to the humidifier, the mass flow rate of salt water, and the mass flow rate of the process air. There existed an optimum rotation speed for the fan corresponding to ...
DESALINATION ELSEVIER
Desalination 130 (2000) 169-175 www.elsevier.com/locale/desal
Experimental investigation of a solar desalination unit with humidification and dehumidification "a*
Y.J. D a l , H.F. Z h a n g b "Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, PR China email: yjdai@sh 163.net l'Institute of Air-conditioning and Solar Energy, Northwestern Polytechnic University, Xi 'an 710072, PR China Received 9 May 2000; accepted 10 July 2000
Abstract
A solar desalination unit with humidification and dehumidification is presented. Experiments on the unit were conducted. It was found that the performance of the system was strongly dependent on the temperature of inlet salt water to the humidifier, the mass flow rate of salt water, and the mass flow rate of the process air. There existed an optimum rotation speed for the fan corresponding to an optimum mass flow rate of air with respect to both thermal efficiency and water production. The unit worked perfectly and the thermal efficiency was above 80%. Other low-grade heat resources such as waste heat can also be utilized to drive the desalination process. It was expected that the unit would be of great potential for use in desalination in arid areas and isolated islands.
Kevwords: Desalination; Humidification and dehumidification; Heat mass transfer; Water production
1. Introduction
Shortages of water occurring at places with hot climate may make the application of solar energy for water desalination practical. It is well known that solar desalination exhibits considerable economic advantages over other salt-water desalination processes because of cost-free energy, reduced operating costs and its simple structure. *Corresponding author.
Corrols Wilson developed the first still using solar energy, which supplied the fresh water need for a mining district. Thereafter, the technology of solar desalination made rapid progress in many countries [1]. Delyannis and Delyannis [2] and Delyannis [3] reviewed the development of solarassisted desalination historically at different periods of time. Early devices of solar desalination had drawbacks of poor utilization of condensation heat and low thermal efficiency due to the large heat losses from the glass cover, under which the condensation occurs. Moreover,
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it is impossible for them to lift the evaporation temperature and decrease the condensation temperature at the same time since evaporation and condensation take place at the same space. Multi-stage flash (MSF) and reverse osmosis (RO) are two important desalination processes used at present. Large-size installations producing millions of gallons of fresh water are in operation using these techniques, which need large amounts of basic thermal and/or electrical energy for fresh water production from seawater or brackish water. Mink [4] has investigated a solar still with heat recycling. Fath [5] and Ei-Bahi [6] have studied the effect of condenser on a solar still. Gomkale [7] has verified the ability of solar stills using as water supply devices in villages oflndia. It has been shown that the humidificationdehumidification process could be an efficient and economical method of desalination [8]. Different from the work reported in [8], the process used in this work is an open-air cycle type in which air is forced to pass through the humidifier and condenser by a fan, and fresh water is produced in the condenser due to the condensation of water vapor. In China, fresh water is highly scarce in arid remote areas and on some islands. Small- or medium-scale water-producing units using lowgrade heat resources are of great importance since conventional desalination methods or water transportation are uneconomical [3]. Research on small or medium desalination units has been undertaken at Northwestern Polytechnic University of China. Two prototypes using a humidification-dehumidification process have been developed. At typical conditions, the smaller one is expected to produce 10kg fresh water per hour and the larger one is expected to produce 100 kg fresh water per hour. The aim of this paper is to report the experimental results of the units and analyze system performance.
2. Prototype and working principle A solar desalination unit with a humidification and dehumidification cycle, which is configured mainly by a solar collector, humidifier and condenser, is shown in Fig. 1. Seawater is heated up to TAby a solar collector which receives solar energy and turns it to thermal energy, and then is sprayed to form falling film at the surface of the honey comb wall in the humidifier by a sprinkler. Fig. 2 shows the configuration of honey-comb paper, which is porous and durable for wetting. The channel is formed by two kinds of wave line shape paper; one is 45 ° and the other is 60 ° . The paper constitutes the packing materials of the humidifier. Driven by a fan, the process air is forced to pass through the humidifier where it becomes hot and humid because of the heat and mass exchange between seawater and moist air. Then the process air passes through the condenser cooled by cold seawater where water vapor condenses and turns into fresh water. Moist air out of the condenser becomes humid and cold. The condenser is specially designed to meet the need for producing fresh water. Cold seawater flows in the tube channel and fresh water is produced on the condensation surface in the condenser at the same time. The remaining seawater drawn from humidifier becomes cold and is collected at the bottom basin. The water is fed to the solar collector again, since it is not so much concentrated and is still warm. This way the fresh water can be produced continually. The main characteristics of the systems are: • the process air flows in a straight channel to avoid large pressure losses and ensure good effects of evaporation and condensation, • recycling of hot unevaporated seawater reduces loss of thermal energy, • condensation heat is recovered efficiently by cooling water which is then supplied to the solar collector., • other low-grade heat resources, such as waste heat, gas/oil/coal burning, etc, can also be utilized efficiently.
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Y.J. Dai, 11.F. Zhang I Desalination 130 (2000) 169-175
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Fig. 1. Schematic of solar desalination unit with humidification and dehumidification.
Fig. 2. Schematic of honey-comb packing material in humidifier.
water productivity. Different from the productivity of a conventional solar still, which is water production per unit time per unit surface of condenser, water productivity (Mp) here stands for the water production per unit time per unit surface to receive solar irradiation, and represents the ability for this desalination unit to produce water. Thermal efficiency stands for the ratio of the amount of useful energy to total heat input per unit time per unit surface to receive solar irradiation; in other words, it is the ratio of the minimum energy to obtain a fixed amount of water to the practical heat input.
3. Indexes
~. -- MI.,'hI2Q~
Two coefficients to evaluate the system performance are defined: thermal efficiency and
Here, hrg is enthalpy of vaporization of water, and
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Y.J. Dai, H.F. Zhang / Desalination 130 (2000.) 169-175
Qr is heat input. Particularly, the reason that this unit does not adopt the conventional productivity is due to the different configuration of the condenser, namely, the glass cover for a conventional solar still, but a fin-tube-type heat exchanger for this unit.
hot seawater, rotation speed of the fan, temperature and humidity of air at different points, etc. A data acquisition system was used to collect the data. Thermal couples were used to measure all temperatures, and a meter for wet bulb temperature was used to measure the humidity ratio. A rotation meter was adopted to record the rotation speed.
4. E x p e r i m e n t The solar desalination unit with a humidification and dehumidification cycle described in this paper uses moist air for working fluids. Fresh water is produced by means of state variation of moist air in the humidifier and the condenser. An experiment on a unit with 100kg fresh water production was conducted in the laboratory located at Northwestern Polytechnic University. The unit is 1 m × 1 m × 1.5 m. Here water vapor with a high temperature and pressure from a boiler is used as the heat resource instead of solar energy in order to acquire the test results more rapidly. The humidifier is 0.6m long. The following parameters had to be measured: temperatures of inlet and outlet water of the humidifier, mass flow rate of cooling water, temperature of cooling water, mass flow rate of
5.Resultsanddiscussion Corresponding to the experiment, the thermal efficiency used here does not account for the factor of solar collector, and is represented by 1"1. Moreover, Mv water production per unit hour of the desalination unit, together with 1"1,are applied to analyze the system performance thereafter. Fig. 3 shows the effect of mass flow rate of feed water to the humidifier. Both Mv and 1] increase with the increase of hi+. There exists a quasi-linear relationship between Mp and hi.,.,or r ! and ni~ when th+ varies in a given range. Because of the complicated heat and mass transfer taken place in the honeycomb humidifier, the process air is warmed and humidified when it passes through the surface of falling film formed along
x experlmentalresult01) / ]1.o ...........curvefltrelult ~ l • expedmenteldata(M,) ~ ~o.e curvefit result ~ .....................~.........."i
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Fig. 3. Effect of mass flow rate of water on Mo and TI.
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EJ. Dai, H.F. Zhang / Desalination 130 (2000) 169-175
the wall of honeycomb paper. The greater the rhs, the higher the temperature and the humidity ratio of air will be. This is beneficial to lift water production. Fig. 4 shows the effect of rotation speed of the fan on thermal efficiency of the system. Rotation speed is the indication of mass flow rate of air: the higher the rotation speed, the more the flow rate of air. Two cases were tested, one conducted at 85°C (inlet salt water to humidifier) and the other at 65°C. There are optimum points corresponding to the rotation speed for the two cases. The lower the temperature of inlet water to the humidifier, the smaller the optimum rotation
speed will be. Under optimal operating conditions, thermal efficiency is about 0.85. Fig. 5 shows the effect of rotation speed on water production, Mp. There are also optimum values with respect to rotation speed for the two cases. The optimum value of rotation speed at 85°C (inlet salt water to the humidifier) is about 850 rpm, while the value at 65°C is about 1000 rpm. In practical operation the mass flow rate of air is adjusted by the changing rotation speed of the fan. The specified rotation speed of the fan is 1400rpm. The real relationship between rotation speed and mass flow rate of air is given in Fig. 6. Higher or lower mass flow rate 110-
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Fig. 5. Effect of rotation speed on Mp. • experimental data(1260rpm) x experimental data(840rpm) - curve fit result(1260rpm) ...........curve fit result(840rpm)
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YJ. DaL H.F Zhang / Desalination 130 (2000) 169-175 •
• experimental data(1260rpm) y experimental data(840rpm) 100 ............ curv efit result(1260rpm)
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above value (no account of solar collector) times the efficiency of solar collector, that is to say, 1],=1] "1]s. Here, 1], is the thermal efficiency of the solar desalination unit, and 1],, is the efficiency of the solar collector. Water productivity of the solar desalination unit can be obtained from the following equation:
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Fig. 8. Effect of temperature of inlet water to humidifier on ~ , .
of air is not good for increasing both water production and thermal efficiency. Figs. 7 and 8 show that both thermal efficiency and water production increase dramatically with the temperature of inlet water into a humidifier under two different rotation speeds. Temperature has a strong influence on water production and thermal efficiency if the conditions of condensation are not changed. This is because the process air acquires more heat, and more water is evaporated into the air. As a result, the force to drive the heat and mass transfer is increased. It is one of the special advantages over the traditional devices in which evaporation and condensation take place at the same space, that this unit is able to increase the evaporation temperature while keeping the condensation temperature constant since evaporation and condensation are not interfered with any longer. The desalination process can also be driven by other low-grade heat resources such as waste heat, because it works perfectly within the temperature range of 70-90°C. If the conversion process from solar irradiation to thermal energy is taken into account, the thermal efficiency of the system should equal the
Assuming that 1]., is 0.8, 1] is 0.85, and the average density of solar irradiation (/) is 700W/m 2 (8h/d), then 1], is 0.68, which means that the water productivity for the described solar desalination unit is about 6.2 kg/m2/d. For a waste heat powered system, it is unnecessary to consider these problems. The experimental results can be directly utilized. The mass flow rate of cooling water in the condenser, concentration of the recycled salt water (since it is still warm), and other parameters which affect the system performance will be discussed in another paper.
6. Conclusions
A solar desalination unit with a humidification and dehumidification cycle was tested. The unit improved the effect of evaporation and overcame the difficulty of increasing the evaporation temperature and decreasing the condensation temperature at the same time by using a falling film humidifier with a large evaporation surface and forced convection to enhance the heat and mass transfer. An optimum mass flow rate of air existed under given conditions. Reuse of the concentrated seawater within a limited range and recovery heat from cooling water is good for saving energy. Other low-grade heat resources such as waste heat can also drive the unit. The thermal efficiency of the system is about 0.85
Y.J. Dai, H.F. Zhang / Desalination 130 (2000) 169-175
under optimal operating conditions. It was proved that the system is of potential to be used as a desalination unit of seawater.
7. Symbols -
-
I -M;
Qr
--
--
T
Enthalpy of vaporization of water, kJ kg- 1 Density of solar irradiation, W m -2 Water productivity, kg m -2 hWater production per hour, kg hMass flow rate of feed water, kg h-1 Heat input, kJ hTemperature, °C
Greek
--
Tt,.
--
~u
--
Thermal efficiency without account of the solar collector Efficiency of solar collector Efficiency of solar desalination unit
175
Acknowledgement
This work was undertaken at the Northwestern Polytechnic University, Xi'an, P.R. China, and grateful acknowledgment is given to the staff and students of NPU who contributed to the work. This paper was sponsored by the Fund of the Post-doctoral Science Foundation of China and Research Project of Key Fundamental Science, China (No. G2000026309).
References
[1] T.S.Gong, New Energy (China), 14(10) (1992) 7. [2] A. Delyannis and E. Delyannis, Desalination, 45 (1983) 361. [3] E. Delyannis,Desalination,67 (1987) 3. [4] G. Mink, M.M. Aboabboudand E. Karmazsin, Solar Energy, 62 (1998) 309. [5] H.E.S. Fath and S.M. Elsherbiny, Energy Convers. Mgmt., 34(1) (1993) 63. [6] A. EI-Bahiand D. Inan, Desalination, 123 (1999) 79. [7] S.D.Gomkale,Desalination, 69 0988) 177. [8] N.Kh. Nawayseh and M. Mehdi Farid, Energy Conversion Mgmt., 40 (1999) 1423.
