This is why I started off referencing external combustion engines like Stirling engines. These tend to have very low maintenance needs as the combustion occurs

This is why I started off referencing external combustion engines like Stirling engines. These tend to have very low maintenance needs as the combustion occurs outside of the pistons, so there is no fouling of the moving parts. One could use self lubricating materials like graphite rods as your piston heads and have quite high reliable components. The stirling engines that NASA utilizes are considered maintenance free. As far as efficiency gains, we're talking 25-40% of the energy being produced as electricity and the majority of the rest being turned into hot water for residential use, either by one household or a whole city block. This doesn't sound like a terribly marginal gain. Specially if the energy production is done during the hours when traditional solar installations are not producing which are generally when folks are running their heating.