Solar energy is major renewable energy resource which can potentially fulfill 100% energy demand of the world while releasing no polluting agents to the atmosphere in contrast to the conventional fossil fuels. However, due to its intermittent nature, solar energy requires effective storage of energy for utilizing during the night and cloudy weather. A solar pond is a promising solution because it has its own energy storage which is suitable for low temperature application like building heating and cooling. This paper presents a thermal analysis of a salt gradient solar pond while extracting heat from the lower convective zone. A mathematical model of surface area is developed. Efficiency analysis is performed numerically using a MATLAB code for steady temperature difference of 30 °C as well as 20 °C across the gradient layer for three different pond sizes of depths 1.5 m, 1.0 m, and 0.5 m. The thermal efficiency of first pond of 1.5 m depth varies from around 21% in summer to 11% in winter. Thermal efficiency of solar pond drops significantly by reducing its size and non-convective zone thickness. Annual average efficiencies are 21%, 19%, and 9.5% for the three ponds of 1.5 m, 1.0 m, and 0.5 m depths, respectively. So it is recommended to prefer a pond of 1.5 m over others. However, the efficiency of smaller the pond can be significantly improved by compromising on quality the of thermal energy, efficiency of 0.5 m pond rises to 17% when operating at temperature just 20 °C above ambient, compared with 9.5% for 30 °C above ambient. Solar pond therefore proves to be suitable for effectively utilizing solar energy and can present an effective solution for low temperature energy needs like space heating.

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CaCO3 is being studied for its application in thermal energy storage. However, it has drawbacks of slow reaction rate during calcination and incomplete reversible carbonation which limit its use. In this paper, SiO2 has been studied as a dopant for CaCO3 to improve its cyclic performance. CaCO3 samples were loaded with different concentrations of SiO2 and its effect on the thermal energy density of CaCO3 was determined. Afterwards, the effect of the dopant on the heat storage process of the synthesized composite along with kinetics of decarbonation reaction was investigated. Cyclic tests were performed to determine the reusability of the material. It was found that the addition of dopant helped to increase the rate of decarbonation reaction, thereby making the heat storage process more efficient as compared to pure CaCO3. The activation energy values are 255.9, 280.1, 244.9 and 234.8 kJ/mol for 5%, 15%, 30% and 0% doped SiO2 samples, respectively. Furthermore, thermal energy storage density increases when the amount of dopant decreases in the samples such as the 30% and 5% doped samples have gravimetric energy storage densities of 339.85 and 759.24 J/g, respectively. It was observed that the large quantities (15% and 30%) of dopant had introduced a new phase of Ca3SiO5 during CaCO3 decomposition.