Most renewable energy sources depend on the sun and so vary with time and ambient conditions. Hence a consistent supply of renewable energy requires energy storage. The main approaches to storing renewable energy are described and quantified. Pumped hydro, compressed air and flywheels are discussed. Storing heat in the fabric of buildings and hot water using sensible heat are described. The increasing importance of phase change materials to store energy through latent heat is recognized. Battery technology is developing very fast; the principles of lithium-ion batteries are explained, together with their advantages and disadvantages. The various materials currently used for the positive electrode are listed. The electrochemistry of various battery technologies is summarized as well as how a large number of cells are connected to form are a useful store of energy. The principle of flow batteries is demonstrated and approaches to the estimation of the lifetime of a lithium-ion battery discussed. The chapter is supported by 10 examples, 16 questions with answers and full solutions in the accompanying online material. Further reading and online resources are identified.

This chapter discusses a multitude of energy storage mechanisms that include pumped storage hydro (PSH) systems and various forms of battery storage, as well as other forms of energy storage with varying levels of technical and commercial maturity. The role and importance of energy storage is changing with the introduction of renewable energy generation such as wind and solar photovoltaics whose output is inherently variable. This increasing generation variability has created a need for energy storage to provide energy balancing. This chapter discusses the different requirements for energy balancing within renewable-based power systems over various timescales. The requirements for balancing services will be met by different forms of energy storage, highlighting the need for a portfolio of energy storage technologies. Energy storage also provides other benefits for modern power systems including to provide network and systems services and to enhance system flexibility and resilience. This chapter concludes by exploring issues related to the integration of energy storage into electricity grids and reviews social research related to energy storage uptake.