Storing renewable energy in mineshaft weights may be cheaper than using batteries

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

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.

“Our idea is very simple. We use excess energy to lift a massive weight or weights to the top of a shaft. These can then be stacked and released when required, delivering energy rapidly back to the grid. The beauty of this is that this can be done multiple times a day for many years, without any loss of performance. This makes it very competitive against other forms of energy storage – including lithium ion batteries,” said Gravitricity managing director Charlie Blair.

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 climate emergency means we need to find new ways to capture and store green energy so we can use it when we need it,” Blair added.

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 that Gravitricity’s multi-weight system offers a better long-term cost of energy storage than batteries or other alternatives.

The report says 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.”

Report author Oliver Schmidt, who specialises in energy storage at Imperial College London, said: “The study has been rigorous in comparing all technologies on a level playing field. We have investigated a number of sensitivities, including discount rate, project duration and expected cost reductions using industry-specific learning rates.

“Gravitricity’s new system has a relatively low upfront capex and a design life of over 25 years. This explains why it is so competitive against other storage options. It is sensitive to the discount rate and the modelled project lifespan.  If we model a shorter response duration, a lower discount rate or a longer project life, then Gravitricity looks even more competitive.  I don’t expect Gravitricity to displace all lithium batteries on grids, but it certainly looks like a compelling proposition.”

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 stations.

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