Energy Storage

14 Nov 2019

In Search for Cheaper, Longer Energy Storage, Mountain Gravity could Eventually Top Lithium-Ion

14 Nov 2019  by Matthew Bandyk   

Mountain gravity energy storage could be a viable way to store electricity for longer durations and at larger scales than lithium-ion battery storage can, according to a study recently published in the academic journal Energy.

The idea of gravity as a form of storage is an example of ongoing research into additional storage options beyond lithium-ion batteries. Despite large cost reductions over the past several years, some experts still view lithium-ion systems as not economically-efficient enough at scale to fully back up the amount of renewable energy expected to come onto the grid due to states’ long-term clean energy goals.

Using mountains for storage could be combined with hydropower and prove to be economically attractive for microgrids, islands and areas with high electricity costs, the study published by the Austrian scientific group International Institute for Applied Systems Analysis​ (IIASA) said.

The researchers propose that a motorized system similar to a ski lift could pull containers full of sand to a crane at the top of a mountain. The sand can then be sent back down the mountain propelled only by the force of gravity, generating electricity in the process.

The basic concept is similar to a gravity storage technology proposed by the Swiss company Energy Vault, which recently received a greater than $100 million equity investment from SoftBank’s Vision Fund. That technology generates electricity through gravity by lowering concrete blocks in a tower.

Lithium-ion battery storage is the fastest-growing storage type and utilities across the U.S. have procured battery storage as a way to back up intermittent renewable energy. But the length of time that they can deploy energy — typically four hours or shorter for — may not be long enough for the greater and greater amounts of solar and wind resources needed to come online to meet emissions reductions goals.

"High-renewables grids, as mandated by many states, will require extremely long durations of storage, potentially on the order of 10-20 hours to shift variable solar power to cover nights and cloudy days, and weeks or even months to shift energy from high-wind months to lower-wind periods," Wood Mackenzie head of energy storage Daniel Finn-Foley told Utility Dive. He noted that lithium-ion batteries "scale up poorly," with costs effectively doubling every time the duration of a lithium-ion battery system doubles.

The authors of the IIASA study claim that mountain gravity energy storage (MGES) can open up possibilities for long-term storage in new locations. Pumped hydropower storage, one of the most common forms of energy storage currently in service, is an example of long-term storage and can deploy stored energy for around 6 to 20 hours.

But pumped hydro, which pumps water from a lower reservoir to a higher reservoir and then releases the water back to the lower reservoir to generate electricity, is limited by geography and high capital costs. "One of the benefits of this system is that sand is cheap and, unlike water, it does not evaporate — so you never lose potential energy and it can be reused innumerable times," making MGES a potential option for dry regions, IIASA researcher Julian Hunt, one of the authors, said in a statement.

"Additionally, [pumped hydro] plants are limited to a height difference of 1,200 meters, due to very high hydraulic pressures. MGES plants could have height differences of more than 5,000 meters," Hunt said.

But the "tipping point" where other storage technologies can outcompete lithium-ion batteries has not been reached, and lithium-ions are being used for eight-hour storage applications, much longer than the more typical four hours, according to Finn-Foley.

"As we scale up to much longer durations, there is room for gravity-based storage, along with other emerging technologies, to compete, but lithium-ion has built a massive head start by piggybacking on the economies of scale amassed by building out capacity for electric vehicles and consumer electronics," he said.

Using the example of the Hawaiian island Molokai, the study’s modeling finds that MGES, wind, solar and batteries could supply the island’s energy demand by themselves. MGES plays the role of filling in the gaps between peak generation and the short-term battery cycles, the study said.

MGES, however, would have to compete with existing options targeting that gap. "It is not clear to me that that grid operators are willing to pay for the benefits of using storage in that timescale, as at the moment, in island countries for example, they can meet those needs by using oil-fired generators that are already running less if these islands are adding more renewables," Yayoi Sekine, BloombergNEF energy storage analyst, told Utility Dive. "That may change if they are looking to run 100% renewable, though."

In an email, Energy Storage Association VP of Policy Jason Burwen did not comment specifically on MGES but said "there are many potential storage technologies under development right now, and you can’t predict what type of storage will dominate the market 20 years from now. There’s enormous room for innovation and step-changes in performance and cost, which makes this space exciting and worth public investment."

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