I expect that it necessarily has rather low efficiency, so it would be best suited for off-grid / low-equipment situations where you care more about heat than electricity. Say you can make a reusable long stip of stuff that you unroll in the sunshine during the day, and then coil up in your tent and "ignite" one end at night and it takes 8-10 hours for the reaction to reach the other end. Or depending on how hot it can get, maybe even a reusable "campfire" that doesn't need fuel and can be a bit safer than a real fire.