Humidification-Dehumidification (HDH) Desalination System with Air-Cooling Condenser and Cellulose Evaporative Pad - MDPI

water
Article
Humidification–Dehumidification (HDH)
Desalination System with Air-Cooling Condenser and
Cellulose Evaporative Pad
Li Xu * , Yan-Ping Chen, Po-Hsien Wu and Bin-Juine Huang
 Mechanical Engineering Department, National Taiwan University, 708 Engineering Building, No.1 Section 4
 Roosevelt Rd., Taipei 106, Taiwan; r05522117@ntu.edu.tw (Y.-P.C.); god583god@hotmail.com (P.-H.W.);
 bjhuang38@gmail.com (B.-J.H.)
 * Correspondence: lixus@ntu.edu.tw
 



 Received: 27 October 2019; Accepted: 30 December 2019; Published: 2 January 2020 


 Abstract: This paper presents a humidification–dehumidification (HDH) desalination system with an
 air-cooling condenser. Seawater in copper tubes is usually used in a condenser, but it has shown the
 drawbacks of pipe erosion, high cost of the copper material, etc. If air could be used as the cooling
 medium, it could not only avoid the above drawbacks but also allow much more flexible structure
 design of condensers, although the challenge is whether the air-cooing condenser can provide as much
 cooling capability as water cooling condensers. There is no previous work that uses air as cooling
 medium in a condenser of a HDH desalination system to the best of our knowledge. In this paper we
 designed a unique air-cooling condenser that was composed of closely packed hollow polycarbonate
 (PC) boards. The structure was designed to create large surface area of 13.5 m2 with the volume
 of only 0.1 m3 . The 0.2 mm thin thickness of the material helped to reduce the thermal resistance
 between the warm humid air and cooling air. A fan was used to suck the ambient air in and out of the
 condenser as an open system to the environment. Results show that the air-cooling condenser could
 provide high cooling capability to produce fresh water efficiently. Meanwhile, cellulous pad material
 was used in the humidifier to enhance the evaporative process. A maximum productivity of 129
 kg/day was achieved using the humidifier with a 0.0525 m3 cellulous pad with a water temperature
 of 49.5 ◦ C. The maximum gained output ratio (GOR) was 0.53, and the maximum coefficient of
 performance (COP) was 20.7 for waste heat recovery. It was found that the system performance was
 compromised as the ambient temperature increased due to the increased temperature of cooling air;
 however, such an effect could be compensated by increasing the volume of the condenser.

 Keywords: desalinization; humidification; dehumidification; air cooling; cellulose

1. Introduction
 Fresh water is very important for human life regarding development in industry, agriculture
and livelihood. The scarcity of fresh water in some countries and areas has driven the research and
development in desalination in recent decades. Various types of technology have been developed
for desalination, such as multi-stage flash (MSF), multi-effect desalination (MED), reverse osmosis
(RO), and humidification and de-humidification (HDH) [1]. Some of these technologies have been
commercialized at large scales; however, there are the drawbacks such as high capital cost and high
operation cost [2]. HDH systems are advantageous in many ways, such as flexible scales, simple
layout, low temperature operation, low capital cost and operation cost, and could be combined with
renewable energy resources such as solar energy [3,4].
 Many research groups have presented their design and results of HDH desalination systems in
recent decades. The performance of these system depends on fluid properties and conditions, including

Water 2020, 12, 142; doi:10.3390/w12010142 www.mdpi.com/journal/water

Water 2020, 12, 142 2 of 14

flow rate and temperature, and also relate to the configuration and materials of either humidifier or
dehumidifier. W.F. He et al. [5] presented the analysis of an open-air open-water HDH system with a
packing bed dehumidifier. Results showed that higher humidifier and dehumidifier effectiveness can
benefit the thermodynamic performance and production cost. Ahmed et al. [6] proposed an analytical
and numerical scheme for thermodynamically balanced HDH systems. Ghazi et al. [7] evaluated the
characteristics of the HDH desalination process as a function of operating conditions such as air flow
rate and cooling water temperature. They reported a water production increase upon the increase in the
air flow rate and decrease in the cooling water temperature. Zehui Chang et al. [8] presented a novel
multi-effect solar desalination system based on HDH and used a porous balls humidifier to increase
the contact surface area. They showed that the multi-effect solar HDH desalination system could be
potentially improved through the optimization of its design and operation. Abdelrahman et al. [9]
assessed a cross-flow HDH system experimentally. They found that the productivity greatly depended
on either the hot water temperature or the water flow rate, and there existed an optimal condition
for the performance of their system in different cases. Hossam A. Ahmed et al. [10] presented an
experimental study with packed aluminum sheets as the material in the humidifier, and showed
results such as humidify efficiency, enthalpy effectiveness and performance coefficient. They found
that the air temperature at the humidifier inlet has a smaller effect on productivity compared to the hot
water temperature. A.E. Kabeel et al. [11] combined the HDH desalination system and a solar dryer.
The latter provided additional vapor to the HDH system while drying food. E.H. Amer et al. [12]
obtained experimental and theoretical results in different materials of humidifier. Their mathematical
model was formulated by applying mass and energy balance on a control-volume to find the maximal
productivity among different packing materials. G. Prakash Narayan et al. [13] presented the simulation
results for different types of HDH system, such as CWOA (closed-water, open-air) HDH, CAOW
(closed-air, open-water) HDH and others. They used the second law of thermodynamics to analyze the
optimal conditions of each system. G. Prakash Narayan et al. [14] also presented analysis to obtain the
optimization of effectiveness and other measures. Dahiru Lawal et al. [15] presented simulation results
by adopting a heat pump in a HDH system. Their results showed that the performance of the HDH
system could be significantly increased by using a heat pump. W.F. He et al. [16] also studied a HDH
system with a heat pump. The simulation results showed that the best performance as 82.12 kg/h for
the water production and 5.14 for the GOR.
 The cooling medium in the dehumidifier in these studies was usually sea water. Sea water in
heat exchangers can cause problems, such as the erosion of parts from the high salt content and pipe
blocking from impurities in the sea water. Copper material is usually used for the cooling system,
which increases the cost of the system. Therefore, there is high motivation to use an alternative cooling
medium that can also provide a sufficient cooling capability. In this paper we proposed a unique
air-cooling condenser design that can efficiently cool down the warm humid air and produce the fresh
water efficiently.
 Using air as the cooling medium in the condenser of a HDH system is challenging and no previous
work has been reported to the authors’ knowledge. Air has a much lower mass density, lower heater
capacity and lower thermal conductivity than water. Several means have been applied in the design
to enhance the heat transfer rate, including maximizing the surface area of heat transfer, using thin
thickness of material to reduce thermal resistance between the cooling air and the warm humid air, etc.
The condenser is an open system to the environment and a fan was used to suck the ambient air into
the condenser and the heated air to the ambient without using any pipes. The flow rate of the cooling
air can be increased independently to increase the heat transfer rate. There was no copper material or
any other metal materials in the condenser, and therefore the total system cost was reduced.
 Another feature of our system is the use of a cellulose pad as the filling material in the humidifier.
Most conventional humidifiers use plastics as the filling material. The surface of plastic material is
usually hydrophobic, which means the wettability of the water on the plastic surface is poor. Water
forms streams instead of films on the plastic surface, and thus the contact area between the air and

bound cardboard structure and of a cross-fluted type. The capillary force results in the strong
 absorption of water into the cellulose pad, which makes the cellulose pad a water dispersing device
 once saturated. Such design greatly increases the contact area between air and water and can generate
 vapor very efficiently [17,18].
 The12,
Water 2020, rest
 142 of the paper is organized as follows.—Section 2 describes the experimental setup, 3 of 14
 Section 3 contains the thermodynamic analysis, Section 4 presents the results and discussion, and
 finally Section 5 is a short conclusion.
water is much less than the surface area of the plastic fillings. Therefore, a cellulose pad is used as
an alternative filling
 2. Experiment Setupmaterial in the humidifier. The cellulose pad is designed as a cellulose-bound
cardboard structure and of a cross-fluted type. The capillary force results in the strong absorption of
waterOurinto humidifier–dehumidifier (HDH)the
 the cellulose pad, which makes system waspad
 cellulose a closed-water-open-air (CWOA)
 a water dispersing device oncesystem and
 saturated.
 the schematic
Such diagram
 design greatly of the experiment
 increases the contact setup is shownair
 area between inand
 Figure 1. The
 water andstorage tank contained
 can generate vapor veryhot
 water that was
efficiently [17,18]. heated by an electric heater. Hot water was pumped into a tank on top of the
 humidifier
 The restand a big
 of the chuck
 paper of spongeaswas
 is organized placed at the exit
 follows.—Section of the tank
 2 describes to help evenly
 the experimental distribute
 setup, Sectionhot
 3
 water throughout the humidifier. The humidifier contained a cellulose pad with a large surface
contains the thermodynamic analysis, Section 4 presents the results and discussion, and finally Section 5 area
isina direct contact with the hot water, which generated vapor efficiently. The rejected water was
 short conclusion.
 collected at the bottom tank and cycled back to the storage tank by another pump.
2. Experiment Setup air was carried to the dehumidifier through an air duct under the drive of the
 The hot humid
 blower located at the dehumidifier side. Cooling air from the ambient was driven by a fan located at
 Our humidifier–dehumidifier (HDH) system was a closed-water-open-air (CWOA) system and
 the top of the dehumidifier and the flow direction was from the bottom to the top of the dehumidifier.
the schematic diagram of the experiment setup is shown in Figure 1. The storage tank contained
 The dehumidifier was composed of closely packed hollow polycarbonate (PC) boards that allowed
hot water that was heated by an electric heater. Hot water was pumped into a tank on top of the
 humid air to flow between the PC boards while allowing the cooling air to flow inside the channels
humidifier and a big chuck of sponge was placed at the exit of the tank to help evenly distribute hot
 of hollow PC boards in a cross direction. The vapor in the humid air condensed on the walls of the
water throughout the humidifier. The humidifier contained a cellulose pad with a large surface area in
 PC boards and was collected by the container underneath the dehumidifier. After passing the
direct contact with the hot water, which generated vapor efficiently. The rejected water was collected
 dehumidifier, the cooled humid air flew out of the system as an exhaust.
at the bottom tank and cycled back to the storage tank by another pump.

 Figure 1. Schematic diagram of experiment setup of the air-cooling humidifier–dehumidifier
 Figure 1. Schematic diagram of experiment setup of the air-cooling humidifier–dehumidifier (HDH)
 (HDH) system.
 system.
 The hot humid air was carried to the dehumidifier through an air duct under the drive of the
blower located at the dehumidifier side. Cooling air from the ambient was driven by a fan located at
the top of the dehumidifier and the flow direction was from the bottom to the top of the dehumidifier.
The dehumidifier was composed of closely packed hollow polycarbonate (PC) boards that allowed
humid air to flow between the PC boards while allowing the cooling air to flow inside the channels
of hollow PC boards in a cross direction. The vapor in the humid air condensed on the walls of
the PC boards and was collected by the container underneath the dehumidifier. After passing the
dehumidifier, the cooled humid air flew out of the system as an exhaust.
 A detailed description of the experimental setup is as follows. Three electric heaters with 2 kW
each were used to heat the water in the storage tank. A control system was employed to keep the

A detailed description of the experimental setup is as follows. Three electric heaters with 2 kW
each were used to heat the water in the storage tank. A control system was employed to keep the
water temperature constant at the experiment conditions and kept the amount of water in the storage
tank constant with feed-in water. The hot water temperature was limited to 50 °C, as the expected
Water 2020, 12, 142 4 of 14
temperature of heated water by the photovoltaic thermal (PVT) collector was about in this range. The
power of the two pumps that cycled the hot water were 50 W and 70 W, respectively.
waterThetemperature
 size of theconstant
 celluloseatpad
 the experiment conditions
 in the humidifier, and kept
 as shown the amount
 in Figure of water
 2a, was 50 cmininthe storage
 height, 35
tank constant with feed-in water. The hot water temperature was limited to 50 ◦ C, as the expected
cm in width and 30 cm in length, with a volume of 0.0525 m . The size of the condenser made of
 3

temperature of heated(PC)
hollow polycarbonate water by the
 boards photovoltaic
 in the dehumidifier thermal (PVT)50collector
 was about was about
 cm in height, 40 cmin in this
 widthrange.
 and
The
50 cmpower of thewith
 in length, twoapumps
 volumethat cycled
 of 0.1 the hollow
 m3. The hot water PCwere
 boards 50 contains
 W and 70channels
 W, respectively.
 with a cross section
 The ×size
of 6 mm 6 mm,of the
 as cellulose
 shown inpad in the
 Figure 2b.humidifier,
 Twenty-eight as shown
 pieces in of Figure
 PC board 2a, were
 was 50 cm in height,
 assembled 35 cm
 in parallel
in width and 30 cm inoflength, 3
with an even spacing 5 mmwith a volume
 by fixture andof 0.0525asmshown
 epoxy, . The in
 size of the2c.
 Figure condenser
 The totalmade
 surfaceof contact
 hollow
polycarbonate
area was about(PC) 13.5boards
 m2. Theinsystem
 the dehumidifier
 was designed wassuch
 about 50 the
 that cm in
 hotheight,
 humid40air cm in width
 flowed and 50the
 between cmPC in
length, 3
boards,with
 while a the
 volume of 0.1
 cooling airmflowed
 . The through
 hollow PC theboards
 channelscontains
 insidechannels with a in
 the PC boards cross
 the section of 6 mm
 cross direction,
× 6 mm, direct
without as shown in Figure
 contact between 2b. the
 Twenty-eight
 two fluids.pieces of PC board were assembled in parallel with an
even The
 spacing of 5 mm and
 temperature by fixture
 humidity and were
 epoxy,measured
 as shownby in aFigure
 T-type2c.thermocouple
 The total surface
 and contact area was
 psychrometer at
about 2
 13.5points
 m . The system was designed such that theathot
multiple across the section of the air duct and thehumid
 exit of air
 theflowed betweenThe
 dehumidifier. theair
 PCvelocity
 boards,
while the cooling
was measured byair
 an flowed through
 anemometer. Thetheflow
 channels
 rate ofinside the PC
 the water wasboards in theby
 measured cross direction, without
 a measuring cup.
directAcontact between the two fluids.
 photograph of the actual system was shown in Figure 3.

 Figure 2. (a) Photo of the cellulose pad in humidifier; (b) Photo of a piece of hollow polycarbonate (PC)
 Figure 2. (a) Photo of the cellulose pad in humidifier; (b) Photo of a piece of hollow polycarbonate
 board with the internal spacing of 6 mm by 6 mm; (c) 3D schematic structure of a stack of PC boards
 (PC) board with the internal spacing of 6 mm by 6 mm; (c) 3D schematic structure of a stack of PC
 assembled with a spacing of 5 mm by fixture and epoxy.
 boards assembled with a spacing of 5 mm by fixture and epoxy.
 The temperature and humidity were measured by a T-type thermocouple and psychrometer at
multiple points across the section of the air duct and at the exit of the dehumidifier. The air velocity
was measured by an anemometer. The flow rate of the water was measured by a measuring cup.
 A photograph of the actual system was shown in Figure 3.

3. Thermodynamic Analysis
 Several parameters were used to evaluate the performance of our HDH desalination system.
The applied principles of thermodynamics are mass and energy conservation and the entropy increase
in each process. A diagram for thermodynamic analysis was shown in Figure 4.
 The mass ratio (MR) is a ratio between the hot water mass rate and the air mass rate as follows:
 mw,i
 MR = (1)
 ma

where ma is the mass rate of air and mw,i is the mass rate of water in the humidifier inlet.

Water 2020, 12, 142 5 of 14
Water 2020, 12, x FOR PEER REVIEW 5 of 14

 Figure 3. Photograph of the experiment setup.

3. Thermodynamic Analysis
 Several parameters were used to evaluate the performance of our HDH desalination system. The
applied principles of thermodynamics are mass and energy conservation and the entropy increase in
each process. A diagram for thermodynamic analysis was shown in Figure 4.
 Figure 3. Photograph
 Figure 3. Photograph of
 of the
 the experiment
 experiment setup.
 setup.

3. Thermodynamic Analysis
 Several parameters were used to evaluate the performance of our HDH desalination system. The
applied principles of thermodynamics are mass and energy conservation and the entropy increase in
each process. A diagram for thermodynamic analysis was shown in Figure 4.

 Figure 4. Diagram
 Figure 4. Diagram of
 of the
 the air-cooling
 air-cooling HDH
 HDH system
 system for
 for thermodynamic
 thermodynamic analysis.
 analysis.

 The
 The gained output
 mass ratio (MR)ratio
 is a(GOR) is commonly
 ratio between used
 the hot to evaluate
 water theand
 mass rate performance of rate
 the air mass a HDH system.
 as follows:
It is defined as the ratio of the latent heat of generated fresh water over the input heat Qin (while the
 ,
work input of pumps and fans is negligible) as =
 follows (1)
 
where is the mass rate of air and , is the mass md hrate
 f g of water in the humidifier inlet.
 GOR = (2)
 The gained Figure
 output 4. Diagram of the
 ratio (GOR) isair-cooling
 commonlyHDH usedQsystem for thermodynamic
 in evaluate
 to the performance analysis.
 of a HDH system.
It is defined as the ratio of the latent heat of generated fresh water over the input heat (while the
whereThe md mass
 is theratio
 mass(MR)
 rate of produced water the
 in dehumidification, h f g and
 is latent heat of water and Qin is
work input of pumps andisfans
 a ratio between
 is negligible) hot water mass rate
 as follows the air mass rate as follows:
heat input of this system. Qin is evaluated with the energy , balance of the water storage tank as
 = ℎ (1)
 =  (2)
 Qin = mw,i ·hw,i − mw,o ·hw,o + me ·h f ,0 (3)
where is the mass rate of air and , is the mass rate of water in the humidifier inlet.
where is the mass rate of produced water in dehumidification, ℎ is latent heat of water and
whereThe mw,igained
 and moutput
 w,o are the
 ratiohot waterismass
 (GOR) flow rate
 commonly usedat totheevaluate
 inlet andthe outlet of humidifier,
 performance respectively,
 of a HDH system.
 is heat input of this system. is evaluated with the energy balance of the water storage tank
It is hdefined
and w,i and h areratio
 asw,othe the enthalpy of the
 of the latent hot
 heat ofwater at thefresh
 generated inlet water
 and outlet
 over of thethe heat respectively,
 humidifier,
 input (while the
as
m e is the
work evaporation
 input of pumpsrate andin humidifier
 fans which
 is negligible) asequals
 follows to the feed water rate at steady state condition,
and h f ,0 is the enthalpy of the feed water at ambient temperature. ℎ
 = (2)
 
where is the mass rate of produced water in dehumidification, ℎ is latent heat of water and
 is heat input of this system. is evaluated with the energy balance of the water storage tank
as

Water 2020, 12, 142 6 of 14

 The recovery ratio (RR) is a ratio of the mass rate of condensed water in the dehumidifier over the
mass rate of evaporated water. RR is defined as
 md
 RR = (4)
 me

where me is the mass rate of evaporated water in the humidifier and calculated by mass conservation as

 me = ma (wa,2 − wa,1 ) (5)

where wa,1 is the absolute humidity at the inlet of the humidifier and wa,2 is the absolute humidity at
the outlet of the humidifier.
 The coefficient of performance (COP) is a coefficient to obtain performance of a system, typically
used in cooling systems such as heat pumps. It is defined as

 md h f g
 COP = (6)
 Win

where Win is the total power consumption of this system, excluding Qin .
 To evaluate the performance of our system, we defined the effectiveness of the HDH system.
The commonly used effectiveness is diverse in the simultaneous heat and mass exchanger or heat
exchanger. We considered that the performance of HDH greatly depends on the fluid property
described as enthalpy, so the definition of effectiveness for HDH is as [14]:

 ∆H
 ε= (7)
 ∆Hmax

where ∆H is the actual enthalpy change of either water steam or air stream, and ∆Hmax is the maximum
possible enthalpy change.
 The above Equation is based on mass and energy conservation, so the effectiveness is the ratio
of real enthalpy change over maximum possible enthalpy change, which could be achieved in either
stream. We name the enthalpy of any stream(x) in ideal state hx,ideal . Thus, the humidifier effectiveness
(εh ) could be defined as the following [13]:
  
  ma (ha,2 − ha,1 ) mw,i hw,i − mw,o hw,o 
 
 εh = max
  
 , (8)
 
 
 m h − m h
 
  ma h
 a,2,ideal − ha,1 w,o
  
 w,i w,i w,o,ideal 

 The max symbol is to make sure that the definition is not against the second law of thermodynamics.
Similarly, the dehumidifier effectiveness (εd ) could be defined as
  
  ma (ha,2 − ha,1 ) mw,i hw,i − mw,o hw,o 
 
 εh = max
  
 , (9)
 
 
 m h − m h
 
  ma h
 a,2,ideal − ha,1 w,o
  
 w,i w,i w,o,ideal 

 T +T
  
where the temperature of h f is assumed to be a,2 2 a,3 and md h f is calculated when the air
 ideal
temperature at the dehumidifier outlet (Ta,3,ideal ) is set to be the ambient temperature.

4. Results and Discussion
 Table 1 lists some selected previous work with similar configuration of HDH systems to compare
with ours. The hot water temperature in our system is set at 40 to 50 ◦ C to simulate hot water from
a photovoltaic thermal (PVT) collector. Taking into account the size difference of the lab systems,
a calculation was done by setting the feed water flow rate to be the same and modifying the productivity
proportionally. At the temperature range of 40–50 ◦ C results showed that the productivity of our
system was the highest among the listed previous work in Table 1.

Water 2020, 12, x FOR PEER REVIEW 7 of 14

productivity proportionally. At the temperature range of 40–50 °C results showed that the
productivity of our system was the highest among the listed previous work in Table 1.
Water 2020, 12, 142 7 of 14

 Table 1. Selected previous work on HDH systems.
 Table 1. Selected previous work on HDH systems.
 Top Temperature Feed Water Carrying Air Flow Cooling Medium Max
 Reference
 of Hot
 TopWater Flow Rate Rate and Temperature Productivity
 Feed Water Carrying Air Cooling Medium Max
 Temperature 0.02 kg/s 0.0014~0.0028 kg/s Water cooling; 10~20 Reference
 35~45 °C Flow Rate Flow Rate3 and Temperature 7.6 kg/day
 Productivity [7]
 of Hot Water (75 kg/hour) (5~10 nm /hour) °C
 0.085~0.115
 0.02 kg/s 0.0014~0.0028 kg/s cooling;19 °C 26.4 kg/day (1.1
 Watercooling;
 35.5~50
 35~45 ◦ C°C 0.045~0.068 kg/s Water 7.6 kg/hour)
 kg/day [19]
 [7]
 kg/s
 (75 kg/h) (5~10 nm /h)
 3 10~20 ◦ C
 0.005~0.045
 0.085~0.115 Water cooling; 24~33 26.4 kg/day
 25.94~36.75
 35.5~50 ◦ C °C 0.0049~0.0294
 0.045~0.068 kg/s kg/s Water cooling; ◦ 10.25 kg/day [20]
 kg/s kg/s °C 19 C (1.1 kg/h)
 [19]
 0.005~0.045 Water
 Water cooling;
 cooling;
 25.94~36.75 ◦ 0.012~0.023 0.0049~0.0294 kg/s 34.8 kg/day
 44.6~68.9 °CC kg/skg/s 0.040~0.043 kg/s 24~33 ◦ C not
 Temperature 10.25 kg/day [20]
 [21]
 (1.45 kg/hour)
 Waterspecified
 cooling;
 0.012~0.023 34.8 kg/day
 44.6~68.9 ◦ C 0.040~0.043 kg/s Water cooling;
 Temperature [21]
 60~75 °C 0.2 kg/s
 kg/s 0.177 kg/s 92 kg/h)
 (1.45 kg/day [9]
 not specified
 30+/−2 °C
 Water cooling;
 Air cooling; 16~27 Current
 45~50◦ C
 60~75 °C 0.2 0.16
 kg/skg/s 0.177 kg/skg/s
 0.036 92129
 kg/day
 kg/day [9]
 30+/−2°C◦ C work
 45~50 ◦ C 0.16 kg/s 0.036 kg/s Air cooling; 16~27 ◦ C 129 kg/day Current work
4.1. Part 1. Effect of Mass Ratio
 4.1. Part 1. Effect of Mass Ratio
 The amount of fresh water produced per day, or productivity, is an important measure of a HDH
desalination system.
 The amount The water
 of fresh correlation between
 produced productivity
 per day, and the
 or productivity, is mass ratio of measure
 an important hot water ofand
 a HDHair
(MR) is shown
 desalination in Figure
 system. The5.correlation
 Productivity, measured
 between by the produced
 productivity and the massfreshratio
 water, increased
 of hot as MR
 water and air
increased, which was attributed to the increasing amount of vapor carried by the air at higher water
 (MR) is shown in Figure 5. Productivity, measured by the produced fresh water, increased as MR
flow rates. which
 increased, As the was
 water flow rateto
 attributed increased, the contact
 the increasing amount area
 of between the hot
 vapor carried bywater and
 the air the cellulose
 at higher water
pad
 flowincreased, since
 rates. As the a larger
 water flowvolume of the cellulose
 rate increased, pad area
 the contact was between
 soaked. Itthe
 is also shownand
 hot water in Figure 5 that
 the cellulose
apad
 higher water since
 increased, temperature
 a larger improved
 volume of the productivity, as the
 cellulose pad was evaporation
 soaked. rate
 It is also increased
 shown at higher
 in Figure 5 that
water
 a highertemperatures. The increase
 water temperature in productivity
 improved was more
 the productivity, as significant at higher
 the evaporation ratewater temperatures,
 increased at higher
as indicated
 water by the slope
 temperatures. The of the trend
 increase lines.
 in productivity was more significant at higher water temperatures,
 Table 2 lists
 as indicated by thetheslope
 experiment data lines.
 of the trend for some example runs.

 Figure 5. Productivity as a function of mass ratio (MR).
 Figure 5. Productivity as a function of mass ratio (MR).
 Table 2 lists the experiment data for some example runs.

Water 2020, 12, 142 8 of 14
Water 2020, 12, x FOR PEER REVIEW 8 of 14

 Table 2. Experiment data of some example runs with varying MR.
 Table 2. Experiment data of some example runs with varying MR.
 Hot Water Water
 Water Mass Air Air Mass Produced Ambient Ambient
 Run # Hot
 Run
 Water
 Temperature Rate mw ,i Mass
 Mass Rate ma MR Water md GOR Ambient
 Produced Temperature Ambient
 Relative
 Temperature
 (◦ C) MR Water m GOR Temperature T0
 T 0 (◦ C) Relative
 Rate(kg/s) Rate m(kg/s) (kg/day) Humidity
 d
 # mw,i a
 (°C) (kg/day) (°C) Humidity
 1 40.1 (kg/s)
 0.14 (kg/s)0.038 3.7 72 0.45 16.0 86%
 12 49.6
 40.1 0.14
 0.14 0.038 0.0373.7 3.8 72 122 0.45 0.52 16.0 15.0 91%
 86%
 23 50.5
 49.6 0.10
 0.14 0.037 0.0413.8 2.5 122 115 0.52 0.53 15.0 14.7 81%
 91%
 34 49.5
 50.5 0.19
 0.10 0.041 0.0422.5 4.5 115 129 0.53 0.54 14.7 15.0 81%
 81%
 4 49.5 0.19 0.042 4.5 129 0.54 15.0 81%

 The measured average temperature at the outlet of the humidifier Ta,2 increased as MR increased,
 The measured average temperature at the outlet of the humidifier Ta,2 increased as MR increased,
as shown in the left y-axis of Figure 6. The absolute humidity wa,2 in the air duct, as shown in the
as shown in the left y-axis of Figure 6. The absolute humidity wa,2 in the air duct, as shown in the right
right y-axis of Figure 6, represented the amount of the vapor generated by the humidifier. It was
y-axis of Figure 6, represented the amount of the vapor generated by the humidifier. It was expressed
expressed as vapor mass over air mass and was calculated from the measured temperature and relative
as vapor mass over air mass and was calculated from the measured temperature and relative
humidity across the air duct at the outlet of the humidifier. The results in Figure 6 show a positive
humidity across the air duct at the outlet of the humidifier. The results in Figure 6 show a positive
correlation between the generated vapor and the mass ratio, i.e., the higher the mass ratio, the higher
correlation between the generated vapor and the mass ratio, i.e., the higher the mass ratio, the higher
the temperature and the absolute humidity at the exit of the humidifier.
the temperature and the absolute humidity at the exit of the humidifier.

 Figure 6. Left y-axis shows the average air temperature in the air duct Ta,2 as a function of MR;
 Figure 6. Left y-axis shows the average air temperature in the air duct , as a function of MR; Right
 Right y-axis shows the absolute humidity in the air duct (wa,2 ) as a function of MR. The data were
 y-axis shows the absolute humidity in the air duct ( , ) as a function of MR. The data were calculated
 calculated based on the measured temperature and relative humidity in the air duct and averaged
 based on the measured temperature and relative humidity in the air duct and averaged across the
 across the section.
 section.
 The gained output ratio (GOR), as an important measure of HDH system performance, varied
with The gained
 the mass output
 ratio, ratio (GOR),
 as shown as an
 in Figure 7. important measure
 A higher mass ratioofresulted
 HDH system performance,
 in a higher varied
 GOR. A higher
with the mass ratio, as shown in Figure 7. A higher mass ratio resulted in a higher GOR.
water temperature also improved GOR. The results show that the recovery ratio (RR) was relatively A higher
water temperature
constant also improved
 as MR varied, as shown in GOR. The8.results
 Figure show
 In other that the
 words, the recovery
 capabilityratio (RR) was the
 to transform relatively
 vapor
constant as MR varied, as shown in Figure 8. In other words, the capability to transform the vapor
into water in the condenser was constant. The same portion of the vapor could be extracted in the
humidifier, in spite of the amount of vapor carried by the air. This was probably attributed to the

Water 2020, 12, 142 9 of 14
Water 2020, 12, x FOR PEER REVIEW 9 of 14

 into water
high cooling
Water 2020,
 in the condenser
 capability
 12, x FOR
 wascondenser,
 of the
 PEER REVIEW
 constant. The
 andsame
 it wasportion of the vapor
 evidenced by thecould be extracted
 constant in the
 dehumidifier
 9 of 14
 humidifier, in spite of the amount of vapor carried
effectiveness which will be discussed in a later section. by the air. This was probably attributed to the high
 cooling
high capability
 cooling of the condenser,
 capability and it was
 of the condenser, andevidenced by the constant
 it was evidenced dehumidifier
 by the effectiveness
 constant dehumidifier
which will be discussed in a later section.
effectiveness which will be discussed in a later section.

 Figure 7. Gained output ratio (GOR) as a function of MR.
 Figure 7. Gained output ratio (GOR) as a function of MR.
 Figure 7. Gained output ratio (GOR) as a function of MR.

 Figure 8. Recovery ratio (RR) as a function of MR.
 Figure 8. Recovery ratio (RR) as a function of MR.
 The humidifier effectiveness εh and dehumidifier effectiveness εd , as defined in Equations (8) and (9),
 varied with MR as shown in Figure 9. The humidifier effectiveness εh was positively correlated with MR.
 The humidifier effectiveness Recovery
 Figure 8. and dehumidifier
 ratio (RR) aseffectiveness , as defined in Equations (8)
 a function of MR.
 A higher mass rate of hot water resulted in a larger contact area, which raised the air temperature and
and (9), varied with MR as shown in Figure 9. The humidifier effectiveness was positively
 The humidifier
correlated with MR. effectiveness
 A higher mass rate
 andofdehumidifier effectiveness
 hot water resulted , as
 in a larger defined
 contact in Equations
 area, (8)
 which raised
andair
the (9),temperature
 varied withand
 MRtheasabsolute
 shown humidity
 in Figure as
 9. the
 Thehot
 humidifier
 water mixedeffectiveness was positively
 with the incoming air in the
correlated with MR. A higher mass rate of hot water resulted in a larger contact area, which raised
the air temperature and the absolute humidity as the hot water mixed with the incoming air in the

Water 2020, 12, 142 10 of 14

Water 2020, 12, x FOR PEER REVIEW 10 of 14
the absolute humidity as the hot water mixed with the incoming air in the humidifier. The dehumidifier
humidifier.
effectivenessTheεd did
 dehumidifier much change with
 not show effectiveness didMR.
 notThe
 show muchwas
 reason change with MR.
 probably The the
 because reason was
 cooling
probably
capabilitybecause the cooling
 of the condenser wascapability ofto
 sufficient the condenser
 condense thewas
 vaporsufficient to the
 under all condense the vapor
 experiment under
 conditions.
all the
This experiment
 may conditions.
 be attributed This may
 to the efficient beexchange
 heat attributed to the efficient
 between heat exchange
 the hot humid between
 air and the the
 cooling airhot
 in
humid air and
the unique the of
 design cooling air in the unique design of the condenser.
 the condenser.

 Figure 9. Humidifier effectiveness εh and dehumidifier effectiveness εd as a function of MR.
 Figure 9. Humidifier effectiveness and dehumidifier effectiveness as a function of MR.
 One of the reasons we think that the air-cooling condenser could work is that the ambient air
as the
 Onecooling
 of themedium
 reasons wehasthink
 no energy cost
 that the and the system
 air-cooling can could
 condenser take as much
 work air as
 is that theitambient
 needs toaircool
 as
down the warm humid air, while a water-cooling condenser needs to match the flow
the cooling medium has no energy cost and the system can take as much air as it needs to cool down rate of water in
the condenser
the warm humid with that
 air, in the
 while humidifier, and
 a water-cooling limits how
 condenser fast the
 needs cooling
 to match thewater
 flow(or preheated
 rate of water water)
 in the
could flow. In other words water-cooling condensers recycle the heat, but compromise
condenser with that in the humidifier, and limits how fast the cooling water (or preheated water) on the cooling
capability
could flow.ofInthe condenser.
 other The air-cooling
 words water-cooling condenser
 condensers worked
 recycle more
 the heat, butindependently,
 compromise onand theitcooling
 could
condense the
capability water
 of the vapor efficiently
 condenser. with a large
 The air-cooling surface worked
 condenser area. more independently, and it could
condense the water vapor efficiently with a large surface area.
4.2. Part 2. Effect of Power Consumption on COP
4.2. Part
 The2.coefficient
 Effect of Power Consumption(COP)
 of performance on COP is an important measure to evaluate how much useful
thermal energy we can obtain provided
 The coefficient of performance (COP) is the amount of workmeasure
 an important required.toInevaluate
 our case,how it was calculated
 much useful
as the ratio of the latent heat of fresh water over the work of the whole system,
thermal energy we can obtain provided the amount of work required. In our case, it was calculated including pumps,
fans
as theetc., but
 ratio ofexcluding Qin for
 the latent heat the application
 of fresh water overof waste
 the workheat recovery.
 of the The maximal
 whole system, includingCOP of 20.7fans
 pumps, has
been achieved with our system. The correlation between the COP and the total
etc., but excluding for the application of waste heat recovery. The maximal COP of 20.7 has been power consumption (Pt )
is shown in Figure 10. The power consumption of the
achieved with our system. The correlation between the COP and the totalhumidifier fan (P ) and dehumidifier
 H power consumption (PtDH fan (P ) is)
varied
shown to in find
 Figure the10.
 maximum
 The power COP, while the power
 consumption of the consumption
 humidifier fanof(Pthe rest of the system (pumps)
 H) and dehumidifier fan (PDH)
was kept constant. The humidifier fan was the blower that delivered
varied to find the maximum COP, while the power consumption of the rest of the system the vapor-carrying air (pumps)
 through
the system
was and the The
 kept constant. dehumidifier
 humidifierfan fanwaswasthe
 thefan that delivered
 blower the cooling
 that delivered air into the dehumidifier.
 the vapor-carrying air through
The left half of Figure 10 shows that the COP was positively
the system and the dehumidifier fan was the fan that delivered the cooling air correlated to PDHintowhen PH was kept
 the dehumidifier.
constant.
The This
 left half of was probably
 Figure 10 showsbecause the COP
 that the higher wasPDH resulted correlated
 positively in a highertocooling
 PDH when capability
 PH wasofkept the
dehumidifier
constant. Thisand wasincreased
 probablythe pure water
 because production
 the higher rate significantly.
 PDH resulted in a higherThe right capability
 cooling half of Figure 10
 of the
shows that the COP was negatively correlative with P
dehumidifier and increased the pure water production Hrate significantly. when P DH was kept constant. The reason
 The right half of Figure 10
was
shows that the COP was negatively correlative with PH when PDH was keptmuch,
 probably that a larger P H did not increase the fresh water production constant.but The
 the total
 reasonpower
 was
consumption
probably thatincreased,
 a larger Presulting a lower COP. The data of some experimental runs are presented in
 H did not increase the fresh water production much, but the total power
Table 3.
consumption increased, resulting a lower COP. The data of some experimental runs are presented in
Table 3.

Water 2020, 12, 142 11 of 14
 Water 2020, 12, x FOR PEER REVIEW 11 of 14

 Figure
 Figure 10.10.The
 Thecoefficient
 coefficient of
 of performance
 performance (COP)
 (COP)ofofthe system
 the systemversus the the
 versus totaltotal
 power consumption
 power of
 consumption
 of the
 the system.
 system.TheThepower
 powerconsumption
 consumptionofofthe
 thehumidifier
 humidifier fan and
 fan de-humidifier
 and de-humidifier fanfan
 is specified as as
 is specified a a
 secondary axis. Pure water production is also shown as a reference.
 secondary axis. Pure water production is also shown as a reference.

 Table3.3.Experiment
 Table Experimentdata
 dataof
 of some
 some example runswith
 example runs withvarying
 varyingpower
 powerconsumption.
 consumption.

 HotHot
 Water Water
 Water Produced
 Produced Blower
 Blower Fan
 Fan
 Power
 Powerof of Total
 Total Power
 Power
 Run Water MassRate Two
 Run # Temp. Mass Water
 Water mdmd Power
 Power PH Power
 Power Two Consumption GOR
 Consumption GORCOPCOP
 # (Temp.
 ◦ C) mRate mw,i Pumps
 w ,i (kg/s) (L/h)
 (L/h) PH (W)
 (W) PPDH
 DH (W)
 (W) Pumps (W) (W)
 (W)
 (°C) (kg/s) (W)
 1 1 48.9
 48.9 0.19
 0.19 3.53
 3.53 44
 44 47
 47 4040 131131 0.50 18.0
 0.50 18.0
 2 2 49.0
 49.0 0.19
 0.19 3.33
 3.33 33
 33 47
 47 4040 120120 0.51 18.6
 0.51 18.6
 3 3 49.1
 49.1 0.19
 0.19 3.33
 3.33 21
 21 47
 47 4040 108108 0.50 20.7
 0.50 20.7
 4 4 49.1
 49.1 0.19
 0.19 2.74
 2.74 21
 21 35
 35 4040 96 96 0.46
 0.46 19.1
 19.1
 5 5 49.0
 49.0 0.19
 0.19 0.60.6 21
 21 24
 24 4040 85 85 0.11
 0.11 4.84.8

 Another measure of the system performance is the performance ratio (PR), recently developed
 Another
 using measure
 an exergy of the
 approach bysystem
 K. C. Ng performance
 et al. [22]. Theis the performance
 calculated result ofratio
 PR (PR),
 for our recently
 system developed
 is about
using an exergy
 9.8, based approach
 on the Equationby Ng, in
 given K.C.,
 the et al. [22]. The calculated result of PR for our system is about
 article.
9.8, based on the Equation given in the article.
 4.3. Part 3. Effect of Ambient Temperature
4.3. Part 3. Effect of Ambient Temperature
 We did experiments at different ambient temperatures and found that the ambient temperature
 We did experiments
 influenced at different in
 the system performance ambient
 two main temperatures
 aspects. One and
 wasfound that theair
 the carrying ambient temperature
 temperature and
influenced
 the other was the cooling air temperature. The two effects influenced the system performance inthe
 the system performance in two main aspects. One was the carrying air temperature and
other was the
 opposite cooling
 ways. air temperature.
 The former one resulted Thein atwo effects
 larger amountinfluenced
 of vapor the system performance
 generation in opposite
 in the humidifier and
ways. The temperature
 a higher former one resulted in a of
 at the outlet larger amount ofasvapor
 the humidifier generation
 the ambient in the humidifier
 temperature increased.andThe alatter
 higher
 one, on theatother
temperature hand, of
 the outlet resulted in a lower
 the humidifier ascooling
 the ambientcapability of cooling
 temperature air in theThe
 increased. dehumidifier
 latter one, and
 on the
 hence lower productivity as the ambient temperature increased. The net effect
other hand, resulted in a lower cooling capability of cooling air in the dehumidifier and hence lower of the two factors could
 be evaluated
productivity by the
 as the correlation
 ambient of GOR
 temperature and ambient
 increased. The nettemperature.
 effect of theAstwoshown
 factorsin could
 Figurebe11, GOR
 evaluated
bydecreased as the ambient
 the correlation of GOR and temperature
 ambientincreased,
 temperature. implying that theinlatter
 As shown Figureeffect
 11, was
 GORmore significant
 decreased as the
ambient temperature increased, implying that the latter effect was more significant than the formerinone.
 than the former one. Although the results suggest that it is more desirable to operate the system
 cool weather
Although or at night,
 the results suggest thethat
 system
 it is performance
 more desirable could be compensated
 to operate the system orin
 improved by increasing
 cool weather or at night,
 the size of the dehumidifier, more specifically by adding more PC boards
the system performance could be compensated or improved by increasing the size of the dehumidifier, to further increase the
 surface area of the condenser.
more specifically by adding more PC boards to further increase the surface area of the condenser.

Water 2020, 12, 142 12 of 14
 Water 2020, 12, x FOR PEER REVIEW 12 of 14

 Figure11.11.Effect
 Figure Effectofofambient
 ambienttemperature T0Ton
 temperature GOR.
 0 on GOR.

4.4. Part 4. Cost Comparison
 4.4. Part 4. Cost Comparison
 The cost comparison includes two aspects—the material cost and manufacturing cost of
 The cost comparison includes two aspects—the material cost and manufacturing cost of the air-
the air-cooling condenser and a water-cooling condenser with the same volume. The former
 cooling condenser and a water-cooling condenser with the same volume. The former used the
used the polycarbonate boards which are readily available as greenhouse covers on the market.
 polycarbonate boards which are readily available as greenhouse covers on the market. The
The manufacturing process was to cut the boards and stack them together with epoxy. The latter used
 manufacturing process was to cut the boards and stack them together with epoxy. The latter used
copper material which is commonly seen in other HDH systems. The manufacturing process involves
 copper material which is commonly seen in other HDH systems. The manufacturing process involves
piping, welding, and screw mounting. The total cost of the air-cooling condenser was about 100 USD,
 piping, welding, and screw mounting. The total cost of the air-cooling condenser was about 100 USD,
which is about half of the cost of the water-cooling condenser.
 which is about half of the cost of the water-cooling condenser.
5. Conclusions
 5. Conclusions
 We have presented a HDH desalination system with an air-cooling condenser and a cellulose
 We have
evaporative pad.presented
 A maximum a HDH desalination
 productivity of 129system
 kg/day,with
 GOR anofair-cooling
 0.53 and COP condenser and aachieved.
 of 20.7 were cellulose
 evaporative
 The main pad. A maximum
 features of our system productivity of 129
 are (1) efficient air kg/day, GOR ofhollow
 cooling through 0.53 and COP of 20.7
 polycarbonate were
 boards
 achieved.
(better than traditional water cooling); (2) the use of the cellulous pad as a humidifier (more efficient
and moreThe compact);
 main features
 (3) highof our system are
 productivity per(1) efficient
 unit volumeairofcooling through
 dehumidifier; (4)hollow
 high COP polycarbonate
 for waste
 boards (better than traditional water cooling);
heat recovery; (5) lower cost of the condenser. (2) the use of the cellulous pad as a humidifier (more
 efficient and more compact); (3) high productivity per unit volume of dehumidifier;
 It was found that productivity, GOR and humidifier effectiveness increased as MR increased (4) high COP for
 waste heat recovery; (5) lower cost of the condenser.
and water temperature increased, while dehumidifier effectiveness did not vary much with MR or
 It was found The
water temperature. thatsystem
 productivity, GOR was
 performance and compromised
 humidifier effectiveness
 as the ambientincreased as MRincreased,
 temperature increased
 and water temperature increased, while dehumidifier effectiveness did not vary
due to the increased temperature of the cooling air; however, such an effect could be compensated much with MRbyor
 water temperature.
increasing the volume of Thethesystem performance
 condenser, or specificallywasbycompromised
 adding more PC as boards
 the ambient temperature
 to further increase
 increased, due to the increased
the surface area of the condenser. temperature of the cooling air; however, such an effect could be
 compensated by increasing the volume of the condenser, or specifically by adding more PC boards
Author Contributions:
 to further Conceptualization
 increase the surface area ofandtheinvestigation:
 condenser. L.X. and B.-J.H.; Experiments setup and tests: Y.-P.C.
and P.-H.W.; Writing: L.X.; Funding acquisition: L.X. All authors have read and agreed to the published version of
 Author
the Contributions: Conceptualization and investigation: L.X. and B.-J.H.; Experiments setup and tests: Y.-
 manuscript.
Funding: P.-H.W.;
 P.C. and FundingWriting:
 of thisL.X.;
 workFunding acquisition:
 is provided L.X.Research and Development Division of National
 by the
Taiwan University.
 Funding: Funding of this work is provided by the Research and Development Division of National Taiwan
 University.

Water 2020, 12, 142 13 of 14

Acknowledgments: We would like to sincerely thank Yi-Fei Chen and Jung-Fu Yeh’s suggestions on the
experiment setup.
Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature
Symbol Physical Quantity Unit
hf Enthalpy of distilled water kJ/kg
hfg Latent heat of water in vapor(liquid) kJ/kg
ha,1 Enthalpy of air at inlet of humidifier kJ/kg
ha,2 Enthalpy of air at outlet of humidifier kJ/kg
ha,3 Enthalpy of air at outlet of dehumidifier kJ/kg
hc,i Enthalpy of cooling air at inlet of dehumidifier kJ/kg
hc,o Enthalpy of cooling air at outlet of dehumidifier kJ/kg
h f ,0 Enthalpy of water at ambient temperature kJ/kg
hw,i Enthalpy of water at inlet of humidifier kJ/kg
hw,o Enthalpy of water at outlet of humidifier kJ/kg
hx,ideal Enthalpy of any stream(x) in ideal state kJ/kg
ma Mass rate of air kg/s
mc Mass rate of cooling air kg/s
md Produced fresh water rate kg/day
me Evaporation rate in humidifier kg/s
mw,i Mass rate of hot water at inlet of humidifier kg/s
mw,o Mass rate of water at outlet of humidifier kg/s
Qin Heat input to heat the water kW
T0 Ambient temperature ◦C

Ta,1 Air temperature at inlet of humidifier ◦C

Ta,2 Air temperature in the air duct ◦C

Tw Water temperature ◦C

wa,1 Absolute humidity at inlet of the humidifier kg vapor/kg dry air
wa,2 Absolute humidity in the air duct kg vapor/kg dry air
Win Total power consumption of the system excluding heaters kW
εh Humidifier effectiveness -
εd Dehumidifier effectiveness -

References
1. Wakil, M.; Burhan, M.; Ang, L.; Choon, K. Energy-water-environment nexus underpinning future desalination
 sustainability. Desalination 2017, 413, 52–64.
2. Al-Karaghouli, A.; Kazmerski, L.L. Energy consumption and water production cost of conventional and
 renewable-energy-powered desalination processes. Renew. Sustain. Energy Rev. 2013, 24, 343–356. [CrossRef]
3. Sharon, H.; Reddy, K.S. A review of solar energy driven desalination technologies. Renew. Sustain. Energy Rev.
 2015, 41, 1080–1118. [CrossRef]
4. Kasaeian, A.; Babaei, S.; Jahanpanah, M.; Sarrafha, H.; Alsagri, A.S.; Ghaffarian, S.; Yan, W.-M. Solar
 humidification-dehumidification desalination systems: A critical review. Energy Convers. Manag. 2019, 201,
 112129. [CrossRef]
5. He, W.F.; Chen, J.J.; Han, D.; Luo, L.T.; Wang, X.C.; Zhang, Q.Y.; Yao, S.Y. Energetic, entropic and economic
 analysis of an open-air, open-water humidification dehumidification desalination system with a packing bed
 dehumidifier. Energy Convers. Manag. 2019, 199, 112016. [CrossRef]
6. Ahmed, M.A.; Qasem, N.A.A.; Zubair, S.M. Analytical and numerical schemes for thermodynamically
 balanced humidification-dehumidification desalination systems. Energy Convers. Manag. 2019, 200, 112052.
 [CrossRef]
7. Al-enezi, G.; Ettouney, H.; Fawzy, N. Low temperature humidification dehumidification desalination process.
 Energy Convers. Manag. 2006, 47, 470–484. [CrossRef]

Water 2020, 12, 142 14 of 14

8. Chang, Z.; Zheng, H.; Yang, Y.; Su, Y.; Duan, Z. Experimental investigation of a novel multi-effect solar
 desalination system based on humidi fi cation e dehumidi fi cation process. Renew. Energy 2014, 69, 253–259.
 [CrossRef]
9. Aburub, A.; Aliyu, M.; Lawal, D.U.; Antar, M.A. Experimental investigations of a cross-flow humdification
 dehumidification desalination system. Int. Water Technol. J. 2017, 7, 198–208.
10. Ahmed, H.A.; Ismail, I.M.; Saleh, W.F.; Ahmed, M. Experimental investigation of
 humidification-dehumidification desalination system with corrugated packing in the humidi fier.
 Desalination 2017, 410, 19–29. [CrossRef]
11. Kabeel, A.E.; Abdelgaied, M. Experimental evaluation of a two-stage indirect solar dryer with reheating
 coupled with HDH desalination system for remote areas. Desalination 2018, 425, 22–29. [CrossRef]
12. Amer, E.H.; Kotb, H.; Mostafa, G.H. Theoretical and experimental investigation of humidification–
 dehumidification desalination unit. Desalination 2009, 249, 949–959. [CrossRef]
13. Narayan, G.; Sharqawy, M.H.; Lienhard V, J.H.; Zubair, S.M. Thermodynamic analysis of humidifi cation
 dehumidifi cation desalination cycles. Desalin. Water Treat. 2010, 16, 339–353. [CrossRef]
14. Narayan, G.; Mistry, K.H.; Sharqawy, M.H.; Zubair, S.M.; Lienhard, J.H. Energy effectiveness of simultaneous
 heat and mass exchange devices. Front. Heat Mass Transf. 2010, 1, 023001. [CrossRef]
15. Lawal, D.; Antar, M.A.; Khalifa, A.; Zubair, S.M.; Al-Sulaiman, F. Humidification-dehumidification
 desalination system operated by a heat pump. Energy Convers. Manag. 2017, 161, 128–140. [CrossRef]
16. He, W.F.; Han, D.; Ji, C. Investigation on humidification dehumidification desalination system coupled with
 heat pump. Desalination 2018, 436, 152–160. [CrossRef]
17. Hu, S.S.; Huang, B.J. Study of a high efficiency residential split water-cooled air conditioner. Appl. Therm. Eng.
 2005, 25, 1599–1613. [CrossRef]
18. Hou, T.; Hsieh, Y.; Lin, T.; Chuang, Y.; Huang, B. Cellulose-pad water cooling system with cold storage.
 Int. J. Refrig. 2016, 69, 383–393. [CrossRef]
19. Yamal, C.; Solmus, I. A solar desalination system using humidification—dehumidification process:
 Experimental study and comparison with the theoretical results. Desalination 2008, 220, 538–551. [CrossRef]
20. Nafey, A.S.; Fath, H.E.S.; El-Helaby, S.O.; Soliman, A. Solar desalination using
 humidification—dehumidification Part II. An experimental investigation. Energy Convers. Manag.
 2004, 45, 1263–1277. [CrossRef]
21. Hermosillo, J.; Arancibia-bulnes, C.A.; Estrada, C.A. Water desalination by air humidification: Mathematical
 model and experimental study. Sol. Energy 2012, 86, 1070–1076. [CrossRef]
22. Ng, K.C.; Shahzad, M.W.; Son, H.S.; Hamed, O.A. An exergy approach to efficiency evaluation of desalination.
 Appl. Phys. Lett. 2017, 110, 184101. [CrossRef]

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