Solar energy can be converted into different forms of energy, either to thermal energy or to electrical energy. Solar energy is converted directly into electrical power by photovoltaic modules, while solar collector converts solar energy into thermal energy. Solar collector works by absorbing the direct solar radiation and converting it into thermal energy, which can be stored in the form of sensible heat or latent heat or a combination of sensible and latent heats. A theoretical study has been carried out to rate the various thermal energy storage commonly used in solar air heaters. During the investigations rock bed storages have been found to be low type thermal heat storage, while phase change materials have been found to be high heat thermal storages. Besides this, a few other heat storing materials have been studied and discussed for lower to higher ratings in terms of thermal performance purposely for solar heaters. 1. Introduction Since solar radiation is an inherently time-dependent energy resource, storage of energy is essential if solar is to meet energy needs at night or during daytime periods of cloud cover and make a significant contribution to total energy needs. Since radiant energy can be converted into a variety of forms and feasible to be stored such as; thermal energy, chemical energy, kinetic energy, or potential energy. Generally, the choice of the storage media is related to the end use of the energy and the process employed to meet this application. The optimum capacity of the storage device for a given solar process depends on the time dependence of the solar availability, the nature of the load, the cost of auxiliary energy, and the price of the process components. These factors must all be weighed carefully for a particular application to arrive at the system design (including storage size), which minimizes the final cost of delivering energy [1, 2]. Storage of solar energy is important for the future success of solar energy utilization. The major problem is the selection of materials having suitable thermophysical characteristics in which solar energy in the form of heat can be stored [3]. The materials can be divided into two broad types (Figure 1): those that store energy in the form of sensible heat and those that undergo a change of state or physical-chemical change at some temperature within the practical range of temperature provided by the solar heat collectors as likely 90 to 120°F [4]. If we talk about the thermal heat storages purposely for solar thermal applications, then (i) SHS: as sensible heat in solids (e.g.,
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