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Dropping A Weight Down A Disused Coal Mine Is The New Way To Store Energy - And It Might Just Work

 

 

By Mike Scott


February 10, 2018 - Energy storage has always been a field that generates some far-out ideas. Even the most conventional technique, pumped hydro, is a bit counter-intuitive – pumping water from a low lake to a higher one so that you can use it to turn hydro-electric turbines.


And giant batteries? Taking power from electric cars and returning it to the grid? Turning off thousands of refrigerators at the same time? A few years ago, all these ideas lived firmly in the realms of science fiction. Today, they are a reality.


Now another crazy idea has reared its head. A U.K.-based startup, Gravitricity, has just received a £650,000 ($901,000) grant from Innovate UK, the national innovation agency, for a plan that involves using disused mine shafts to store energy.


Essentially, the Gravitricity system is a huge "clock weight," the company says. It plans to suspend a 3,000-tonne cylindrical weight in mine shafts from 150m to 1,500m deep. The weight is attached to a series of winches that can lift it, and when electricity is needed, the weight is dropped to drive a turbine that creates electricity. It is then winched back to the top of the shaft using cheap, off-peak electricity.


Gravitricity says the technology “offers some of the best characteristics of lithium batteries and pumped storage, without the need for a nearby mountain with a lake or loch at the top.”


The technology can go from zero to full power in less than a second, has an efficiency of between 80% and 90% and can either run rapidly at high power for 15 minutes, or for up to eight hours at lower power. It has a 50-year design life with no limit on how often it can be used and it does not degrade, unlike batteries, the company says. In addition, it is easy to build, can be sited near existing transmission networks and is much cheaper than lithium-ion batteries.


“Over the 12 months from January 2018, we will be undertaking sub-system design and deploying a 250kW concept demonstrator. We aim to trial our first full-scale prototype in 2019 or 2020 at a disused mine in the U.K.,” it adds.


Once it has proved the concept in existing mine shafts, it plans to drill purpose-built shafts close to power lines.


This is not the first attempt to use gravity to generate power. An American company called ARES has developed a system that uses trains filled with rocks on a hillside, pushing them to the top of the hill out of peak demand and generating electricity by releasing the train and letting it roll to the bottom.


Another company, Energy Cache, planned to do the same thing using ski lifts full of weights, but little has been heard of it recently. There are also proposals to fill mines with water and use them as underground pumped hydro facilities, but the logistics of those make them a more complex undertaking than Gravitricity’s scheme.


A different approach has been taken by another U.K. company, Highview Power. It uses off-peak electricity to freeze nitrogen, a process it calls liquid air, and releases it at peak times to produce power. Its system has the advantage that it can be easily incorporated into industrial facilities.


A final approach that may become commonplace in years to come is to use off-peak renewable energy to create hydrogen from water. The hydrogen can then be stored either on site in tanks or injected into the natural gas network. It can be used to run fuel cells or it can be burnt in engines to create power.


 

There seem to be as many different ways to store energy as there are forms of energy. Each one seems crazier than the last, yet many of them are pretty effective and may be coming to a mine shaft, a hillside or an industrial facility near you soon. 

 

A disused coal mine in Wales, UK

 

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