The share of electricity generated by intermittent renewable energy sources is increasing (now at 26% of global electricity generation) and the requirements of affordable, reliable and secure energy supply designate grid-scale storage as an imperative component of most energy transition pathways. The most widely deployed bulk energy storage solution is pumped-hydro energy storage (PHES), however, this technology is geographically constrained. Alternatively, flow batteries are location independent and have higher energy densities than PHES, but remain associated with high costs and short lifetimes, which highlights the importance of developing and utilizing additional larger-scale, longer-duration and long-lifetime energy storage alternatives. In this paper, we review a class of promising bulk energy storage technologies based on thermo-mechanical principles, which includes: compressed-air energy storage, liquid-air energy storage and pumped-thermal electricity storage. The thermodynamic principles upon which these thermo-mechanical energy storage (TMES) technologies are based are discussed and a synopsis of recent progress in their development is presented, assessing their ability to provide reliable and cost-effective solutions. The current performance and future prospects of TMES systems are examined within a unified framework and a thermo-economic analysis is conducted to explore their competitiveness relative to each other as well as when compared to PHES and battery systems. This includes carefully selected thermodynamic and economic methodologies for estimating the component costs of each configuration in order to provide a detailed and fair comparison at various system sizes. The analysis reveals that the technical and economic characteristics of TMES systems are such that, especially at higher discharge power ratings and longer discharge durations, they can offer promising performance (round-trip efficiencies higher than 60%) along with long lifetimes (>30 years), low specific costs (often below 100 $ kWh⁻¹), low ecological footprints and unique sector-coupling features compared to other storage options. TMES systems have significant potential for further progress and the thermo-economic comparisons in this paper can be used as a benchmark for their future evolution.

This is the accepted manuscript for a paper published in Applied Energy Volume 130, 1 October 2014, Pages 648–657, DOI: 10.1016/j.apenergy.2014.02.07

Several emerging electrical energy storage technologies make use of packed-bed reservoirs to store thermal energy for subsequent conversion back to electricity. The present paper describes analysis and optimisation of such reservoirs under transient and steady-state cyclic operation. The focus is on thermodynamic issues, but a simple costing model is also included in order to determine the influence of cost factors on the main design parameters. A major part of the paper is devoted to segmentation (or layering) of the packed beds, which has previously been proposed as a means of simultaneously attaining high storage efficiency and full utilisation of the reservoirs. As illustrative examples, three different reservoirs are modelled, corresponding to the hot and cold thermal stores of a pumped thermal energy storage system, and a larger thermal store suitable for integration with adiabatic compressed air energy storage.