BATTERY MADE FROM A PILE OF ROCKS

Wind and solar power have seen extraordinary growth in the United States over the past decade, so much so that a major focus now is finding a way to save this renewable electricity for the proverbial rainy day. While the price of lithium batteries has fallen, there’s still a market opportunity for a simple and inexpensive way to store energy.

When it comes to simple and inexpensive, it’s hard to beat a box of rocks.

“We wanted to find a way to store energy inexpensively for the transition to renewable, intermittent wind and solar energy,” said Walter Gerstle, who co-founded of CSolPower in Albuquerque in 2019. “Thermal energy storage in gravel seems to be the perfect solution.”

His company has collaborated with Sandia National Laboratories to develop a system that uses rocks and gravel as the storage medium.

Thermal energy storage is by no means a new idea. Steam accumulators, which store heat in tanks of pressurized hot water and retrieve it by lowering the pressure in the tank to flash steam, have been around since the 19th century. The concept of storing excess energy from renewable power facilities in the form of superheated solids has become the focus of a lot of recent development work. For instance, Antora Energy, a startup in Sunnyvale, Calif., uses electric coils embedded in blocks of graphite to raise their temperature to 1,800 °C. The blocks can store that heat for hours or days at a time, and then release it for industrial use, replacing heat from burning natural gas.

The process involved placing pea gravel in a container that can be heated or cooled using air to store/release thermal energy. To make the system even simpler, resistive heating elements acted on air within the container rather than directly on the solid gravel. Hot air flowing through the container raised the temperature of the gravel to greater than 500 °C. To extract the heat, ambient air was pumped in and hot air was pumped out.

“One of the advantages of thermal energy storage in rocks is that the system can be built anywhere,” Gerstle said. “It can be commodified and doesn’t require extensive permitting. We believe it can be implemented more quickly and economically than other approaches.”

Engineers assembled a compact 100-kilowatt-hour test rig at the National Solar Thermal Test Facility, using photovoltaic panels, to evaluate how well the rock bed performed. The system sustained this temperature for a duration of up to 20 hours, confirming the team’s predictions and modeling efforts.

The biggest surprise was “how well the thermally self-insulated gravel performed when the air is kept from moving within the packed bed,” Gerstle said. That points to potential for similar systems to provide energy storage ranging from hours to months.

The simplicity, ease, and low cost of this storage system could make it an attractive storage option, especially for small-scale needs (for example, residential). It should also be relatively simple to scale it up to utility-scale, depending on the size of the area served.

When the project reaches its end of life or is decommissioned, “you’re just left with a bunch of gravel that you could repurpose for other applications, or distribute back into the landscape in an eco-friendly way, and you’re not having to deal with the recycling that you’re going to see with other electrochemical style energy storage technologies,” said Sandia mechanical engineer and team member Luke McLaughlin.

Unlike lithium-ion batteries and some high-temperature thermal storage systems, the output of the CSolPower-Sandia thermal storage system is heat, not electricity. (The temperature of the heated gravel is too low to efficiently power a gas turbine.)

But heat has many uses, such as drying vegetables or driving some chemical processes, and a proverbial box of rocks would be easy to install on industrial sites. At the residential scale, the system could be used to store surplus electricity during the day as heat and then use it to warm water and homes during the night.

Several greenhouses in northern New Mexico are lined up to use the rock bed for thermal energy storage, Gerstle noted, adding, “Instead of curtailing solar energy production, we would store it and use it during cold nights to keep the greenhouses warm enough to grow plants year-round.”

The team plans to refine its gravel-based technology into a small-scale commodity item first, and then scale up to utility-scale, long-duration energy storage.

“Our aim is to develop this technology and take it to places where you can use wind and photovoltaic energy sources to charge the system,” Gerstle said.