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I think EnergyNumbers's Answer is an excellent one, but could leave some people a bit mystified by what exactly a "direct ray" is and what exactly its relevance is. The essential idea here is that a Fresnel lens is an imaging machine: it puts a curvature on a low aberration wavefront so that that wavefront converges. Its working depends on there being ...


The system you describe, a water tower with pumps and turbines, is a version of a pumped hydro storage system. The amount of energy stored in a pumped hydro storage system is defined by the elevation, i.e. the height that the water is lifted; multiplied by the volume of water that is lifted (assuming that the height the water will fall, is the same as the ...


Rare earth elements aren't really used much in PV at all. The market is dominated by silicon cells (~95% of the market). The next big player is Cadmium Telluride (~5% of the market). The rest are pretty insignificant. Also, note that rare-earth elements are (with the exception of Promethium) not particularly rare.

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