Throughput efficiency of a pumped-thermal system integrating exergy storage into wind turbines

Abstract

The throughput efficiency of a system encompassing wind-powered generation and storage describes the ratio between total electrical energy output and mechanical energy input over a period. Invariably the value of throughput efficiency declines monotonically as increased fractions of energy are passed through storage. Systems that integrate storage with the wind turbine (such that exergy is stored prior to the generation of electricity) could be useful in future Net-Zero energy systems if a high proportion of the input exergy handled by these systems is passed through storage. This paper analyses, for the first time, the achievable throughput efficiency of one class of such systems based on pumped thermal energy storage using a reversible Joule (Brayton) cycle in which most (or all) of the energy used for compression comes directly from the wind turbine rotor and in which most (or all) of the energy drawn out from expansion feeds directly into an electrical generator. The combined operation of the compressor and expander serves firstly as a speed-increasing mechanical transmission with continuously variable speed-ratio and secondly as a means of either driving exergy into storage or recovering it from storage. The relationship between throughput efficiency and the fraction of input mechanical energy passed through storage is shown in this paper to comprise two straight-line portions. The first straight-line portion applies when low proportions of input exergy pass through storage. In those cases, all of the compressor work is provided directly from the wind turbine rotor and all of the expander power is fed into the generator. The second straight-line portion begins when some portion of expander work must be redirected into compression in order to pass higher proportions of input exergy through storage.