Half a croissant, on a plate, with a sign in front of it saying '50c'
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Power Farming

Have your farm and heat it too.
  [vote for,

Solar farm + greenhouse. Build a massive greenhouse over farmland or forest. Use large dielectric hot mirrors over the greenhouse to reflect infrared light to solar power towers in the manner of a solar farm. Visible light passes through to feed plants. Waste heat can be piped back into the system to maintain optimum growing temperatures.

I think excess waste heat could also be used with an organic waste treatment plant to purify water, maintain humidity, and produce compost.
LimpNotes, Feb 26 2015

Someone is doing it with PVs https://www.youtube...ue=18&v=lfKYfQbOvlc
[LimpNotes, Feb 20 2018]

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       One, partial, problem with the idea is that infrared is not somehow magical heat. Solar thermal plants convert all of the sunlight into heat including the visible and UV spectrum, all of which is higher energy than infrared. Only about maybe half of the sun's energy comes into the atmosphere as IR, and significantly less than that comes to the earth's surface, owing to the absorption spectra of water.   

       This ignores the relative cost increase of selective pass filters relative to broad spectrum mirrors.   

       Practically speaking, you'd want to reflect all light that isn't commonly used for photosynthesis. Unfortunately, that only adds the Green and UV bands to your reflected spectrum. What that does to your total energy availability is harder to determine, but best guess is that you're going to be down around 30% as efficient as a broad spectrum concentration system, at best.   

       Please note that this doesn't rule out the possibility entirely, but it does swing the cost/benefit analysis somewhat against it.
MechE, Feb 26 2015

       [MechE] I agree completely with your points. But the faults in this system are largely a result of viewing this as a low performing power-plant, rather than an overly-efficient farm. I imagine a future (well into the future) where the resources of food and power will compete with each other for square footage and this is a means for getting the two to cooperate.

I'd argue that food is considerably more important than power in the survival sense, and that the cost for these mirrors could easily be much lower. Currently they are used in high precision optics and are generally custom built. A hot mirror is simply a piece of glass with several thin films deposited onto it. I see no reason why they couldn't be mass-produced at considerable cost savings if a demand were evidenced by an idea such as this.

The structural elements would add to the cost too, but offsetting this is the benefits of year-round growing and higher efficiency in water usage and heat-retention within the greenhouse. Efficiency and cost/benefit needs to be whole picture, and while I agree that at present this project is too expensive to consider, I think there will be a time when it will make sense.
LimpNotes, Feb 26 2015


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