Experimental investigation of a weir-type cascade solar still with built-in latent heat thermal energy storage system
Introduction
Supplying of fresh water is an urgent need for cleaning, drinking, agriculture and domestic usages. Nowadays, remote areas and arid zones suffer from water scarcity. Surveys reveal that about 79% of available water is salty, only 1% is fresh and the rest 20% is brackish [1]. Desalination is a process in which fresh water is produced from saline water. Solar stills are used for water desalination in remote areas and rural places with low congestion and limited demand. Solar radiation is free, abundant, available and need no transportation. Distillation is a natural phenomenon. Solar energy heats water source, evaporates it and condenses by clouds and back to the earth as rainfall. Solar stills are simulating this natural process in a small scale. Solar energy can directly or indirectly be used for desalination. Direct solar stills use the solar energy to produce distillate directly in the solar collector and the systems that combine conventional desalination system with solar collector are called indirect systems [2]. Many reviews on different types of solar stills can be found in the literature [2], [3], [4], [5].
Several configurations of basin solar stills such as simple basin, wick type, and stepped solar still were investigated by many researchers. Performance of basin type of solar still have been developed by Rahim [6], [7], Bahadori and Edlin [8] and Tiwari et al. [9]. El-Sebaii [10] investigated theoretically and experimentally a shallow solar-pond integrated with a baffle plate. El-Sebaii et al. [11] designed a baffle suspended absorber. Velmurugan et al. [12] investigated the integration of fins at the basin of the still and compared with other types and found that daily productivity was increased from 1.88 to 2.8 kg/m2. Stepped solar stills and wick type ones were investigated by some researchers [13], [14], [15]. Aybar [13] tested an inclined solar still with and without wicks. The results showed that the wicks increase the fresh water generation by two or three times of bare plate. Souza et al. [16] have investigated the effect of inclination of tilt evaporator and observed that higher efficiencies were attained when the inclination of the collector was adjusted monthly. El-Bassuoni [17] presented the design, analysis and performance of ordinary and new modified cascade solar stills. Radhawan [18], El-Sebaii and Al-Ghamdi et al. [19] studied the transient performances of stepped solar still and a single basin solar still with phase change material (PCM) as a storage medium, respectively, and reported that the basin still integrated with heat storage system is more efficient for water provision during the lack of sunlight, especially at night because of storage capacity of basin water mass and higher temperature difference between water and glass cover due to the reduction in ambient temperature. On the other hand, a considerable amount of heat was stored within the PCM during sunshine hours instead of wasting it to surroundings. During solidification of the PCM, the stored heat discharges to keep the basin water at a temperature enough to produce distillated water. Most of surveys on inclined solar still have been carried out on wick type stills. In spite of advantages of these systems, construction costs, rapid deterioration of wick materials and unsuitable distribution of saline water prevented the practical use of these types of solar stills.
In the present study, two cascade solar stills with latent heat thermal energy storage system (LHTESS) and without LHTESS were designed based on the optimum inclination through the year for Zahedan city in Iran. Thermal performances of stills were compared in typical sunny (23/05/2009) and partially cloudy (13/05/2009) days. Furthermore, the effect of flow rate on total productivity of the stills was investigated in five sunny consecutive days through 23 to 27 of May, 2009.
Section snippets
Experimental set up
A cross sectional view of schematic diagram of cascade solar still is shown in Fig. 1.
Aluminum sheet is used for the construction because of better resistance to corrosion in low and medium temperatures, low weight and easy formation. The stills consist of stepped absorber plate covered with the inclined glass on the top with an air gap of about 2.5 cm that creates an enclosure. Absorber consists of 15 steps with 60 cm width and horizontal and a vertical surface with the lengths of 5 and 3 cm,
Results and discussion
An experimental study in sunny (23/05/2009) and cloudy (13/05/2009) days were carried out to compare the thermal performance of cascade solar still with and without LHTESS. Fig. 3A and B illustrates the temperature distribution of water Tw, absorber plate Tab and glass cover Tg temperatures of the still with LHTESS as well as the PCM temperature TPCM, in typical sunny and cloudy days, respectively.
Based on the obtained results from the Fig. 3A, absorber, water and glass temperatures increase
Conclusions
Two cascade solar stills with and without LHTESS was constructed for comparison of the stills productivity in sunny and cloudy days. The effect of flow rate was also investigated on the total productivity of stills during five sunny days. The experiments were conducted in typical days under weather conditions of Zahedan city which is located in the east of Iran. The concluded results are presented as follows:
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Using weir on each step of the stills leads to a forced flow and increases the
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