Solar Thermal Energy Storage

Over the past few decades, considerable research efforts have been devoted to improve the usage of renewable energy resources. Till date, major energy demands have been addressed by fossil fuels, and the limited resource of these precious fuels is continuously depleting at an alarming rate. Increase in the energy demands, deficiency of fossil fuels, and influence of pollution on the environment have forced us to opt for renewable energy resources. Solar energy is a natural source of energy that is not depleted by its use. It is a promising option for replacing conventional energy resources partially or totally, but it is transient, intermittent, and unpredictable in nature. Because of this sporadic nature of solar energy across a given interval of hours, days, and season, various practical problems arise. Variable DNI causes power plants to shut down for few hours of the day or to run at part load most of the time. This creates a demand for an effective subsystem which is capable of storing energy when available solar energy overshoots the demand during the interval of radiant sunshine, and to make it accessible during night or season. A similar problem arises for waste heat recovery systems where accessibility of waste heat and usage period are not the same, and thus creates a need for thermal energy storage (TES) for energy conservation. TES has tempted a lot of researchers to improve its high energy storage capacity and efficiency. If solar energy system is not run with TES, a considerable section of energy demand has to depend on conventional resources which in result reduce the annual solar fraction. TES helps to reduce dependency over conventional resource by minimizing energy waste. TES is mainly described by the parameters like capacity, power, efficiency, storage period, charge and discharge time, and cost. There are different storage mechanisms by which energy can be stored: sensible, latent, and chemical reactions. In sensible-type storage, energy is stored by increasing the temperature of solid or liquid storage media (e.g., sand-rock minerals, concrete, oils, and liquid sodium). These materials have excellent thermal conductivity and are cheaper, but due to low heat capacity, it increases system size. In latent-type storage, energy is stored/released during phase change; thus, it has higher storage capacity than sensible, but suffers from the issue of low thermal conductivity. As the solid–liquid phase change process of pure or eutectic substances is isothermal in nature, it is beneficial for the application having limitations with working temperature. In chemical-type TES, heat is absorbed/released due to breakdown or formation of chemical bonds. The technology is not much developed and has limited application due to possibility of degradation over time and chemical instability. TES can also be classified as active and passive depending upon the solid or liquid energy storage medium. Active TES is further classified as direct active and indirect active depending on whether the storage fluid and the heat transfer fluid (HTF) are same or some other HTF is required to extract heat from solar field. The discussion in this chapter includes basic heat transfer models, along with experimental studies by different research groups on various TES. Finally, methods and design criteria that can improve the system performance are discussed.