Recent investments and patent applications show that people are increasingly interested in a class of energy storage concepts that seem simple but have not yet gained commercial appeal.
Open-pit mine: the next game of energy storage?
Recently, with the news of strategic investment and new patents, the speculative field of gravity energy storage has been boosted.
The Swiss-U.S. startup Energy Vault is one of the most high-profile participants in gravity storage so far. It has received financial support from Cemex Ventures, the corporate venture capital arm of the world's second largest building materials giant, and promises to use the following methods Help deploy Cemex's "strategic network".
At the same time, the University of Nottingham and the World Association for Sustainable Energy Technology confirmed a patent application for a concept called EarthPumpStore, which uses abandoned mines as a gravity storage asset.
The University of Nottingham stated in a press release that the technology is only implemented in 150,000 abandoned open-pit mines in China, and it is estimated that it can provide 250 terawatt-hours of storage capacity.
These announcements show that people are becoming more and more interested in a seemingly simple but not yet widely accepted concept of energy storage.
Most gravity storage concepts are based on the idea of using backup power to lift the weight, so energy can be recovered by letting the weight drop again when needed.
In the case of Energy Vault, the blocks were made of concrete and were lifted by a 33-story crane. At the same time, EarthPumpStore envisaged pulling containers filled with compacted soil to both sides of the open-pit mine.
Gravity is also the force that supports pumped storage, which is the most common and cost-effective form of energy storage in the world. However, pumped storage development is slow and costly, requires sites with specific topographical features, and often involves major licensing barriers.
Proponents of the new gravity storage option claim that their technology is faster, easier, and cheaper to install and deploy.
For example, the University of Nottingham estimates that the cost per installed kilowatt-hour of EarthPumpStore is about US$50, while the cost of pumped storage is US$200 and the cost of battery storage is US$400.
The university also stated that EarthPumpStore can achieve a round-trip efficiency of more than 90%, while the efficiency of pumped storage is between 50% and 70%, and the energy storage density is as high as eight times. Other sources have made similar claims.
For example, in 2017, a study conducted by Imperial College London for Heindl Energy, a developer of gravity energy storage technology, concluded that the Heindl concept can achieve an average energy storage cost of US$148 per MWh, compared to the cost of pumped storage. It is 206 US dollars.
The study concluded: “Based on the given data, gravity storage is the most cost-effective for large-capacity power storage, followed by pumped storage and compressed air storage.”
Given the apparent simplicity and cost-effectiveness of gravity storage, it is strange that this concept has not yet taken off. One of the first companies that proposed the concept of gravity was Advanced Rail Energy Storage (ARES), a Santa Barbara company founded in 2010.
ARES plans to lift heavy objects based on rail cars to the hillside, and in 2016 it finally obtained approval from the U.S. Bureau of Land Management for the proposed 50 MW and 12.5 MWh project in Nevada. At that time, ARES expected the project to be up and running in early 2019.
However, as of August last year, the company is still obtaining permission and delaying its launch date to 2020. Other hopes for gravity storage seem to have made equally slow progress, even though two British companies received funding last year.
Energy SRS, a collaboration between five British companies and the University of Bristol, received £727,000 (approximately US$922,000 at today's exchange rate) from Innovate UK, a government research and innovation agency
The funds are used for prototypes, and Energy SRS hopes to expand the scale by 2020. At the same time, another start-up company, Gravitricity, received a separate grant of £650,000 ($824,000 today) for innovation in the UK for the construction of a 250-kilowatt mine prototype based on the concept of gravity.
Gravitricity is also scheduled to be fully implemented next year.
Daniel Finn-Foley, principal analyst at Wood Mackenzie Power & Renewables, said that concerns about the safety, scalability, and round-trip efficiency of lithium-ion batteries may lead to growing interest in alternatives such as gravity storage.
"As states continue to mandate carbon-free energy, this may be a key technology in the long run," he said. "I doubt whether the 100% vision can be solved by discarding lithium-ion batteries anywhere, so seeing the emergence of new technologies will be the key."