Could the Solution to Renewable Energy Storage be Beneath our feet?

The UK has a long history with the coal industry. The engine of the Industrial Revolution was fuelled by the dirty black rock and led to this small island nation becoming a superpower and a world leader in engineering. By 1920, 1.19m people worked in coal mines stretching from the Central Belt of Scotland to Kent. However, the industry suffered a steep decline in the 1950s and the next 3 decades saw the decimation of King Coal. By the 21st century hardly any mines remained open with the UK instead relying on imports from Russia, The US and Columbia. A remarkable milestone in the transition to low-carbon energy generation was reached on 21st April 2017 when the UK went a whole day without burning coal for electricity for the first time in 135 years.

An estimated 59,000 mine shafts lie dark and disused today. The closures of these mines left some communities enduring decades of unemployment, but this could all be about to change, and the whirr and clank of motors and winches could be heard again.

Disused mine shafts around the UK could soon be given a new lease of life and used as giant gravity batteries, capable of reacting to grid demands in under a second. This novel form of energy storage under development by a Scottish engineering company could be used like chemical batteries to rapidly respond to fluctuating grid demands.


Edinburgh based start-up, Gravitricity which has just received a £650,000 grant from Innovate UK is developing a surprising new source of “gravity energy”. They hope to use Britain’s long abandoned mines to make better use of clean electricity at half the cost of lithium-ion batteries.

Britain’s least expensive “virtual battery” could be produced by hoisting and dropping 12,000-tonne weights (half the weight of the Statue of liberty) down disused mine shafts according to London’s Imperial College. Gravitricity plans to supply enormous weights and winches for these disused mine shafts. When energy is plentiful, the weights will be winched towards the surface, in much the same way that water is driven uphill in pumped hydro storage. However, unlike pumped hydro, the system should be able to respond to variations in demand almost instantly as well as having a potential degradation-free operational lifespan of 50 years. Surplus power will be drawn from the grid to raise the weights closer to ground level. When the time comes to inject energy back into the grid, the weights can be released for a burst of power generation by dropping the weights hundreds of metres down vertical shafts.

The scheme imitates the way that hydropower projects work. Hydropower projects have played a key role in helping to balance the electricity grid since the Dinorwig project in Wales began operating in the mid-1970s.

Imperial’s report found that electricity released by a typical 10MW lithium-ion battery project, capable of releasing 24MW of electricity per hour, would cost $367 (£283) per megawatt-hour over its lifetime compared with a cost of $171 (£132)/MWh for electricity from a Gravitricity project.

Oliver Schmidt, the lead author of Imperial’s report said:

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

The system was first developed by Gravitricity’s founder, Peter Fraenkel, who also invented the world’s first full-scale tidal energy turbines. The design was bought by the German industrial firm Siemens.

Gravitricity’s managing director, Charlie Blair said:

“So far there is a lot of focus on batteries, but our idea is quite different. Gravitricity uses a heavy weight – up to 2000 tonnes – suspended in a deep shaft by cables attached to winches. When there is excess electricity, for example on a windy day, the weight is winched to the top of the shaft ready to generate power.

This weight can then be released when required – in less than a second – and the winches become generators, producing either a large burst of electricity quickly, or releasing it more slowly depending on what is needed.

As we rely more and more on renewable energy, there is an increasing need to find ways to store that energy – so we can produce quick bursts of power exactly when it is needed. 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.”

According to Blair, the system would be able to operate for decades without degradation and could have a lifespan of around 50 years. Models from 1 to 20MW will be offered, with a part-scale demonstrator planned for later this year.

A full-scale project would drop 24 weights totalling 12,000 tonnes to a depth of 800 metres to produce enough electricity to power 63,000 homes for more than an hour.

Gravitricity has said it could increase this period by carefully controlling the winches to allow the weights to fall at a slower rate leading to the release of electricity over a longer period.

Miles Franklin, lead engineer at Gravitricity, says:


“The system is a bit like solid-state pumped hydro. The basic concept is the same as that inside an 18th-century grandfather clock. You’re winding up a weight to store some energy in the position of that weight… that weight descending then powers the system for a period.” 

Miles Franklin also said:

“It’s not the case that we’ll be able to employ the same numbers of people as a mine once it is operating, but there will be a long period of installation which can employ as much local labour as possible, and then in the longer term it does bring some new life to those kind of old mining assets.”  

However, Charlie Blair says that the focus will shift to digging new shafts in the future which he feels would make a really big difference. This way systems could be built where storage is most needed closer to cities for example instead of in mine shafts which are most often found in rural areas.

Gravitricity hopes to keep to a target date for a part-scale demonstration model later this year with a full-scale operating prototype lined up for 2020.

They are building an above-ground 250kW demonstrator, aiming to prove simulations of start-up behaviour, stable operation and predicted one-second response times. The team says the technology could eventually operate in the 1MW to 20MW peak power range, with output duration from 15 minutes to eight hours. The company predicts impressive system efficiency of 80-90%.

Currently, Gravitricity is looking to partner with investors including those who can bring mining expertise to the team. They are also investigating several disused mine shafts both in the UK and South Africa.

This new energy-storage technology is part of a wave of projects looking underground to prevent a future energy crisis. Together they could help to keep the lights on and make sure renewable energy has the era-defining impact that coal once had.

The new technology is aimed at solving renewable energy’s greatest problem which is how to store electricity generated by intermittent sources such as wind and solar. Failing to create enough storage after switching from fossil fuels to renewables could otherwise lead to frequent power shortages during periods of high demand.

Find out more about solar here

Find out more about wind turbines here.