Gravity – the solution to energy storage?

Heindl’s Gravity Storage, which uses the gravitational power of a huge mass of rock to store large amounts of electricity.

Lithium-ion batteries seem to be used everywhere – from tablets and smart phones, to electric vehicles, to large-scale stationary storage systems like Tesla’s mega-battery in South Australia. Key to this success is their modularity – the ability to stack any number of battery cells together to create storage systems of any size. As a result, over the past 20 years, the cost of lithium-ion batteries has plummeted. Initially, this was brought about by the mass manufacturing of lithium-ion battery cells for consumer electronics. It made them an attractive option for use in electric vehicles, which further accelerated these cost reductions, making lithium-ion technology viable even for large-scale stationary storage.

Yet, modularity is also the weakness of lithium-ion batteries, particularly when it comes to large-scale storage. Scale effects (meaning cost reductions from building larger products) are limited when having to stack multiple small cells to build large storage systems. That’s because, as manufacturing costs decrease, the cost of the raw materials becomes the main cost driver. These raw material costs are not only expensive for electrochemical batteries, they are also the same for each additional cell (no scale effect).

Could new technologies that use low-cost raw materials undercut the lithium-ion monopoly?

Heindl Energy and Gravitricity are companies that develop new electricity storage technologies based on gravity. Their technologies are designed for different applications in the electricity system – bulk storage and frequency response.

Heindl’s Gravity Storage can store large amounts of electricity by pumping water below a cylinder-shaped rock to lift it when electricity is plentiful. Then, when electricity is needed, gravity pulls the rock down, pushing the water through turbines to generate electricity for multiple hours. Gravitricity has developed a technology that can release large amounts of electricity in a short time, for example to balance national electricity demand and supply on a second-by-second basis. The system is operated by dropping a heavy weight down a vertical shaft when power is needed, and pulling it up when not.

The Gravitricity system, showing a cylindrical heavy weight suspended in a vertical shaft.

Both concepts are based on gravity – the fundamental physical force that is experienced by all particles with a mass. In comparison, electromagnetism, the force behind electrochemical reactions in lithium-ion batteries, is experienced only by electrically charged particles. So, while the developers of gravitational energy storage can use any heavy material, electrochemical batteries need materials that are easily charged and discharged, often precious metals, which are rare and costly.

So, could gravity be the answer to large-scale energy storage?

It all comes down to scale. As raw materials are cheap, the main cost drivers for gravitational energy storage are equipment and construction. These costs are subject to scale effects and increase at a much lower rate than a respective increase in storage capacity. For example, doubling the rock radius of Heindl’s Gravity Storage increases the energy stored 16-fold, while construction costs only increase 4-fold. Doubling the maximum speed of the falling weight in a Gravitricity system would double its power, but increase costs only by around 30%. In comparison, doubling the energy or power capacity of a lithium-ion storage system would increase costs by around 50% each.

This explains why Heindl’s Gravity Storage and Gravitricity are so competitive when it comes to their lifetime cost, also called levelized cost of storage (LCOS) – as analysed by Storage Lab (the energy storage insights firm I set up during my PhD). Furthermore, gravity-based storage concepts can make good use of existing infrastructure, such as old mine shafts, and mature equipment, such as water turbines, that has a long lifetime compared with electrochemical batteries.

Comparison of Gravitricity’s LCOS in a frequency response application (US$/kW) to other suitable electricity storage technologies.

Comparison of Heindl’s Gravity Storage’s LCOS in a bulk storage application (US$/MWh) to other suitable electricity storage technologies.

The main uncertainty for both technologies is that they are commercially unproven – and pilot projects aren’t planned until 2020. Thus, the key criterion for success is whether their cost estimates can live up to commercial scale construction and operation. If they do, then they could be a serious contender to lithium-ion batteries for large-scale stationary storage and could accelerate the shift to an affordable low-carbon energy system.