Design and Experimental Study on a New Closed-Cycle Desalination System Based on Ambient Temperature

: The use of seawater desalination technology to solve water shortages in energy- and resource-scarce regions has attracted widespread attention worldwide. In this paper, the performance of a closed-cycle humidiﬁcation–dehumidiﬁcation desalination system with a heat pump was experimentally investigated. The system is a closed-cycle system, which includes humidiﬁers, a heat pump, dehumidiﬁers, and an air heat exchanger. The heat pump is used by the system to carry energy. The e ﬀ ects of di ﬀ erent parameters on the system performance were studied. Scale and economic analyses of the system were conducted to explore the application prospects of the system. The maximum gained output ratio of the system was 4.82. The maximum freshwater production was 960 kg / h, and the cost per kilogram of freshwater was USD 0.03, which are more considerable compared with other systems. This system provides an e ﬀ ective way to save energy in remote areas with energy shortages and freshwater resource shortages.


Introduction
The world economy has developed rapidly, the scale of industrial activities has continued to increase, and the population has accelerated, resulting in global water shortages [1]. By consuming the least amount of energy to produce freshwater, desalination systems can play a significant role in areas where water resources are scarce.
Traditional desalination methods include MED (multi-effect distillation), MSF (multi-stage flash evaporation), VC (vapor compression), RO (reverse osmosis), ED (electrodialysis) [1], solar distillation [2], and nanofiltration, which usually require high construction costs and large amounts of fossil fuels [1]. HDH (humidification-dehumidification) desalination technology is a promising option to ensure the safe supply of freshwater in regions with scarce water resources [3]. In the humidifier, moist air is humidified by vaporized seawater, then it flows into the dehumidifier, condenses, and discharges freshwater [4]. An HDH desalination plant has the advantages of convenient maintenance, small footprint, long service life, and excellent compatibility with renewable energy [5]. HDH technology has been proven to be an inexpensive and reliable desalination system [6].
The heat source of HDH systems in many studies is mainly from solar radiation. Many scholars have performed numerous different studies on the application of solar energy in HDH desalination plants.
The design of the solar HDH systems needs to be improved in order to improve the efficiency and economy of the system according to the studies conducted by Abdelmoez et al. [7], and Sathyamurthy et al. [8]. Zamen et al. [9] optimized the performance of the HDH desalination process through mathematical programming but increased the cost. Liu et al. [10] and Zubair et al. [11] studied solar-based HDH desalination systems, and their results show that the performance of the system is greatly affected by solar radiation.
In addition to solar power plants, heat pump units can also effectively provide heat for HDH systems and provide cooling for hot and humid air condensation. Compared with the traditional HDH system, studies on the use of heat pump technology in HDH are lacking.
Xu et al. [12] constructed a system combining a heat pump with humidification-dehumidification (HDH) and established mathematical models to improve system performance. He et al. [13], Faegh et al. [14], Wu et al. [15], Lawal et al. [16], and Zhang et al. [17] conducted a series of experiments and numerical studies on the system combining a heat pump and HDH. The influence of different operating parameters on the system was studied.
A new compact closed-cycle HDH desalination system, with the following features, is experimentally studied in this paper: • The entire system is in a closed state. Compared with other systems, this system uses a more compact structure, which can make full use of the energy and can minimize heat loss.

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The system is designed with an air heat exchanger, which can recover more heat.

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The flow of air is opposite to that of seawater, which improves the heat and mass transfer efficiency of the humidifier.

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The system only needs to consume electricity, does not need to consume steam, and is not affected by solar radiation.
The different parameters of the system are studied, including the temperature of seawater entering the humidifier (T s1 ), the flow rate of seawater entering the humidifier (f s1 ), and the air flow rate in the plant (F a4 ). On this basis, the maximum production efficiency, humidifier efficiency, the gained output ratio, and economic cost are studied and compared with other HDH systems.
This system provides a better method for the popularization of desalination technology in remote areas with energy shortages and freshwater resource shortages.

Materials and Methods
The system included humidifiers, a heat pump, dehumidifiers, and an air heat exchanger. Figures 1 and 2 show the schematic and a detailed view of the system. The entire system was in a closed state, including air circulation, seawater circulation, and refrigerant circulation.
Air circulation: The air is initially heated in the air heat exchanger. A fan blows air into the humidifier from the bottom. The air is heated again in the humidifier. In the humidifier, the air carries water vapor formed by the evaporation of seawater. The air enters the air heat exchanger after forming saturated air. Then, the air enters the dehumidifier. In the dehumidifier, air transfers heat to the refrigerant. The air from the dehumidifier re-enters the air heat exchanger.
In the air heat exchanger, saturated air transfers a part of the heat to the air blown out by the dehumidifier, which improves the sensible heat of the air blown out by the dehumidifier. The freshwater is produced in air heat exchangers and dehumidifiers. Figure 3 shows the air humidity diagram under an experimental condition-the air circulation process in the system. A1, A2, A3, and A4 represent the air conditions at the humidifier outlet, dehumidifier inlet, dehumidifier outlet, and humidifier inlet, respectively.
Seawater circulation: Seawater is pumped into the circulation tank. Seawater is sprayed onto the titanium pipes (condenser of the heat pump) through the circulation pump in the humidifier. A large amount of water is vapor formed by the endothermic evaporation of seawater. Water vapor is carried by air into the air heat exchanger. The remaining seawater returns to the circulation tank.
Refrigerant circulation: The humidifier is combined with the condenser of the heat pump and the dehumidifier is combined with the evaporator of the heat pump. The refrigerant absorbs energy in the dehumidifier and releases energy in the humidifier.
In the air heat exchanger, a system cooling module is designed. When the system runs for too long, causing heat build-up, the module will cool the system. The module also has a dehumidifying effect.  Air circulation: The air is initially heated in the air heat exchanger. A fan blows air into the humidifier from the bottom. The air is heated again in the humidifier. In the humidifier, the air carries water vapor formed by the evaporation of seawater. The air enters the air heat exchanger after forming saturated air. Then, the air enters the dehumidifier. In the dehumidifier, air transfers heat to the refrigerant. The air from the dehumidifier re-enters the air heat exchanger.
In the air heat exchanger, saturated air transfers a part of the heat to the air blown out by the dehumidifier, which improves the sensible heat of the air blown out by the dehumidifier. The freshwater is produced in air heat exchangers and dehumidifiers. Figure 3 shows the air humidity diagram under an experimental condition-the air circulation process in the system. A1, A2, A3, and A4 represent the air conditions at the humidifier outlet, dehumidifier inlet, dehumidifier outlet, and humidifier inlet, respectively.   Air circulation: The air is initially heated in the air heat exchanger. A fan blows air into the humidifier from the bottom. The air is heated again in the humidifier. In the humidifier, the air carries water vapor formed by the evaporation of seawater. The air enters the air heat exchanger after forming saturated air. Then, the air enters the dehumidifier. In the dehumidifier, air transfers heat to the refrigerant. The air from the dehumidifier re-enters the air heat exchanger.
In the air heat exchanger, saturated air transfers a part of the heat to the air blown out by the dehumidifier, which improves the sensible heat of the air blown out by the dehumidifier. The freshwater is produced in air heat exchangers and dehumidifiers. Figure 3 shows the air humidity diagram under an experimental condition-the air circulation process in the system. A1, A2, A3, and A4 represent the air conditions at the humidifier outlet, dehumidifier inlet, dehumidifier outlet, and humidifier inlet, respectively.  Seawater circulation: Seawater is pumped into the circulation tank. Seawater is sprayed onto the titanium pipes (condenser of the heat pump) through the circulation pump in the humidifier. A large amount of water is vapor formed by the endothermic evaporation of seawater. Water vapor is carried by air into the air heat exchanger. The remaining seawater returns to the circulation tank.
Refrigerant circulation: The humidifier is combined with the condenser of the heat pump and the dehumidifier is combined with the evaporator of the heat pump. The refrigerant absorbs energy in the dehumidifier and releases energy in the humidifier.
In the air heat exchanger, a system cooling module is designed. When the system runs for too long, causing heat build-up, the module will cool the system. The module also has a dehumidifying effect.

Humidifier
The system contained two sets of humidifiers. The humidifier consisted of five parts-namely the upper cover, the bottom funnel, the spray pipes and titanium pipes, and the shell. Spray pipes were arranged in the upper part of the inner chamber, which was ringed with seawater. Titanium pipes were arranged in the middle, which was ringed with refrigerant.

Heat Pump
In the heat pump, the humidifier is the condenser of the heat pump, and the dehumidifier is the evaporator. The theoretical COP (coefficient of performance) of the heat pump was 5.6, and the refrigerant model used in the refrigerant cycle of the system was R134a. The theoretical input power of this heat pump was 92 KW.

Air Heat Exchanger
The air heat exchanger was composed of aluminum material, and the air duct material was polypropylene. The diameter of the air ducts entering the air heat exchanger was 80 cm, and the diameter of the two outlet air ducts of the air heat exchanger was 60 cm. In the air heat exchanger, the total heat transfer area was 100 m 2 .
In the system cooling module, the heat transfer area was 80 m 2 .

Dehumidifier
The structure of the dehumidifier was the same as that of the air heat exchanger, but the dimensions were different. The diameter of the refrigerant pipes was 20 cm, which was made of copper. The diameter of the air duct was 60 cm. The heat transfer area was 115 m 2 .

Humidifier
The system contained two sets of humidifiers. The humidifier consisted of five parts-namely the upper cover, the bottom funnel, the spray pipes and titanium pipes, and the shell. Spray pipes were arranged in the upper part of the inner chamber, which was ringed with seawater. Titanium pipes were arranged in the middle, which was ringed with refrigerant.

Heat Pump
In the heat pump, the humidifier is the condenser of the heat pump, and the dehumidifier is the evaporator. The theoretical COP (coefficient of performance) of the heat pump was 5.6, and the refrigerant model used in the refrigerant cycle of the system was R134a. The theoretical input power of this heat pump was 92 KW.

Air Heat Exchanger
The air heat exchanger was composed of aluminum material, and the air duct material was polypropylene. The diameter of the air ducts entering the air heat exchanger was 80 cm, and the diameter of the two outlet air ducts of the air heat exchanger was 60 cm. In the air heat exchanger, the total heat transfer area was 100 m 2 .
In the system cooling module, the heat transfer area was 80 m 2 .

Dehumidifier
The structure of the dehumidifier was the same as that of the air heat exchanger, but the dimensions were different. The diameter of the refrigerant pipes was 20 cm, which was made of copper. The diameter of the air duct was 60 cm. The heat transfer area was 115 m 2 .

Circulation Tank
The circulating tank material was made of polypropylene with a thickness of 8 mm. The volume of the circulating tank was 5 m 3 . The outside of the circulation tank was wrapped with thermal insulation material. A liquid level gauge was introduced to control the entry of external seawater.

Pumps and Fans
The power of seawater circulation was controlled by a 1.5 KW frequency conversion pump, and the inlet water of the circulation tank was controlled by a 0.75 KW frequency conversion pump. The air circulation was controlled by a variable frequency fan with a power of 11 KW. A frequency conversion fan was introduced to control the air dehumidification and heat dissipation module.

Air Ducts, Water Pipes, and Refrigerant Pipes
The air ducts and water pipes were made of 5 mm thick polypropylene. The material of the refrigerant pipe was titanium.

Other Instruments
Anemometers, thermohygrometers, thermal resistances, and ultrasonic flowmeters were used to monitor and record different indicators. Table 1 records the relevant parameters of the instruments.

Results and Discussions
The experiment studied the different parameters of the system. The optimal operating conditions of the system were studied. This paper studied three input parameters, including the temperature of seawater entering the humidifier (T s1 ), the flow rate of seawater entering the humidifier (f s1 ), and the air flow rate in the plant (F a4 ). The best overall performance was obtained. The GOR (gained output ratio), under the optimal working condition of the system, was studied. The feasibility and economics of the system were summarized and compared.

Seawater Temperature
Fixed parameters: F a4 = 12,000 m 3 /h, and f s1 = 32 m 3 /h. The temperature of the seawater (T s1 ) in the circulation tank was controlled by a thermostat. In the experiment, T s1 varied between 44 and 52 degrees Celsius. Freshwater productivity is shown in Figure 4. Freshwater production increased from 550 kg/h to 960 kg/h. It can be seen from Figure 5 that T a1 , T a3 , ϕ a1 , and ϕ a3 all increase with the increase in T s1 . In the dehumidifier, with the increase in T s1 , the temperature difference between T a1 and T a3 changed slightly. The heat exchange efficiency of the dehumidifier did not vary greatly. However, with the increase in ϕ a1 and ϕ a3 , the enthalpy difference between the air at the outlet of the humidifier and the outlet of the dehumidifier continued to increase, which is the main reason for the increase in productivity. T s1 and T a1 also affected the heating capacity and condensing capacity of the heat pump unit. With the increase in T s1 and T a1 , the heat capacity and cold capacity of the heat pump unit increased. Saturated air with a higher temperature and humidity ratio entering the dehumidifier can result in a greater yield of freshwater. The total productivity increases with the increase in T s1 . T s1 increased from 42 to 52 • C, and freshwater productivity increased by 75%. Therefore, the temperature of the seawater had an obvious impact on productivity.
Processes 2020, 8, x FOR PEER REVIEW 6 of 13 productivity increased by 75%. Therefore, the temperature of the seawater had an obvious impact on productivity.

Air Flow Rate
Fixed parameters: Ts1 = 52 °C, fs1 = 32 m 3 /h. A fan that could control the speed by adjusting the voltage was introduced to control the air flow rate (Fa4). As Fa4 increased, as shown in Figure 6, there was a change in freshwater productivity. Figure 7 describes the changes in Ta1, Ta3, φa1, and φa3. It can be seen from Figure 6 that when Ts1 and fs1 were unchanged, as Fa4 increased, Ta1 and Ta3 both had a downward trend. With the increase in Fa4, both φa1 and φa3 were relatively stable. When Fa4 increased to a specific value, φa1 decreased. The air temperature and relative air humidity changed in the opposite direction to Fa4. This shows that, with the increase in Fa4, the heat loss in the dehumidifier was smaller, and the heat exchange efficiency also increased. Due to the reduction in heat loss in the dehumidifier and the increase in Fa4, the combined effect of the two led to an upward trend in the freshwater production rate. In the process of increasing the air flow rate from 8000 m 3 /h to 13,000 m 3 /h, the air flow rate increased by 63%, and the freshwater productivity increased by 12%. The productivity of freshwater increased slightly. Therefore, freshwater productivity is not sensitive to changes in air flow rate. productivity increased by 75%. Therefore, the temperature of the seawater had an obvious impact on productivity.

Air Flow Rate
Fixed parameters: Ts1 = 52 °C, fs1 = 32 m 3 /h. A fan that could control the speed by adjusting the voltage was introduced to control the air flow rate (Fa4). As Fa4 increased, as shown in Figure 6, there was a change in freshwater productivity. Figure 7 describes the changes in Ta1, Ta3, φa1, and φa3. It can be seen from Figure 6 that when Ts1 and fs1 were unchanged, as Fa4 increased, Ta1 and Ta3 both had a downward trend. With the increase in Fa4, both φa1 and φa3 were relatively stable. When Fa4 increased to a specific value, φa1 decreased. The air temperature and relative air humidity changed in the opposite direction to Fa4. This shows that, with the increase in Fa4, the heat loss in the dehumidifier was smaller, and the heat exchange efficiency also increased. Due to the reduction in heat loss in the dehumidifier and the increase in Fa4, the combined effect of the two led to an upward trend in the freshwater production rate. In the process of increasing the air flow rate from 8000 m 3 /h to 13,000 m 3 /h, the air flow rate increased by 63%, and the freshwater productivity increased by 12%. The productivity of freshwater increased slightly. Therefore, freshwater productivity is not sensitive to changes in air flow rate.

Air Flow Rate
Fixed parameters: T s1 = 52 • C, f s1 = 32 m 3 /h. A fan that could control the speed by adjusting the voltage was introduced to control the air flow rate (F a4 ). As F a4 increased, as shown in Figure 6, there was a change in freshwater productivity. Figure 7 describes the changes in T a1 , T a3 , ϕ a1 , and ϕ a3 . It can be seen from Figure 6 that when T s1 and f s1 were unchanged, as F a4 increased, T a1 and T a3 both had a downward trend. With the increase in F a4 , both ϕ a1 and ϕ a3 were relatively stable. When F a4 increased to a specific value, ϕ a1 decreased. The air temperature and relative air humidity changed in the opposite direction to F a4 . This shows that, with the increase in F a4 , the heat loss in the dehumidifier was smaller, and the heat exchange efficiency also increased. Due to the reduction in heat loss in the dehumidifier and the increase in F a4 , the combined effect of the two led to an upward trend in the freshwater production rate. In the process of increasing the air flow rate from 8000 m 3 /h to 13,000 m 3 /h, the air flow rate increased by 63%, and the freshwater productivity increased by 12%. The productivity of freshwater increased slightly. Therefore, freshwater productivity is not sensitive to changes in air flow rate.

Seawater Flow Rate
Fixed parameters: Fa4 = 12,000 m 3 /h, Ts1 = 52 °C. The fs1 was controlled by a water pump. In the experiment, fs1 varied from 28 m 3 /h to 36 m 3 /h. The effect of fs1 on the system's freshwater productivity is shown in Figure 8. It can be seen that, with the increase in fs1, freshwater productivity continued to increase. When fs1 reached a specific value, the freshwater productivity remained stable. Figure 9 describes the changes in Ta1, Ta3, φa1, and φa3. When Ts1 and Fa4 remained unchanged, with the increase in fs1, there was little change in Ta1, Ta3, and φa3, while φa1 was consistent with the changing trend of system productivity. The heat exchange efficiency of the dehumidifier did not vary greatly. The increase in φa1 led to an increase in air enthalpy and specific humidity, thereby increasing the freshwater productivity of the system.

Seawater Flow Rate
Fixed parameters: Fa4 = 12,000 m 3 /h, Ts1 = 52 °C. The fs1 was controlled by a water pump. In the experiment, fs1 varied from 28 m 3 /h to 36 m 3 /h. The effect of fs1 on the system's freshwater productivity is shown in Figure 8. It can be seen that, with the increase in fs1, freshwater productivity continued to increase. When fs1 reached a specific value, the freshwater productivity remained stable. Figure 9 describes the changes in Ta1, Ta3, φa1, and φa3. When Ts1 and Fa4 remained unchanged, with the increase in fs1, there was little change in Ta1, Ta3, and φa3, while φa1 was consistent with the changing trend of system productivity. The heat exchange efficiency of the dehumidifier did not vary greatly. The increase in φa1 led to an increase in air enthalpy and specific humidity, thereby increasing the freshwater productivity of the system.

Seawater Flow Rate
Fixed parameters: F a4 = 12,000 m 3 /h, T s1 = 52 • C. The f s1 was controlled by a water pump. In the experiment, f s1 varied from 28 m 3 /h to 36 m 3 /h. The effect of f s1 on the system's freshwater productivity is shown in Figure 8. It can be seen that, with the increase in f s1 , freshwater productivity continued to increase. When f s1 reached a specific value, the freshwater productivity remained stable. Figure 9 describes the changes in T a1 , T a3 , ϕ a1 , and ϕ a3 . When T s1 and F a4 remained unchanged, with the increase in f s1 , there was little change in T a1 , T a3 , and ϕ a3 , while ϕ a1 was consistent with the changing trend of system productivity. The heat exchange efficiency of the dehumidifier did not vary greatly. The increase in ϕ a1 led to an increase in air enthalpy and specific humidity, thereby increasing the freshwater productivity of the system.
During the change in seawater flow rate, when the seawater flow rate increased from 28 m 3 /h to 32 m 3 /h, the freshwater productivity increased from 800 to 960 kg/h. The seawater flow rate increased by 14% and the freshwater productivity increased by 20%. However, when the seawater flow rate was greater than 32 m 3 /h, the increase in the seawater flow rate had less impact on freshwater productivity and was affected by the heat exchange efficiency. Within a certain range, seawater flow had a greater impact on freshwater productivity.  During the change in seawater flow rate, when the seawater flow rate increased from 28 m 3 /h to 32 m 3 /h, the freshwater productivity increased from 800 to 960 kg/h. The seawater flow rate increased by 14% and the freshwater productivity increased by 20%. However, when the seawater flow rate was greater than 32 m 3 /h, the increase in the seawater flow rate had less impact on freshwater productivity and was affected by the heat exchange efficiency. Within a certain range, seawater flow had a greater impact on freshwater productivity.

Humidification Performance
Humidifier performance can be calculated using the following formula [18]: In all test scenarios, when the optimal conditions were reached, the humidity at the measured humidifier outlet reached saturation. Therefore, the humidifier efficiency of the proposed system was 100%.  During the change in seawater flow rate, when the seawater flow rate increased from 28 m 3 /h to 32 m 3 /h, the freshwater productivity increased from 800 to 960 kg/h. The seawater flow rate increased by 14% and the freshwater productivity increased by 20%. However, when the seawater flow rate was greater than 32 m 3 /h, the increase in the seawater flow rate had less impact on freshwater productivity and was affected by the heat exchange efficiency. Within a certain range, seawater flow had a greater impact on freshwater productivity.

Humidification Performance
Humidifier performance can be calculated using the following formula [18]: In all test scenarios, when the optimal conditions were reached, the humidity at the measured humidifier outlet reached saturation. Therefore, the humidifier efficiency of the proposed system was 100%.

Humidification Performance
Humidifier performance can be calculated using the following formula [18]: In all test scenarios, when the optimal conditions were reached, the humidity at the measured humidifier outlet reached saturation. Therefore, the humidifier efficiency of the proposed system was 100%.

System Production Efficiency
In this system, when the operating conditions were seawater temperature, air flow rate, and seawater flow rate at 52 • C, 12,000 m 3 /h, and 32 m 3 /h, the maximum productivity of the system was 960 kg/h. The compressor reached the best working condition when evaporator pressure was 0.5 MPa and the condenser pressure was 16 MPa. Compared with the work of other researchers, the productivity was significantly improved.

Performance Comparison with Different HDH Desalination Systems
The general index for evaluating the performance of seawater desalination systems is GOR. The following expression can calculate GOR [19]: During the experiments, the power of electrical appliances, such as fans, water circulation pumps, cold water circulation pumps, seawater pumps, and heat pumps, were measured. Figures 4, 6 and 8 show the change in GOR under different conditions. In this experiment, seawater temperature has a significant influence on GOR. Air flow rate and seawater flow rate have relatively little influence on GOR. After analysis of this experiment, the maximum value of GOR was 4.82. Table 2 lists some reported performance parameters of other desalination systems. The freshwater productivity per unit of time of the proposed system is greater than most desalination systems, and the production cost is highly competitive. The proposed system saves energy and improves production efficiency.

Cost Estimate
Through cost estimation, the critical value of the system can be more intuitively reflected. Table 3 shows the cost of the system. The operating life (n) of the system is assumed to be ten years. The system operating time (t) is assumed to be 20 h per day. The electricity cost (e) is 0.127 US$/KWh [17]. Through the calculation of the following formula, the cost per kilogram of fresh water can be obtained [24], which is shown in Table 4. The cost per kilogram of freshwater is USD 0.03. Compared with other systems in terms of energy and capacity, the system has certain advantages and competitiveness.

Conclusions
A new closed-cycle HDH desalination system was established. The effects of different parameters on the system performance were studied. The GOR of the system was studied. The system was economically evaluated, and the feasibility of the system studied was analyzed. Our conclusions are as follows: • The condenser of the heat pump was combined with the humidifier, and the evaporator of the heat pump was combined with the dehumidifier. This design reduces energy consumption during heat exchange.

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The maximum freshwater production is 960 kg/h. This system used a heat pump, which did not need to absorb heat from too much radiation and can be used in areas with limited resources. This shows that the system can guarantee the production of fresh water and is not affected by environmental conditions. • The temperature of the seawater has an obvious impact on productivity.

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As the air flow rate increases, the freshwater productivity will reach a maximum. As a result, the freshwater productivity will not be sensitive to changes in air flow rate.

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Within a certain range, freshwater productivity reaches a maximum value under a certain seawater flow rate, and seawater flow has a greater impact on freshwater productivity.

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The efficiency of the humidifier obtained by this system is 100%. The gained output ratio of performance is 4.82. Compared to other HDH systems, this system requires less energy per unit of freshwater.

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The cost of freshwater per kilogram is approximately USD 0.03. The economy of this system is considerable. The proposed system saves energy and improves production efficiency.

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The air ducts and water pipes of the system cause a certain amount of heat loss. It is very important to design a more compact system structure. The system can be designed in a container, which is convenient for industrial production and convenient for transportation. Funding: This research received no external funding.

Acknowledgments:
The authors acknowledge the support and funds received from Anhui Tongsu Environment Protection Technology Co., Ltd.

Conflicts of Interest:
The authors declare no conflict of interest.