Gravitricity Ltd is an energy storage company based in Edinburgh, UK. The company is developing innovative, long-life underground technologies that help solve the problem of intermittent renewable energy by storing excess energy during periods of low demand and releasing it back when required.

Founded in 2011 and developing their solution since 2016. It has raised a total of £5.1m from private investors and over £2.6m in innovation grants.

The company is developing two separate energy storage technologies:

1) GraviStore - Underground gravity energy storage technology using the force of gravity to offer some of the best characteristics of lithium-ion batteries and pumped hydro storage.

How does it work? - GraviStore is based on a simple principle: raising and lowering a heavy weight to store and release energy. Electricity is stored in the form of potential energy by raising the weight. Power is then generated by lowering the weights to turn a generator.

How big is the system? - Single weight system is ideally suited to shafts in the range of 300-700m and designed to use weight configurations totalling 1,000+ tonnes which are suspended by a number of cables, each of which is engaged with an electric winch capable of lifting its share of the weight.

How much energy can it store? - Gravitricity are developing full-scale single weight systems with a power output of 1-8MW (a typical single module is 2MW with a 600t mass), with a typical duration of 30 mins (depends on shaft depth, can be adjusted to meet grid or site demands) providing 1-4MWh energy. Future multi-weight systems could have a much greater capacity (power 1-8MW, duration up to 12 hours).

2) H2FlexiStore - Underground hydrogen storage technology using the geology of the earth to contain pressurised fuel gas, allowing safe, large-scale storage, close to the point of demand.

What is it? How does it work? - Gravitricity are developing an innovative lined rock shaft (LRS) system, similar to the lined rock cavern method. The LRS high-pressure hydrogen storage method uses the surrounding rock mass to support the gas pressure, minimising the amount of containment material required. A bespoke shaft is sunk for each case and a series of lining materials are installed in the shaft which contain the hydrogen within a gas tight layer, transferring the load from the internal pressure to the rock mass.

How much hydrogen / energy can it store? - Intended storage capacity is between 15 to 100 tonnes, with the ability to co-locate multiple shafts to give larger storage volumes. A 6m x 365m shaft at 200 bar would provide storage for 3.3GWh of raw energy (enough to refuel 3,125 FCEV buses) or 1.6GWh of electricity storage.

What pressure is the hydrogen stored at? - The maximum operating pressures will be up to approx. 220 bar, with a minimum pressure of approx. 30 bar, with the remaining hydrogen (approx. 15%) acting as a cushion gas.

Price depending on installation size and characteristics.

GraviStore

• High Efficiency - Predicted full-scale efficiency of up to 80%.

• Long Lifetime - 50 year+ design life with no cycle limit or degradation.

• Fast response - Has the same fast response time as Li-ion batteries, being able to go from zero to full power in less than one second.

• Extends life of existing infrastructure - Can be applied in thousands of unused and soon to be decommissioned mine shafts worldwide, offering a potential route to a just transition for declining industrial sectors.

H2FlexiStore

• Purity - Purity of hydrogen is maintained due to gas tight lining (unlike salt caverns).

• Safety - If hydrogen permeates lining it is contained or dispersed by surrounding rock mass.

• Location - Much less geographically constrained than other geological storage like salt caverns and depleted oil and gas reservoirs.

• Intermediate scale - Provides a buffer store up to 100 tonnes, with a 40 x smaller footprint than equivalent above ground storage (for 65 tonnes).

GraviStore

• Flexibility - As the world generates 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. GraviStore can run fast to deliver high power, or more slowly for longer duration. This flexibility is a major strength of the technology.

• Frequency control - Gravistore can respond with full power in less than a second, being able to stabilize the grid when needed.

H2FlexiStore

• Grid Scale Energy Storage - H2FlexiStore can provide distributed mid-scale buffer storage for green hydrogen, which is produced using renewable electricity. The hydrogen can be used to provide zero carbon GWh scale electricity storage for daily peaks, long duration and inter-seasonal requirements.

• Location flexibility - Lined rock shafts can be situated where needed and don't require specific underground geological conditions like salt caverns. This allows storage of hydrogen where needed, close to the point of demand.