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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.
Someone is doing it with PVs
[LimpNotes, Feb 20 2018]
||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
||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] 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.