The Stirling cycle engine post reminded me of these solar furnaces. Under the right conditions they can produce a great deal of heat without much in the way of complicated machinery. A few generate enough heat to melt steel, and smaller versions can be used for cooking or heating water.
https://www.nrel.gov/csp/facility-hfsf.html
Here's one used for research in the US
If you're ever in Uzbekistan you can visit the largest solar furnace in the world
https://en.wikipedia.org/wiki/Solar_furnace_of_Uzbekistan
Here's one of the original large furnaces in France
https://en.wikipedia.org/wiki/Mont-Louis_Solar_Furnace
Ussr really out there building crazy shit.