Weighty support for Gravitricity3 July 2018
Gravitricity’s ingenious design makes use of disused mine shafts to store and generate energy on demand. It has now teamed up with lifting specialist Huisman to build a 250 kW demonstrator.
Gravitricity has teamed up with lifting, drilling and subsea specialists Huisman to develop a scaled demonstrator of its innovative gravity-fed energy storage system during 2018, and has made plans for a long-term partnership to bring gravity- fed technology to the market. The Edinburgh firm has signed an R&D agreement with the Dutch multi-national to develop a concept demonstrator and test it in the Netherlands and Scotland early next year. Following this they plan to scale up to fully commercial 20 MW systems. The project partners believe Gravitricity can be a strong competitor in the frequency response market, where there is a requirement from grid operators for large, fast and frequent bursts of power to stabilise increasing amounts of renewables on the grid. “This 250kW prototype will help us validate our simulations about how the technology works in practice and will give us the opportunity to utilise Huisman’s experience in winches and cranes,” commented Gravitricity managing director Charlie Blair. “At the same time Huisman and our engineers will begin detailed design of the winch drive modules for our full-size 4 MW demonstrator, which will be deployed in a UK mine shaft in 2020.”
The Gravitricity technology consists essentially of a giant weight that drops down into the mine shaft to create power on demand using gravity. The principle is simple: raising and lowering a heavy weight to store energy and release it on demand, the same principle that powers pendulum clocks, which use a weighted pendulum that acts as a power generator to keep the clock running. A weight up to 3000 tons is suspended in a mine shaft, and energy is generated or expended by lowering and raising the weight. The power used to raise the weight is stored in the form of potential energy.
The system is capable of generating up to 20 MWe in shafts of up to 5000 ft. It can last 50 years without degradation, and it is cost- effective because it uses existing infrastructure.
There are several advantages inherent to this kind of mechanical storage. It can be available immediately on demand – it takes only one second to drop the weight or it can be released slowly for sustained energy. Also there is no problem with the disposal and degradation inherent to battery systems. The system is responsive (zero to full power in less than one second) efficient (between 80 and 90 %) it is versatile – it can run slowly at low power or fast at high power – and it is cost effective, with levelised costs well below those of lithium batteries. Each full sized unit can be configured to produce between 1 and 20 MW peak power, with output duration from 15 minutes to 8 hours.
Gravitricity is still in the development phase, but the startup has already received $1 million in a grant from Innovate UK. If the system takes off, it could mean a degree of revival for communities that are struggling as coal mines decline.
A cylindrical weight is suspended in a deep shaft by a number of synthetic ropes each of which is engaged with a winch capable of lifting its share of the weight. Mechanical energy 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 shaft.
The shaft can be a disused mineshaft brought back into use, or a purpose-sunk shaft. Shaft depths can be from 150 m for new shafts, to 1500 m for existing mines. The biggest single cost is the shaft, and initially Gravitricity intends to prove the technology using existing mine shafts. The grid connection is through power electronics to permit rapid switching between generation and absorption of power. The system can deliver reactive as well as resistive power to help with grid stability.
While the weight system can be used on its own, the energy storage capacity of the system can be much increased when the shaft is used as a pressure vessel for compressed air. This involves adding a pressure-tight ‘lid’ to the top of the shaft and lining the shaft to prevent leakage. The winches and generators are normally contained in the pressurised space so that only electrical cables need to penetrate the pressure vessel walls.
The International Energy Agency estimates that in order to limit global warming to below 2°C, the capacity of storage connected to the grid worldwide should increase from 140 GW in 2014 to 450 GW by 2050. Bloomberg New Energy Finance in its report Global Energy Outlook 2016 forecast over $100 billion of global spend on energy storage to 2030. Gravitricity says that its system can provide balancing services such as frequency response and reserve to the high voltage transmission grid in a region, as well as appealing to network-constrained users and generators, 33kV distribution networks and major power users looking for fast response. It can also be sited at locations where energy storage is particularly required: on constrained distribution grids, and on micro-grids where the technology can provide long-term system balancing.