Cryogenic energy storage plant could provide valuable back-up

The UK’s first cryogenic power storage plant, which uses liquid nitrogen to store and release energy, is scheduled to open next month.

Its operator, Highview Power Storage, said the system could provide a relatively cheap way of storing power, particularly from intermittent sources such as wind turbines, to better match the supply of electricity to demand.

The pilot facility near Slough has been providing electricity to the National Grid since April last year by evaporating liquid nitrogen stored at -200ºC to drive turbine generators.

The full system returns about 50 per cent of the energy put in, rising to 70 per cent efficiency if it uses waste heat from another source

Highview is now installing equipment to re-liquefy and compress the nitrogen using electricity from the grid, creating a closed cryogenic system that can store energy at times of surplus and release it when it is needed, while re-using the cold air exhaust.

‘We think the system has some pretty important benefits, one of which is capital costs, which is a very important driver in this market,’ Highview’s chief operating officer, Toby Peters, told The Engineer.

The company estimates the captial cost of cryogenic energy storage will be less than $1,000 per kW when the technology is mature, one quarter of the costs of sodium-sulphur batteries and between half and quarter of that of pumping water uphill into reservoirs.

‘Pumped hydro is the gold standard but there aren’t many mountains close to London,’ said Peters. ‘[Our technology is] modular and scalable and you can move it.

‘Because of the cycle we can harness waste heat and specifically low-grade waste heat. And we generate cold as we operate. When you think about data centres, the application demand for cold is very big.’

The full system returns about 50 per cent of the energy put in, rising to 70 per cent efficiency if it uses waste heat from another source, such as a power station.

This is similar to the efficiency of the much less energy-dense compressed-air storage plants and compares to 70 to 85 per cent for batteries and 65 to 75 per cent for pumped hydro.

The Slough plant is hosted by Scottish and Southern Energy and part-funded by a £1.1m grant from the Department of Energy and Climate Change (DECC).

Highview has largely used existing technology to construct the liquefaction plant and power generator, which evaporates the liquid nitrogen using waste heat from an adjacent biomass power plant. It can also use ambient heat.

The cold energy is then captured using a specially designed cold buffer that uses technology similar to that used in the steel and chemical industries, where cold is often stored in beds of sand or gravel.

The cold can then be used to re-liquefy the nitrogen when it is next needed to store more energy, doubling the efficiency of the electricity-generation process.

Highview’s cold store allows the two sides of the asymmetric liquefaction and generation system to be used when needed, said chief technology officer Rob Morgan.

‘The means for storing cold are well established. What we’ve found is the way you integrate these two parts of the process to run intermittently at different times.’

Using only the generator, the system could provide an almost immediate back-up power source to industrial sites with data centres, which consume around five per cent of the UK’s electricity, being an obvious market because of the cold energy produced.

But with its cold storage and liquefaction facilities, the system could be scaled up to store and release hundreds of megawatts as a way of saving surplus electricity produced at off-peak times, particularly that from renewable sources.

‘If you want a green grid you need energy storage,’ said Peters. ‘It’s the whole issue of strategic policy for energy security and economics. It’s the time-shifting of energy to support not just intermittent sources but also must-run plants.’

Highview is now planning a commercial-scale 3.5MW power generator due to be operational by the end of next year, which will be 10 times larger than the pilot plant, with a fully integrated liquefaction system to follow.