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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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The major one is Indium. Indium Tin Oxide is used as a transparent conductive electrode used to coat the top of the cell. Indium production is a potential bottleneck when it comes to expansion of the PV manufacturing industry and currently costs around $700/kg. Less than 1000 tonnes per year is produced. However, it looks like it may well be replaced with ... 1 The system you describe, a water tower with pumps and turbines, is a version of a pumped hydro storage system. The amount of energy stored in a pumped hydro storage system is defined by the elevation, i.e. the height that the water is lifted; multiplied by the volume of water that is lifted (assuming that the height the water will fall, is the same as the ... 2 I think EnergyNumbers's Answer is an excellent one, but could leave some people a bit mystified by what exactly a "direct ray" is and what exactly its relevance is. The essential idea here is that a Fresnel lens is an imaging machine: it puts a curvature on a low aberration wavefront so that that wavefront converges. Its working depends on there being ... 0 They are not generally used with the photovoltaic cells as they concentrate the sun light and heat to a temperature so high that many of the cells get, for lack of a better word, fried. 0 There is indeed an optimal arrangement for extracting the most usfeul energy from moving Newtonian fluids. The highest efficiency possible is$\frac{16}{27}\$ - this is known as the Betz Limit or Betz Coefficient, after Albert Betz, who rediscovered it in 1920 after Fred Lanchester first calculated it in 1915. The ratio of exit speed to entry speed is 1:3 ...

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Lets say your receiver is a copper plate, if you leave it in sunshine in the conditions you describe you get certain temperature, lets say 50 c, lets say you calculate the area of your dish an it is 1 square meter, now you place the piece of copper close to the focal point, and you can see a concentrated bright dot, and you measure the area of that dot, lets ...

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