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

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; [email protected] (Y.-P.C.); [email protected] (P.-H.W.); [email protected] (B.-J.H.) * Correspondence: [email protected]

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 Water 2020, 12, x FOR PEER REVIEW 3 of 14

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 water is much less than the surface area of the plastic fillings. Therefore, a cellulose pad is used as an alternative filling material 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 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]. Water 2020The, 12rest, 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 an2. Experiment alternative filling Setup material 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 waterOur into humidifier–dehumidifier the cellulose pad, which makes(HDH) the system cellulose was pada closed-water-open-air a water dispersing device (CWOA) once system saturated. and Suchthe schematic design greatly diagram increases of the experiment the contact setup area between is shown air in andFigure water 1. The and st canorage generate tank contained vapor very hot ewaterfficiently that [17 was,18]. heated by an electric heater. Hot water was pumped into a tank on top of the humidifierThe rest and of the a big paper chuck is organized of sponge as was follows.—Section placed at the 2exit describes of the tank the experimental to help evenly setup, distribute Section hot3 containswater throughout the thermodynamic the humidifier. analysis, The Section humidifier4 presents contained the results a cellulose and discussion, pad with and a large finally surface Section area5 isin a direct short conclusion.contact with the hot water, which generated vapor efficiently. The rejected water was collected at the bottom tank and cycled back to the storage tank by another pump. 2. ExperimentThe hot humid Setup 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 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 Figure1. 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 Water 2020, 12, x FOR PEER REVIEW 4 of 14

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. waterThe temperature size of the constant cellulose at pad the experimentin the humidifier, conditions as shown and kept in Figure the amount 2a, was of water50 cm inin the height, storage 35 tankcm in constant width and with 30 feed-in cm in water.length, Thewith hot a volume water temperature of 0.0525 m was3. The limited size of to the 50 condenser◦C, as the expectedmade of temperaturehollow polycarbonate of heated (PC) water boards by the in the photovoltaic dehumidifier thermal was about (PVT) 50 collector cm in height, was about 40 cm inin thiswidth range. and The50 cm power in length, of the with two a pumps volume that of 0.1 cycled m3. The the hollow hot water PC were boards 50 contains W and 70 channels W, respectively. with a cross section of 6 mmThe × size 6 mm, of the as celluloseshown in pad Figure in the 2b. humidifier, Twenty-eight as shownpieces inof FigurePC board2a, waswere 50 assembled cm in height, in parallel 35 cm inwith width an even and 30spacing cm in of length, 5 mm withby fixture a volume and ofepoxy, 0.0525 as m shown3. The in size Figure of the 2c. condenser The total made surface of contact hollow polycarbonatearea was about (PC) 13.5 boardsm2. The in system the dehumidifier was designed was such about that 50 th cme inhot height, humid 40 air cm flowed in width between and 50 the cm PC in length,boards, withwhile a the volume cooling of 0.1 air m flowed3. The through hollow PC the boards channe containsls inside channels the PC boards with a in cross the sectioncross direction, of 6 mm without6 mm, direct as shown contact in Figure between2b. Twenty-eightthe two fluids. pieces of PC board were assembled in parallel with an × evenThe spacing temperature of 5 mm and by fixture humidity and were epoxy, measured as shown by in a Figure T-type2c. thermocouple The total surface and contactpsychrometer area was at aboutmultiple 13.5 points m2. The across system the section was designed of the air such duct that an thed at hot the humid exit of airthe flowed dehumidifier. between The the air PC velocity boards, whilewas measured the cooling by airan flowedanemometer. through The the flow channels rate of inside the water the PC was boards measured in the by cross a measuring direction, withoutcup. directA contact photograph between of the the actual two fluids. 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 Figure3.

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 Figure4. 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, x FOR PEER REVIEW 5 of 14

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. FigureFigure 3.3. Photograph ofof thethe experimentexperiment setup.setup.

Figure 4.4. Diagram of the air-cooling HDH system for thermodynamic analysis.analysis.

TheThe gainedmass ratio output (MR ratio) is a ( GORratio) between is commonly the hot used water to evaluate mass rate the and performance the air mass of rate a HDH as follows: system. It is defined as the ratio of the latent heat of generated fresh water over the input heat Qin (while the 𝑚, work input of pumps and fans is negligible) as𝑀𝑅 follows = (1) 𝑚 where 𝑚 is the mass rate of air and 𝑚 is the massmdh ratef g of water in the humidifier inlet. , GOR = (2) The gainedFigure output 4. Diagram ratio (GOR of the) is air-cooling commonly HDH usedQ system into evaluate for thermodynamic 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 𝑄 (while the where m is the mass rate of produced water in dehumidification, h is latent heat of water and Q is workThe inputd mass of pumpsratio (MR and) is fans a ratio is negligible) between the as followshot water mass ratef g and the air mass rate as follows:in heat input of this system. Q is evaluated with the energy balance of the water storage tank as in 𝑚, 𝑀𝑅 =𝑚ℎ (1) 𝐺𝑂𝑅 = 𝑚 (2) Q = m h mw,𝑄o hw,o + me h (3) in w,i· w,i − · · f ,0 where 𝑚 is the mass rate of air and 𝑚, is the mass rate of water in the humidifier inlet. where 𝑚 is the mass rate of produced water in dehumidification, ℎ is latent heat of water and whereThemw gained,i and m outputw,o are theratio hot (GOR water) is masscommonly flow rate used at to the evaluate inlet and the outlet performance of humidifier, of a HDH respectively, system. 𝑄 is heat input of this system. 𝑄 is evaluated with the energy balance of the water storage tank andIt is hdefinedw,i and hasw ,theo are ratio the enthalpyof the latent of the heat hot ofwater generated at the fresh inlet water and outlet over ofthe the input humidifier, heat 𝑄 respectively, (while the as mworke is the input evaporation of pumps rateand infans humidifier is negligible) which as equals follows to the feed water rate at steady state condition, and h f ,0 is the enthalpy of the feed water at ambient temperature. 𝑚ℎ 𝐺𝑂𝑅 = (2) 𝑄 where 𝑚 is the mass rate of produced water in dehumidification, ℎ is latent heat of water and 𝑄 is heat input of this system. 𝑄 is evaluated with the energy balance of the water storage tank as

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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 m RR = d (4) me where me is the mass rate of evaporated water in the humidifier and calculated by mass conservation as

me = ma(wa w ) (5) ,2 − a,1 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

mdh 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,ihw,i mw,ohw,o  ε = max − , −  (8) h   ma h h mw,ihw,i mw,ohw,o,ideal  a,2,ideal − a,1 − 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,ihw,i mw,ohw,o  ε = max − , −  (9) h   ma h h mw,ihw,i mw,ohw,o,ideal  a,2,ideal − a,1 − T +T where the temperature of h is assumed to be a,2 a,3 and m h is calculated when the air f 2 d f ideal temperature at the dehumidifier outlet (Ta,3,ideal) is set to be the ambient temperature.

4. Results and Discussion Table1 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 Table1. Water 2020, 12, x FOR PEER REVIEW 7 of 14 productivity proportionally. At the temperature range of 40–50 °C results showed that the productivityWater 2020, 12, 142of our system was the highest among the listed previous work in Table 1. 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 HotTop Water 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 Rate and Temperature Productivity7.6 kg/day [7] of Hot Water (75 kg/hour) (5~10 nm3/hour) °C 0.085~0.115 26.4 kg/day (1.1 35.5~50 °C 0.02 kg/s 0.0014~0.00280.045~0.068 kg kg/s/s WaterWater cooling; cooling; 19 °C [19] 35~45 ◦C kg/s 3 7.6kg/hour) kg/day [7] (75 kg/h) (5~10 nm /h) 10~20 ◦C 0.085~0.1150.005~0.045 Water cooling; 24~33 26.4 kg/day 25.94~36.7535.5~50 C °C 0.045~0.0680.0049~0.0294 kg/s kg/s Water cooling; 19 C 10.25 kg/day [19 [20]] ◦ kg/skg/s °C ◦ (1.1 kg/h) Water cooling; 0.005~0.0450.012~0.023 Water cooling; 34.8 kg/day 25.94~36.7544.6~68.9 °C◦C 0.0049~0.02940.040~0.043 kg kg/s/s Temperature not 10.25 kg/day [20[21]] kg/skg/s 24~33 ◦C (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 TemperatureWater cooling; [21] 60~75 °C◦ kg 0.2/s kg/s 0.177 kg/s (1.4592 kgkg/day/h) [9] not specified30+/−2 °C WaterAir cooling; cooling; 16~27 Current 60~7545~50◦ C°C 0.2 0.16 kg/s kg/s 0.177 0.036 kg/ skg/s 92129 kg /kg/dayday [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 desalinationThe amount system. of fresh The watercorrelation produced between per day,productivity or productivity, and the is mass an important ratio of measurehot water of and a HDH air (desalinationMR) is shown system. in Figure The 5. correlation Productivity, between measured productivity by the produced and the mass fresh ratio water, of hotincreased water andas MR air increased,(MR) is shown which in was Figure attributed5. Productivity, to the increasing measured amount by the of produced vapor carried fresh by water, the air increased at higher as waterMR flowincreased, rates. whichAs the waswater attributed flow rate to increased, the increasing the contact amount area of between vapor carried the hot by water the air and at higherthe cellulose water padflow increased, rates. As thesince water a larger flow volume rate increased, of the cellulose the contact pad areawas betweensoaked. It the is also hot watershown and in Figure the cellulose 5 that apad higher increased, water sincetemperature a larger improved volume of the productivi cellulose padty, wasas the soaked. evaporation It is also rate shown increased in Figure at higher5 that watera higher temperatures. water temperature The increase improved in productivity the productivity, was more as significant the evaporation at higher rate water increased temperatures, at higher aswater indicated temperatures. by the slope The of increase the trend in productivitylines. was more significant at higher water temperatures, as indicatedTable 2 lists by the the slope experiment of the trend data lines.for some example runs.

Figure 5. Productivity as a function of mass ratio (MR). Figure 5. Productivity as a function of mass ratio (MR). Table2 lists the experiment data for some example runs.

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Table 2. Experiment data of some example runs with varying MR. Table 2. Experiment data of some example runs with varying MR.

Hot Water WaterWater Mass AirAir Mass Produced Ambient Ambient Hot Water Produced Ambient Ambient RunRun # Temperature MassRate mw,i MassRate ma MR Water md GOR Temperature Relative Temperature( C) (kg/s) (kg/s)MR Water m(kgd /day)GOR TemperatureT ( C)T0 RelativeHumidity # ◦ Rate mw,i Rate ma 0 ◦ (°C) (kg/day) (°C) Humidity 1 40.1(kg/s) 0.14 (kg/s)0.038 3.7 72 0.45 16.0 86% 1 240.1 49.6 0.14 0.14 0.038 0.0373.7 3.872 1220.45 0.5216.0 15.0 86% 91% 2 349.6 50.5 0.14 0.10 0.037 0.0413.8 2.5122 1150.52 0.5315.0 14.7 91% 81% 3 450.5 49.5 0.10 0.19 0.041 0.0422.5 4.5115 1290.53 0.5414.7 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 Figure6. 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 Figure6, 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 Figure6 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 𝑇, 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 (𝑤 ) 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 withThe the massgained ratio, output as shownratio (GOR in Figure), as an7. Aimportant higher mass measure ratio of resulted HDH system in a higher performance,GOR. A highervaried waterwith the temperature mass ratio, also as shown improved in FigureGOR. The7. A results higher show mass thatratio the resulted recovery in ratioa higher (RR) GOR was. relativelyA higher waterconstant temperature as MR varied, also improved as shown inGOR Figure. The8 .results In other show words, that thethe capabilityrecovery ratio to transform (RR) was the 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

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into water in the condenser was constant. The same portion of the vapor could be extracted in the highWater 2020cooling, 12, x FORcapability PEER REVIEW of the condenser, and it was evidenced by the constant dehumidifier9 of 14 effectivenesshumidifier, in which spite ofwill the be amount discussed of vapor in a later carried section. by the air. This was probably attributed to the high highcooling cooling capability capability of the condenser,of the condenser, and it was and evidenced it was evidenced by the constant by the dehumidifier constant dehumidifier effectiveness effectivenesswhich will be which discussed will be in adiscussed later section. 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 Figure9. The humidiﬁer e ﬀectiveness εh was positively correlated with MR. The humidifier effectivenessFigure 8.𝜀 Recovery and dehumidifier ratio (RR) as effectiveness a function of MR 𝜀,. as defined in Equations (8) 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 𝜀 was positively correlatedThe humidifier with MR. effectivenessA higher mass 𝜀 rate and of dehumidifier hot water resulted effectiveness in a larger 𝜀, ascontact defined area, in Equationswhich raised (8) theand air (9), temperature varied with and MR the as absolute shown humidityin Figure as9. theThe hot humidifier water mixed effectiveness with the incoming𝜀 was positively 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.effectiveness Theεd diddehumi not showdifier effectiveness much change 𝜀 with didMR not. Theshow reason much was change probably with MR because. The thereason cooling was probablycapability because of the condenser the cooling was capability sufficient of to the condense condenser the was vapor sufficient under allto thecondense experiment the vapor conditions. under Thisall the may experiment be attributed conditions. to the e ffiThiscient may heat be exchange attributed between to the efficient the hot humidheat exchange air and thebetween cooling the air hot in humidthe unique air and design the ofcooling the condenser. air in the unique design of the condenser.

Figure 9. Humidifier effectiveness εh and dehumidifier effectiveness εd as a function of MR.

Figure 9. Humidifier effectiveness 𝜀 and dehumidifier effectiveness 𝜀 as a function of MR. One of the reasons we think that the air-cooling condenser could work is that the ambient air as theOne cooling of the medium reasons we has think no energy that the cost air-cooling and the systemcondenser can could take aswork much is that air asthe it ambient needs to air cool as thedown cooling the warm medium humid hasair, no whileenergy a cost water-cooling and the system condenser can take needs as much to match air as the it flow needs rate to ofcool water down in the condenserwarm humid with air, that while in the a water-cooling humidifier, and condenser limits how needs fast theto match cooling the water flow (orrate preheated of water water)in the condensercould flow. with In other that wordsin the water-coolinghumidifier, and condensers limits how recycle fast the heat,cooling but water compromise (or preheated on the coolingwater) couldcapability flow. of In the other condenser. words water-cooling The air-cooling condensers condenser recycle worked the heat, more but independently, compromise on and the itcooling could capabilitycondense theof the water condenser. vapor effi Theciently air-cooling with a large cond surfaceenser worked 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. PartThe 2. coe Effectfficient of Power of performance Consumption ( COPon COP) is an important measure to evaluate how much useful thermalThe energycoefficient we canof performance obtain provided (COP the) is amount an important of work measure required. to In evaluate our case, how it was much calculated useful thermalas the ratio energy of the we latent can obtain heat ofprovided fresh water the amount over the of work work of required. the whole In our system, case, including it was calculated pumps, Q COP asfans the etc., ratio but of excludingthe latent heatin for of fresh the application water over ofthe waste work heat of the recovery. whole system, The maximal including pumps,of 20.7 fans has been achieved with our system. The correlation between the COP and the total power consumption (Pt) etc., but excluding 𝑄 for the application of waste heat recovery. The maximal COP of 20.7 has been is shown in Figure 10. The power consumption of the humidifier fan (PH) and dehumidifier fan (PDH) achieved with our system. The correlation between the COP and the total power consumption (Pt) is varied to find the maximum COP, while the power consumption of the rest of the system (pumps) shown in Figure 10. The power consumption of the humidifier fan (PH) and dehumidifier fan (PDH) wasvaried kept to constant.find the maximum The humidifier COP, fanwhile was the the power blower consumption that delivered of the rest vapor-carrying of the system air (pumps) through wasthe system kept constant. and the The dehumidifier humidifier fan fan was was the the fan blow thater delivered that delivered the cooling the vapor-carrying air into the dehumidifier. air through Thethe system left half and of the Figure dehumidifier 10 shows fan that was the theCOP fanwas that positively delivered correlated the cooling to air PDH intowhen the dehumidifier. PH was kept constant. This was probably because the higher PDH resulted in a higher cooling capability of the The left half of Figure 10 shows that the COP was positively correlated to PDH when PH was kept dehumidifier and increased the pure water production rate significantly. The right half of Figure 10 constant. This was probably because the higher PDH resulted in a higher cooling capability of the dehumidifiershows that the andCOP increasedwas negatively the pure correlativewater production with P Hratewhen significantly. PDH was keptThe right constant. half of The Figure reason 10 was probably that a larger PH did not increase the fresh water production much, but the total power shows that the COP was negatively correlative with PH when PDH was kept constant. The reason was consumption increased, resulting a lower COP. The data of some experimental runs are presented in probably that a larger PH did not increase the fresh water production much, but the total power Tableconsumption3. 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

FigureFigure 10. 10.The The coe coefficientfficient of performance (COP (COP) of) of the the system system versus versus the thetotal total power power consumption consumption of ofthe the system. system. The The power power consumption consumption ofof the the humidifier humidifier fan fan and and de-humidifier de-humidifier fan fan is specified is specified as asa a secondarysecondary axis. axis. Pure Pure water water production production isis also shown as as a a reference. reference.

TableTable 3. 3.Experiment Experiment datadata ofof somesome example ru runsns with with varying varying power power consumption. consumption.

Hot Water Power of Hot Water Water ProducedProduced Blower FanFan Power of TotalTotal Power Power Run Water Mass Two Run # Temp. Mass Rate WaterWater mdm d PowerPower PH PowerPower Two ConsumptionConsumption GORGOR COP COP # Temp. Rate mw,i Pumps (◦C) mw,i (kg/s) (L/h)(L/h) PH(W)(W) PPDHDH (W)(W) Pumps (W) (W)(W) (°C) (kg/s) (W) 11 48.948.9 0.19 0.19 3.53 3.53 44 44 47 47 40 40 131 131 0.500.50 18.018.0 22 49.049.0 0.19 0.19 3.33 3.33 33 33 47 47 40 40 120 120 0.510.51 18.618.6 33 49.149.1 0.19 0.19 3.33 3.33 21 21 47 47 40 40 108 108 0.500.50 20.720.7 44 49.149.1 0.19 0.19 2.74 2.74 21 21 35 35 40 40 96 96 0.460.46 19.119.1 55 49.049.0 0.19 0.19 0.6 0.6 21 21 24 24 40 40 85 85 0.110.11 4.84.8

Another measure of the system performance is the performance ratio (PR), recently developed usingAnother an exergy measure approach of the by system K. C. Ng performance et al. [22]. The is thecalculated performance result of ratio PR (PR),for our recently system developedis about using9.8, based an exergy on the approach Equation by given Ng, in K.C., the etarticle. al. [22 ]. The calculated result of PR for our system is about 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 influencedWe did experimentsthe system performance at different in ambient two main temperatures aspects. One and was found the carrying that the air ambient temperature temperature and influencedthe other the was system the cooling performance air temperature. in two main The aspects. two effects One wasinfluenced the carrying the system air temperature performance and in the otheropposite was the ways. cooling The former air temperature. one resulted The in a two larger effects amount influenced of vapor the generation system performance in the humidifier in opposite and ways.a higher The temperature former one resultedat the outlet in a of larger the humidifier amount of as vapor the ambient generation temperature in the humidifier increased. andThe alatter higher temperatureone, on the at other the outlethand, ofresulted the humidifier in a lower as cooling the ambient capability temperature of cooling increased. air in the The dehumidifier latter one, and on the otherhence hand, lower resulted productivity in a lower as the cooling ambient capability temperature of coolingincreased. air The in thenet dehumidifiereffect of the two and factors hence could lower productivitybe evaluated as theby the ambient correlation temperature of GOR increased. and ambient The nettemperature. effect of the As two shown factors in couldFigure be 11, evaluated GOR bydecreased the correlation as the ambient of GOR andtemperature ambient increased, temperature. implying As shown that the in latter Figure effect 11, wasGOR moredecreased significant as the ambientthan the temperature former one. increased, Although implying the results that suggest the latter that e itff ectis more was moredesirable significant to operate than the the system former in one. Althoughcool weather the results or at night, suggest the that system it is performance more desirable could to operate be compensated the system or inimproved cool weather by increasing or at night, thethe system size of performance the dehumidifier, could bemore compensated specifically or by improved adding more by increasing PC boards the to size further of the increase dehumidifier, 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

Figure 11. Effect of ambient temperature T on GOR. Figure 11. Effect of ambient temperature 0T0 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 evaporativeWe have pad. presented A maximum a HDH productivity desalination of 129 system kg/day, withGOR anof air-cooling 0.53 and COP condenserof 20.7 wereand a achieved. cellulose evaporativeThe main pad. features A maximum of our system productivity are (1) effi cientof 129 air kg/day, cooling throughGOR of hollow 0.53 and polycarbonate COP of 20.7 boards were (betterachieved. than traditional water cooling); (2) the use of the cellulous pad as a humidifier (more efficient and moreThe compact);main features (3) high of our productivity system are per (1) unit efficient volume air of cooling dehumidifier; through (4) hollow high COP polycarbonatefor waste heatboards recovery; (better (5) than lower traditional cost of the water condenser. cooling); (2) the use of the cellulous pad as a humidifier (more efficientIt was and found more that compact); productivity, (3) highGOR productiviand humidifierty per unit e volumeffectiveness of dehumidifier; increased as (4)MR highincreased COP for andwaste water heat temperature recovery; (5) increased, lower cost while of the dehumidifier condenser. effectiveness did not vary much with MR or water temperature.It was found Thethat system productivity, performance GOR wasand compromisedhumidifier effectiveness as the ambient increased temperature as MR increased, increased dueand to water the increased temperature temperature increased, of thewhile cooling dehumidi air; however,fier effectiveness such ane didffect not could vary be much compensated with MR by or increasingwater temperature. the volume ofThe the system condenser, performance or specifically was by compromised adding more PCas boardsthe ambient to further temperature increase theincreased, surface areadue ofto the the condenser. increased 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 Authorto further Contributions: increase theConceptualization surface area of and the investigation: 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 theAuthor manuscript. Contributions: Conceptualization and investigation: L.X. and B.-J.H.; Experiments setup and tests: Y.- Funding:P.C. and P.-H.W.;Funding Writing: of this L.X.; work Funding is provided acquisition: by the L.X. Research and Development Division of National TaiwanFunding: University. 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 -

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