The Scottish startup Gravitricity is planning a pilot gravitational energy storage project at an industrial site in the Port of Leith (Edinburgh). The manufacturing work of the project has been started on key equipment for a project to showcase how old mineshafts could be used as batteries to store renewable energy.
The company is spending about £1 million building an energy storage demonstrator of its technology and hopes to begin testing in next spring, in Scotland. The new gravity-based energy storage system offers some of the best characteristics of lithium batteries and pumped storage.
The Gravitricity system suspends weights of 500 – 5000 tonnes in a deep shaft by a number of cables, each of which is engaged with a winch capable of lifting its share of the weight. Electrical power is then absorbed or generated by raising or lowering the weight. The weight is guided by a system of tensioned guide wires to prevent it from swinging and damaging the shaft. The winch system can be accurately controlled through the electrical drives to keep the weight stable in the hole.
The demonstrator will stand 16 m tall, lifting and dropping two 25-tonne weights in order to generate 250 kW. The electricity discharged could power about 30,000 homes for two hours. The winches and control system are being constructed by Huisman in the Czech Republic, while Kelvin Power is fabricating the lattice tower in Leicester, UK.
“This grid-connected demonstrator will use two 25-tonnes weights suspended by steel cables. In our first test, we’ll drop the weights together to generate full power and verify our speed of response. We calculate we can go from zero to full power in less than a second – which can be extremely valuable in the frequency response and back-up power markets,” said Miles Franklin, lead engineer at Gravitricity.
“We will then run tests with the two single weights, dropping one after the other to verify smooth energy output over a longer period. Together, this two-month test program will confirm our modeling and give us valuable data for our first full-scale project.”
In the final stage, the company expects to be able to deliver peak outputs of 1 to 20 megawatts in a period of 15 minutes to 8 hours. And that function for up to 50 years with no loss of performance. To recharge this giant mechanical battery, electricity from renewable sources power the winches to lift the weights back to the top. In all, the system has an efficiency of between 80 and 90%.
But that is still a long way off. If the technology proves itself with the prototype that is now planned, the company aims to deploy a first full-scale prototype in 2022 or 2023 at a disused mine in the UK. After all, the startup received a £640,000 grant from Innovate UK, the British government’s innovation agency for the energy storage system and a crowdfunding campaign recently raised over £1.5m for technology development.