Solar energy desalination for arid coastal regions: development of a humidification dehumidification seawater greenhouse

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1 Solar Energy 75 (2003) Solar energy desalination for arid coastal regions: development of a humidification dehumidification seawater greenhouse M.F.A. Goosen a, *, S.S. Sablani a, C. Paton b, J. Perret a, A. Al-Nuaimi c, I. Haffar a, H. Al-Hinai d, W.H. Shayya e a Department of Bioresource and Agricultural Engineering, Sultan Qaboos University, P.O. Box 34, Al-Khod 123, Muscat, Oman b Seawater Greenhouse Ltd, 2A Greenwood Road, London E8 1AB, UK c Department of Civil and Architectural Engineering, Sultan Qaboos University, Al-Khod 123, Muscat, Oman d Department of Mechanical and Industrial Engineering, Sultan Qaboos University, Al-Khod 123, Muscat, Oman e 134 Marshall Hall, Morrisville State College, Morrisville, NY 13408, USA Received 6 June 2003; accepted 10 July 2003 Abstract The long-term aim of our research is to develop humidification dehumidification desalination technology for farms in arid coastal regions that are suffering from salt-infected soils and shortages of potable groundwater. The specific aim of our current study was to determine the influence of greenhouse-related parameters on a process, called Seawater Greenhouse, which combines fresh water production with growth of crops in a greenhouse system. A thermodynamic model was used based on heat and mass balances. The dimension of the greenhouse had the greatest overall effect on the water production and energy consumption. A wide shallow greenhouse, 200 m wide by 50 m deep gave 125 m 3 d 1 of fresh water. This was greater than a factor of two compared to the worst-case scenario with the same area (50 m wide by 200 m deep), which gave 58 m 3 d 1. Low power consumption went hand-in-hand with high efficiency. The wide shallow greenhouse consumed 1.16 kw h m 3, while the narrow deep structure consumed 5.02 kw h m 3. Analysis of the local climate indicated that the structure should be built facing the NE direction. We are also in the process of building a commercial size Seawater Greenhouse at a site by the sea. The aim is to demonstrate the technology to local farmers and companies in the Arabian Gulf. The system will allow for the reclamation of salt-infected land by not relying, at all, on groundwater resources. Ó 2003 Elsevier Ltd. All rights reserved. 1. Introduction * Corresponding author. Tel.: ; fax: addresses: theog@squ.edu.om, mattheus@omantel. net.om (M.F.A. Goosen). There is a growing realization that the long-term solution to a shortage of potable water lies in a coordinated approach involving water management, purification, and conservation (Goosen and Shayya, 1999). A key feature to this approach is the development of environmental friendly and sustainable water purification techniques (Goosen et al., 2000; Hamed et al., 1993). While the most common desalination methods are based on fossil fueled thermal and membrane processes, alternative techniques, such as solar desalination, are also being considered (Delyannis and Belessiotis, 1999). Solar methods are well suited for the arid and sunny regions of the world as in North Africa and the Arabian Peninsula. A variety of solar desalination devices have been developed. It has become apparent that a key feature in improving overall efficiency is the need to gain a better understanding of the thermodynamics of the processes X/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi: /j.solener

2 414 M.F.A. Goosen et al. / Solar Energy 75 (2003) and how the designs can be made more efficient. While a system may be technically very efficient it may not be economic. Materials of construction, plant size and location, and operating costs must all be taken into account. Therefore, both efficiency and economics need to be considered when choosing a solar desalination system. The Seawater Greenhouse is a new development that produces fresh water from sea water, and cools and humidifies the growing environment, creating optimum conditions for the cultivation of temperate crops (Paton and Davis, 1996; Sablani et al., in press). The concept has been developed in collaboration with a number of eminent research organizations and tested with a prototype on the Canary Island of Tenerife. In many countries that suffer a chronic shortage of water, such as those of the Middle East and North Africa, over 80% of all fresh water consumed is used for agriculture. As fresh water resources are finite, there is an inexorable pressure to reduce agricultural use of water to meet the growing demand for domestic and industrial use. The Seawater Greenhouse provides a solution by creating a growing environment that substantially reduces the amount of water required for irrigation, together with a new source of fresh water. The main objective of our study was to determine the influence of greenhouse-related parameters on a desalination process that combines fresh water production with the growth of crops in a greenhouse. A thermodynamic model was used based on heat and mass balances. The thermodynamic and economic efficiencies of solar water desalination systems were also briefly reviewed. 2. Thermodynamic and economic considerations in solar desalination Good overviews of solar desalination schemes are given by Goosen et al. (2000), Kumar and Tiwari (1998), Chaiba (2000) and Hamed et al. (1993). A multiple-effect distiller is generally more effective thermodynamically than a single-effect distiller since the former uses the available heat energy more than once. Single and double-basin solar stills can also be coupled with a flat-plate collector to enhance the distillate output. In a study by Yadav and Jha (1989), a transient analysis was presented of a double-basin solar still coupled with a flatplate solar collector. A paper by Kumar et al. (1989) was devoted to the development of a transient model to study the performance of a double-basin solar still. The still was integrated with a heat exchanger in order to enhance its distillate output per unit area. The increase in efficiency of multiple-effect solar desalination systems must be balanced against the increase in capital and operating costs when compared to a simpler single-basin still. The efficiency of a multipleeffect solar still can be increased, for example, by inclining the glass cover surface towards the sun and installing weirs (i.e. grooves) on the upper surface of the glass to hold and warm the saline water before it enters the still. The addition of flat-plate collectors and heat exchangers to transfer waste heat from local industrial plants to the solar still provides an additional way of enhancing the productivity of the system. The earliest solar distillation plant on record was designed and built in 1872 by Charles Wilson in Chile (Talbert et al., 1970). A more recent example of a humidification dehumidification system is a pilot plant built at Kuwait University (Delyannis and Belessiotis, 1999). The system consisted of a salt gradient solar pond, 1700 m 2 in surface area, used to load the air with humidity. An air-dehumidification method suitable for coastal regions was also described by Khalid (1993). Paton and Davis (1996) used the humidification dehumidification method in a greenhouse structure for desalination and for crop growth (Fig. 1). Their Seawater Greenhouse produced fresh water and crops in one unit. The temperature differences between the solid surfaces heated by the sun and cold water drawn from below the sea surface was the driving force in the system. The Greenhouse acted as a solar still while providing a controlled environment suited for the cultivation of crops. Mathematical models (i.e. thermodynamic models) need to be employed to optimize the desalination system performance. Several humidification dehumidification pilot plants have been built around the world. Lower operating costs in the form of alternative energy sources (e.g. waste heat or wind energy) were found to be key factors in their economic viability. Cost estimates of solar desalination systems were reported by Delyannis and Belessiotis (1999). They noted that solar energy conversion plants are capitalintensive enterprises. According to the authors, a general economic analysis is not easy to accomplish since only few studies report on the price of water or focus on economics. The problem is compounded by the fact that most of them are constructed from local materials using local personnel. In such a situation, prices differ considerably from one location to another. Hoffman (1992) presented a detailed theoretical cost analysis based on operational data for solar driven desalination plants. He concluded that the price of water from solar-powered desalination plants ranges from 0.52 to 1.59 $ m 3. Voivontas et al. (1999) analyzed water management strategies based on advanced desalination schemes (such as reverse osmosis and electrodialysis) powered by renewable energy sources (such as wind and solar energy). A framework was presented for developing a decision procedure that monitors water shortage problems and identifies the availability of renewable energy resources

3 M.F.A. Goosen et al. / Solar Energy 75 (2003) Fig. 1. Seawater Greenhouse. Seawater trickles down the front wall evaporator, through which air is drawn into the Greenhouse. Dust, salt spray, pollen and insects are filtered out leaving the air humidified and cool; Sunlight is filtered to remove radiation that does not contribute to photosynthesis; Air passes through a second evaporator and is further humidified to saturation point. Saturated air passes through the condenser, which is cooled using sea water. Distilled water condenses and is piped to storage. to power desalination plants. The cost of alternative solutions, taking into account energy costs or profits by selling to a grid, was estimated. Emphasis was given to the market forces and the relationships among technology prices and market potential. 3. Methodology A software program developed by Light Works Limited, England was used to model thermodynamic analysis of the humidification/dehumidification Seawater Greenhouse system. The computer program consisted of several modules: Seapipe, Airflow, Evaporator 1, Roof, Planting Area, Evaporator 2, Condenser (air/ water heat exchanger). Weather data for the year 1995 obtained from the Meteorological Office situated at Muscat, Sultanate of Oman, were used. The software needs a weather data file and a bathymetric (seawater temperature) file. These are specific to a location. The file contained transient data on solar radiation on a horizontal surface, dry bulb temperature and relative humidity of air, wind speed and wind direction. The bathymetric file contained temperature of the seawater at distance along the sea bed from the coast. The software program predicts the inside air conditions and water production for a given configuration/dimension of the greenhouse, and weather and bathymetric data. The program allows many parameters to be varied. These variables can be grouped into following categories: Greenhouse (i.e. dimension of the greenhouse, and its orientation, roof transparency of each layer, height of front and rear evaporative pads, height of the planting area, condenser); Seawater pipe; Air exchange. Three parameters i.e. dimension of the greenhouse, rooftransparency and height of the front evaporator were taken as variables. These parameters were varied as follows: dimensions of greenhouse (width length) Area was kept constant at 10 4 m 2 ;50 200, , , and 200 m 50 m; roof transparency and ; height of the front evaporator 3 and 4 m. The parameters kept constant were: height of planting area ¼ 4 m; height of the rear evaporator ¼ 2 m; height of the condenser ¼ 2 m, orientation of greenhouse ¼ 40 N; seawater pipe diameter ¼ 0.9 m, length ¼ 5000 m; volumetric flow ¼ 0.1 m 3 /s; pit depth ¼ )3 m, height ¼ 7.5 m, wall thickness ¼ 0.1 m; air change ¼ 0.15 (fraction)/min; fin spacing ¼ m and depth ¼ m. Three climatic scenarios were considered. In the temperate version the temperature in the growing area is cool and the humidity high. This version is suited to lettuces, French beans, carrots, spinach, tomatoes, strawberries and tree saplings, for example. In the tropical version the temperature in the growing area is warm and the humidity very high. Examples of suitable crops include aubergines, cucumbers, melons, pineapples, avocados, peppers and pineapple. The design is similar to that of the temperate version, but the airflow is lower. The oasis version allows for a diversity of crops. This version is separated into temperate and tropical sections of equal area. The areas covered by these

4 416 M.F.A. Goosen et al. / Solar Energy 75 (2003) Greenhouses were 1080 m 2 (temperate and tropical) and 1530 m 2 (oasis). 4. Results and discussion 4.1. Simulation studies The overall water production rate increased from 65 to 100 m 3 d 1 when the width to length ratio increased from 0.25 to 4.00 (Table 1). Similarly the overall energy consumption rate decreased from 4.0 to 1.4 kw h m 3 when the width to length ratio increased from 0.25 to Analyses showed that the dimensions of the greenhouse (i.e. width to length ratio) had the greatest overall effect on water production and energy consumption. A5 2 3 full factorial design was employed with five dimensions (width and length) of greenhouse, two roof transparencies and three heights of the front evaporator. A total of 30 simulation runs were carried out with 1 year of weather data (Table 1). The water production and power consumption data were analyzed using Statistical Analysis System (SAS) program. Analysis of variance (ANOVA) procedure was used to detect the significance of the dimension of the Greenhouse, transparency of roof materials and height of the front evaporator. The overall effects of roof transparency and evaporator height on water production were not significant. It was possible for a wide shallow greenhouse, 200 m wide by 50 m deep with an evaporator height of 2 m, to give 125 m 3 d 1 of fresh water. This was greater than a factor of two compared to the worst-case scenario with the same overall planting area (50 m wide by 200 m deep) and same evaporator height, which gave 58 m 3 d 1. For the same specific cases, low power consumption went hand-in-hand with high efficiency. The wide shallow greenhouse consumed 1.16 kw h m 3, while the narrow deep structure consumed 5.02 kw h m 3. Table 1 Results of the simulation runs for Oman using Waterworks software and 1 year s data Run # Greenhouse dimensions Evaporator Width (m) Length (m) height (m) Roof transparency (%) Water production (m 3 /day) Power consumption (kw h/m 3 )

5 M.F.A. Goosen et al. / Solar Energy 75 (2003) Performance of seawater greenhouse for various climate scenarios It can be seen in Table 2 that while the tropical version produced water most efficiently (i.e. lowest power consumption), it also produced the smallest surplus of water over that transpired by the crop inside. In this case the fan and pumps are run at their lowest rate to maintain high temperatures and humidity while still producing water in all conditions. Table 2 Water production and energy consumption Model Temperature outside ( C) Temperature inside ( C) Annual water production (m 3 ) Power consumption (kw h/m 3 Max Min Max Min Total Surplus water produced) Temperate Tropical Oasis Fig. 2. Annual temperature profile (top) and humidity profile (bottom) Seeb Airport 1997.

6 418 M.F.A. Goosen et al. / Solar Energy 75 (2003) Total fresh water production for the three climate scenarios was also calculated. One year s detailed meteorological data from Seeb Airport, Muscat was entered into the model to test the performance sensitivity for the various designs. The model results predicted that the Seawater Greenhouse would perform efficiently throughout the year, but with measurable variations in performance between the alternative versions. For example, the water production rate and energy efficiency results from the simulations using optimized and constant values for fan and pump speeds showed that the temperate scenario had almost double the water production rate per hectare compared to the tropical scenario (i.e. 20,370 m 3 /ha compared to 11,574 m 3 /ha) while the power consumption for the former was only slightly higher (i.e. 1.9 and 1.6 kw h/m 3, respectively) Design considerations for local climatic conditions As the Seawater Greenhouse uses renewable energy for climate control and to make fresh water, the first step in its design for any location is the analysis of the local climate. The UK Meteorological Climate Services Unit supplied the data file from Seeb Airport, Muscat. These files contained observations at 0.00, 3.00, 6.00, 9.00, 12.00, and h, covering a whole year (1997). Rainfall varied from less than 50 mm in central Oman rising to 300 mm in the Northern Mountains. Rainfall showed wide year on year and monthly variations, with sustained periods of above average and below average precipitation. Muscat had an average annual rainfall of 100 mm, nearly all of which fell between October and April. Temperature and humidity are high throughout the year, and the period May to September (i.e. day 120 to 280) is the hottest season with virtually no rain (Fig. 2, top). The peaks of high humidity usually occurred from sunset to sunrise with a corresponding reduction in humidity in the day (Fig. 2, bottom). The wind distribution below showed a marked pattern of prevailing wind from the Northeast with a smaller component from the Southwest (Fig. 3). Note that the length of each bar is proportional to the number of observations from that direction and not wind speed. The yearly average wind speed is 2.7 m/s although gusts of up to 50 m/s are recorded from 30 N. It is advantageous to point the Greenhouse into the wind such that the front wall evaporator is at right angles to the prevailing wind direction. This helps to drive the ventilation process and reduces fan power consumption. The choice of which direction is optimum is made easier by separating the daytime prevailing wind from that of the night. These showed that the wind nearly always comes off the sea from the NE during the day, and blows from the SW at night. This indicates that the structure should be built facing the NE direction. Fig. 3. Average annual wind distribution, Seeb Airport Closing remarks There are several benefits for the development of the humidification dehumidification Seawater Greenhouse system in arid regions. It provides for additional water supplies for other purposes such as the development of environmental projects. It also allows for the reclamation of salt-infected land by not relying, at all, on groundwater resources. In addition, it gives the opportunity to develop a high value agricultural sector that is sustainable in the long term and immune to climatic variations. A commercial size Seawater Greenhouse is currently being built at our SQU Desalination Research Laboratory site by the sea. The aim is to demonstrate the technology to local farmers and companies in the Arabian Gulf. We believe that this technology will be of real benefit to coastal farms, worldwide, that are struggling with the problem of saline intrusion. It will provide a real alternative to increasing shortages of groundwater. Acknowledgements This study has been made possible through funding from the Middle East Desalination Research Center (MEDRC) through contract number 97-A-005b. We also wish to acknowledge the support of H.M. Strategic Research Fund SR/AGR/BIOR/02/01 (SQU) to S. Sablani and J. Perret. References Chaiba, M.T., An overview of solar desalination for domestic and agricultural water needs in remote arid areas. Desalination 127,

7 M.F.A. Goosen et al. / Solar Energy 75 (2003) Delyannis, E., Belessiotis, V., Solar desalination for remote arid zones. In: Goosen, M.F.A., Shayya, W.H. (Eds.), Water Management, Purification and Conservation in Arid Climates, Volume II: Water Purification. Technomic Publishing, Lancaster, PA, pp Goosen, M.F.A., Shayya, W.H., Water management purification and conservation in arid climates. In: Goosen, M.F.A., Shayya, W.H. (Eds.), Water Management, Purification and Conservation in Arid Climates, Volume I: Water Management. Technomic Publishing, Lancaster, PA, pp Goosen, M.F.A., Sablani, S.S., Shayya, W.H., Paton, C., Al- Hinai, H., Thermodynamic and economic considerations in solar desalination. Desalination 129, Hamed, O.A., Eisa, E.I., Abdalla, W.E., Overview of solar desalination. Desalination 93, Hoffman, D., The application of solar energy for largescale water desalination. Desalination 89, Khalid, A., Dehumidification of atmospheric air as a potential source of fresh water in the UAE. Desalination 93, Kumar, A.M., Tiwari, G.N., Optimization of collector and basin areas for a higher yield for active solar stills. Desalination 116, 1 9. Kumar, A., Singh, M., Anand, J.D., Transient performance of a double-basin solar still integrated with a heat exchanger. Energy 14 (10), Paton, A.C., Davis, The seawater greenhouse for arid lands. In: Proc. Mediterranean Conf. on Renewable Energy Sources for Water Production, Santorini, June. Sablani, S., Goosen, M.F.A., Paton, C., Shayya, W.H., Al- Hinai, H., in press. Simulation of fresh water production using a humidification dehumidification Seawater Greenhouse. Desalination. Talbert, S.G., Eibling, J.A., Lof, G.O.G., Wong, C.-M., Sieder, E.N., Manual on solar distillation of saline water. Research and Dev. Progress Report No. 546, April. United States Department of the Interior Contract No Voivontas, D., Yannopoulos, K., Rados, K., Zervos, A., Assimacopoulos, D., Market potential of renewable energy powered desalination systems in Greece. Desalination 121, Yadav, Y.P., Jha, L.K., A double-basin solar still coupled to a collector and operating in the thermosiphon mode. Energy 14 (10),