I’d like to believe that an apple did hit Newton on the head, and that led him to discovering gravity. The story is just more fun that way. But whether or not the apple actually fell on his head, according to the manuscript in the archives of the Royal Society, an apple definitely seems to have been involved. Newton’s Principia, explaining the phenomenon of gravity was first published in Latin in 1687. And it created quite a ruckus.

Now, over 300 years later, climate change is hitting us on the head in no uncertain terms, and gravity seems to be making a comeback. As we rapidly transition towards producing more and more energy from intermittent renewable sources like wind and solar, we must find efficient and effective ways to store this energy at scale, and reuse it when needed. I’ve been curious about sustainable ways to store energy over long periods of time, so I started looking into what’s out there. This post is focused on exciting gravity-based solutions and the companies working on them.

The underlying ‘gravity’ concept is similar to that of traditional pumped hydropower. To put it very simply, energy is used to pump water up to a height. When the water is released, and it passes through a turbine, it releases this energy. The challenge with pumped hydro, however, is that it requires a suitable location with water and appropriate surrounding topography (hills for reservoirs at different heights for example) for us to be able to harness it.

Novel solutions, using this concept, but replacing water with other materials (like bricks and weights)b, or creating the possibility to tap into hydropower in more widely available locations are being developed.

Why are these gravity-based technologies potential competitors to large, grid-scale battery systems?

  • These installations are modular, flexible and scalable. When the capacity of a single installation is reached, they can easily be installed in multiples — we could very well have gravity-storage parks, just like windfarms.
  • They have lower ‘valuable’ resource intensity. The installations are made from steel, concrete and composites, which are cheaper and more easily accessible than lithium, zinc, and other metals required for batteries. As they do not require complex or advanced materials, decommissioning and recycling at end-of-life are relatively straightforward.
  • They are designed to be in service for 40–50 years, during which they would only see mechanical wear-and-tear. This high asset longevity can contribute significantly to achieving a lower cost per kWh of storage capacity.
  • Existing infrastructure like decommissioned mine-shafts can be used for some of these concepts, making deployment easier. In the case of hydropower, some of these new concepts make it possible to utilise much smaller slopes and even artificial tanks and reservoirs.
  • These systems can be combined with other energy storage options like hydrogen, thermal storage or even batteries.
  • The bricks and weights used have the potential to be carbon ‘sinks’. Using materials like concrete from CarbonCure or Carbicrete, or composites made from recycled industrial waste could help add another dimension to their impact.

Who is building these technologies and what are the systems like?

Here’s a non-exhaustive list –

Energy Vault(Switzerland, founded 2017):
Energy Vault’s solution replaces the water in pumped hydro with custom made 35-ton composite blocks made from low-cost materials. The concept video reminds me of playing Tetris, but with real-life bricks going in two directions — ‘recharging’ when the bricks are taken up and placed on top of the tower and ‘discharging’ when they are brought back down. Energy Vault estimates the round-trip energy efficiency to be between 80–90%. The plants are designed for ranges of 20–35–80 MWh storage capacity and a 4–8MW of continuous power discharge for 8–16 hours. They are also developing solutions to offer full scalability from 1 MWh modules up to multi-GWh of storage capacity. Their first demo site is in Tinicio, Switzerland.

Gravitricity(UK, founded 2011):
Gravitricity’s system suspends weights with the help of cables in a deep shaft. Each cable is engaged with a winch capable of lifting its share of the weight. Electrical power is consumed when raising the weight and generated by lowering it. The system is estimated to have an 80–90% efficiency, can utilize decommissioned mine shafts (> 300m depth) and has a design life of up to 50 years, enabling a low cost of storage per kWh. Their first 250 kW demo in Edinburgh recently generated (shore-)power for the first time, and a full-scale, 4 MW plant is planned to start later this year. Gravitricity recently announced plans to add compressed hydrogen and thermal storage to their installations.
study (in 2019) by Imperial College London estimated that Gravitricity’s technology could store energy at half the cost of Lithium-ion batteries. Even considering the drop in battery prices over the last year, this is a significant advantage.

Advanced Rail Energy Storage (USA, founded 2010):
Sheldon Cooper (or any ‘trainiac’) would absolutely love this one!
ARES’ fixed motor, chain-drive system uses electricity to drive mass cars uphill on fixed rail tracks, converting electricity into potential energy. When power is required, this process is reversed and the mass cars go downhill; the electric motors operate as generators, converting the potential energy back into electricity. The amount of energy stored depends on the number of mass cars and the (perpendicular) height of the slope. Storage capacity can be increased by adding more tracks, and duration by increasing track length. Each mass car is 25% longer and about twice as wide as a standard twenty-foot container (TEU) and weighs approx. 320 tons. A 50 MW GravityLine with 10 multi-rail tracks and 210 mass cars is being built in Nevada.

Gravity Power (USA):
The GPM (Gravity Power Module) uses a very large piston suspended in a deep, water-filled shaft, with sliding seals to prevent leakage around the piston and a return pipe connecting to a pump-turbine at ground level. The piston is made up of reinforced rock (or low-cost concrete). The dropping piston forces water down the storage shaft, up the return pipe and through the turbine, producing electricity. To store energy, electricity drives the generator in reverse, spinning the pump to force water down the return pipe and into the shaft, lifting the piston. Each shaft can be used to store several MWh of energy. Water needs to be filled into the shaft once at the start of operations; it can then be reused.

New Energy Let’s Go (Germany):
Their concept, originally developed by Heindl Energy GmbH, involves creating a cylindrical well (in bedrock) in which a large rock mass is pushed up and down with the help of hydraulic pumps. Excess renewable electricity can be used to push water under a movable rock piston, and when electricity is needed, the rock mass is lowered, pushing the water under it through a turbine to generate power. Estimated diameter of a 1 GWh plant is 150m and that of an 8 GWh plant is 250m. New Energy Let’s Go is an interim investor who has acquired the patents and is seeking a strategic investor to continue development.

RheEnergise (UK, founded 2019):
The company is developing ‘HD Hydro’ or high-density hydropower technology that makes it possible to reach the same power output on hills that are 60% lower than in projects using water. This is achieved by using an environmentally benign high-tech fluid (R-19 TM) with a density 2.5x that of water that has been engineered to be non-reactive and non-corrosive. This unlocks several new potential sites, reduces plant construction costs and has the potential to significantly speed up deployment.

Easy Hydro (UK, founded 2019):
Easy Hydro, a spin-out of Trinity College Dublin, makes small-scale (1–200 kW) modular hydropower generating sets suitable for harnessing energy from man-made water networks (e.g. drinking water or irrigation networks) as well as natural ones (rivers, lakes). The installation requires a 10m water column (or 1 bar pressure) and a flow rate of 4 litres/sec. The magnitude of the power output is based on the pressure and flow rates. Easy Hydro uses off-the-shelf water pumps with a reversed flow direction, instead of custom hardware. This enables them to keep costs low, and makes installations easy to repair.

How does the (design) scale of these different solutions compare?

Energy storage solutions can be compared on two aspects. Energy capacity measures the total amount of energy that can be stored in a system (units – kWh, MWh, GWh, etc.) And power capacity measures the maximum amount of power that can be delivered by a system (units – kW, MW, GW, etc.)

For instance, an energy storage installation with an 8 MW, 80 MWh configuration can provide a maximum continuous power of 8 MW for 10 hours, or it can provide a lower, 4 MW power for 20 hours. This design however, cannot provide power higher than 8 MW.

From the examples above, ‘energy towers’ could be made taller or ‘gravity lines’ could be made to go to higher altitudes to increase the energy capacity. The power capacity of the installations could be manipulated by changing the speed at which the bricks / water / mass cars are displaced.

And this is how the energy capacity of individual installations from the above companies (approximately) compares:

Comparison of Energy Capacity of different gravity-based energy storage technologies

In conclusion, for any energy storage solution to be successful, it has to be easily deployable, scalable and low-cost. At sufficient scale, gravitational storage solutions could (theoretically for now) easily compete with batteries on cost, and offer a more environmentally sustainable solution when we compare the entire lifecycle of the technology. This is worth accelerating the work on gravity-based solutions, don’t you think?

Written by Prasanna Colluru.

Contact Prasanna on LinkedIn.

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