CPV solar power system


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There are relatively few CPV applications still pursuing Si cells. The idea was somewhat 'hot', no pun intended, when there was a silicon shortage going on, but silicon prices have dropped as new capacity has come online and the only successful CPV concepts being marketed which I know of are based on triple junction Ge/GaAs/InGaP or similar cells. These are also rather heat sensitive, and the two main effects that drop the efficiency of CPV solar systems using them to ~24% from the 39% efficiency of a bare cell are optical losses and heat. Generating the high heat that a reasonably efficient CPV system would need, say 500C, in close proximity to these cells would be tricky to say the least. Also, with a Ge cell in the materials stack, these can use some of the IR spectrum, and do it with a decent efficiency, so I'm not sure what part of the spectrum you'd really want to push away. I suppose, in an ideal hypothetical situation you'd want to shed the long IR, the UV, and then part of the gaps between the three cells since at the upper limit of the the gap in their spacing you'd have more heat being created in the solar cell than electricity.

So, with those frequencies filtered out, the solar cell would run at a higher efficiency relative to the filtered spectrum, but at a lower power output overall. So the question then is can you divert and absorb these parts of the spectrum in such a way that you can then generate power off the heat they provide such that the net efficiency, and, in particular, the cost per kWe is lower than simply dumping the entire solar inverter on the cell. I think you'd find the answer is 'no', since you're talking about building two independent power producing system and adding some very complex optics. I think a more reasonable approach to maximizing the useful power out would be to liquid cool the backplane the cells are mounted on, and make direct use of the thermal energy gained for hot water or heating. This would be fairly cheap and easy to add on, but it wouldn't get you the high delta-T needed for efficiency heat engine operation unless the cells were absolutely cooked to uselessness.

So, short answer, no, I don't know of anyone who is taking the approach of using thermal cogeneration with a CPV system. I have seen hybrid solar thermal absorber/PV panels, for production of both power and domestic hot water, but I've only seen that in fixed panel inverter systems, not tracking CPV. Routing a heat carrying fluid in a moving system adds a lot of weight and complexity, and this is probably why it's not done. Doing spectrum reshaping and trying to reuse the shed part of the spectrum to do generation via a heat engine adds a lot of optical and power generation complexity, so I think that's probably unrealistic unless it can be done very, very cheaply, which I can't imagine a way to do. Certainly, advanced optical materials could provide an answer to how to efficiently pull parts of the spectrum out of the concentrator path, and I've seen some very clever things done with IR photonic crystals, but if I had to bet on applications of advanced optical materials to CPV solar power inverter systems, I'd bet on up-converters or down-converters to remap the spectrum to maximize the efficiency of the PV cell before I'd bet on thermal cogeneration using rejected parts of the spectrum. Dr. Martin Green at UNSW has done some research on use of photonic up inverter and down inverter, so you should be able to find some information on that if you search his publications.

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