Thermo-mechanical technologies offer promise as grid-scale electricity storage

A new international review of bulk electricity storage technologies highlights the potential of thermo-mechanical energy storage

Thermo-mechanical energy storage (TMES) technologies can offer a reliable, low-cost solution as grid-scale electricity storage, according to a comprehensive review led by researchers at Imperial College London.

The research, published in Progress in Energy, examines recent progress in the advancement of a range of TMES technologies, including compressed-air energy storage, liquid-air energy storage and pumped-thermal electricity storage.

Using a combined approach comprising validated thermodynamic models and estimates from multiple costing approaches, the researchers compared the technical and economic characteristics of these technologies and assessed their competitiveness against other bulk energy storage options such as flow batteries and pumped-hydro energy storage.

“This is the first time a detailed techno-economic analysis of the main thermo-mechanical energy storage options has been performed for a large range of sizes under a unified modelling framework,” says Andreas Olympios from the Department of Chemical Engineering.

“This gives added value and confidence to the comparisons we have made and to the conclusions drawn from those comparisons.”

The need for storage 

Variable renewable energy sources such as wind and solar now account for just over a quarter of global electricity generation, a share that is growing steadily, creating new challenges for electricity grids.

As their name suggests, variable renewables are not continuously available; sometimes they produce more energy than is needed and sometime less, so some form of energy storage is required to ensure a reliable supply at the time when this is needed. 

Pumped-hydro energy storage is by far the most common form of grid-scale storage in use today. It involves pumping water from a low-elevation to a high-elevation reservoir when excess electricity is available and demand is low, and then releasing the water through turbines to generate electricity when demand is high.

Only around 20% of the energy stored is lost in the process and large amounts of energy can be supplied to the grid for relatively long periods of time (up to 12 hours).

Pumped-hydro energy storage systems, however, have limitations, requiring large amounts of water and a suitable location with a significant height difference between potential reservoirs. There are also concerns around their impact on local ecosystems.

Electrochemical storage solutions, on the other hand, are not location-constrained but present their own challenges when used for grid-scale storage. 

Lead-acid batteries, for example, have prohibitively short lifetimes, while lithium-ion batteries, commonly used in personal electronics and electric vehicles, are costly to deploy for situations when long storage durations are required by the grid. 

Flow batteries are the most promising electrochemical solution for grid-scale storage, offering longer discharge durations and lower costs than lithium-ion batteries, as well as high efficiencies. However, they are still at early stages of development, and face challenges with complicated system requirements and disposal/recycling issues.   

A promising alternative

The review finds that TMES technologies show ‘significant potential’. Although these solutions are at an early stage of development, they can offer promising performance along with low costs, long lifetimes, low ecological footprints, and lack the geographical constraints of, for example, pumped-hydro energy storage.

The paper reviews progress in three main TMES technology classes:

• Compressed-air storage systems, which store electricity in the form of high-pressure compressed air;
• Liquid-air energy storage systems, which store electricityin the form of low-pressure liquified air;
• Pumped-thermal electricity storage systems, which store electricityas heat and cold, by inducing a temperature difference between two thermal reservoirs;

Of these, compressed-air energy storage is the most commercially mature technology and recent advancements suggest that efficiencies of up to 80% are possible, which is comparable with pumped-hydro energy storage. 

Liquid-air and pumped-thermal systems have seen increased attention and development breakthroughs in recent years. Despite their slightly lower efficiencies (45-75%), they have competing costs and can be discharged for periods far beyond what is possible economically with batteries.

Other than compressed-air energy storage systems with underground storage caverns, TMES technologies have no geographical constraints, and their life expectancies are significantly longer than any large-scale battery system.

They also have additional advantages. They can, for example, be used to provide heating and cooling as well as electricity, potentially making use of or delivering heat to/from industrial processes. 

It is this unique combination of benefits that makes TMES systems such a promising solution for large-scale electricity storage, according to the authors of the paper, but more work is needed to improve efficiencies and reduce system costs.

“The future prospects of TMES technologies are closely linked to research and development aimed at further increasing efficiency and, more importantly, reducing cost,” says Professor Christos Markides, who leads this research and Imperial’s Clean Energy Processes (CEP) Laboratory.    

“This depends strongly on the development - either innovation or evolution - of higher-performance and lower-cost components that this work has highlighted and that we are therefore currently investigating.”

It is hoped that the review can be used as a benchmark for the future evolution of TMES technologies so that their full potential can be realised.

'Progress and prospects of thermo-mechanical energy storage—a critical review' by Andreas V. Olympios et al. was published in Progress in Energy, 12 March 2021.

This review was conducted by members of Project 2 of IDLES, whose work focuses on characterising current and new/future energy technologies in terms of cost and performance, for inclusion in whole-energy system modelling.