I was searching for a way to collect solar energy from a roof without photovoltaic, which left mainly thermal engines. Then I found about the free-piston Stirling generator developed by NASA. Its is interesting because of its reliability : some models are operating continuously for decades. In addition, a simplified version can be home-build in one hour.

This concept uses oil to store and transfer heat energy, with the two loops arranged in a thermosiphon configuration to eliminate the need for pumps. The overall efficiency could be higher than PV panels if the Stirling engine is adapted to the temperature differential at steady state.