Evaluation of Single Slop Solar Still Integrated With Evaporative Cooling System for Brackish Water Desalination

The thermal performance of a single slope (SS) solar still integrated with a condenser supplied with cooled water from an evaporative cooling system under hot and humid climatic conditions were studied and analyzed. Results of this study shows that the solar energy available inside the still with and without condenser resulted in an increase in the brackish water temperatures of 8.1 oC. It also increased the temperature of the water vapor inside the still by 12.7 oC. Due to increase in the intensity of solar radiation and consequently the heat energy stored inside the single slope solar still, the productivity of distilled water was 5941.4 ml/m day. The thermal efficiency of the (SS) still without using condenser was on, an average, 54.4%. Connecting the solar still to a condenser led to an increase in the productivity and thermal efficiency to 55.41% and 30.1%, respectively. The obtained data also revealed that the pH (7.9), EC (43.4 μs/cm), and TDS (30.9 ppm) values were lower than those of the brackish water (8.1, 1,436 μs/cm, and 1,370 ppm, respectively).


Introduction
Water is one of the most abundant natural resources on earth, and it covers three-quarters of the earth's surface.However, approximately 97% of the earth's water is salt water in the seas and oceans, and a tiny 3% is freshwater.This small percentage of the earth's water, which supplies the needs of humans, animals, and plants, mainly exists in groundwater, lakes, and rivers.Approximately 25% of the world does not have access to the necessary quality and quantity of freshwater, and more than 80 countries face severe water problems (Badran & Abu-Khader, 2007).
The global freshwater shortage, particularly in remote areas, presents an international problem.The problem is more severe in desert countries, such as the Gulf Cooperation Council (GCC) countries and, in particular, the Kingdom of Saudi Arabia (KSA), where freshwater shortages are a serious problem (Radhwan, 2004).Many of these countries, however, enjoy abundant and free high-intensity solar energy, so solar desalination might be an ideal technology that could provide some freshwater for both domestic and agricultural use.Solar distillation is one of the many processes that can be used for water purification.Among all the alternative energy resources, solar energy is one of the most promising of all the sources because of its potential to provide for our future energy needs.Many developing countries that normally could not afford to use desalinated water are likely to have much greater water needs due to population growth.These countries, in general, also receive a greater amount of solar radiation.For example, the daily average solar radiation flux incident on Saudi Arabia is 5-8 kWh/m 2 (Al-Ansari, 2013).
Solar radiation can be used as a source of heat energy in a process in which brackish or seawater is evaporated and then condensed as pure water (Hepbasli & Alsuhaibani, 2011).The Kingdom of Saudi Arabia (KSA) lies between the latitude angles of 17.5°N and 31°N and the longitude angles of 36.6°E and 50°E.This means that KSA is located in the heart of one of the world's most productive solar regions, which receives the most potent type of sunlight.The annual average solar radiation falling on the Arabian Peninsula is approximately 2.2 MWh/m 2 (Hepbasli & Alsuhaibani, 2011).KSA is an arid desert country, and there are no perennial rivers or streams or permanent freshwater lakes.The rainfall is both scarce and infrequent and is associated with high evaporation and sandy land dissipation rates.The available groundwater supplies are deeply buried and do not replenish themselves (Radhwan & Fath, 2005).The many practical uses of solar energy in Saudi Arabia include lighting, cooling, cooking, water heating, crop and fruit drying, water desalination, operating irrigation pumps and meteorological stations, road and tunnel lighting, traffic lights, and road instruction signals (Hepbasli & Alsuhaibani, 2011).
A solar pond (SP) is a stable pool of salt water in which the water salinity increases in the middle layer from its top to the bottom with a gradient that prevents convective temperature mixing.Heat is passively collected and stored in the lower convective zone because the middle layer is a nonconvective zone (Arjunan et al., 2009).Most commercial multistage flash (MSF) units operate with a top brine temperature of 90-110°C, heated by steam, whereas the solar pond operates in the range of 30-95°C.Therefore, in solar pond-assisted multistage flash (MSF) systems, the first stage of MSF heat exchangers is changed to a liquid-liquid heat exchanger instead of a steam-liquid heat exchanger (Micale et al., 2009).Because a solar pond combines solar collection and storage, it overcomes the intermittent nature of solar energy.However, the solar pond has to be oversized for winter conditions, necessitating that some of the surplus summer heat be wasted.As an alternative, waste heat energy from other sources (gas turbine, for example) may be used during periods of insufficient sunshine (Agha, 2009).These types of hybrid solar pond systems could store extra waste heat energy, such as from gas turbine exhaust during peak times to lower the freshwater production cost and the solar pond size (Tiwari et al., 2003;Drawish & Alsairafi, 2004;Kalogirou, 2005;Li et al., 2013).
The thermal performance of a single basin solar still with the entering brine flowing between double glazing was investigated by Abu-Arabi et al. (2002).The main purpose of their arrangement was to lower the glass temperature and thus increase the temperature difference between the brine water and glass cover.This arrangement resulting in improved performance because a faster rate of evaporation from the basin of brine water was achieved.Utilization of solar energy with two different types of solar stills and the factors that influence the productivity of solar stills was studied by Al-Hayek and Badran (2004).They found that the productivity and efficiency of an asymmetric greenhouse-type still (ASGHT) having mirrors on its inside walls were higher than those of the symmetric greenhouse type still (SGHT).
Greenhouse technology is a breakthrough in agriculture production technology that integrates market-driven quality parameters with production system profits (Kumar et al., 2009).Protected cultivation in greenhouses has become the favored way to develop the agriculture sector due to the harsh climate and the scarcity and poor quality of water resources on the Arabian Peninsula (Hepbasli & Alsuhaibani, 2011).In 2006, there was 5150 ha of greenhouses in Saudi Arabia producing 487,614 metric tons of vegetables (Alhamdan & Al-Helal, 2009).Cooling is considered a basic necessity for greenhouse crop production in tropical and subtropical regions to overcome the problems of high temperatures during summer months (Kumar et al., 2009).Evaporative cooling involves no change in the enthalpy (heat content) of the air mixture.Rather, as water evaporates, it takes heat from the air, thus reducing its temperature.Evaporative cooling systems are based on the conversion of sensible heat into the latent heat of evaporated water, with the water supplied mechanically.Evaporative cooling has been used to improve human, plant, and animal comfort for many years in thermal environmental control applications.It remains one of the least expensive techniques that can be used to bring the air dry-bulb temperature into a more comfortable range.It is a reliable method and requires minimum power consumption (Ahmed et al., 2011).Greenhouses located in dry, desert environments benefit greatly from evaporative cooling systems because large amounts of water can be evaporated into the incoming air, resulting in significant temperature drops.The fan-pad system consists of a fan on one sidewall and a pad on the other sidewall of the greenhouse.The principle of evaporative cooling is applied by running a water stream over the pad, followed by drawing air through the pad using fans on the opposite side.The fan-pad cooling produces two changes in the condition of air exiting the pads.The air becomes cooler and its humidity is also raised (Sethi & Sharma, 2007).As a result, the water draining from the cooling pads will have a lower temperature.Abdel-Rehim and Lashine (2012) concluded that combining the solar still with the air-conditioning system could increase the condensate output from the solar still while meeting the cooling load needs.Fath and Hosny (2002) used shielded separate condenser to increase the productivity of solar still.
The main objectives of the present work are to investigate and to evaluate the various parameters affecting both the thermal efficiency and the productivity of single slope solar still, and to study the effect of integration using condenser supplied with cooled water from an evaporative cooling system on the performance of the solar stills.

Materials and Methods
A single slope solar still was designed, constructed, and tested during May 2013 at the Agricultural and Veterinary training and Research Station of King Faisal University, Saudi Arabia (Latitude angle 25.3ºN, longitude angle 49.5ºE, and mean altitude above sea level 172 m).The geometric characteristics of the single slope still are as follows: width, 1.0 m; length, 2.0 m; still rafter angle, 35º; rafter length, 1.22 m; gable height, 0.7 m; basin height, 0.2 m; and basin surface area, 2.0 m 2 , as demonstrated in Figure 1.

Case 2
For the du condensati surfaces) w net.org/jas re 6. Various te reveals that the re, the particle perature was u ence between t perature, it cau 9 am), the gl ty due to the s ensity of the so ase in heat en r varied as time ctivity of freshw 3.5 ml/m 2 hr) i 74.4 ml/m 2 hr s that has a dir The cooled water temperature from the evaporative cooling system reservoir going to the condenser ranged from 18 to 25°C.The means of various parameters within the solar still and the ambient air temperature during another five successive days were measured and are listed in Table 1.The daily average intensity of solar radiation inside the solar still was 7.369 kW/m 2 , which gave hourly average solar radiation intensities of 669.9 W/m 2 .In spite of the fact that the maximum value of the solar radiation intensity during the experimental period occurred at and around noon, the brackish water and water vapor temperatures still increased until reaching the maximum values in the afternoon (14.00 hr) because some heat energy accumulated inside the solar stills in the form of sensible heat.The temperatures of the glass covers of the solar still also increased from early morning until they approached the maximum values in the afternoon because part of the heat from the water was transferred to the glass cover by free convection using air, and the remaining part was transferred by radiation.The glass covers transferred the heat into the atmosphere by convection and radiation.Lowering the glass cover temperatures below the water and outside air temperatures helped increase the rate of heat and mass transfer.The temperature difference between the glass covers of the still and the basin brackish water increased, which increased the natural circulation of the air mass inside the two stills.The natural circulation increased both the convective and evaporative heat transfer between the basin brackish water and glass cover.Figure 8 illustrates the productivity of freshwater for the solar still with and without using a condenser.The productivity rate of freshwater varied as time passes from early morning until late afternoon for both systems.
Using the condenser with the solar still resulting in an increase in the condensed freshwater rate because the cooler inner surface of the condenser increased the rate of condensation.The daily average productivity of freshwater for the single slope solar still connected to a condenser increased from 5.9414 to 9.2333 L/m 2 day, which gave an increasing rate of 55.41%.Because the inner surface temperatures of the two condensers were much smaller than the water vapor temperature, they caused an increasing condensation rate on the inner surfaces.In the early hours of the morning (7-8 am), the temperature difference between the inside parameters (brackish water and water vapor temperatures) of the solar still and the inner surface of the condenser was lower than at other times, which resulted in low freshwater productivity.Moreover, produced distilled water can be added to the evaporative cooling system reservoir that will help in reducing salts build up on the cooling pads (pads clogging) and enhance the pads life suggesting that this system can be integrated to the evaporative cooling system.Total Dissolved Solids (TDS), ppm 1370 30.9

Conclusions
In the present research work, several conclusions can be drawn: 1) The increase in either the intensity of solar radiation and/or the ambient air temperature can lead to an increase in the productivity of a single slope solar still.
2) As the intensity of the solar radiation inside the solar still is increased, the productivity of freshwater increased due to the increase in heat energy gained from brackish water vaporization inside the still.
3) The maximum productivity and efficiency for single slope solar still occurred in the early afternoon due to the high heat energy accumulated inside the solar still at this time.
4) The technique of using a condenser that provides cooled water to increase the productivity of potable water significantly increases the productivity and volumetric thermal efficiency of the solar still.

Table 1 .
Hourly average ambient air temperature (T a ), intensity of solar radiation (R), water temperature (T w ), water vapor temperature (T v ), condenser water temperature (T Cond ), and glass cover temperature (T g ) for the single slope solar still during the experimental tests daysLocal standard time T a , °C R, W/m 2 T w , °C T v , °C T g , °C T Cond , °C

Table 2 .
Quality parameters (pH, EC, and TDS) of the solar distilled water and brackish water during the experimental tests