What is Gravity energy storage (GES)?

GES stores unprecedentedly large quantities of energy by moving mass against gravity to create potential energy that can be used, or 'discharged' at any time. This technology ticks all of the boxes for grid-scale energy storage as it is highly salable and theoretically suited to both power applications such as regulation and also energy-centric applications like energy shifting.

GES is very much in the development stages with a variety of methods being developed. Some of these are:

• Using disused mine shafts to suspend weights on a winch
• Using raised platforms or geographical features to suspend weights on a winch
• Lifting massive rocks using hydraulic pumps
• Moving high mass train carriages along railways built on hills

Below we take a brief look at each of these methods, that are being developed by different companies.

Gravitricity

Gravitricity is a UK-based start-up which earlier this year won £650,000 of government backing, is developing a solution based on disused mine-shafts.

The key characteristics of this solution according to Gravitricity are:

• 50-year design life
• Response time to full power: < 1s
• AC Round-Trip Efficiency (RTE): 80-90%
• Levelised cost: US$ 141 /kW (claimed)
• Systems are modular between 1 and 20MW peak power
• Output duration is from 15 minutes to 8 hours.

StratoSolar

StratoSolar, from California, are working on a solution that stores energy by raising masses (hundreds of tonnes) from the ground to a buoyant platform 20,000 meters above sea-level using electric motor/generator driven winches. The platform may also have solar PV across the top of it's surface.

The key characteristics of this solution according to StratoSolar are:

• 30-year design life with 15,000 cycles
• Response time: 'fast'
• AC Round-Trip Efficiency (RTE): 85%
• Capital Cost: $125/kWh and will fall with production volume (claimed)
• Scalable from kW to TW by adding platform elements and weights
• No geographic site dependence on hills or caverns

Heindl Energy

Heindl Energy is a German engineering firm who are developing a GES solution based on the hydraulic lifting of a large rock mass. Using electrical pumps, water is pumped beneath a movable rock piston, thereby lifting the rock mass. During times of insufficient generation of renewable power, the water which is under high pressure from the rock mass, is routed to a turbine, as in conventional hydroelectric plants, and generates electricity using a generator.

The key characteristics of this solution according to Heindl Energy are:

• 60-year + design life
• AC Round-Trip Efficiency (RTE): 80% +
• Capital cost: 200 USD/kWh
• Running costs: < 1% of capex per year
• Capacity: 8 GWh or more
• Size: The rock piston should have a diameter >100m (250m is optimal for costs)
• More than 40% of land is suitable for this solution

ARES - Advanced Rail Energy Storage

ARES is an American company with a very different approach. They use locomotives on a train track going up and down a hill. This solution is typically comprised of two electric locomotives and multiple weighted cars, that ascend and descend a rail line at slow speeds to either take electricity off the grid (ascend) or supply electricity to the grid (descend)

The key characteristics of this solution according to ARES are:

• 50-year design life
• Able to provide fast response ancillary services
• AC Round-Trip Efficiency (RTE): 80%
• Levelised cost: unknown
• Small system size: 100 MW with 2 hours capacity (200 MWh)
• Large system size: 3 GW with 8 hours capacity (24 GWh)

Summary

With performance metrics comparable to Lithium Ion batteries and with better cost and sustainability characteristics, the GES concept does seem to have a future. While it is early days and it remains to be seen whether 20km high buoyant platforms or 250m wide floating rocks will be things, one thing is certain - the need for these types of mass storage solutions is growing fast.