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Question: what about a combined photovoltaic plus heat-engine/thermal based solar system? Couldn't that achieve greater efficiency than either alone, and why is it not heavily used?

That's an interesting question. A photovoltaic only uses about 10% of the photons because they have to be in the right frequency band. The others are mostly just absorbed, since the panels look black or dark blue. But maybe the absorbtion is at the electrodes. A mesh electrode and a reflective backing might allow most of the photons that are not absorbed by the photovoltaic to be reflected to a boiler.

--to me the most obvious scheme is a "hotbox" (insulated box with black interior, glass front, gets hot inside due to greenhouse effect) with photovoltaic cells inside. The PV cells would have to be ones which can work when hot or not, and you can get hot water via pipes passing thru the box. Then you generate both electricity and hot water instead of just one. But should be greater efficiency and lower cost. Why is this not done? Another scheme which works best at high altitudes (direct sun, not scattered light) involves cylindrical shaped concentrator solar cells at the focus of a parabolic trough mirror; then pass fluid thru the pipe. It is expensive to fabricate cylinder-shaped solar cells. And tracking parabolic mirrors, while not exactly high-tech, are also not trivial to build using supplies from hardware store. I think this second scheme has been built (?).