This chapter covers the basics of energy storage, i.e., why it is needed, when it is used, how it is used, its benefits, and the types of energy storage technologies. Special attention is given to thermal energy storage due to its usage in a variety of guises in renewable power applications.

Solar thermal technology provides a wide range of opportunities for climate-resilient global development. High-temperature concentrating solar thermal power (CSP) systems are used to generate flexible, dispatchable renewable electricity in large-scale grid-connected systems and could also soon be used as a heat source for industrial processes such as for desalinated water, fuels, chemical products and refined ores. Most CSP electricity systems include thermal energy storage units, allowing output to continue for hours after sunset. Solar thermal systems, which rely on heating up a working medium to operate, are distinct from solar photovoltaic (PV) technologies that directly convert solar photons into electric current. In addition to CSP, low-temperature solar thermal systems, used for domestic hot water and other applications, are briefly reviewed.