Experimental study of an innovative solar water desalination system utilizing a passive vacuum technique

Solar Energy 75 (2003) 395–401 www.elsevier.com/locate/solener

Experimental study of an innovative solar water desalination system utilizing a passive vacuum technique S. Al-Kharabsheh, D. Yogi Goswami

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Solar Energy and Energy Conversion Laboratory, Mechanical and Aerospace Engineering Department, University of Florida, 220 MEB, P.O. Box 116300, Gainesville, FL 32611-6300, USA Received 30 June 2003; received in revised form 1 August 2003; accepted 4 August 2003

Abstract A solar desalination system based on an innovative passive vacuum concept, utilizing low-grade solar heat, was studied experimentally. The system uses the natural means of gravity and atmospheric pressure to create a vacuum, under which liquid can be evaporated at much lower temperatures and with less energy than conventional techniques. A vacuum equivalent to 3.7 kPa (abs) or less can be created depending on the ambient temperature at which condensation will take place. The system consists of a heat source, an evaporator, a condenser, and injection, withdrawal and discharge pipes. The effect of various operating conditions (withdrawal rate, depth of water body and temperature of the heat source) were studied experimentally and compared with theoretical results. The experimental results agreed well with the theoretical predictions. It was found that the effects of withdrawal rate and the depth of water in the evaporator were small while the effect of heat source temperature was significant. Ó 2003 Published by Elsevier Ltd.

1. Introduction Solar energy may be used to supply the required energy for a desalination process either in the form of thermal energy or electricity. The most widely used solar desalination system is a simple solar still, where the heat collection and distillation processes take place in the same equipment. The main disadvantage of a simple solar still is its low efficiency, which rarely exceeds 50%, averaging value of 30–40% (Delyannis and Belessiotis, 2001). The daily solar still production is about 3–4 l/m2 (Kalogirou, 1997). Simple solar stills have been studied extensively to improve their efficiency. A theoretical analysis by Dunkle (1961), and the relations that he derived for the heat and mass transfer within the still, formed the basis for many research efforts since then. Dunkle found out that the mass transfer rate depends on the temperature difference between the water surface and

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Corresponding author. Tel.: +1-352-392-0812; fax: +1-352392-1071. E-mail address: [email protected]fl.edu (D.Y. Goswami). 0038-092X/$ - see front matter Ó 2003 Published by Elsevier Ltd. doi:10.1016/j.solener.2003.08.031

the glass cover. In order to increase this temperature difference some researchers (Boukar and Harmim, 2001; Kumar and Tiwari, 1996; Lawrence and Tiwari, 1990; Tiwari et al., 1996; Yadav and Jha, 1989; Yadav, 1993; Yadav and Prasad, 1995) studied the effect of coupling the solar still to a flat plate solar collector. Another way to increase the temperature difference is to reduce the temperature of the glass cover.The temperature difference between the saline water surface and the transparent cover could be increased by adding a condenser to the still, thus increasing the heat sink capacity, hence the still performance (El-Bahi and Inan, 1999; Fath and Elsherbiny, 1993; Khalifa, 1985; Saatci, 1984). Evaporation at a low temperature using vacuum conditions, leads to a good improvement in the system efficiency as the evaporation rate increases with the reduction of pressure. System productivity higher than that from similar solar desalination systems operating under atmospheric pressure has been reported by many researchers (Abu-Jabal et al., 2001; Jubran et al., 2000; Low and Tay, 1991; Tay et al., 1996; Uda et al., 1994). The present study utilizes vacuum conditions for evaporation and condensation, where a vacuum is created in

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a unique way proposed by Sharma (1994) using natural forces of gravity and atmospheric pressure. This paper presents experimental test results of the new water desalination system under passively created vacuum conditions and compares them with the theoretical ones.

2. System description and operating principle The proposed desalination system consists of a solar heating system, and an evaporation chamber and a condenser at a height of about 10 m above ground level, connected via pipes to a saline water supply tank and a fresh water tank, respectively. Fig. 1 shows a schematic of the system. A vacuum is created by balancing the hydrostatic and the atmospheric pressures in the supply and discharge pipes. The evaporation chamber has provisions to feed the cold fluid directly to the chamber and provide solar or other low-grade thermal energy through a closed loop heat exchanger as well as withdrawing the concentrated brine. The incoming cold fluid and withdrawn brine pass through a tube-in-tube heat exchanger in order to extract the maximum possible heat from the hot brine. The evaporation chamber is connected to a condenser, which dissipates the heat of condensation to the environment. It is known that the vapor pressure of seawater is about 1.84% less than that of fresh water in the temperature range of 0–100 °C. This means that if the top of the two chambers (saline water evaporator and fresh water condenser) are connected while maintained at the same temperature, water will distill from the fresh water side to the saline water side. In order to maintain the distillation of potable water from the saline water the vapor pressure of the saline water must be kept above

that of the fresh water, which is done by increasing the temperature of the saline water by solar energy. To start up the unit, it is filled completely with water initially. The water is then allowed to drop down out of the unit under the influence of gravity. Depending on the barometric pressure, water falls to a level of about 10 m from the ground level, leaving behind a vacuum above the water level in the unit. Vacuum enables the distillation of water at a lower temperature level, requiring less thermal energy. This heat can be provided from solar collectors, which will operate at a higher efficiency because of lower collector operating temperatures. Simple flat plate collectors may be used to heat the saline water in the evaporator. As saline water in the evaporator starts evaporating, its salinity increases which tends to decrease evaporation rate, so it becomes necessary to withdraw the concentrated brine at a certain flow rate and inject saline water at a rate equivalent to the withdrawal plus evaporation rates. The withdrawn water will be at a temperature equal to that of the evaporator, so it becomes necessary to recover the energy from it. A tube-in-tube heat exchanger is used for this purpose, where injected water flows inside the inner tube and withdrawn water will flow in the annulus in a counter-current direction. Under the influence of vacuum conditions at the saline water surface in the evaporator, water can be injected by the atmospheric pressure; hence no pumping power is required. This makes the proposed system a continuous process type, unlike a flat basin solar still which is usually a batch process. The withdrawn concentrated brine can be concentrated further and used to construct a solar pond, which may be used as a solar energy collection and storage system. The system will require periodic cleaning by flushing and restarting it, so that the non-condensable gases are not allowed to accumulate destroying the vacuum.

3. Experimental setup

Fig. 1. Schematic of the system.

A small-scale system was designed and built. A photo of the evaporator condenser is shown in Fig. 2, and the experimental setup is shown in Fig. 3. The experimental system has the following specifications. The heat exchanger coil for heat input to the saline water is a copper tube of 2.4 m length and 1.27 cm outside diameter. The evaporator is a cylinder of 0.2 m2 cross sectional area, 0.2 m height, with a truncated cone on top of it. The evaporator has a provision for feed water, through a 1.27 cm diameter copper tube, enclosed by 2.54 cm CPVC pipe that is used for withdrawing the concentrated brine. The two pipes form a tube-in-tube heat exchanger. The condenser is a 10.16 cm copper tube of 0.5 m length, 0.25 cm thickness. On its lateral surface, 10 copper fins of 25.4 cm diameter and 0.0635 cm

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Fig. 2. Photo of the evaporator–condenser.

Fig. 3. Photo of the experimental setup.

thickness are soldered 4 cm apart. The other end of the condenser is connected to a condensate receiver via a 1.27 cm PVC pipe.

temperature profiles from the experiments. A measurement uncertainty analysis was conducted using standard deviation method to calculate the uncertainty of directly measured values within a given level of confidence (taken as 90%). For quantities calculated from measured quantities, the method of propagation of errors was used, where the total uncertainty was calculated as the combination of uncertainties of individual components (Wilson, 1952). The obtained uncertainty limits are given in Table 1. The first set of tests were performed for the heat source temperature of 60 °C, a withdrawal rate of 0.1 kg/ h, and the depth of water in the evaporator as 0.08 m. This test was repeated six times. Fig. 4 shows the temperature profiles of the saline water as a function of time. Also included in the figure are the ambient temperature profiles during those tests. As can be seen from the figure, saline water temperature profiles show the same trend, and at steady state conditions the difference between the maximum and the minimum saline water temperatures is about 2.1 °C, or about 4%. This difference can be attributed to the variation in the ambient temperature during the tests and the measurement uncertainty. The water outputs during these six tests are shown in Fig. 5. These tests were conducted under the same experimental conditions except for the ambient temperature, which was different for each day because of the

4. Results A number of tests were performed covering various combinations of operating conditions. The results are presented in this section. Theoretical results are also presented which are obtained using a simulation model presented in earlier papers by the authors (Al-Kharabsheh and Goswami, 2003a,b). The theoretical results were based on the measured heat source and ambient

Table 1 Measurement uncertainty limits Quantity

Uncertainty

Flow rate (kg/min) Output (kg/day) Heat input (W)

5.3% 6.8% 5.25%

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Fig. 4. Saline water and ambient temperatures for six tests (each line style represents one test day). Source temperature ¼ 60 °C; withdrawal rate ¼ 0.1 kg/h; and evaporator water depth ¼ 0.08 m.

Fig. 5. Theoretical and experimental output for six tests. Source temperature ¼ 60 °C; withdrawal rate ¼ 0.1 kg/h; and evaporator water depth ¼ 0.08 m.

outdoor tests. This explains the difference between the results from various tests. The first day, which has the lowest ambient temperature, has a significantly higher output (about 0.9 kg) than the other days. It is seen from these results that the ambient temperature affects the system output significantly. The lower the ambient temperature, the higher the output. A similar trend exists with respect to the fresh water temperature. Also shown in Fig. 5 are the theoretical simulation results based on measured temperature profiles of the heat source fluid and the ambient. The experimental and theoretical results agree very well. The maximum difference is about 0.049 kg, or about 5%, which is within the measurement uncertainty. It was noticed that the experimental values were always less than the theoretical ones. This may be attributed partly to the fact that the theoretical model assumes that all molecules evaporated from the saline water inside the evaporator will reach the condenser and condense as liquid water, whereas in real life a number of those molecules might fall back to the pool. Also the model assumes that the fins are an integral part of the condenser, whereas they were soldered to the condenser surface which would reduce the rate of

heat transfer from the condenser, hence the system output would be reduced. Another possible reason is that the model assumes the heat loss from the system to be by natural convection only, whereas it might be higher because of wind. Both experimental and theoretical outputs are broken down into daytime output (time during which heat is supplied to the system) and nighttime output (time after the heat is no longer supplied to the system), as shown in Fig. 5. The night time output is a result of the heat stored in the system during the initial hours of operation, and varies slightly for different tests, depending on the saline water temperature at the end of the test and the ambient temperature. The maximum and minimum experimental night time outputs (excluding the first day) were 0.319 and 0.290 kg, respectively. The saline water temperatures at the end of these two days were 49.7 and 49.4 °C, respectively, but the average ambient temperatures were 27 and 30 °C, respectively. Additional results from the previous six tests are presented in Figs. 6 and 7. Fig. 6 shows how the saline water temperature and the heat input vary with time both experimentally and theoretically. Also shown in the figure are the heat source and ambient temperatures. The saline water temperature increased with time and reached a steady state value of about 50 and 48 °C, for experimental and theoretical results, respectively. The higher value obtained from the experiments may be due to the fact that the temperature was measured at a distance of 10 cm from the evaporator wall, where the temperature may be slightly higher than that near the wall, where the heat loss takes place. In the theoretical model, the saline water temperature was assumed to be uniform throughout the evaporator. The heat input to the system was high initially, to raise the temperature of the saline water and the evaporator material (stored as sensible heat). This stored heat becomes useful at night time. As the system reached steady state, the experimental and theoretical values of the

Fig. 6. Experimental and theoretical saline water temperature and heat input with time. Source temperature ¼ 60 °C; withdrawal rate ¼ 0.1 kg/h; and evaporator water depth ¼ 0.08 m.

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Fig. 7. Variation of the output, output rate, and energy and exergy efficiencies with time. Source temperature ¼ 60 °C; withdrawal rate ¼ 0.1 kg/h; and evaporator water depth ¼ 0.08 m.

energy input were about 109 and 103 W, respectively. The difference is about 5%, which is within the uncertainty of measurements. Fig. 7 shows the variation of the theoretical output rate, accumulated output, and the energy and exergy efficiencies with time, based on the measured temperature profiles of the heat source fluid and the ambient. The accumulated output during the six hours test reached a value of 0.495 kg compared to 0.462 kg obtained experimentally. The difference is about 6.6%, which is within the measurement uncertainty. At steady state the output rate was 0.115 kg/h compared to 0.108 kg/h obtained experimentally, a difference of about 6%. The energy and exergy efficiencies were low at the beginning of the test and increased with time. When the system reached steady state conditions the energy and exergy efficiencies reached values of 74% and 79%, respectively. The effect of water depth in the evaporator on the system performance is shown in Fig. 8. Three different depths were considered: 0.06, 0.08, and 0.1 m. For these tests, the heat source temperature was set at 60 °C and the withdrawal rate was 0.1 kg/h. The effect of water depth should not be significant, but there are some differences due to the variation in the ambient temperatures. The saline water temperature for the test day corresponding to a water depth of 0.1 m was about 47 °C, as compared to about 50 °C for the other two days. However, the test with water depth of 0.1 m gave the highest accumulated output of about 0.908 kg, with the output rate of about 0.124 kg/h under steady state conditions, compared to an accumulated output of about 0.750 kg and an output rate of about 0.118 kg/h for the other two test days. This is because the average ambient temperature for that day was 21 °C compared to about 27 °C for the other two days resulting in a temperature differential of 26 and 23 °C, respectively. The agreement between the experimental and theoretical

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Fig. 8. Effect of depth of water body on the system performance.

output results for each depth is within the experimental uncertainty. Although no comparison of experimental outputs and output rates can be made for the present concept with the conventional solar still, the fact that the experimental results agree with the theoretical predictions, allows us to use a comparison based on theoretical simulations. Theoretical simulation showed that this new concept would give as much as two times the output of a simple flat basin solar still for the same input and evaporator area. (Al-Kharabsheh and Goswami, 2003b) The effect of withdrawal rate is shown in Fig. 9. Three different withdrawal rates were considered: 0, 0.1, and 0.5 kg/h. For these tests, the heat source temperature was 60 °C and the depth of water body was 0.08 m. The average ambient temperatures during these tests were almost the same. The effect of withdrawal rate was found to be very small. With the increase in the withdrawal rate from 0 (batch process) to 0.5 kg/h, the output rates and the accumulated outputs decreased slightly from 0.119 kg/h and 0.763 kg to 0.116 kg/h and 0.755 kg, respectively. The effect of heat source temperature is shown in Fig. 10. Three different temperatures were considered, 40, 50 and 60 °C. For these tests, the withdrawal rate was 0.1 kg/h, and the depth of water body was 0.08 m. Although the ambient temperatures were different for the different tests, the effect of heat source temperature is significant.

Fig. 9. Effect of withdrawal rate on the system performance.

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References

Fig. 10. Effect of heat source temperature on the system performance.

The output rate, accumulated output and saline water temperature all increased with the heat source temperature as shown in the figure. The theoretical results agree very well with the experimental results, except for the saline water temperature. As explained earlier, the disagreement in these temperatures is mainly because the measurement was made at 10 cm away from the evaporator wall while the theoretical analysis assumed a well mixed uniform temperature in the evaporator.

5. Conclusions An innovative solar water distillation system, that uses a vacuum created by natural forces, was studied experimentally. The system is a continuous type as opposed to a batch type conventional solar still. Because of passively created vacuum conditions, it requires lower temperatures for distillation, which can be easily provided by flat plate solar collectors. The experimental results agree well (within the experimental uncertainty) with the theoretical simulation results. Although it was not possible to compare the experimental results of this innovative system with a conventional solar still, based on the agreement of the experimental and theoretical results, theoretical simulations can be used for comparison. Based on theoretical simulations the present system would perform much better than a simple flat basin solar still. A multi-effect system based on the same principle, which would utilize the latent heat of condensation from one stage to evaporate a part of water in the next stage, would improve the performance even further. The effects of water depth in the evaporator, concentrated brine withdrawal rate, and heat source temperature on the system performance were studied. The effect of water depth in the evaporator and the withdrawal rate of concentrated brine (in the range considered) were found to be small, whereas, the effect of heat source temperature was found to be significant.

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