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desorption method and condensation method for extracting water from air
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Procedia Engineering 00 (2017) 000–000 Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
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Procedia Engineering 205 (2017) 2069–2075
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1910th International Symposium on22Heating, Air Conditioning, ISHVAC2017, 19OctoberVentilation 2017, Jinan,and China 22 October 2017, Jinan, China
Study of composite scheme of absorption/desorption method and Study of composite scheme of absorption/desorption method and condensation method for extracting water from air condensation method for extracting water from air Xiuyuan Hao**, Shibin Geng, Li Yuan, and Bowen Luo Xiuyuan Hao , Shibin Geng, Li Yuan, and Bowen Luo College of Defence Engineering, PLA Univ. of Sci. & Tech. , Nanjing, China College of Defence Engineering, PLA Univ. of Sci. & Tech. , Nanjing, China
Abstract Abstract This paper introduced main methods of extracting water from air and analyzed operation characteristics and limitations of This paper introduced main methodsscheme of extracting water from air and analyzed operation characteristics and limitations of condensation method. The composite of absorption/desorption method and condensation method for extracting water from condensation method. composite schemethe ofwater absorption/desorption method and condensation method forthe extracting water from air was presented. TheThe parameters affecting collection in the desiccant-wheel were analyzed, and results showed that air wasthe presented. The parameters the water collection in is the5r/h, desiccant-wheel were analyzed, andthe themoisture results showed that when regeneration temperatureaffecting is 100℃，the rotation speed and the angle is 180 degrees, absorption when the temperature is 100℃，the rotationofspeed is 5r/h, wheel and the angle iswith 180condensation degrees, the system moisture runner hasregeneration the best effect, Then the key technical problems the desiccant matching areabsorption presented runner hasplate the best effect, Then the key technical problemsrecover of the desiccant wheel and three fin heat exchangers are used to effectively the energy in thematching system, with whichcondensation can avoid thesystem offsetare andpresented improve and three plate fin heatThe exchangers are usedatofeasible effectively thewater energy thehigh-temperature system, which can the offset area, and improve the system efficiency. scheme provide way recover to collect forinthe andavoid low-humidity such as the system efficiency. The scheme provide a feasible way to collect water for the high-temperature and low-humidity area, such as Gobi and desert. Gobi and desert. © 2017 The Authors. Published by Elsevier Ltd. © 2017 2017 The The Authors. Published Elsevier Ltd. © Authors. Published by by Ltd. committee of the 10th International Symposium on Heating, Ventilation and Air Peer-review under responsibility of Elsevier the scientific Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. Air Conditioning. Conditioning. Keywords: Absorption/desorption method；Condensation method；Desiccant-wheel；Extracting water from air; Keywords: Absorption/desorption method；Condensation method；Desiccant-wheel；Extracting water from air;
Nomenclature Nomenclature T T φR φR ω ω m m
temperature, ℃ temperature, ℃ angle, ° dehumidification dehumidification wheel speed, r/h angle, ° wheel speed, r/h condensation water of the unit mass air, g/kg condensation water of the unit mass air, g/kg
* Corresponding author. Tel.: +86-13776601313 . * Corresponding Tel.: +86-13776601313 . E-mail address:author. [email protected] E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 2017responsibility The Authors. of Published by Elsevier Ltd. of the 10th International Symposium on Heating, Ventilation and Air Conditioning. Peer-review©under the scientific committee Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.096
2
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Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075
1. INTRODUCTION The fresh water is an important resource for human survival and development, but about 80 countries and regions, accounted for 40% of the world's population, are serious water shortage, especially in the mountains, Gobi, desert, islands, etc. There are four traditional ways to obtain water: (1) Collection of atmospheric precipitation, such as Israel's residents collecting roof rainwater, and the water cellar in western region of China; (2) Exploitation of underground water and surface water, it is the main way of water supply, because of environmental pollution and serious exploitation, resulting in those freshwater resources increasingly reduced; (3) Sea water desalination, that is suitable for the coast, but this method is not feasible for inland areas; (4) Vehicle transportation, this method is adopted in the special area and it’s cost is very high. Therefore, how to obtain the potable water by a special way has become a new research direction. There are three kinds of states of water in the nature word, such as vapor, liquid and solid state. The atmosphere becomes the transfer station of water cycle under the action of solar radiation and the earth gravity, the water transforming three states. It is estimated that the amount of water in the air is 10 times as much as the total amount of water on the earth surface, which amounts to 14000m3. In the dry desert, air moisture content is also more than 10g/m3 because of the role of atmospheric circulation, which provides a theoretical basis for extracting water from air. This paper focuses on the technical method of combining with absorption/desorption method and condensation method for extracting water from air, using desiccant-wheel to enrich air moisture content, and then extracting potable water by condensation method, that aim is to provide a feasible method for water extracting in the low humidity areas, such as mountains, deserts and other serious water shortage. 2. The Research Status of Extracting Water from Air at Home and Abroad At present, the air-to-water equipment using for areas with high air humidity has been developed in the United States, Germany, Canada, Israel, China and other countries. Its principles are condensation method, solar adsorption refrigeration method, and solar semiconductor refrigeration method. 2.1. Condensation method The condensation method mainly uses the refrigerant circulation system to make the air flow through the evaporator below the dew point temperature to obtain condensate water. In 1993, Khalil [1] et al. studied the extraction of water from high temperature, high humidity air by freezing condensation. Luo [2] , a senior engineer, developed the "battlefield survival to protect the air intake equipment". In the condition of 5 ℃ -43 ℃ the relative humidity is higher than 40%, more than 30 kg water can be extracted from the air, and the water quality can fit in with the national drinking water standards by multistage filtration, disinfection and adding some trace elements. Zhang [3] of the Logistics Engineering College based on the kind of the air-to-water equipment, and thought that the unit time flow through the evaporator air volume, compressor speed and evaporator surface temperature are closely related. The interaction between the parameters is a complex system with large interference, variable load and high degree of nonlinearity. An adaptive fuzzy PID control technique is proposed to improve the effluent of the equipment. Evaporative condenser
Solar collectors
Wet air Water collector Adsorbents
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075 Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
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Fig. 1. Solar energy adsorption refrigeration method
2.2. Solar adsorption refrigeration method As shown in Fig. 1, the French scientist Jannot Y [4] proposed a solar adsorption refrigeration condensation method that first uses the solar radiation during the day to heat the adsorbent bed to desorb its internal adsorbent and at the other end condensate in the condenser. Under the atmospheric cooling effect at night, adsorption bed started the adsorption process, which made the continuous evaporation, adsorption of evaporative condenser in refrigeration, when the evaporator surface temperature is lower than the dew point temperature, the vapor will condense. Under the action of gravity, through the collection plate to store and store to the storage tank, the air intake process had achieved. 2.3 Solar semiconductor refrigeration method The basic principle of solar energy semiconductor refrigeration method [5] is that the battery is charged with photovoltaic power generation during the day and supplies power to the semiconductor refrigeration component, causing temperature of the cold plate falls below the dew point of the air. Under the action of the fan, the air over the cold plate, and the water vapor condensates in the cold plate surface. Because of the narrow flow passage between the cold plates, the airflow velocity is faster. Condensed water droplets formed by air flow to the condensate water area, flow velocity of small, and water droplets entrained in the air density greater than the density of air, will sink to the bottom of the water condensing chamber, so as to realize the separation of air and water droplets. At the same time, the heat recovery device is adopted to make the air entering the water intake device to be pre cooled. However, there are two problems in this technology: on the one hand, the conversion efficiency of solar cells is low and the loss is large. On the other hand, the efficiency of water intake is limited by the circulation area. Zhang using solar photovoltaic cells and semiconductor cooling film, put forward the development of portable air sampler. However, the water extraction equipment, water intake is small, high cost, only suitable for use in the field emergency conditions. 3 Principle and Shortcomings of Condensation Method
H/kj·kg-1
The condensation method of extracting water from air is achieved by the evaporator in the refrigeration system. The wet air crosses through the evaporator under the action of the fans, when the evaporator surface temperature tw is lower than the dew point temperature of the wet air td, the water vapor will condense on its surface, form small droplets or water film, and finally pool into the water tray. The dew point temperature determines the amount of moisture in the air, thereby affecting the potential of extracting water. In the enthalpy humidity chart, the process of wet air condensation is from the initial state point W to the last state point L. In this process, the water vapor in the air is continuously condensed into liquid water, and the condensation water of the unit mass air is m=dW-dL, that is, the difference between the moisture content of the two state points [6] , as shown in figure 2. W
Φ=100% L
dL
dW
d/g·(kg Dry air)-1
Fig. 2. Wet air cooling dehumidification process
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075 Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
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During the cooling and condensation process, a large part of the sensible heat released by the wet air to the dew point temperature consumes the cooling capacity of the evaporator, resulting in low efficiency and low water consumption. On this basis, Geng proposed energy-saving refrigeration dehumidifier, which is in the middle of the evaporator and condenser with a plate-fin heat exchanger equipment as a cold recovery device. Using low-temperature air-out of the evaporator to preheat the high temperature inlet air, the proportion of the latent heat from wet air vapor condensation to liquid water has a substantial increased in the total cooling capacity of the evaporator, which can greatly improve the water efficiency. The key to condensation method of extracting water from air is that the air dew point temperature is higher than the evaporator temperature of the refrigeration equipment. Otherwise, if the air moisture content and dew point temperature are low, the technology cannot obtain condensate. In the desert area, the air temperature is high, and the relative humidity is low, and the dew point temperature is only about 2 ℃; the air temperature even as low as below 0 ℃in the plateau. Therefore, the freeze-condensation method of air intake technology is not suitable for dry plateau, desert areas. 4 Principle and Analysis of Composite Scheme In dry areas, the air containing less moisture, it is difficult to realize the direct use of freezing condensation water. In this paper, firstly, the water vapor in the air is enriched by the method of absorption/desorption method, and then using condensation method to extract water. The power source required for the water intake process uses the rich solar energy of the desert area. 4.1 Principle of composite scheme
Solar panels
1
T/℃
Composite scheme of solar air water intake includes desiccant-wheel and refrigeration system. The desiccant-wheel is a solid moisture absorbent as the core component to adsorb and desorb moisture in the air, with high moisture absorption, continuous operation, compact and energy saving and other characteristics. Especially the use of solar energy and industrial waste heat and other low-grade energy, can be used for dry air moisture enrichment [7] . The scheme of system structure as shown in Figure 3, the evaporator compression refrigeration system of the air inlet is communicated with the air outlet runner regeneration, and the air outlet of the solar air heater is communicated with the regeneration air inlet of the desiccant-wheel. The condenser outlet is connected to the inlet of the solar heater. And the process of the enthalpy diagram shown in Figure 4, the general air treatment process is as follows: The fresh air (state point 1) passes through the condenser to obtain the condensed heat released by the refrigerant, and the air temperature rises to the state point 2, then heating by the solar heater to become the higher temperature regenerative air (state point 3). When the fresh air passes through the dehumidification runner, the temperature of the fresh air is lowered and becomes the high temperature and high humidity air (state point 4); The fresh air through the evaporator cooling to the dew point temperature (state point 5), water vapor liquefied, Where 6-7 indicates that the moisture in the fresh air is absorbed and the temperature is reduced. Fresh air
Condenser
2
3 Desiccant
Heater
3
7
wheel
4
1（6）
5 Fresh air
6
4
2
Evaporator
5
Exhaust air
7
d/g/kg Dry air
Xiuyuan HaoHao et al./ Engineering 00 (2017) 000–000 Xiuyuan et Procedia al. / Procedia Engineering 205 (2017) 2069–2075 Fig. 3. Physical model of water intake program
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Fig. 4. System enthalpy chart
4.2 Performance Analysis of desiccant-wheel The runner consists of two parts: the moisture absorption zone and the regeneration zone. The runner is filled with honeycomb-shaped flow channel, and the flow channel contains silica gel adsorbent. When the fresh air passes through the flow path of the moisture absorption zone under the action of the fan, the water vapor in the air is absorbed by the adsorbent due to the low water vapor pressure on the surface of the adsorbent. With the rotation of the runner, the saturated moisture flow channel is transferred to the regeneration zone, and the high temperature regenerative air flows through these honeycomb flow passages. Since the high temperature regenerated air corresponded to the water vapor pressure below the adsorbent surface water vapor Sub-pressure, so that the moisture in the silica gel is removed, and the runner regain the moisture absorption capacity. The regenerated air flowing through the runner also becomes an air with a relatively high moisture content, and the moisture enrichment in the dry air is realized. The temperature and moisture content of the fresh air, the temperature and moisture content of the regeneration air, the speed of the suction runner and the dehumidification angle and the heat of the adsorbent will have a complicated effect on the hygroscopic effect of the absorbent wheel. Increasing the regeneration temperature ensures that the water desorption is more thoroughly and the air with higher moisture content is obtained, but the energy utilization rate will decrease after a certain range. The relevant experimental studies show that: When the regeneration temperature is 100 ℃, the moisture absorption coefficient is the highest. The absorption wheel speed ω determines the residence time of the adsorbent in the moisture absorption zone and the regeneration zone. When the rotation speed is too fast, the desorption of the adsorbent is not sufficient, resulting in a decrease in the capacity of the rotor, On the other hand, when the speed is too slow, the adsorbent near the regeneration zone is too saturated, and the moisture absorption capacity is reduced. When the speed is 5r/h, the efficiency of the rotor is the highest; Dehumidification angle φR determines the ratio of the moisture absorption area and the regeneration zone. When φR is too small, the adsorbent cannot be fully regenerated. When φR is too large, the adsorbent moisture absorption is insufficient. When the wheel uses of dehumidification angle of 0.5, as shown in Figure 5, that half of the runner for the moisture zone, the other half of the regeneration zone, the amount of vapor will reach the maximum enrichment.
Hygroscopic zone
φR
ω
Regeneration zone
Fig. 5. Desiccant wheel physical model
4.3 Coupling analysis of composite scheme of absorption/desorption method and condensation method The desorption effect of desiccant wheel is positively correlated with the air temperature in the regeneration zone, that is, the higher the inlet air temperature, the better the desorption effect; In order to fully absorb the enriched vapor, it needs to consume a large amount of energy to improve the inlet air temperature of the regeneration zone of the desiccant wheel. At the same time, the desiccant capacity of the desiccant wheel is negatively correlated with the inlet
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075 Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
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air temperature in the hygroscopic zone, that is, the higher the inlet air temperature, the less the moisture absorption capacity. The desiccant-wheel significantly reduce the moisture absorption capacity in a high temperature and low humidity environment. It should try to increase the regeneration temperature, while reducing the fresh air temperature of the moisture absorption zone. In the proposed program，the temperature of the air entering the evaporator is high and the enthalpy is large, and there are some cold and heat offset in some links, which result in the low energy efficiency ratio. On this basis, an improvement program is proposed——using the new three plate fin heat exchanger to recover the waste heat and cooling process, as shown in Figure 6. By using the first heat exchanger, making the use of high temperature air from desiccant-wheel regeneration zone to preheat regeneration fresh air and reducing the regeneration of heating energy consumption, and also precool the high temperature and humidity air go into the evaporator to improve the efficiency of condensate water and larger the amount of water. By using the second heat exchanger, the high temperature and high humidity air from the desiccant-wheel regeneration zone is pre-cooled by the first heat exchanger and then through the second heat exchanger, transfers heat with the evaporator cool air and remove part of the sensible heat further after the precooling, and then go into the evaporator, which can further improve the capacity of steam compression refrigeration system cooling water and unit operating efficiency. By using the third heat exchanger, before the outdoor air from the high temperature and low humidity environment go into the dehumidification wheel moisture absorption area, first through the third heat exchanger, and transfers heat with cool air from the two heat exchanger, after precooling the outdoor air, and then by moisture absorption zone of the dehumidification wheel for moisture enrichment, to improve the efficiency of the whole system of extracting water. The capacity of the desiccant-wheel is less than the capacity of the evaporator to handle air volume in unit time. In order to solve the problem of the two air volume mismatch, the amount of fresh air can be introduced before the evaporator, so as to adjusting the proportion of the fresh air by the valve. The enthalpy profile of the air conditioning process is shown in Figure 7. 4 First heat exchanger Desiccant wheel
Condenser 3
2
3
Fresh air
Heater 4
Evaporator
11 Fresh air 12 Third heat exchanger
Second heat 7 exchanger
6
5
14 13
Fresh 1 air
Exhaust air
5
1
9
14 T（℃）
Solar panels
6
7
2 1（12）
8 13
11 10 9
8
10
Humidity ratio（g/kg）
Figure 6. Physical model of extracting water from air
Figure 7. Psychrometric chart of system
5 CONCLUSIONS There are many domestic and foreign research on the methods of extracting water from air, but most of them are used for high humidity. However, for the low moisture region, such as Gobi, desert and other areas, this paper puts forward the combination of the absorption/desorption method and the condensation method. Firstly, the moisture in the air is enriched by the desiccant-wheel, then using the condensation method to extract water. This paper presents a set of feasible scheme for the air intake in the dry desert and plateau area.
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075 Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
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6 ACKNOWLEDGEMENTS The author would like to give his thanks to undergraduate student, X Y Hao for his assistance in the experiment. References [1] A Khalil. Dehumidification of atmospheric air as a potential source of fresh water in the UAE. Desalination, 1993, 93(1-3) 587-596． [2] J. Luo, W. Zhang, X, Bai. Development and application of a field water maker. Journal of HV&AC. 2004, 34(4) 42-45 [3] H. Zhao et al. The characteristic research on solar water-getting tube for extracting water from air with adsorption. Renewable Energy Resources, 2014, 32(3) 259-264. [4] Y. Jannot. Production d'eau par condensation de l'humidité atmosphérique. Evaluation du procédé en zone sahélienne. Revue Générale De Thermique, 1993, 384 705-709. [5] J. Ye et al. Study on using solar-energy semiconductor refrigeration to dew water from air. Journal of Anshan University of Science and Technology, 2004, 27(4) 2 82-285. [6] H. Geng et al. A review on water extraction from air. CHEMICAL INDUSTRY AND ENGINGEERING PROGRESS, 2011, 8: 1664-1669. [7] M. P. Maiya et al. Application of desiccant wheel to control humidity in air-conditioning systems. Applied Thermal Engineering, 2004, 24(17– 18) 2777-2788.
ScienceDirect ScienceDirect
Procedia Engineering 00 (2017) 000–000 Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 205 (2017) 2069–2075
10th International Symposium on Heating, Ventilation and Air Conditioning, ISHVAC2017, 1910th International Symposium on22Heating, Air Conditioning, ISHVAC2017, 19OctoberVentilation 2017, Jinan,and China 22 October 2017, Jinan, China
Study of composite scheme of absorption/desorption method and Study of composite scheme of absorption/desorption method and condensation method for extracting water from air condensation method for extracting water from air Xiuyuan Hao**, Shibin Geng, Li Yuan, and Bowen Luo Xiuyuan Hao , Shibin Geng, Li Yuan, and Bowen Luo College of Defence Engineering, PLA Univ. of Sci. & Tech. , Nanjing, China College of Defence Engineering, PLA Univ. of Sci. & Tech. , Nanjing, China
Abstract Abstract This paper introduced main methods of extracting water from air and analyzed operation characteristics and limitations of This paper introduced main methodsscheme of extracting water from air and analyzed operation characteristics and limitations of condensation method. The composite of absorption/desorption method and condensation method for extracting water from condensation method. composite schemethe ofwater absorption/desorption method and condensation method forthe extracting water from air was presented. TheThe parameters affecting collection in the desiccant-wheel were analyzed, and results showed that air wasthe presented. The parameters the water collection in is the5r/h, desiccant-wheel were analyzed, andthe themoisture results showed that when regeneration temperatureaffecting is 100℃，the rotation speed and the angle is 180 degrees, absorption when the temperature is 100℃，the rotationofspeed is 5r/h, wheel and the angle iswith 180condensation degrees, the system moisture runner hasregeneration the best effect, Then the key technical problems the desiccant matching areabsorption presented runner hasplate the best effect, Then the key technical problemsrecover of the desiccant wheel and three fin heat exchangers are used to effectively the energy in thematching system, with whichcondensation can avoid thesystem offsetare andpresented improve and three plate fin heatThe exchangers are usedatofeasible effectively thewater energy thehigh-temperature system, which can the offset area, and improve the system efficiency. scheme provide way recover to collect forinthe andavoid low-humidity such as the system efficiency. The scheme provide a feasible way to collect water for the high-temperature and low-humidity area, such as Gobi and desert. Gobi and desert. © 2017 The Authors. Published by Elsevier Ltd. © 2017 2017 The The Authors. Published Elsevier Ltd. © Authors. Published by by Ltd. committee of the 10th International Symposium on Heating, Ventilation and Air Peer-review under responsibility of Elsevier the scientific Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. Air Conditioning. Conditioning. Keywords: Absorption/desorption method；Condensation method；Desiccant-wheel；Extracting water from air; Keywords: Absorption/desorption method；Condensation method；Desiccant-wheel；Extracting water from air;
Nomenclature Nomenclature T T φR φR ω ω m m
temperature, ℃ temperature, ℃ angle, ° dehumidification dehumidification wheel speed, r/h angle, ° wheel speed, r/h condensation water of the unit mass air, g/kg condensation water of the unit mass air, g/kg
* Corresponding author. Tel.: +86-13776601313 . * Corresponding Tel.: +86-13776601313 . E-mail address:author. [email protected] E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 2017responsibility The Authors. of Published by Elsevier Ltd. of the 10th International Symposium on Heating, Ventilation and Air Conditioning. Peer-review©under the scientific committee Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 10th International Symposium on Heating, Ventilation and Air Conditioning. 10.1016/j.proeng.2017.10.096
2
2070
Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075
1. INTRODUCTION The fresh water is an important resource for human survival and development, but about 80 countries and regions, accounted for 40% of the world's population, are serious water shortage, especially in the mountains, Gobi, desert, islands, etc. There are four traditional ways to obtain water: (1) Collection of atmospheric precipitation, such as Israel's residents collecting roof rainwater, and the water cellar in western region of China; (2) Exploitation of underground water and surface water, it is the main way of water supply, because of environmental pollution and serious exploitation, resulting in those freshwater resources increasingly reduced; (3) Sea water desalination, that is suitable for the coast, but this method is not feasible for inland areas; (4) Vehicle transportation, this method is adopted in the special area and it’s cost is very high. Therefore, how to obtain the potable water by a special way has become a new research direction. There are three kinds of states of water in the nature word, such as vapor, liquid and solid state. The atmosphere becomes the transfer station of water cycle under the action of solar radiation and the earth gravity, the water transforming three states. It is estimated that the amount of water in the air is 10 times as much as the total amount of water on the earth surface, which amounts to 14000m3. In the dry desert, air moisture content is also more than 10g/m3 because of the role of atmospheric circulation, which provides a theoretical basis for extracting water from air. This paper focuses on the technical method of combining with absorption/desorption method and condensation method for extracting water from air, using desiccant-wheel to enrich air moisture content, and then extracting potable water by condensation method, that aim is to provide a feasible method for water extracting in the low humidity areas, such as mountains, deserts and other serious water shortage. 2. The Research Status of Extracting Water from Air at Home and Abroad At present, the air-to-water equipment using for areas with high air humidity has been developed in the United States, Germany, Canada, Israel, China and other countries. Its principles are condensation method, solar adsorption refrigeration method, and solar semiconductor refrigeration method. 2.1. Condensation method The condensation method mainly uses the refrigerant circulation system to make the air flow through the evaporator below the dew point temperature to obtain condensate water. In 1993, Khalil [1] et al. studied the extraction of water from high temperature, high humidity air by freezing condensation. Luo [2] , a senior engineer, developed the "battlefield survival to protect the air intake equipment". In the condition of 5 ℃ -43 ℃ the relative humidity is higher than 40%, more than 30 kg water can be extracted from the air, and the water quality can fit in with the national drinking water standards by multistage filtration, disinfection and adding some trace elements. Zhang [3] of the Logistics Engineering College based on the kind of the air-to-water equipment, and thought that the unit time flow through the evaporator air volume, compressor speed and evaporator surface temperature are closely related. The interaction between the parameters is a complex system with large interference, variable load and high degree of nonlinearity. An adaptive fuzzy PID control technique is proposed to improve the effluent of the equipment. Evaporative condenser
Solar collectors
Wet air Water collector Adsorbents
Xiuyuan Hao et al. / Procedia Engineering 205 (2017) 2069–2075 Xiuyuan Hao et al./ Procedia Engineering 00 (2017) 000–000
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Fig. 1. Solar energy adsorption refrigeration method
2.2. Solar adsorption refrigeration method As shown in Fig. 1, the French scientist Jannot Y [4] proposed a solar adsorption refrigeration condensation method that first uses the solar radiation during the day to heat the adsorbent bed to desorb its internal adsorbent and at the other end condensate in the condenser. Under the atmospheric cooling effect at night, adsorption bed started the adsorption process, which made the continuous evaporation, adsorption of evaporative condenser in refrigeration, when the evaporator surface temperature is lower than the dew point temperature, the vapor will condense. Under the action of gravity, through the collection plate to store and store to the storage tank, the air intake process had achieved. 2.3 Solar semiconductor refrigeration method The basic principle of solar energy semiconductor refrigeration method [5] is that the battery is charged with photovoltaic power generation during the day and supplies power to the semiconductor refrigeration component, causing temperature of the cold plate falls below the dew point of the air. Under the action of the fan, the air over the cold plate, and the water vapor condensates in the cold plate surface. Because of the narrow flow passage between the cold plates, the airflow velocity is faster. Condensed water droplets formed by air flow to the condensate water area, flow velocity of small, and water droplets entrained in the air density greater than the density of air, will sink to the bottom of the water condensing chamber, so as to realize the separation of air and water droplets. At the same time, the heat recovery device is adopted to make the air entering the water intake device to be pre cooled. However, there are two problems in this technology: on the one hand, the conversion efficiency of solar cells is low and the loss is large. On the other hand, the efficiency of water intake is limited by the circulation area. Zhang using solar photovoltaic cells and semiconductor cooling film, put forward the development of portable air sampler. However, the water extraction equipment, water intake is small, high cost, only suitable for use in the field emergency conditions. 3 Principle and Shortcomings of Condensation Method
H/kj·kg-1
The condensation method of extracting water from air is achieved by the evaporator in the refrigeration system. The wet air crosses through the evaporator under the action of the fans, when the evaporator surface temperature tw is lower than the dew point temperature of the wet air td, the water vapor will condense on its surface, form small droplets or water film, and finally pool into the water tray. The dew point temperature determines the amount of moisture in the air, thereby affecting the potential of extracting water. In the enthalpy humidity chart, the process of wet air condensation is from the initial state point W to the last state point L. In this process, the water vapor in the air is continuously condensed into liquid water, and the condensation water of the unit mass air is m=dW-dL, that is, the difference between the moisture content of the two state points [6] , as shown in figure 2. W
Φ=100% L
dL
dW
d/g·(kg Dry air)-1
Fig. 2. Wet air cooling dehumidification process
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During the cooling and condensation process, a large part of the sensible heat released by the wet air to the dew point temperature consumes the cooling capacity of the evaporator, resulting in low efficiency and low water consumption. On this basis, Geng proposed energy-saving refrigeration dehumidifier, which is in the middle of the evaporator and condenser with a plate-fin heat exchanger equipment as a cold recovery device. Using low-temperature air-out of the evaporator to preheat the high temperature inlet air, the proportion of the latent heat from wet air vapor condensation to liquid water has a substantial increased in the total cooling capacity of the evaporator, which can greatly improve the water efficiency. The key to condensation method of extracting water from air is that the air dew point temperature is higher than the evaporator temperature of the refrigeration equipment. Otherwise, if the air moisture content and dew point temperature are low, the technology cannot obtain condensate. In the desert area, the air temperature is high, and the relative humidity is low, and the dew point temperature is only about 2 ℃; the air temperature even as low as below 0 ℃in the plateau. Therefore, the freeze-condensation method of air intake technology is not suitable for dry plateau, desert areas. 4 Principle and Analysis of Composite Scheme In dry areas, the air containing less moisture, it is difficult to realize the direct use of freezing condensation water. In this paper, firstly, the water vapor in the air is enriched by the method of absorption/desorption method, and then using condensation method to extract water. The power source required for the water intake process uses the rich solar energy of the desert area. 4.1 Principle of composite scheme
Solar panels
1
T/℃
Composite scheme of solar air water intake includes desiccant-wheel and refrigeration system. The desiccant-wheel is a solid moisture absorbent as the core component to adsorb and desorb moisture in the air, with high moisture absorption, continuous operation, compact and energy saving and other characteristics. Especially the use of solar energy and industrial waste heat and other low-grade energy, can be used for dry air moisture enrichment [7] . The scheme of system structure as shown in Figure 3, the evaporator compression refrigeration system of the air inlet is communicated with the air outlet runner regeneration, and the air outlet of the solar air heater is communicated with the regeneration air inlet of the desiccant-wheel. The condenser outlet is connected to the inlet of the solar heater. And the process of the enthalpy diagram shown in Figure 4, the general air treatment process is as follows: The fresh air (state point 1) passes through the condenser to obtain the condensed heat released by the refrigerant, and the air temperature rises to the state point 2, then heating by the solar heater to become the higher temperature regenerative air (state point 3). When the fresh air passes through the dehumidification runner, the temperature of the fresh air is lowered and becomes the high temperature and high humidity air (state point 4); The fresh air through the evaporator cooling to the dew point temperature (state point 5), water vapor liquefied, Where 6-7 indicates that the moisture in the fresh air is absorbed and the temperature is reduced. Fresh air
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Heater
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wheel
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1（6）
5 Fresh air
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d/g/kg Dry air
Xiuyuan HaoHao et al./ Engineering 00 (2017) 000–000 Xiuyuan et Procedia al. / Procedia Engineering 205 (2017) 2069–2075 Fig. 3. Physical model of water intake program
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Fig. 4. System enthalpy chart
4.2 Performance Analysis of desiccant-wheel The runner consists of two parts: the moisture absorption zone and the regeneration zone. The runner is filled with honeycomb-shaped flow channel, and the flow channel contains silica gel adsorbent. When the fresh air passes through the flow path of the moisture absorption zone under the action of the fan, the water vapor in the air is absorbed by the adsorbent due to the low water vapor pressure on the surface of the adsorbent. With the rotation of the runner, the saturated moisture flow channel is transferred to the regeneration zone, and the high temperature regenerative air flows through these honeycomb flow passages. Since the high temperature regenerated air corresponded to the water vapor pressure below the adsorbent surface water vapor Sub-pressure, so that the moisture in the silica gel is removed, and the runner regain the moisture absorption capacity. The regenerated air flowing through the runner also becomes an air with a relatively high moisture content, and the moisture enrichment in the dry air is realized. The temperature and moisture content of the fresh air, the temperature and moisture content of the regeneration air, the speed of the suction runner and the dehumidification angle and the heat of the adsorbent will have a complicated effect on the hygroscopic effect of the absorbent wheel. Increasing the regeneration temperature ensures that the water desorption is more thoroughly and the air with higher moisture content is obtained, but the energy utilization rate will decrease after a certain range. The relevant experimental studies show that: When the regeneration temperature is 100 ℃, the moisture absorption coefficient is the highest. The absorption wheel speed ω determines the residence time of the adsorbent in the moisture absorption zone and the regeneration zone. When the rotation speed is too fast, the desorption of the adsorbent is not sufficient, resulting in a decrease in the capacity of the rotor, On the other hand, when the speed is too slow, the adsorbent near the regeneration zone is too saturated, and the moisture absorption capacity is reduced. When the speed is 5r/h, the efficiency of the rotor is the highest; Dehumidification angle φR determines the ratio of the moisture absorption area and the regeneration zone. When φR is too small, the adsorbent cannot be fully regenerated. When φR is too large, the adsorbent moisture absorption is insufficient. When the wheel uses of dehumidification angle of 0.5, as shown in Figure 5, that half of the runner for the moisture zone, the other half of the regeneration zone, the amount of vapor will reach the maximum enrichment.
Hygroscopic zone
φR
ω
Regeneration zone
Fig. 5. Desiccant wheel physical model
4.3 Coupling analysis of composite scheme of absorption/desorption method and condensation method The desorption effect of desiccant wheel is positively correlated with the air temperature in the regeneration zone, that is, the higher the inlet air temperature, the better the desorption effect; In order to fully absorb the enriched vapor, it needs to consume a large amount of energy to improve the inlet air temperature of the regeneration zone of the desiccant wheel. At the same time, the desiccant capacity of the desiccant wheel is negatively correlated with the inlet
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air temperature in the hygroscopic zone, that is, the higher the inlet air temperature, the less the moisture absorption capacity. The desiccant-wheel significantly reduce the moisture absorption capacity in a high temperature and low humidity environment. It should try to increase the regeneration temperature, while reducing the fresh air temperature of the moisture absorption zone. In the proposed program，the temperature of the air entering the evaporator is high and the enthalpy is large, and there are some cold and heat offset in some links, which result in the low energy efficiency ratio. On this basis, an improvement program is proposed——using the new three plate fin heat exchanger to recover the waste heat and cooling process, as shown in Figure 6. By using the first heat exchanger, making the use of high temperature air from desiccant-wheel regeneration zone to preheat regeneration fresh air and reducing the regeneration of heating energy consumption, and also precool the high temperature and humidity air go into the evaporator to improve the efficiency of condensate water and larger the amount of water. By using the second heat exchanger, the high temperature and high humidity air from the desiccant-wheel regeneration zone is pre-cooled by the first heat exchanger and then through the second heat exchanger, transfers heat with the evaporator cool air and remove part of the sensible heat further after the precooling, and then go into the evaporator, which can further improve the capacity of steam compression refrigeration system cooling water and unit operating efficiency. By using the third heat exchanger, before the outdoor air from the high temperature and low humidity environment go into the dehumidification wheel moisture absorption area, first through the third heat exchanger, and transfers heat with cool air from the two heat exchanger, after precooling the outdoor air, and then by moisture absorption zone of the dehumidification wheel for moisture enrichment, to improve the efficiency of the whole system of extracting water. The capacity of the desiccant-wheel is less than the capacity of the evaporator to handle air volume in unit time. In order to solve the problem of the two air volume mismatch, the amount of fresh air can be introduced before the evaporator, so as to adjusting the proportion of the fresh air by the valve. The enthalpy profile of the air conditioning process is shown in Figure 7. 4 First heat exchanger Desiccant wheel
Condenser 3
2
3
Fresh air
Heater 4
Evaporator
11 Fresh air 12 Third heat exchanger
Second heat 7 exchanger
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5
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Fresh 1 air
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1
9
14 T（℃）
Solar panels
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7
2 1（12）
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11 10 9
8
10
Humidity ratio（g/kg）
Figure 6. Physical model of extracting water from air
Figure 7. Psychrometric chart of system
5 CONCLUSIONS There are many domestic and foreign research on the methods of extracting water from air, but most of them are used for high humidity. However, for the low moisture region, such as Gobi, desert and other areas, this paper puts forward the combination of the absorption/desorption method and the condensation method. Firstly, the moisture in the air is enriched by the desiccant-wheel, then using the condensation method to extract water. This paper presents a set of feasible scheme for the air intake in the dry desert and plateau area.
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6 ACKNOWLEDGEMENTS The author would like to give his thanks to undergraduate student, X Y Hao for his assistance in the experiment. References [1] A Khalil. Dehumidification of atmospheric air as a potential source of fresh water in the UAE. Desalination, 1993, 93(1-3) 587-596． [2] J. Luo, W. Zhang, X, Bai. Development and application of a field water maker. Journal of HV&AC. 2004, 34(4) 42-45 [3] H. Zhao et al. The characteristic research on solar water-getting tube for extracting water from air with adsorption. Renewable Energy Resources, 2014, 32(3) 259-264. [4] Y. Jannot. Production d'eau par condensation de l'humidité atmosphérique. Evaluation du procédé en zone sahélienne. Revue Générale De Thermique, 1993, 384 705-709. [5] J. Ye et al. Study on using solar-energy semiconductor refrigeration to dew water from air. Journal of Anshan University of Science and Technology, 2004, 27(4) 2 82-285. [6] H. Geng et al. A review on water extraction from air. CHEMICAL INDUSTRY AND ENGINGEERING PROGRESS, 2011, 8: 1664-1669. [7] M. P. Maiya et al. Application of desiccant wheel to control humidity in air-conditioning systems. Applied Thermal Engineering, 2004, 24(17– 18) 2777-2788.