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University researchers say gravity energy storage could be significantly cheaper than li-ion batteries

23 October 2019

A system that stores excess renewable energy in weights suspended above disused mineshafts could be cheaper than lithium ion batteries, according to a report by independent analysts at Imperial College London.

Artist's impression of gravity energy site - Image: Gravitricity
Artist's impression of gravity energy site - Image: Gravitricity

The idea behind the technology is simple. Excess energy from renewables when demand is low is used to lift a massive weight or weights to the top of a shaft. These can then be stacked and released when demand is high, delivering energy rapidly back to the grid.

This can be done many times a day for many years, without any loss of performance and makes it very competitive against other forms of energy storage – including lithium ion batteries.

Up to 24 weights totalling 12,000 tonnes can be stacked in a mine shaft to capture power and can then be released with sub-second response time.

The scheme mimics pumped-storage hydropower projects such as Dinorwig in Wales and Cruachan in Scotland, which have played a key role in helping to balance the UK electricity grid.

Oliver Schmidt, the lead author of Imperial’s report, said the model, developed by UK energy startup Gravitricity, is the most price competitive energy storage option because it has a relatively low upfront cost and a potential lifespan of more than 25 years.

The report found that electricity released by a typical 10MW lithium-ion battery project, would cost $367 (£283) per megawatt-hour over its lifetime compared with a cost of $171 (£132)/MWh for electricity from a Gravitricity project.

Schmidt said: “I don’t expect Gravitricity to displace all lithium batteries on grids, but it certainly looks like a compelling proposition.”

The proposed system received a £650,000 grant from Innovate UK last year, and patent owner Gravitricity has teamed up with Dutch winch specialist Huisman to build a 250kW scale prototype.

“The climate emergency means we need to find new ways to capture and store green energy so we can use it when we need it,” Gravitricity managing director Charlie Blair said.

While the developers will target former mine shafts initially, in the longer term they plan to sink purpose-built shafts wherever required. They are currently in discussion with mine owners in the UK, South Africa, Finland, Poland and the Czech Republic.

The analysts at Imperial Consultants said the Gravitricity system performs particularly well in the ‘peak-shaving’ market, a service which ensures the balancing of electricity demand and supply during peak demand periods to reduce the strain on power system infrastructure.

This typically requires a storage system to discharge for two to six hours, twice a day. The analysis was based on a 10MW maximum capacity delivering 24MWh – this translates to around 2.5 hours discharge duration at maximum power output.

The system is also well suited to the fast frequency response market, which requires multiple short cycles and high power availability.

“’The report validates our belief that energy storage with a long life can be built into the infrastructure of the grid, potentially saving billions in grid upgrades,” Blair said.

“Mechanical systems such as ours are very cost-effective, as we can cycle several times per day with no performance degradation, enabling us to deliver a range of different services to grid operators. This means we can also stack up different revenue streams to improve our business case.”

Recent figures have shown that renewables are producing record levels of electricity for the UK, but their intermittency can cause problems for the National Grid when compared to baseload power.

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