Energy Storage Is Going Underground
If you say energy storage today, most people think you are talking about batteries. The intermittency of renewable energy sources such as solar and wind means sometimes there is more electricity available than is needed. Batteries can take that excess electricity and store it until such time as it can be put to work. But there are other ways of storing electricity that rely on potential energy.
An example of potential energy is a freight train parked at the top of a mountain. If there are generators connected to its wheels, they can create electricity as the train rolls downhill. When there is excess electricity available, it can push that same train back up the mountain.
Some energy storage systems are long duration and some are not. A train sitting on top of a mountain could store all that potential energy for weeks, months, or even years. A battery often loses some of its charge over time, as anyone who has left a car parked in the garage over the winter understands all too well. Long duration storage is great in theory, but not if the electricity it produces costs a dollar per kilowatt-hour. The point is that no system is ideal for every use case.
The Los Angeles Times reported this week that a number of local governments have signed a $775 million contract to buy electricity from the world’s largest underground energy storage facility over the next 25 years. “We need a diverse fleet of resources. This new technology is a critical component of that,” said Robert Shaw, chief operating officer at Central Coast Community Energy. “That’s how we get to 100% renewables.” Julia Souder, executive director of the Long Duration Energy Storage Council added, “If you want clean, renewable energy every hour of every day of every month of every year, you need long duration energy storage.”
The project developer is Hydrostor, which is based in Toronto. It plans to drill three shafts thousands of feet into the ground, then send down miners to dig out a series of rows and columns. When the project is ready to go in 2028, the underground caverns will have a collective volume equivalent to two football fields in length and width and about 100 yards high. When renewable electricity is plentiful, Hydrostor will use some of it to push air down into the caverns. When Central Coast Community Energy needs electricity, the company will open a valve and use the high pressure air to spin turbines to generate it.
Until now, compressed air storage has mostly been used in places with naturally occurring underground salt domes where companies can pump down water to dissolve the salt and hollow out large caverns. The only two compressed air systems in operation today are at salt domes in Alabama and Germany. Those projects depend in part on natural gas to heat the compressed air as it leaves the salt caverns because its temperature would otherwise drop significantly as it expands, reducing efficiency.
Hydrostor says it has figured out how to capture and reuse the heat generated when air is compressed, which will eliminate the need for natural gas. It also says it knows how to make the mechanics work in areas where caverns must be dug out of hard rock, rather than salt. The problem is, it’s never been done before.
Yet Goldman Sachs studied the proposal and invested $250 million in Hydrostor last year. The Canadian Pension Plan followed with a $25 million investment a few months later. Aaron Marks, a senior analyst at energy research firm Wood Mackenzie, says he has “some degree of confidence” in Hydrostor. “As a first of its kind plant, does that mean its technology is going to work perfectly, with no problems? I doubt it,” Marks told the LA Times. “But that’s not a strike against the technology.”
Compressed air storage is much less efficient than battery storage. It produces just 60% to 65% of the electricity it consumes, which suggests the electricity it provides could be fairly expensive. It also can only supply electricity for about 8 hours at a time. But Central Coast Community Energy thinks it is still worthwhile at a time when the price of lithium is driving up the price of battery storage. In addition, the system should have a useful life of decades, which is much longer than the service life of lithium ion batteries.
CCCE has agreed to accept 200 megawatts of electricity from Hydrostor. The entire project is projected to provide 500 megawatts in total. CCCE decided to invest in compressed air storage partly to avoid price volatility in battery storage technology and partly to help prove the viability of a technology that the agency’s elected officials believe will ultimately benefit customers. “It takes somebody to make that initial investment,” Shaw said. “Our board has given that direction.”
Eric Gimon, a senior fellow at the San Francisco-based research firm Energy Innovation, told the LA Times he is skeptical about compressed air storage playing a major role on the power grid, but is glad the technology is being given a chance to succeed. “It’s certainly good to expand the quiver of tools,” he said.
The Energy Storage Power Of Sand
Remember that train at the top of a mountain? The International Institute for Applied Systems Analysis in Austria is proposing a similar system, but using sand lowered into abandoned mines. The weight of the sand would be used to turn generators on the way down the mine shaft. Excess electricity would raise the sand back up to the surface when renewable energy is abundant. The proposal and analysis were published recently in the journal Energies.
Here’s the abstract:
Low-carbon energy transitions taking place worldwide are primarily driven by the integration of renewable energy sources such as wind and solar power. These variable renewable energy (VRE) sources require energy storage options to match energy demand reliably at different time scales. This article suggests using a gravitational-based energy storage method by making use of decommissioned underground mines as storage reservoirs, using a vertical shaft and electric motor/generators for lifting and dumping large volumes of sand.
The proposed technology, called Underground Gravity Energy Storage (UGES), can discharge electricity by lowering large volumes of sand into an underground mine through the mine shaft. When there is excess electrical energy in the grid, UGES can store electricity by elevating sand from the mine and depositing it in upper storage sites on top of the mine. Unlike battery energy storage, the energy storage medium of UGES is sand, which means the self-discharge rate of the system is zero, enabling ultra-long energy storage times.
Furthermore, the use of sand as storage media alleviates any risk for contaminating underground water resources as opposed to an underground pumped hydro storage alternative. UGES offers weekly to pluriannual energy storage cycles with energy storage investment costs of about 1 to 10 USD/kWh. The technology is estimated to have a global energy storage potential of 7 to 70 TWh and can support sustainable development, mainly by providing seasonal energy storage services.
According to a IIASA blog post, “UGES generates electricity when the price is high by lowering sand into an underground mine and converting the potential energy of the sand into electricity via regenerative braking and then lifting the sand from the mine to an upper reservoir using electric motors to store energy when electricity is cheap. The main components of UGES are the shaft, motor/generator, upper and lower storage sites, and mining equipment. The deeper and broader the mine shaft, the more power can be extracted from the plant, and the larger the mine, the higher the plant’s energy storage capacity.”
“To decarbonize the economy, we need to rethink the energy system based on innovative solutions using existing resources. Turning abandoned mines into energy storage is one example of many solutions that exist around us, and we only need to change the way we deploy them,” says Behnam Zakeri, study co-author and a researcher in the IIASA Energy, Climate, and Environment Program.
The Takeaway
Compressed air storage and gravity storage systems are so low tech, they seem almost laughable compared to battery storage. And they are incapable of reacting in milliseconds to fluctuations in voltage or frequency the way batteries do. That eliminates one important revenue stream that can help make them profitable. If there is a lesson here, it is that there are many ways to store excess renewable energy to keep the lights on after the sun sets and when the winds don’t blow.
If the goal is to have an adequate supply of reliable, dependable renewable energy so we can retire the many fossil fuel powered generating stations in the world, we will need all the tools in our energy storage toolbox as possible.
Just as no electric car meets the needs of every driver, no energy storage system is ideal for each use case. If moving sand up and down a mine shaft works in West Virginia and compressed air storage works in the San Joaquin valley in California, then hooray for both and hooray for all the other energy storage solutions floating around out there. Let’s not let perfect be the enemy of good. Sometimes good may be good enough.
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