Mechanical energy storage

This chapter considers energy stored in the form of mechanical kinetic and potential energy. This includes well-established pumped hydroelectric storage (pumped hydro) and flywheels as well as more recent concepts of gravity and buoyancy energy storage. While other sources may consider compressed air energy storage (CAES) as mechanical energy storage by the compression and expansion of gas, there is significant thermal aspect to that technology that warrants its inclusion in the chapter on heat engine-based systems elsewhere in this book. Pumped hydro is a proven commercial technology where water is pumped (energy input) from a low reservoir to a high reservoir for storage, then the water in the high reservoir is allowed to return to the low reservoir through a turbine to extract energy. Flywheels, also proven commercially in several applications, store kinetic energy of a rotating mass: energy is added by driving the flywheel shaft with a motor, and energy is extracted by allowing the shaft to drive a generator. Gravity and buoyancy energy storage concepts are fundamentally similar in that they deal with relative positioning of a static load in a potential energy field. This chapter discusses the technical details and current applications of these technologies, including aspects of design and performance.