of night-time     cover      without
                                     losses. Kumar and night
                                                         Roseninsulation
                                                                 [35] undertook  cover;     (4) transparent
                                                                                   a comparative    performance insulation
                                                                                                                   investigation with single
glass cover; and    (5) insulating
             of rectangular    ICSSWHs,    baffle   plate
                                              assessing     with
                                                        various     single
                                                                  heat         glass cover.
                                                                        loss reduction            They
                                                                                          strategies. The found
                                                                                                           following that
                                                                                                                       five the
                                                                                                                             casesTIM covered
             were assessed: (1) single glass cover without night insulation; (2) single glass cover with night
system reached lower absorber plate temperatures than single and double glazed cases due to the
             insulation cover; (3) double glass cover without night insulation cover; (4) transparent insulation with
system receiving
             single10–15%
                     glass cover; lessandsolar   radiation.
                                           (5) insulating baffleHowever,        the TIM
                                                                  plate with single    glass layer    did help
                                                                                              cover. They  foundretain
                                                                                                                   that theheat
                                                                                                                              TIM in the water
store during covered
             the night,      more
                        system   reachedso than    the double
                                            lower absorber         glazing and
                                                             plate temperatures        significantly
                                                                                    than                 more
                                                                                          single and double       than
                                                                                                               glazed      thedue
                                                                                                                       cases    single glazing.
             to the system receiving 10–15% less solar radiation. However, the TIM layer did help retain heat in
The single and double glazing perform in a similar manner during periods of insolation but at night the
             the water store during the night, more so than the double glazing and significantly more than the
double glazed   systems
             single   glazing.exhibits
                                 The single greater    heat retention.
                                                and double    glazing performFigurein a 4similar
                                                                                           illustrates
                                                                                                  mannerthe     bulkperiods
                                                                                                             during     wateroftemperatures
of the ICSSWH systems over a 24 h period showing that all five cases perform well. However, it is
             insolation   but  at  night  the double  glazed  systems    exhibits greater   heat retention. Figure   4 illustrates
             the bulk water temperatures of the ICSSWH systems over a 24 h period showing that all five cases
clear that Cases  2 and 3 maintain the highest temperatures and collector efficiencies, ranging from 57.1
             perform well. However, it is clear that Cases 2 and 3 maintain the highest temperatures and collector
to 79.4% andefficiencies,
              57.6 to 81.2%,rangingrespectively         (Figure
                                       from 57.1 to 79.4%   and 57.64).
                                                                      to 81.2%, respectively (Figure 4).




                       Figure 4. Daily variation in the bulk water temperature (left) and collector efficiencies for the five
       Figure 4. Daily   variation
                    reviewed           in the
                              cases (right).      bulkglass
                                             (1) single   water
                                                             covertemperature       (left) and
                                                                   without night insulation;        collector
                                                                                             (2) single         efficiencies
                                                                                                        glass cover with night for the five
       reviewed cases   (right).
                    insulation    (1)(3)
                               cover;  single
                                         doubleglass      cover
                                                   glass cover   without
                                                               without night night   insulation;
                                                                             insulation              (2) single
                                                                                        cover; (4) transparent    glass cover
                                                                                                               insulation with with night
       insulation cover; (3) double glass cover without night insulation cover; (4) transparent insulation with
       single glass cover; and (5) insulating baffle plate with single glass cover. Adapted from Kumar and
       Rosen [35].
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, x FOR PEER REVIEW                                                                                                 7 of 26


       single glass cover; and (5) insulating baffle plate with single glass cover. Adapted from Kumar and
Energies 2018, 11, 1615                                                                                  7 of 26
       Rosen [35].

3.2. Auxiliary
3.2. Auxiliary Heating
               Heating
       Currently,average
      Currently,     averagetotal
                                totalhot
                                      hotwater
                                            waterdemand
                                                    demand cannot
                                                              cannot be
                                                                      be met
                                                                          met bybySWHs
                                                                                    SWHs alone
                                                                                           alone duedue tototheir
                                                                                                             their diurnal
                                                                                                                   diurnal
 natureand
nature   andthis
              thisisis aa problem
                          problem for
                                    for all
                                        all solar
                                             solar thermal
                                                   thermal technologies,
                                                             technologies, particularly
                                                                             particularlyinin temperate
                                                                                               temperateclimates
                                                                                                            climatessuch
                                                                                                                      such
 as Scotland.   Garnier     [36] tested   the  thermal    performance    of  an ICSSWH     with
as Scotland. Garnier [36] tested the thermal performance of an ICSSWH with a 1 m absorber area and a 21 m 2 absorber area

 and
50     50 L capacity.
    L capacity.            They discovered
                 They discovered      that thethat
                                                 amountthe amount
                                                           of energyofrequired
                                                                        energy to required
                                                                                     supply atothree-person
                                                                                                 supply a three-person
                                                                                                               household
 household
with          with hot
       a domestic     a domestic     hot water
                            water demand            demand
                                                of 50         of 50 L/person/day
                                                        L/person/day     at 55 C is at
                                                                                ◦        55 °C is approximately
                                                                                       approximately       7.5 kWh/day, 7.5
 kWh/day,    2747    kWh/year.     The  amount      of  energy that the  studied    ICSSWH
2747 kWh/year. The amount of energy that the studied ICSSWH could contribute to this demand    could    contribute  to this
 demand
is shown inis Figure
               shown5;in1107  Figure
                                  kWh, 5; or
                                           1107
                                              40%kWh,
                                                   of theor  40% energy
                                                           yearly  of the demand.
                                                                            yearly energy     demand.
                                                                                       Therefore,          Therefore,60%
                                                                                                     the remaining      the
 remaining
would    need60%
               to bewould     needbytoan
                       supplied        beauxiliary
                                            suppliedheater
                                                        by an for
                                                              auxiliary
                                                                  diurnalheater
                                                                           demand.for diurnal demand.




      Figure 5.5. Domestic
      Figure      Domestic hot
                            hot water
                                water energy
                                      energy requirement
                                             requirement and
                                                         and ICSSWH
                                                              ICSSWH contribution
                                                                      contribution for
                                                                                    for aa three-person
                                                                                            three-person
      household   in 2007. Adapted from Garnier [36].
      household in 2007. Adapted from Garnier [36].


      AAsolar
           solarcombi
                  combisystem
                            system(SCS)(SCS)    is one
                                             is one  way way    to provide
                                                            to provide   thisthis  additional
                                                                              additional    heat heat      requirement,
                                                                                                    requirement,              if multiple
                                                                                                                       if multiple    solar
 solar   thermal     panels      are   unfeasible.     Sarbu     and   Sebarchievici      [37]   summarise
thermal panels are unfeasible. Sarbu and Sebarchievici [37] summarise the major studies surrounding                 the   major    studies
 surrounding
SCS                SCS in the
      in the literature.      Drückliterature.
                                        and Hahne Drück    and
                                                        [38]      Hahne [38]
                                                              conducted         conducted a study
                                                                            a comparative         comparative
                                                                                                           of four study      of four
                                                                                                                      hot water          hot
                                                                                                                                    stores,
 waterconnected
each     stores, each to aconnected        to a 10 m
                             10 m2 flat-plate
                                                      2 flat-plate
                                                    collector        collector
                                                                 area,  where area,     where the
                                                                                 the back-up       heatback-up
                                                                                                          methodheat   is anmethod
                                                                                                                              immersed is an
 immersed      heat   exchanger.        The   authors     found    that, for  a  well  performing
heat exchanger. The authors found that, for a well performing SCS, the most important factors are         SCS,   the   most    important
 factors
good       are good
        thermal          thermaland
                    insulation        insulation
                                          the optimaland configuration
                                                            the optimal configuration
                                                                             of the hot water  of theandhot     water heating
                                                                                                           auxiliary       and auxiliary
                                                                                                                                    loops.
 heating    loops.   By  using    an   SCS   over   a conventional      boiler,  fractional   energy
By using an SCS over a conventional boiler, fractional energy savings of up to 21% can be realised.       savings     of  up  to 21% can
 be realised.
      A study in Jordan looked at the techno-economic feasibility of a SWH system with a built-in
       A study
electric   coil asinanJordan
                          auxiliarylooked     at the
                                         heater        techno-economic
                                                   compared                   feasibility
                                                                  to a gas geyser           of a[39].
                                                                                       system       SWHThe    system
                                                                                                                  author with   a built-in
                                                                                                                             found     that
 electric   coil as  an   auxiliary     heater    compared       to a gas  geyser   system
the SWH with the integrated auxiliary heater was more economic due to the longer operational   [39].   The     author    found    that life
                                                                                                                                         the
 SWH     with    the  integrated       auxiliary     heater    was    more   economic      due    to
expectancy and the number of days where the use of the electric coil is not required. If the number ofthe    longer     operational      life
 expectancy
days             and theisnumber
       that electricity        used toof     days where
                                          provide    the fullthe  use hot
                                                                daily  of the  electric
                                                                           water   needs coil
                                                                                            ofisa not
                                                                                                  familyrequired.
                                                                                                             is 120 or If less
                                                                                                                          the number
                                                                                                                               the actual  of
 days   that  electricity     is  used    to  provide    the   full  daily hot
cost of the SWH system is less than the gas system, over the operational life.   water   needs     of   a  family    is  120   or less   the
 actual cost of the SWH system is less than the gas system, over the operational life.
Legionella/Freezing
 Legionella/Freezing
      Due to bacterial growth and freezing potential, most ICSSWHs are currently connected to a
       Due to
secondary         bacterial
               auxiliary        growth
                            heating         and freezing
                                         system.    Hot water  potential,
                                                                   consumed most byICSSWHs
                                                                                     the end-user  are must
                                                                                                         currently
                                                                                                                 be at connected
                                                                                                                         a temperature  to a
 secondary     auxiliary     heating     system.    Hot    water    consumed      by the   end-user
that is high enough to kill any bacteria that may have formed in a system, the biggest concern being     must    be   at  a temperature
 that is high
Legionella       enough to A
             pneumophila.        kill any bacteria
                                    review    on behalf thatofmay    have formed
                                                               the World              in a system, the
                                                                            Health Organisation           [40]biggest     concern
                                                                                                                states that          being
                                                                                                                                Legionella
 Legionella
thrives   at 36   ◦ C and within
              pneumophila.       A review
                                       a range on of
                                                   behalf
                                                      25–50 of◦the   Worldtemperatures
                                                                C. Water     Health Organisation
                                                                                              are required [40] states
                                                                                                                  to exceedthat 60  ◦ C for
                                                                                                                                Legionella
 thrivesminutes,
several    at 36 °C and      within
                       at least   oncea arange
                                            day, toofkill
                                                      25–50off °C.
                                                               anyWater     temperatures
                                                                     bacteria.  Alternatively,aredrawrequired      to exceed 60 could
                                                                                                             off temperatures        °C for
 several
be          minutes,
    increased             ◦
                  to 70atCleastwhich once    a day,the
                                         destroys     to bacteria
                                                           kill off any   bacteria.
                                                                      instantly.   ThisAlternatively,
                                                                                         is potentiallydraw          off temperatures
                                                                                                               unattainable      through
 couldinsolation
solar     be increased in atotemperate
                                 70 °C which        destroys
                                              climate    such the     bacteria instantly.
                                                                 as Scotland,     especiallyThis       is potentially
                                                                                                 in winter      months.unattainable
                                                                                                                              Therefore,
methods of back-up heating that work in conjunction with SWHs need to be incorporated.
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, x FOR PEER REVIEW                                                                                                       8 of 26


through solar insolation in a temperate climate such as Scotland, especially in winter months.
Energies 2018, 11, 1615                                                                                                                     8 of 26
Therefore, methods of back-up heating that work in conjunction with SWHs need to be incorporated.
      Freezing is also an issue for ICSSWHs due to their high level of exposure to the elements. Smyth
et al.Freezing
       [6] suggestedis also thatansystems
                                    issue forlessICSSWHs
                                                    than 100 mm    duedeep       werehigh
                                                                          to their       at greater
                                                                                              level risk       of freezing
                                                                                                        of exposure         toandthe to  prevent
                                                                                                                                      elements.
damage
Smyth    et to   thesuggested
            al. [6]    collector that and systems
                                            storage less tankthanthe 100depthmmshould
                                                                                    deep were be greater
                                                                                                    at greater   than
                                                                                                                    riskthis.    Schmidt
                                                                                                                          of freezing     andand to
Goetzberger       [25]   predicted      that, for   Northern       European        climates,     the
prevent damage to the collector and storage tank the depth should be greater than this. Schmidt and    risk   of  freezing    only   occurs     for
ICS systems [25]
Goetzberger       withpredicted
                           a specificthat,
                                         collector    volume lower
                                              for Northern        European  thanclimates,
                                                                                    ~70 L/m2the    andriskthat
                                                                                                             of damage
                                                                                                                 freezing due onlyto    freezing
                                                                                                                                     occurs     for
onlysystems
ICS    occurs ifwithmore     than 20%
                          a specific      of the ICS
                                        collector         waterlower
                                                     volume        content than is ~70
                                                                                   frozen.L/m2 and that damage due to freezing
only occurs if more than 20% of the ICS water content is frozen.
3.3. Baffle Plate/Inner Sleeve
3.3. Baffle Plate/Inner Sleeve
      Baffle plates have been used successfully in numerous ICSSWH designs as a method of
      Baffle plates
improved       thermal   have   been used successfully
                              performance         and heat in        numerousVarious
                                                                  retention.          ICSSWH       designs[41–43]
                                                                                                studies         as a methodhave ofutilised
                                                                                                                                     improved    an
thermal
insulating performance
              plate, inserted   andparallel
                                       heat retention.        Variousplate,
                                                 to the absorber          studies which[41–43]    havea utilised
                                                                                            creates                    an insulating
                                                                                                            narrow channel          with aplate,
                                                                                                                                              thin
inserted
layer of parallel
           water which to the can
                                absorber
                                      reachplate,
                                              muchwhichhigher   creates    a narrow than
                                                                   temperatures          channelthe with
                                                                                                      maina water
                                                                                                                thin layer
                                                                                                                         body.of water    which
                                                                                                                                   This system
can  reach much
separates             higher temperatures
              the downward              and upward   than theflow,mainthuswater     body. This
                                                                               decreasing           systeminseparates
                                                                                                 mixing            the storagethe downward
                                                                                                                                      tank and
and   upwardstratification.
promoting         flow, thus decreasing
                                    Once the watermixing     in the storage
                                                          is heated                tank and
                                                                        it is deposited      at promoting
                                                                                                 the top of the    stratification.
                                                                                                                      tank, allowing   Once    the
                                                                                                                                          colder
water   is heated     it  is deposited     at   the  top   of  the   tank,    allowing      colder
water to be drawn up into the channel. Souza et al. [33] studied an ICS system incorporating a         water     to  be  drawn      up  into   the
channel.    Souza
stratification        et al.
                   plate       [33] studied
                            running     parallel antoICS
                                                       thesystem
                                                              absorberincorporating
                                                                            surface, which  a stratification
                                                                                                   was only heated  plate running
                                                                                                                            over theparallel
                                                                                                                                         bottom
to
0.2the
     m.absorber
         It was found surface, thatwhich
                                     there was
                                             wasonly not aheated      over the
                                                              significant           bottom 0.2 in
                                                                               improvement           m.thermal
                                                                                                           It was found        that there
                                                                                                                      stratification,     withwasa
not  a significant
maximum                 improvement
              temperature                   in thermal
                                   difference     of 4.2 °Cstratification,
                                                               being achieved.    withAafollow-up
                                                                                             maximumstudy     temperature
                                                                                                                      by Swiatek  difference     of
                                                                                                                                       et al. [34]
4.2 ◦ C being achieved. A follow-up study by Swiatek et al. [34] found that a shorter stratification plate
found    that a shorter stratification plate (0.75 m compared to 0.9 m) in the 1.3 m rectangular system
(0.75  m compared
improved      thermalto        0.9 m) in thewith
                            stratification,       1.3 m   rectangulartemperature
                                                       a maximum              system improved difference thermal      stratification,
                                                                                                                of 20.1    °C. In contrastwithto  a
maximum                                                   ◦
Souza et al.temperature
                [33], Swiatek    difference
                                     et al. [34]of heated
                                                    20.1 C.aIn   0.2contrast
                                                                      m section  to Souza
                                                                                      in theetmiddle
                                                                                                  al. [33],ofSwiatek
                                                                                                               the tanketand al. [34]   heated
                                                                                                                                   reduced        a
                                                                                                                                                the
0.2  m section
channel    gap fromin the8 middle
                             mm to 5ofmm. the tank and reduced the channel gap from 8 mm to 5 mm.
      Ziapour
      Ziapour and AghamiriAghamiri [44] looked looked at the effect effect ofof the
                                                                                 the channel
                                                                                       channel width
                                                                                                   width on   on collector
                                                                                                                   collector efficiency
                                                                                                                                efficiency in  in a
two trapezoid
      trapezoid ICSSWH
                     ICSSWH system and found that a gap                 gap of of 66 mm
                                                                                      mm hadhad thethe highest
                                                                                                         highest performance.
                                                                                                                     performance. Kumar   Kumar
and Rosen
      Rosen [35]
               [35]studied
                      studieda arectangular
                                      rectangular      ICSSWH
                                                     ICSSWH        withwith     an insulating
                                                                           an insulating      baffle baffle
                                                                                                        plateplate      and single
                                                                                                                 and single             glazing.
                                                                                                                                 glazing.   They
They                                                                                     ◦
foundfound       that absorber
         that absorber                plate temperatures
                               plate temperatures          peaked   peaked
                                                                       at 63 °C at 63
                                                                                    during C during
                                                                                               the day   thethendaysuffered
                                                                                                                      then suffered
                                                                                                                                  heavy heavy
                                                                                                                                           losses
losses
duringduring        the night,
           the night,               dropping
                              dropping             by approximately
                                            by approximately               30 30 °C.◦ C.   However,ininthe
                                                                                        However,                the cylindrical
                                                                                                                      cylindrical ICSSWH
                                                                                                                                       ICSSWH
configuration presented by          by Smyth
                                         Smythet   etal.
                                                      al.[31,45,46]
                                                           [31,45,46]ititwas  wasfound
                                                                                     foundthatthat byby    perforating
                                                                                                        perforating       thethe   baffle
                                                                                                                                baffle     plate,
                                                                                                                                        plate,   to
to avoid
avoid   thethe
             heat heat   build-up
                     build-up          at the
                                    at the  toptop of of
                                                      thethe    channel
                                                            channel      that that  hinders
                                                                                hinders         flow,
                                                                                            flow,        night-time
                                                                                                     night-time           heat
                                                                                                                       heat       losses
                                                                                                                              losses       could
                                                                                                                                       could     be
be  reduced
reduced     byby 20% 20% asas   the
                              the    plateprevents
                                   plate     preventsreverse
                                                           reverseflow flowininthe theabsence
                                                                                         absence of  of aa heat source. The operating operating
principle
principle of of this
                this perforated
                       perforated bafflebaffleplate
                                                 plateisisillustrated
                                                            illustratedin    inFigure
                                                                                 Figure6.6.




       Figure      Operatingprinciple
       Figure 6. Operating     principleofofthe
                                              the heat
                                                heat    retaining
                                                     retaining       ICSSWH
                                                                ICSSWH           design
                                                                              design    during
                                                                                     during     collection
                                                                                            collection (left)(left) and
                                                                                                              and non-
       non-collection  (right)
       collection (right)      periods.
                          periods.      Adapted
                                   Adapted    fromfrom
                                                    SmythSmyth
                                                            et al.et[45].
                                                                      al. [45].
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, 1615                                                                                                        9 of 26



      Sokolov and Vaxman [47] and, in a follow-up study, Vaxman and Sokolov [48] assessed a triangular
      Energies 2018, 11, x FOR PEER REVIEW                                                                                 9 of 26
system with a baffle plate and achieved an efficiency of 53%. Smyth et al. [31] presented results on
an experimental
            Sokolov analysis
                         and Vaxmanof a 1.5
                                         [47]mand,
                                                cylindrical   ICS vessel,
                                                      in a follow-up    study,mounted
                                                                                  Vaxmanwithin      a reflector
                                                                                             and Sokolov      [48] cavity,
                                                                                                                    assessed witha the
upper   third heavily
      triangular    systeminsulated      andplate
                              with a baffle    a perforated     inner
                                                     and achieved    ansleeve    acting
                                                                        efficiency        as a Smyth
                                                                                      of 53%.   baffle. etThey    showed
                                                                                                            al. [31]         that 60%
                                                                                                                     presented
of theresults
        thermal on anenergy    stored within
                         experimental     analysisthe
                                                    of vessel
                                                        a 1.5 mcould    be retained
                                                                  cylindrical            overmounted
                                                                                ICS vessel,      a 16 h non-collection
                                                                                                            within a reflector  period.
      cavity,   with  the   upper  third  heavily  insulated   and  a perforated     inner  sleeve  acting
The most efficient configuration was the 1.5 m vessel, with the top third insulated, no insulated cover      as  a baffle. They
duringshowed     that 60% of the
         the non-charging            thermal
                                  phase,   andenergy    stored within
                                                a perforated             the vessel
                                                                 inner sleeve.        could be retained
                                                                                   It outperformed       theover
                                                                                                               samea 16  h non-
                                                                                                                      configuration
      collection   period.   The  most   efficient configuration    was  the  1.5 m   vessel, with
without the inner sleeve by 1.3% and the 1 m vessel with no insulated upper section and no inner    the  top  third  insulated,
      no insulated cover during the non-charging phase, and a perforated inner sleeve. It outperformed the
sleeve by almost 7%.
      same configuration without the inner sleeve by 1.3% and the 1 m vessel with no insulated upper
      El-Sebaii    [49] studied the thermal performance of a shallow solar-pond, a batch system similar
      section and no inner sleeve by almost 7%.
to ICSSWH,        with [49]
            El-Sebaii     an integrated
                              studied the baffle
                                            thermalplate.    Thermal
                                                       performance    of aperformance       was found
                                                                           shallow solar-pond,       a batchto system
                                                                                                                improve      with the
                                                                                                                        similar
inclusion    of  the  baffle   plate.  The   highest    daily  efficiencies,    up  to  64.3%,    were
      to ICSSWH, with an integrated baffle plate. Thermal performance was found to improve with the      achieved     using     a plate
without   vents    and    positioned    at approximately       two-thirds    of  the  total  height   of  the
      inclusion of the baffle plate. The highest daily efficiencies, up to 64.3%, were achieved using a plate  collector.   However,
whenwithout
        a bafflevents
                   plate andwithpositioned
                                  vents wasatused approximately     two-thirds
                                                       the performance             of dependent
                                                                             is less   the total height
                                                                                                     on itsofposition
                                                                                                                 the collector.
                                                                                                                          within the
pond.However,
        It was alsowhen     a baffle
                         shown       plate
                                  that  thewith  vents
                                              baffle     wasmaterial
                                                      plate   used the had
                                                                         performance     is lessimpact
                                                                               a negligible       dependent     on its position
                                                                                                           on water    temperature
      within the
throughout      the pond.
                     pond,Itranging
                                was alsofrom
                                           shown58 that  the
                                                    ◦ C to   baffle
                                                           59.1      plate
                                                                 ◦ C to 60.3material
                                                                               ◦ C for had
                                                                                        mica,a negligible
                                                                                                aluminium    impact   on water steel,
                                                                                                                and stainless
      temperature throughout the pond, ranging from 58 °C to 59.1 °C to 60.3 °C for mica, aluminium and
respectively. These results conformed to those previously reported by Kaushik et al. [41]. The baffle
      stainless steel, respectively. These results conformed to those previously reported by Kaushik et al.
plate also allowed the pond to retain hot water overnight, reaching temperatures of 71 ◦ C in the late
      [41]. The baffle plate also allowed the pond to retain hot            water overnight, reaching temperatures of
afternoon     and providing water at temperatures of 43 ◦ C in the early morning the following day.
      71 °C in the late afternoon and providing water at temperatures of 43 °C in the early morning the
      following day.
3.4. Fins
      3.4.1907,
      In    Fins Charles L. Haskell [50] patented a solar heater design incorporating ‘struts’ for improved
heat conduction
             In 1907, as  well as
                      Charles      structural
                                L. Haskell  [50]stability
                                                 patented(Figure       7). Since
                                                             a solar heater        then,
                                                                              design      ‘struts’, or‘struts’
                                                                                      incorporating     fins/extended      surfaces,
                                                                                                                for improved
are commonly        usedas
      heat conduction      inwell
                               commercial      evacuated
                                   as structural              tube and
                                                   stability (Figure       flat-plate
                                                                      7). Since         collectors,
                                                                                 then, ‘struts’,      protruding surfaces,
                                                                                                 or fins/extended     out from the
      are commonly
collector                used in commercial
             pipes to enhance       heat transferevacuated
                                                      to the tube    and flat-plate
                                                               working      fluid. Incollectors,
                                                                                       ICSSWHprotruding          out from
                                                                                                    systems, these      finsthe
                                                                                                                              can be
      collector   pipes  to enhance    heat  transfer  to the  working    fluid.  In ICSSWH      systems,
placed directly into the water body, i.e., storage tank, and extend throughout the full depth allowing      these  fins can  be
heat placed     directly intothrough
      to be transferred        the waterconduction
                                          body, i.e., storage
                                                          to thetank,  and
                                                                   areas      extend
                                                                           that  are throughout
                                                                                      not in closethe    full depth
                                                                                                      contact        allowing
                                                                                                                 to the  absorbing
      heat to be transferred through conduction to the areas that are not in close contact to the absorbing
surface. Youcef-Ali [51] conducted an optimisation study evaluating the impact of fin length and
      surface. Youcef-Ali [51] conducted an optimisation study evaluating the impact of fin length and
various    glazing types on thermal performance. The author studied a solar air collector with offset
      various glazing types on thermal performance. The author studied a solar air collector with offset
rectangular     plateplate
      rectangular      fin absorbers    andand
                            fin absorbers     found   that
                                                  found     they
                                                          that     generated
                                                                they  generateda ahigher
                                                                                     higherheater
                                                                                             heatertransfer
                                                                                                      transfer versus
                                                                                                                versus aa flat-
                                                                                                                          flat-plate
alone.plate
        Thealone.
               flat-plate  collector,collector,
                     The flat-plate     with double      glazing,
                                                  with double       produced
                                                                  glazing,         an efficiency
                                                                             produced              of 38%
                                                                                        an efficiency   of 38%against   64%
                                                                                                                  against 64% for the
offsetfor
       absorber
           the offsetplate  withplate
                      absorber     50 mmwithfins.
                                               50 mm fins.




             7. Solar
          Figure
      Figure           water
                  7. Solar    heater
                           water      incorporating
                                  heater incorporatingfins
                                                         fins patented  in1907
                                                              patented in   1907bybyCharles
                                                                                     Charles   L. Haskell.
                                                                                            L. Haskell.      Illustrated
                                                                                                        Illustrated is   is
          the mounted
      the mounted        collector
                     collector  andand  a crosssection
                                     a cross     section(A-B)
                                                         (A-B) to show
                                                                  show thetheinternal
                                                                              internalconfiguration. Adapted
                                                                                        configuration.           from from
                                                                                                           Adapted
          Haskell
      Haskell [50].[50].
Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Energies 2018, 11, x FOR PEER REVIEW                                                                                                          10 of 26
Energies 2018, 11, 1615                                                                                                                       10 of 26

       Gertzos and Caouris [52] optimised the arrangement of structural and functional parts in a flat-
 plateGertzos
        ICSSWHand      experimentally
                            Caouris [52]and         numerically.
                                               optimised                Their aim wasof
                                                                 the arrangement             tostructural
                                                                                                  eliminate the   andstagnation
                                                                                                                          functional   area  nearinthe
                                                                                                                                          parts        a
 geometrical
flat-plate        centre of
              ICSSWH             the stored water
                              experimentally         and body    and provide
                                                            numerically.             structural
                                                                                 Their    aim was   stability.    The authors
                                                                                                        to eliminate                 found thatarea
                                                                                                                            the stagnation          the
 presence
near         of fins had no
       the geometrical              effectofonthe
                                  centre         thestored
                                                       outletwater
                                                                 water bodytemperature        as the structural
                                                                                    and provide        increase instability.
                                                                                                                          heat transferThe rate    was
                                                                                                                                             authors
 insignificant     due    to  their   small   area,   which     was    dwarfed       by  that   of  the
found that the presence of fins had no effect on the outlet water temperature as the increase in heat    tank.    Li   and   Wu    [53]  studied    the
 effect of rate
transfer    extended        fins on improving
                   was insignificant          due toheattheirperformance
                                                                small area, which  in shell-tube      thermalby
                                                                                            was dwarfed             energy
                                                                                                                       that ofstorage
                                                                                                                                 the tank.unitsLi with
                                                                                                                                                   and
 phase   change      materials      (PCMs).     They    found     that  by    incorporating        fins
Wu [53] studied the effect of extended fins on improving heat performance in shell-tube thermal energy   into   the   tube   design    the  charging
 time could
storage   units bewith
                     shortened
                            phase by     up tomaterials
                                      change      20%, depending
                                                               (PCMs).on        the found
                                                                             They     PCM material         used.
                                                                                               that by incorporating           fins into the tube
       Work      conducted          by  Muneer        et  al.  [22],    which      was
design the charging time could be shortened by up to 20%, depending on the PCM material   later    ratified     by    Junaidi     [54], led    to the
                                                                                                                                            used.
 inclusion     and     optimisation        of  elongated        fins   inside     a  box-type       ICSSWH
      Work conducted by Muneer et al. [22], which was later ratified by Junaidi [54], led to the inclusion          to  improve       heat   transfer
 throughout
and   optimisationthe water       body. Junaidi
                          of elongated                [54] carried
                                             fins inside      a box-typeout simulations
                                                                               ICSSWH to improve on both aheat   finned     and un-finned
                                                                                                                       transfer    throughout    SWHthe
 and  determined         that    the  finned    collector     performed         significantly
water body. Junaidi [54] carried out simulations on both a finned and un-finned SWH and determined  better.   Currie      et al. [9]   analysed     the
 optimisation        of  fin   length    and   thickness       for   an   even     heat   distribution
that the finned collector performed significantly better. Currie et al. [9] analysed the optimisation of fin throughout         the    storage    tank
 withoutand
length      disrupting
                 thicknessflow    for patterns.
                                       an even heat The distribution
                                                           prototype subsequently
                                                                               throughout tested           by Garnier
                                                                                                 the storage                 [55] utilised
                                                                                                                     tank without              four 3
                                                                                                                                         disrupting
 mm-thick       aluminium          fins   mounted        vertically      in   a  square     shaped
flow patterns. The prototype subsequently tested by Garnier [55] utilised four 3 mm-thick aluminium      collector.      These      were    found     to
 improve
fins  mounted bothvertically
                       the thermal   in aperformance
                                           square shaped       and    structural
                                                                  collector.     Thesestrength
                                                                                          were found of theto  ICSSWH
                                                                                                                  improve(Figure
                                                                                                                               both the   8).thermal
                                                                                                                                              A 13%
 increase   in   heat    transfer     was   found,     as   a  direct   result     of  the  fin
performance and structural strength of the ICSSWH (Figure 8). A 13% increase in heat transfer wasinstallation.        Mohsen      and    Akash     [56]
 also  studied      the   performance         of  a   box-type      ICSSWH          with   extended
found, as a direct result of the fin installation. Mohsen and Akash [56] also studied the performance of   heat     transfer   fins.    They   found
athat the useICSSWH
   box-type        of fins can  withimprove
                                       extended  theheat
                                                       cumulative         efficiency
                                                             transfer fins.      They of     the system
                                                                                         found      that theby   use 9%,
                                                                                                                       of during
                                                                                                                           fins canthe     monththe
                                                                                                                                       improve        of
 November        in  Jordan.
cumulative efficiency of the system by 9%, during the month of November in Jordan.
       Following aa numerical
      Following            numerical study study by  by Chaabane
                                                          Chaabane et     et al.
                                                                               al. [57],
                                                                                   [57], the
                                                                                          the thermal
                                                                                                thermal performance
                                                                                                             performance of        of aa cylindrical
                                                                                                                                         cylindrical
 ICSSWH, mounted
ICSSWH,        mountedinina aCPC,     CPC,was  was    improved
                                                   improved        bybythethe     inclusion
                                                                             inclusion          of rectangular
                                                                                           of rectangular          radialradial
                                                                                                                            fins.fins.
                                                                                                                                    This This
                                                                                                                                           workwork
                                                                                                                                                  was
based on, and validated by, the experimental data presented by Chaouachi and Gabsi [58]. [58].
 was  based     on,    and    validated     by,   the   experimental         data    presented       by   Chaouachi         and    Gabsi           The
                                                                                                                                             The fins
 fins  create     a   modified        surface     that    enhanced         the    convective        heat
create a modified surface that enhanced the convective heat transfer due to their higher characteristic     transfer       due    to   their   higher
 characteristic
length.               length.inThis
          This resulted                  resulted in asword
                                     a double-edged           double-edged
                                                                       with higher   swordwater with    higher water
                                                                                                    temperatures          andtemperatures
                                                                                                                               reduced thermal     and
 reduced     thermal       losses    during    the    charging      period      but   higher     thermal
losses during the charging period but higher thermal losses during non-collection periods. The authors        losses     during     non-collection
 periods.
also  noted  The     authors also
                a significant             noted a between
                                     correlation       significant  fin correlation
                                                                          length and between
                                                                                           average water  fin length        and average
                                                                                                                     temperature,               water
                                                                                                                                         convective
 temperature,        convective        heat   transfer,     and    heat     retention.     Three     fin
heat transfer, and heat retention. Three fin lengths were modelled, 15, 30 and 45 mm, and in all cases    lengths       were   modelled,        15,   30
 and  45  mm,      and    in  all  cases   the  45   mm     fins  produced         the  best   results
the 45 mm fins produced the best results during day-time operation. As the fin depth increases so         during       day-time      operation.      As
 the fin
does   thedepth     increases
            availability        of so
                                    hotdoes   the temperatures
                                         water;     availability ofover   hot water;
                                                                                 50 ◦ C temperatures
                                                                                          were maintained       overfor 506.5
                                                                                                                            °C were
                                                                                                                               h andmaintained
                                                                                                                                          more than
 for 6.5  h  and     more     than    10  h  for  a  fin  length     of   15   mm    and    45
10 h for a fin length of 15 mm and 45 mm, respectively. During night-time operation, the thermalmm,      respectively.        During      night-time
                                                                                                                                                   loss
 operation, is
coefficient     the    thermal
                   higher           losscases
                               for the    coefficient
                                                 with fins,is higher
                                                                 with afor      the increase
                                                                            slight    cases with      fins,coefficient
                                                                                                  in the      with a slight  withincrease
                                                                                                                                     increasing in the
                                                                                                                                                     fin
 coefficient    with     increasing       fin  depth.     However,         the   higher    temperatures
depth. However, the higher temperatures and longer heat retention gained during the day means a                    and    longer    heat   retention
 gainedoverall
better     duringperformance
                        the day means          a better
                                         for the    finned  overall
                                                               ICS over performance
                                                                             un-finned.for    Thethe    finned
                                                                                                     author          ICS over
                                                                                                                 suggests          un-finned.
                                                                                                                              a night              The
                                                                                                                                          insulation
 authortosuggests
cover       combat the   a night     insulation
                               radiative             cover to combat the radiative heat losses.
                                            heat losses.




                   Figure 8.
                   Figure 8. Exploded view
                                      view of the ICSSWH
                                                  ICSSWH studied.
                                                         studied. Adapted from Garnier
                                                                               Garnier et
                                                                                       et al.
                                                                                          al. [55].
                                                                                              [55].

3.5. Glazing
     Solar water heaters generally include a glazed aperture over the absorber area with an optimised
air gap to suppress upward convection and minimise heat losses, whilst maintaining a high
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