Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Page content transcription ( If your browser does not render page correctly, please read the page content below )
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; firstname.lastname@example.org 2 Resource Efficient Built Environment Lab (REBEL), Edinburgh Napier University, Edinburgh EH10 5DT, UK; email@example.com 3 Scottish Energy Centre, Edinburgh Napier University, Edinburgh EH10 5DT, UK; firstname.lastname@example.org * Correspondence: email@example.com; 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 , 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 . 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 . 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% . 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
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 . . 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 . 1970s . 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 . They collect advantages relative to cost both .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.
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;  2006 ICSSWH water heaters phase change material (PCM); Reflectors; Selective absorber surfaces A comprehensive review on solar Additional insulation; Collector material;  2011 Thermo-syphon water heaters Glazing; Selective absorber surfaces Review of solar water heaters Additional insulation; Baffle Plates;  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;  2013 their market trends Direct/indirect circulation; Air Reflectors Flat-plate; Evacuated tube; Additional insulation; Baffle plates; Recent advances in the solar water  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;  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  2014 on energy and environmental PV/T; PCM surfaces aspects Building integrated solar thermal  2015 PV/T & solar thermal General overview of current technologies collectors—A review Integrated collector storage solar Additional insulation; Baffle plate/inner  2015 water heaters: Survey and recent ICSSWH sleeve; PCM; Thermal diodes developments Performance enhancement of Flat-plate; PV/T; Evacuated tube;  2015 Nanofluids; Selective absorber surfaces solar collectors–A review Compound parabolic Innovation in flat solar thermal Additional insulation; Collector material;  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;  2016 Auxiliary immersion heating; Nanofluids for solar energy applications Concentrating; ICSSWH Recent developments in Additional insulation; Baffle plate/inner  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.  present the most comprehensive review of the technical aspects of ICSSWH systems. Kumar and Rosen  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 . 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
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. . Figure 2. Illustration of the heat transfer processes in an ICSSWH. Adapted from Smyth et al. . 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 , 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 , 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
Energies 2018, 11, 1615 5 of 26 Energies 2018, 11, x FOR PEER REVIEW 5 of 26 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. illustrated in Figure 3 . 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 .of the different TIMs in terms of incident solar irradiance. Adapted from Kaushika and Sumathy . Kaushika and Sumathy  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  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 . 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 .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  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  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%  without thethat, determined TIM. for Reddy and Kaushikaabsorber a single-tube  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  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  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  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  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  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  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.  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.  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
Energies 2018, 11, 1615 6 of 26 top third heavily insulated. Unlike Tripanagnostopoulos et al. , 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.  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. , 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.  and However, theseSwiatek results are et al.  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.  studied tank. the Chaabane effect et al.  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.  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.  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  a maximum and temperature Swiatek et al.  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.  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.  a maximum temperature different of 20.1 C . 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.  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  undertook temperature different of 20.1a °C comparative . 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  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 .
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 . 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  tested the thermal performance of an ICSSWH with as Scotland. Garnier  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 . household in 2007. Adapted from Garnier . 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  summarise thermal panels are unfeasible. Sarbu and Sebarchievici  summarise the major studies surrounding the major studies surrounding SCS SCS in the in the literature. Drückliterature. and Hahne Drück and  Hahne  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. 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 . 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 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  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.
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  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.  collector that and systems storage less tankthanthe 100depthmmshould deep were be greater at greater than riskthis. Schmidt of freezing andand to Goetzberger  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  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.  studied an ICS system incorporating a water to be drawn up into the channel. Souza stratification et al. plate  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.  4.2 ◦ C being achieved. A follow-up study by Swiatek et al.  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 , Swiatek difference et al. of heated 20.1 C.aIn 0.2contrast m section to Souza in theetmiddle al. ,ofSwiatek the tanketand al.  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  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  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. al. .
Energies 2018, 11, 1615 9 of 26 Sokolov and Vaxman  and, in a follow-up study, Vaxman and Sokolov  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.  presented results on an experimental Sokolov analysis and Vaxmanof a 1.5 mand, cylindrical ICS vessel, in a follow-up study,mounted Vaxmanwithin a reflector and Sokolov  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.  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  studied the thermal performance of a shallow solar-pond, a batch system similar section and no inner sleeve by almost 7%. to ICSSWH, with  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. . 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 . 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  patented a solar heater design incorporating ‘struts’ for improved heat conduction In 1907, as well as Charles structural L. Haskell 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  conducted an optimisation study evaluating the impact of fin length and surface. Youcef-Ali  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 ..
Energies 2018, 11, x FOR PEER REVIEW 10 of 26 Energies 2018, 11, 1615 10 of 26 Gertzos and Caouris  optimised the arrangement of structural and functional parts in a flat- plateGertzos ICSSWHand experimentally Caouris 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  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  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. , which was design the charging time could be shortened by up to 20%, depending on the PCM material later ratified by Junaidi , led to the used. inclusion and optimisation of elongated fins inside a box-type ICSSWH Work conducted by Muneer et al. , which was later ratified by Junaidi , led to the inclusion to improve heat transfer throughout and optimisationthe water body. Junaidi of elongated  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  carried out simulations on both a finned and un-finned SWH and determined better. Currie et al.  analysed the optimisation of fin length and thickness for an even heat distribution that the finned collector performed significantly better. Currie et al.  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  utilised tank without four 3 disrupting mm-thick aluminium fins mounted vertically in a square shaped flow patterns. The prototype subsequently tested by Garnier  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  also studied the performance of a box-type ICSSWH with extended found, as a direct result of the fin installation. Mohsen and Akash  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. , , 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 . . 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. . . 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
Pages you may also like
You can also read
Next part ... Cancel