Because they're trying to go way up on temperature. The idea is to get dirt up to 600°C. It's easy to get warm water from solar, and solar water heaters have be

Because they're trying to go way up on temperature. The idea is to get dirt up to 600°C. It's easy to get warm water from solar, and solar water heaters have been around for decades, if not centuries. But that's well below 100°C. If you start from electricity, you can reach any temperature up to plasma. If you're just storing warm water, you can't get much of the energy back. E = (T2-T1)/T2, remember. They've made no progress on getting the energy out of their heated dirt. They want to make a hot dirt powered boiler - run pipes through the dirt, put in water, and get steam. Not hot water, superheated steam. That's the hard part, and it will have to be custom. Small steam turbines are available. Siemens has a whole range.[1] They start around 750KW, at the high end of automotive scale. Siemens can sell you a matching generator. When these guys can power one of those, it's real. Right now, it's three guys. They need VC funding and a really good boiler engineer. The goal should be a working prototype at about 500KW scale. That would be a big enough prototype to get some meaningful efficiency measurements. (Maintenance will be tough. You can't turn the heat source off.) [1] file:///home/john/Downloads/SE-Brochure-Dresser-Rand-Steam-Turbines-2021-pdf_Original_20file.pdf