In the small town of Pixley, California on the western edge of the Sequoia National Forest, a stack of massive bricks delivers zero-carbon heat to Calgren Renewable Fuels. The bricks — called heat batteries — were installed at the biogas production facility in early 2023 as part of Rondo Energy’s first commercial scale demonstration project.
Rondo co-founder and chief innovation officer John O’Donnell had tried lowering Calgren’s carbon footprint before.
“I had failed previously to come up with something that Calgren found acceptable using concentrating solar technologies,” he said in an interview on The Green Blueprint podcast.
Throughout the 2010s, O’Donnell and Rondo co-founder and chief technology officer Pete on Behrens worked at GlassPoint Solar where they delivered industrial heating solutions using solar. And although they workshopped many solutions with Calgren over the years, they never came up with a system that Calgren was willing to install.
But in 2019, von Behrens came up with a design and configuration that would enable bricks to deliver heat at an extremely high temperature — over 1,000 degrees Celsius. If it worked, the brick-based heat batteries could be paired with a turbine to deliver steam, the most common method of delivering heat in industrial processes today.
The concept behind heat batteries isn’t new. “If you put a stone in your oven, you heat it up, you put it in your bed, it’ll keep your feet warm all night. You stored electricity in an unrefined material that was literally dirt cheap,” O’Donnell said.
Rondo uses renewable electricity to heat its bricks, which are configured to allow air to pass through small passages. As the air passes through, it absorbs the heat from the bricks and delivers it as steam to a turbine or for other uses.
“The very small air passages are instrumental to being able to deliver the same outlet temperature all the way through discharge,” said O’Donnell.
By 2022, O’Donnell and von Behren had built a prototype of the bricks they had proved with a 200-kilowatt hour project. Armed with this proof point, they approached Calgren with an idea: deliver all of the refinery’s heat and electricity needs — totaling close to 600 megawatt hours — using heat batteries.
“They said, ‘Great, but show me. I don’t want to be the first taking a technology risk,’” O’Donnell recounted.
Rondo and Calgren agreed on a 2MWh installation to prove the technology worked in the refinery. Then, Rondo faced its first big scaling decision: choosing how to size the bricks.
“We had a structure, but we were implementing that structure with individual cast bricks that each weighed like 50 kilograms,” said O’Donnell. “A 50 kilogram brick is almost perfectly the wrong size. It takes two people to handle it, and there are a lot of them. So to the extent that we could make larger single bricks, we could reduce crane operations or people hours building these things.”
But using larger bricks meant that Rondo would have to delay the project significantly to manufacture them.
“We wound up taking about…six months longer than we thought it was going to take, learning how to make these large, much larger bricks that have fine structure that is not at all common,” O’Donnell said
The decision to use larger bricks from the start at Calgren set Rondo up for future success with 100MWh scale projects. “To the greatest extent possible, we wanted [the batteries] to be the same except one of them has 10 tons, one of them has 500 tons,” said O’Donnell.
Rondo broke ground at Calgren’s Pixley, California refinery in October 2022, and by early 2023, the heat batteries were producing steam. Since then, the unit has been in “relatively continuous operation” with scheduled maintenance and emergency tests taking it offline only occasionally.
“It’s been a place for independent engineers, owners, engineers of customers, and an IE that we engaged to go look at a real operating unit,” said O’Donnell.
Rondo is currently working on projects in Texas, Kentucky, Denmark, Germany, Siberia, Southeast Asia, and California. Although the current 2MWh unit at Calgren produces a fraction of 1% of its total energy needs, the ultimate goal is to scale to 600MWh at the refinery.
“This is just this giant opportunity,” said O’Donnell. ‘It wasn’t always clear right at the beginning, whether that would be true or not.”
This story borrows from an interview that appeared on The Green Blueprint, a Latitude Media podcast.
