Nathan Schroeder, a mechanical engineer at Sandia National Labs, arranges landscaping gravel in a thermal energy storage unit. Sandia is working with CSolPower on optimizing a low-cost solution for storing intermittent renewable energy. (Image: Craig Fritz, Sandia National Laboratories)

Storing energy is one of the key challenges for implementing sustainable but intermittent electricity sources like solar and wind. Engineers at Sandia National Laboratories are collaborating with New Mexico-based CSolPower LLC to develop a very affordable method of accomplishing that storage.

Thermal energy is very energy-dense — you can basically use any material that could withstand high temperatures to store heat and then deliver it when you need it. “Say you have a PV or a wind farm and you're overproducing in the middle of the day. You can take that excess electricity and heat up air using a big resistive heater,” said Sandia engineer Luke McLaughlin. You can then use a blower to push the heated air through a bed of gravel, which is quite porous. That transfers the heat from the air to the rocks. You then close some valves, and the system goes into “hold” mode for hours, days, or weeks, until it needs to be released by reversing the flow. “The rocks have so much surface area, you're blowing through what is basically a big sponge — it’s a lot of area for the heat transfer,” said Nathan Schroeder, a mechanical engineer at Sandia.

When the heat energy is ready to be used, it is transferred back to the air. The heated air can either be dispatched directly to a process or it can be used to create steam to drive a turbine like you would in a traditional power cycle.

The team built a prototype 100-kilowatt-hour test setup and charged the rocks with 500 °C (932 °F) air. During the test, the rocks stayed at temperature for 20 hours. “Since we’re looking at a high-temperature process, we're actually creating a more efficient system than you would have if you were to store electricity with Li-ion batteries — and at a much lower cost,” said Schroeder. “The primary advantage here is the cost of this system because it’s just a bed of gravel, it reduces the need for expensive materials. And there's also not the replacement cost you would have with Li-ion.”

“The installed cost for our thermal storage system is less than $5-10 per kWh thermal, as compared to other energy storage technologies, which are in the range of $150-$200 per kWh electric,” added McLaughlin.

In their current deployment, they’ve used a large agriculture stock tank, which is a six-foot-diameter drum typically used for hay or cattle feed. Depending on the particular installation, you could also dig a hole in the ground or create a pyramid-shaped pile on the surface.

According to McLaughlin, the test system had an efficiency of about 85-90 percent. “And actually, because this is a smaller prototype, you end up with an unfavorable ratio of the volume of gravel to its overall surface area. That results in a larger percentage of heat loss relative to the amount of energy you can store. If you made this really big, it would have less heat loss and you would start to look at round-trip efficiencies upward of 95 percent, maybe even higher,” he said.

One of the early applications is heating greenhouses over the winter so that plants could be grown all year. “That's something that can use a lot of electricity. So, you can put up solar panels, install our storage system and be able to heat the greenhouse overnight with energy collected during the day,” said McLaughlin. He added that greenhouse operators have already shown interest in trying this process.

While the Sandia team continues its research in areas such as the control strategy and how to tie the bed of hot rocks into the process that requires the heat, CSolPower is focused on getting this technology to commercial markets. “We're anticipating that in the coming one to two years we’ll most likely demonstrate one of these beds in a commercial application,” said Schroeder.

This article was written by Ed Brown, Associate Editor, SAE Media Group. For more information, visit here .