alt Hacker News1. The countercurrent heat exchanger achieves exactly that: exhaust gases are cooled while the inflowing fuel mixture is heated up.
2. Thermophotovoltaics in general can operate with any heat source, though this device is clearly optimized for combustion. However, the efficiency is far too low to compete in the large-scale power generation segment. This is almost certainly aimed at light aviation, heavy drones, military applications, etc., where there are not a lot of alternatives that combine small size, high power density and good efficiency.
I suppose for aviation at least this is no less efficient than a gas turbine or a piston, and it's certainly a good deal quieter, has fewer moving parts, and requires less precision engineering than a jet engine. This feels tailor-made for attritable low->medium performance aviation, aka loitering munitions and drones. Strip away the "green" talk, and you're left with something that can burn just about anything (including hydrocarbons like avgas) without the complexity of a turbine.