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Gravity – the solution to energy storage right under our feet?

Battery technology is seemingly the easy solution to the storage solution, but developers of gravity-based ‘virtual batteries’ are starting to make noise that is hard to ignore.

Innovative start-ups such as Energy Vault and the UK’s Gravitricity, have developed methods of storing energy as gravitational potential. Using winch systems to lift high mass blocks with excess electricity, power can be generated in times of high demand by simply releasing the block, allowing it to spin a turbine as it falls.

Edinburgh-based Gravitricity, aims to use redundant mine shafts, with a 3,000-ton weight, the equivalent of about 21 blue whales, to produce a system with a LCOE of under half the price of lithium-ion batteries. Early reports have suggested that a cost of $171 per MWh may be attainable, although there are uncertainties regarding safety and functionality of previously unmonitored mine shafts.

Energy Vault is claiming that it can reach closer to $60 per MWh. The difference between the two is likely to be that the mines which Gravitricity find a use for do not have a cost attached to them, and that Energy Vault has never factored in land costs. Otherwise, they are essentially the same thing.

Analysis by a team in the Centre for Environmental Policy regarding the Gravitricity system, has shown the technology to provide a rapid frequency response to grid operators, offering a suitable partner to renewables in a balanced and carbon-free electricity grid.

The systems also claim a 50-year design life, with no absolute limit, able to produce between 1 and 20 MW of peak power for between 15 minutes and 8 hours. Due to the simplicity of design, the efficiency of the system is reported to reach nearly 90%.

Discussions are currently underway with mine owners in the UK, Finland, Poland, the Czech Republic and South Africa, where mine shafts can reach down over 2000 meters, providing a potential of 65 MWh of energy per system. With the land surrounding mines often uninhabited and uninhabitable, projects provide the possibility of pairing with wind or solar something we have recently seen at retired coal-plants in the US.

Naturally, the potential of such a technology is limited by the number of accessible mine shafts, and hence will not fully displace the lithium battery market. However, as reported a few weeks ago, coal facilities look set to leave countries such as Japan with up to $76 billion in stranded assets. The ability to make use of such facilities, and maintain job provision through the transition to renewables, is an often-overlooked advantage of technology such as gravity storage.

The Gravitricity system, is significantly simpler than that of competitors such as Energy Vault, where complex software and mechanisms are required to construct and deconstruct towers of concrete blocks. Despite limited application, there are few reasons, except up-front cost, not to install a Gravitricity system where possible, as its efficiency far exceeds thermal-based systems such as those using molten salts.

Bloomberg has estimated that the storage market will pull in $620 billion of global spend by 2040, with a significant amount of this set to be invested into new technologies.

Gravitricity is currently preparing a 250 kW concept demo alongside a 4 MW prototype, although there are no fixed plans to implement a system into the UK grid.

The two seemingly similar gravity systems do have quite different drawbacks – Energy Vault has to pile up blocks the size of a tower block, which may bring criticisms around safety and what it does to the aesthetic of a town or region, but you can put it where you want – while Gravitricity requires an existing coal mine, and few of there are close enough to regions of high population, so it runs into transmission costs.

Bringing these technologies to market in a concrete (excuse the pun) fashion will perhaps give them a 2 to 4 year window to grab market share, before improvements in lithium ion, start to make alternatives look unnecessary.

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