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It is not a physics efficiency, it is a cost efficiency. By that, I mean that instead of either building one big PV cell to handle a 20x20 cm2 area, or by having to have 400 PV cells, you can now have a SINGLE cell. The most expensive part of converting light to electricity is the PV cell itself. IBM's trick is be able to concentrate so you don't need as many PV cells.

Yes some energy is lost in the cooling but that percent is much smaller than the cost efficiency gained by reducing the number of cells for the same area from 400 to 1.

I see your point with it being a cost efficiency issue.... However, I noticed that they’re claiming that it produces 70 watts from a concentrated 230 watts.

That’s 30.4% efficiency, a remarkably high efficiency for solar cells. Are these Gallium-Arsenide cells? Those are the only ones I’ve ever seen break 30%. But they’re danged expensive, so minimizing the amount used would be critical.

The other thing that I don’t remember a thing about that I wonder with this is what the effect of temperature on efficiency is on a typical array. The best of both worlds would be if concentrating the sun reduced the amount of cells you needed, and the cells used would increase in efficiency at a higher operating temperature.... You might need to cool them, still, but like a car engine, which runs best at a careful balance well above ambient temps....