Never has improving energy storage capacity had a greater economic and political importance than now. Any transformation will depend not only on meaningful backing through government policy but looking beyond existing battery and pumped hydro storage systems. No one denies the value of these, but they have drawbacks – a short lifespan and reliance on minerals such as lithium in the case of some battery set-ups; and, with hydro, a drastic impact on the environment.

In 2021 the share of global electricity produced by intermittent renewable energy sources was estimated at 26%. The International Energy Agency and World Energy Council say a storage capacity in excess of 250 GW will be needed by 2030. The race is on to find alternatives; and progress is being made on refining new technologies.

The main focus is on thermo-mechanical energy storage (TMES) systems. These are considered the way forward for longer-duration storage, offering high reliability, durability and long lifetimes. They can also deliver useful vectors beyond electricity – heat and cold, for example – to end users.

TMES possibilities include compressed air energy storage (CAES), pumped thermal electricity storage (PTES) and liquid air energy storage (LAES). Last year their potential was examined by leading academics and their findings highlighted by one of the team, Professor Christos Markides, during a recent seminar at Imperial College London. ‘There is no single monolithic (storage) option,’ he told the audience.

Compressed air

The world’s first operating CAES facility, a 290 MW plant in Huntorf, Germany, was built by E.ON-Kraftwerk in 1978 and remains in operation. Its purpose is to help manage grid loads by storing electricity as pressurised air when night-time demand was low. 

Compressed air, which becomes hot during its storage, or ‘charge’ phase, is housed in two underground salt caverns. (The cooler it is, the more you can store.) When electricity is needed, air is released and heated by a turbine combusting natural gas, enabling it to expand. Electricity is produced during two hours of peak demand before the caverns need to be refilled. To increase efficiency, CAES plants are also working on retaining the heat associated with compression.

Off-the-shelf compressors and turbines for gas-fired power plants can help reduce costs. Relying on access to underground space, the environmental impact of CAES is less than that of pumped hydro storage. Other concepts featuring above-ground pressure vessels or underwater storage balloons are also being developed.

Pumped thermal

PTES systems convert excess electricity into thermal energy which is stored in vessels filled with liquid or solid thermal energy storage (TES) material, for example water, oils, molten salts and gravel. When discharging, the energy is used to drive a heat engine, which generates electricity by operating across the temperature difference between the hot and/or cold stores when needed.

Like CAES systems, PTES plants can use standard components such as compressors, turbines and pumps, which reduce development time and resources. The world’s first grid-scale PTES demonstration plant was a UK collaboration, the fruit of a deal signed in 2017 between the Energy Technologies Institute and Newcastle University.

The facility has a storage capacity of 600 kWh and a rated output power of 150 kW. In 2019 a demonstration with turn-round efficiency of 65% was reported, an outcome described by academics Tristan Davenne and Benjamin Peters at the Rutherford Appleton Laboratory as: ‘a great achievement’. 

Liquid air

Liquid-air energy storage (LAES) combines elements of compressed air and pumped thermal. Excess electricity is used to liquefy air, which is then stored in cryogenic (very low temperature) tanks at atmospheric pressure. This avoids the need for expensive pressure-resistant storage.

When electricity is needed, the liquid air is returned to a gaseous state (either by exposure to ambient air or using waste heat from an industrial process), and the gas used to turn a turbine and generate power.

Integrating additional hot and cold stores can increase efficiency, alleviating the lost heat of compression and the cold energy of evaporation during the charging process. LAES technology can simultaneously produce both electricity and cooling energy during regasification/expansion – cold energy can be regarded as a free by-product.

In the UK, a focal point of LAES development is the Highview Power facility at Trafford Energy Park, Manchester, currently under construction and which received a £10mn grant from the Department for Business, Energy and Industrial Strategy (BEIS). It’s being developed in collaboration with power station developer Carlton Power. A 50 MW cryogenic energy storage plant with a minimum of 250 MWh, the plan is for it to use existing substation and transmission infrastructure. 

A demonstrator plant developed by Highview in 2018 at Pilsworth Landfill, Bury, showed how cryogenic energy storage can provide services such as short-term operating reserve (STOR) and support the grid during winter peaks.

The main focus is on thermo-mechanical energy storage systems – these are considered the way forward for longer-duration storage, offering high reliability, durability and long lifetimes.

Global developments

As for developing global market share, TMES systems start from a low base, given that, according to 2021 figures, pumped hydro accounted for more than 96% of world storage capacity. However, academics point to successes at pilot plant-scale or in larger demonstrator plants. They are capable of significantly larger power ratings (often more than 10 MW) and longer discharge times (24 hours) than batteries.

Moreover, their life expectancies range from 20–40 years – a significant advantage compared to any type of large-scale battery system, for which a typical lifespan is 10–15 years. PTES systems were found to have the longest life expectancy, close to 60 years. 

With proper investment in insulation, the self-discharge of TMES systems can be very low – less than 1%. That gives it a further advantage over conventional batteries. However, ‘the early stage of technology development means that cost information is limited’, the study said. ‘To overcome this challenge, costing models were developed, based on components and machines similar to those [systems] considered here, and using consistent cost correlations.’

Efficiency and performance

Where underground caverns are available, CAES was projected to be the cheapest TMES option. But it has the lowest energy density as it requires a large volume to store the pressurised air. PTES and LAES systems are capable of achieving higher energy densities as energy is stored in liquid or solid materials. ‘Although capital costs are likely to be higher than for CAES, these technologies show promise, and are not restricted to sites with large available air storage volumes,’ says the study.

The achievable round trip efficiencies of TMES systems – defined as the ratio of the electricity recovered from discharging to the electricity used for charging – were found to be similar. CAES with heat storage, PTES and LAES are expected to reach levels of 60–70%, 50–75% and 45–70% respectively, the study found. 

‘But although efforts always focus on this key performance indicator, this is arguably not as important as cost. While batteries can achieve higher efficiencies (60–90%), TMES technologies benefit strongly from economies of scales, so often have lower projected costs.’

CAES was considered ‘the most commercially mature’, according to the study. ‘When an air-tight chamber is available, it shows significantly lower power and energy capital costs than the other options at all scales,’ it said. LAES was shown to be affected by the performance of its sub-thermal energy stores, amongst which high grade cold storage (HGCS) was identified as being by far the most important. Improvements could be made through developing new phase-change material for HGCS that can help act as a thermal buffer.

Where next for PTES? High-performance reversible compression/expansion machines would be a ‘game-changer’, enabling significant cost reductions due to the need for fewer components, experts say. PTES systems are currently at a lower technology readiness level and generally require further testing of components, particularly high-temperature compressors, thermal storage systems and control systems development.

Energy hubs

Dr Nina Skorupska, Chief Executive of the Renewable Energy Association, has called for the creation of energy hubs to accommodate and exploit these developments. Academics agree that a great advantage of TMES systems is their ability to integrate with other external heat sources, including waste heat. 

They could be interesting solutions for waste-heat recovery from industrial processes, renewable heat (solar, geothermal, biomass) and/or provide heating alongside electricity for industrial, commercial or district heating/cooling applications. ‘Some TMES technologies can store up to 500°C – that covers a lot of applications,’ according to Markides. 

This joined-up approach is gaining importance in smart grid applications; but, because of the cost factor, more research – though some is already underway – is needed into integrating TMES with systems for smaller applications such as microgrids

Skorupska told the Imperial seminar that government ‘is really beginning to listen’. She said: ‘Funds are being made available – £6.7mn has been committed to storage under the innovation portfolio fund. Is enough going in? Probably not in my book, when you see £180mn going to nuclear innovation. We need to make these technologies fly.’

Darren Jones, Technology Manager at Hitachi Energy, agreed: ‘We have all the technology. It’s the scale of action we need. We’ve had announcements and 10-point plans. We need policy in place, pretty quickly.’

Stuart Nelmes, Engineering Director at Highview Power, said that with the market place being ‘a hotbed of technologies’, the focus should now be on the ‘wider set of services these can provide’. ‘It’s important we keep them funded and that they progress so that we’re ready for when the real demand, and problem, occurs. It’s getting ever closer, particularly with the world climate as it is,’ he concluded.