As the world generates more and more electricity from intermittent renewable energy sources, there is a growing need for technologies which can capture and store energy during periods of low demand and release it rapidly when required. For more information see the IDTechEx report on Batteries for Stationary Energy Storage 2019-2029.
Gravitricity is developing a novel storage technology which offers some of the best characteristics of lithium batteries and pumped storage. Ideally suited to network-constrained users and operators, distribution networks and major power users, the technology operates in the 1MW to 20 MW power range and enables existing grid infrastructure to go further in a renewable energy world.
The patented technology is based on a simple principle: raising and lowering a heavy weight to store and release energy. 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 (patents applied for) 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.
Gravitricity™ technology has a unique combination of characteristics:
- 50-year design life - with no cycle limit or degradation
- Response time - zero to full power in less than one second
- Efficiency - between 80 and 90 percent
- Versatile - can run slowly at low power or fast at high power
- Simple - easy to construct near networks
- Cost effective - levelised costs well below lithium batteries
Each unit can be configured to produce between 1 and 20MW peak power, with output duration from 15 minutes to 8 hours.
The key requirement is a deep hole in the ground; it can be a disused mineshaft brought back into use, or a purpose-sunk shaft. Shaft depths can be from 150m for new shafts down to 1500m for existing mines. The grid connection is through modern power electronics to permit rapid switching between generation and absorption of power and the system can deliver active as well as reactive power to help with grid stability.
While the weight system can be used on its own, the energy storage capacity of the overall system can be much increased when the shaft is used as a pressure vessel, allowing a compressed air energy storage to operate alongside the weight system. This involves adding a pressure-tight "lid" to the top of the shaft and lining the shaft to prevent leakage. The ground provides the bursting resistance other than at the very top of the shaft. The winches and generators will be contained in the pressurized space so that only electrical cables need to penetrate the pressure vessel walls.
Initially Gravitricity will prove the technology using existing mine shafts. Future deployments will be able to utilise existing mines or purpose-built shafts, allowing development wherever storage is required. During 2019/20 Gravitricity is undertaking sub-system design and will be building a 250kW concept demonstrator in 2020 with the aim to deploy the first full-scale prototype in 2021 or 2022 at a disused mine in the UK.
Gravicity ran a crowdfunding campaign in October 2019 and received a very positive response.