Improving the performance of solar still by using PCM as a thermal storage medium under Egyptian conditions
Introduction
Obtaining clean drinking water represents one of the major problems in developing countries. In the last forty years the problem of clean drinking water is one of the challenges facing the world. Clean drinking water is a basic human necessity, and without water life will be impossible. In addition, with an ever increasing population and rapid growth of industrialization, there is a great demand for fresh water, especially for drinking. One of the options used to obtain pure drinking water from saltwater is to use solar desalination plants. Solar stills represent a good option and a simple technique compared to the other distillation methods. The problem facing the solar stills is the low distillate water productivity. To enhance the productivity of the solar still, various research works are being carried out.
Free surface of the basin water is the important factor that affects the productivity of the solar still. Bassam [1] use the sponge cubes in the basin water to increment the free surface of the saline water. This study showed that, using the sponge cubes in the basin water improves the distillate water productivity. Velmurugan et al. [2], [3], [4] found that, the use of the sponges in a single basin still and stepped still improved the productivity by 15.3%.
The evaporation rate in the conventional solar still depends only on the solar radiation, but in the active solar still the evaporation rate depends on some additional external sources such as a solar water heater [5], [6], [7], [8], parabolic trough concentrators [9], a flat plate reflector [10], solar collectors and a storage tank [11], [12] and a pump [13]. These studies showed that, the daily distillate water productivity increases with an increase in the thermal energy in the basin still.
The basin water depth is an important factor affecting the distillate water productivity of the still. Investigations show that the basin water depth is inversely proportional to the distillate water productivity of solar still [14], [15]. The effect of several conditions including ambient temperature, wind velocity and the intensity of solar radiation for ambient conditions and the angle of the glass cover, the saline water depth, the thermal insulation and the difference in temperature between the basin water and condensation surface for the operating conditions on the productivity of the solar still has been studied Xiao et al. [16].
El-Agouz et al. [17] studied the performance of the inclined solar still with and without water closed loop. They found that, the productivity of the inclined solar still with a makeup water is higher than that of a conventional basin-type solar still by 57.2%.
Another technique that can be used to improve the distillate water productivity of solar still is the use of thermal storage material. The thermal storage material can be classified into the latent heat thermal storage material and the sensible heat thermal storage material. The latent heat thermal storage material has a significant advantage over sensible heat thermal storage material, including a large amount of energy storage per unit mass [18]. A wide range of phase change materials can be used for latent heat storage applications, including salt hydrates, paraffin waxes, fatty acids, and sugar alcohols [19], [20]. For use in solar still applications, paraffin waxes represent the most suitable option due to their congruent melting temperatures, availability and low cost. However, they usually suffer from relatively high volumetric expansion ratios and low thermal conductivity values. The poor thermal conductivity can be overcome by using the PCM through geometries with large surface area to volume ratios. A little number of papers studied the use of phase change material (PCM) as a thermal storage material in solar stills. El-Sebaii et al. [21] and Radhawan et al. [22] have theoretically studied transient performances of a single basin still and stepped still with PCM as a thermal storage material, respectively. Mohammad and Farshad [23] studied the behavior of cascade still with latent heat storage material. The mathematical models have been developed to study the behavior of the solar still with and without PCM. The results indicated that, the distillate water productivity of the solar still with PCM was 31% higher than the solar still without PCM. Arunkumar et al. [24] studied the effect of thermal storage material on the productivity of the concentrator-coupled hemispherical basin still. They found that, the distillate water productivity of the still with PCM was 26% higher than the still without PCM.
The objective of the present work is to improve the distillate water productivity of the solar still by using phase change materials (PCMs) as a thermal storage medium. The PCM will act as a latent and sensible heat storage medium. The used PCM in the current work is the Paraffin wax due to its wide availability and low cost. The effect of PCM on the performance of the solar still with PCM is experimentally investigated, and the study results are compared with conventional still, to evaluate the development in the distillate water productivity and the daily efficiency for using PCM, under Egyptian conditions. Also, the estimated cost of the distillate water productivity has been investigated.
Section snippets
Experimental work
The present experimental work of this study was carried out in the Faculty of Engineering, Tanta University, Egypt (Latitude 30.47°N and longitude 31°E), during the period from June to July 2015.
In the present work, PCM is used in the solar still as a thermal storage medium to improve the freshwater productivity of the solar still. Two stills were designed, fabricated and constructed to compare the freshwater productivity of the solar desalination system. One of the stills is the conventional
Experimental procedure
The experimental work was designed and constructed in Faculty of Engineering-Tanta University, Egypt. The experiments were carried out from 6:00 am to 10:00 pm during the period from June to July 2015. The following parameters are measured during the experiments, the solar radiation, ambient temperature, glass cover temperature, basin water temperature, absorber plate temperature, PCM temperature and distilled water temperature. The measurements are taken every hour. The collected freshwater
Mechanism of heat transfer in PCM
The PCM is used as a heat storage medium. When heating the phase change material, at the beginning, the heat is stored in PCM as sensible heat until the PCM temperature reaches its melting temperature. When the PCM is completely melted, the heat is absorbed again in the form of sensible heat. The PCM will represent a source of heat for the basin water during the night and in the periods of low solar radiation intensity.
Error analysis
During the experiments, several parameters were measured in order to evaluate the system performance. The temperatures at different points (ambient temperature, glass cover temperature, basin water temperature, absorber plate temperature and PCM temperature) were measured by K-type thermocouples which are integrated with a PLC. Total solar radiation was measured on the same level of still glass cover by a solarimeter. Wind speed was measured by a vane anemometer. The amount of freshwater
The daily efficiency of the solar stills
The daily efficiency, ηd, for the solar still was obtained by the summation of the hourly condensate production mew, multiplied by the average latent heat hfg at Tw average basin water temperature, hence the results were divided by the daily average solar radiation I(t) over the whole area A of the device:
where; the average latent heat hfg is followed by [27]:
Results and discussion
Depending on the state of the climate conditions the intensity of the solar radiation varied from 220 to 1080 W/m2 and the wind velocity varied from 0.4 to 5.2 m/s. The performances of the solar still with PCM and the conventional solar still are tested at the same basin water depth of 2 cm and under the same ambient conditions.
Absorber plate
The energy balance equation for the absorber plate can be written as [27].
where: kPCM, XPCM and TPCM are the thermal conductivity, thickness and temperature of the PCM, respectively; αp, Tp, mp, cp and Ap are the absorptivity, temperature, mass, specific heat and area of the absorber plate, respectively; h is the convective heat transfer coefficient between the absorber plate and water; and Tw and τw are the temperature and transmissivity of basin
Cost evaluation
The fixed cost of the conventional solar still is about F = 750 LE. To obtain the average value of the cost of freshwater productivity it is assumed that: the expected still lifetime, P is the operating cost, C is the total cost, where, C = F + P. Assume that operating cost P equals 0.3 F per year [28], and for the expected still life 10 years, then C = 750 + (0.3 ∗ 750 ∗ 10) = 3000 LE, where the minimum average daily productivity can be estimated from the analysis of different experimental data, and it is
Conclusions
In this work, the effect of PCM on the behavior of the solar still with PCM was investigated experimentally under the weather conditions of Tanta city (Egypt). The rise in the temperature of PCM for the solar still with PCM is due to the intensity of solar radiation absorbed by the absorber surface, the absorber plate made from copper sheet. After noon, during the period of low intensity of solar radiation and during the night the PCM will represent a source of heat for the basin water to keep
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