Could inexpensive pea gravel from a local landscaping company play a role in decarbonizing heat and electricity? That is a question that Sandia National Laboratories and its partner CSolPower want to answer. The researchers believe that rocks, like pea gravel, could be used to store energy generated by solar panels.
The New Mexico-based CSolPower states on its website that its purpose is “to provide an engineered solution using non-toxic, low-cost materials in conjunction with legacy power plant infrastructure.”
Walter Gerstle co-founded CSolPower in 2019.
In a press release, he said that storing thermal energy in rocks is advantageous because it can be built anywhere.
“It can be commodified and doesn’t require extensive permitting,” he said. “We believe it can be implemented more quickly and economically than other approaches.”
Nathan Schroeder, a Sandia mechanical engineer, said the team has proven the basic concept using propane. Now they are working to demonstrate that it can also be done using a photovoltaic solar array.
To meet that end, the Sandia researchers have set up a 100 kilowatt hour rig.
The team uses renewable energy to heat the pea gravel to 500 degrees Celsius, or more than 900 degrees Fahrenheit.
Luke McLaughlin, a mechanical engineer at Sandia, explained the process to NM Political Report.
He said electricity is sent through a resistant element, which causes energy to dissipate as heat. This warms the air, which then flows over a pile of rocks. The heat from the air transfers to the rocks.
To extract the stored heat, ambient temperature air is used.
“You can put electricity in and then get heat out or you can put in electricity and get heat out and then convert that heat into electricity,” Schroeder said.
Converting the heat to electricity may involve using steam to turn a turbine. Schroeder said that would be a later evolution of the concept or technology that the researchers are testing.
Currently, researchers are using pea gravel, in part because of the affordability. But other rocks may also be used.
“There’s certain rocks that are more expensive than others and some break down with the heating and cooling,” Schroeder said.
The gravel is not only cheap, it is also abundant. But Schroeder said basalt has been used in similar applications. Being a volcanic rock, basalt tends to be more resistant to changing temperatures.
One challenge the team faces is the intermittent nature of solar and wind power.
“How do you quickly get things going so you can put that energy in when it’s available and then on the flip side, once that renewable energy quickly drops off— say clouds roll through or the wind just dies— how do you quickly dispatch that energy that was stored?” McLaughlin said.
He said that is something the researchers hope to address during this project.
Neither Schroeder nor McLaughlin see this technology as fully replacing the current lithium-ion battery storage, but in certain situations it could provide an alternative that may have fewer of the environmental problems associated with the creation of batteries.
McLaughlin said in some situations it can complement lithium batteries while in others it may be a “more reliable, safer and lower cost alternative.”
One example he gave was utility-scale energy storage, which can be expensive to build.
“We see a lot of promise in this technology offering a much lower cost energy storage solution than lithium ion,” he said.
Additionally, he said that roughly 51 percent of energy used in industrial processes is heat rather than electricity and that comes with a significant carbon footprint.
“When you want to decarbonize the processes that require heat, such as cement production, steel making, drying, really any process that requires drying, it’s much more cost effective to store that energy as heat rather than electrons. So we see in that space that this sort of technology will replace lithium solutions,” McLaughlin said.
Schroeder said another advantage of the technology is that it can use materials that are locally sourced. This provides a further advantage of insulating it from the supply chain challenges that have impacted clean energy production.
Many technologies that are used in the energy transition require mining and industrial processes that have carbon emissions. But this technology is “very low carbon” and doesn’t take much to deploy, Schroeder said.
McLaughlin added that it also has advantages once the system reaches the end of its life.
“You’re just left with a bunch of gravel that you could repurpose for other applications, you could 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,” he said.
McLaughlin said based on early research and what is currently known about the materials, the system’s lifespan will likely be fairly long, outlasting the solar panels that are used to charge the technology.
Energy storage is becoming increasingly important as more renewable electricity is placed on the grid.
Schroeder pointed to the Public Service Company of New Mexico, which has been retiring fossil fuel generation and replacing it with solar and wind as well as battery storage.
“As we start getting into higher and higher percentages of renewable penetration, these long-term storage technologies, especially cheap ones, are going to become increasingly important,” Schroeder said.
