Optimizing thermal storage efficiency of a salt gradient solar pond using polyethylene membrane

Abstract

The need for energy has risen greatly all over the world in recent years. One major source of energy is fossil fuel and most countries get access to this fuel through imports. Besides the rise in cost of fossil fuel, it has serious impacts on the environment which include air pollution and increased greenhouse gases in the atmosphere. Furthermore, this source of energy is nonrenewable and is quickly being depleted. Recently, attention has turned to environmentally benign and sustainable sources of energy. Solar radiation constitutes a vast energy source which is abundantly available all over the earth. Solar energy is in many regards viewed as one of the best alternatives to non-renewable sources of energy. One way to collect and store energy is through the use of solar ponds which can be employed to supply thermal energy for various applications such as process heating, water desalination, refrigeration, drying and power generation. Solar ponds consist of three distinct layers of salt solution stratified by their differences in density, usually called salt gradient solar pond. The bottom layer of the solar pond always has the highest salt content and is thus the heat storage layer. This study aimed at constructing model rectangular solar ponds made from transparent Perspex glass material painted black at the bottom to increase absorption of solar radiation. The rates of diffusion and efficiencies of the salt gradient solar pond and one stabilized by a low density polyethylene was determined and compared by studying the kinetics involved using the hourly temperature rise during the day when there was active solar radiation. This was achieved by measuring the salt concentrations of the layers using a refractometer which measures the refractive index of solutions. The efficiency was calculated using the temperature differences between the start and end of the experiment and was measured using thermocouples connected to a digital thermometer. Results show that the efficiency of a polyethylene stabilized pond rises from 0% to about 69% compared to the traditional solar pond with about 52%. This is attributed to the polyethylene used brings about greenhouse effect where the solar radiation absorbed is trapped and therefore the storage zone heat rises more. The best solar pond system with the highest thermal storage efficiency of 69 % was identified from the various combinations of salt concentration of the layers. The results also show that efficiency of the storage zone increases with increasing salinity and the optimal LCZ concentration was found to be 25% salinity, 10% for NCZ and 0% for UCZ. At these concentrations, there is formation of a clear gradient zone which is stable and therefore hinders convection currents to occur.