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Comment by 1053r | Hacker News Reader

Production prices for solar energy are now below $0.80 / watt (http://www.123jump.com/market-update/First-Solar,-Inc-Q4-Ear...), although skyrocketing demand has caused wholesale prices to remain much higher. It seems likely that we may in the next 10 years be in a place where we can turn sunlight into electricity, and then electricity into the exact wavelengths of light used in photosynthesis more efficiently than allowing the plants to sit in the sun.

If this is the case, than we may be able to grow crops more efficiently indoors than out. But we will be replacing acres of crops with acres of solar panels. And it's unclear to me why we would want these buildings to be skyscrapers, which are much more expensive per square foot than large warehouses in the middle of nowhere.

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How in the world did you get +8 mod on this?

That's utter nonsense!

You can only get as much light as you have surface area - so now instead of fields of crops, you have fields of solar collectors!

Did you think there was some magical light multiplier device that could give you more output light than you started with?

Plants already use the best parts of the visible spectrum to collect light. The small amounts of infra-red you might barely be able to collect and frequency up-convert are not worthwhile - especially since you'd need to throw away all the energy from the higher frequencies in order to get it.

Solar cells are terrible as far as efficiency is concerned - and not because of engineering, because that's simply how they work. (Unless you want a multijunction solar cell but those are very very expensive.)

And for engineering challenges there simply doesn't exist in the world enough rare earth elements to make enough solar cells for everyone - and that's just for regular electrical usage, forget about growing plants. We do have enough silicon, but silicon based solar cells have poor efficiency.

I think the idea here is that plants need a narrow set of frequencies and throw the rest away. If solar cells could capture the frequencies plants don't use, convert them to energy, then convert the energy to frequencies plants like, you've increased the efficency of your light-collection area.

You'll also have a garden full of black plants, which is both creepy and cool.

Solar cells work the same way: they capture a narrow frequency, and throw the rest away.

More accurately, below the frequency cutoff they ignore the light, above the cutoff any, extra energy is wasted as heat.

And plants don't need a narrow frequency anyway.

They don't seem to be using the green light, at least.

You might be able to beat crops in that way if you dispensed with photosynthesis and crops altogether and came up with a completely different way to convert solar energy to carbohydrates. Somebody else in this thread said that a wheat field converts 1% of its solar energy to food calories. They didn't cite that and I don't have time to look it up, but if it's true it should be beatable.

Assuming three billion years of evolution is beatable is generally a fool's game. I do like the idea of chlorophyll-tuned lasers, though.

Evolution can be stuck in local maxima determined by some path it went down a long time ago. If life happened to evolve an inherently inefficient metabolism, it would be stuck with it forever because you can't really go back on something that fundamental (we share glycolysis mechanism with bacteria).

And, even if evolution automatically did win, plants did not evolve to feed us. They evolved to reproduce themselves, and we have incidentally hijacked them to use as food sources.

But they did evolve to extract useful energy from solar radiation. Assuming it would be easy to tweak photosynthesis is ridiculous.

Evoltution is great at tweaking, and would be hard to beat, but it might be possible to build something completely different. It would also be hard to tweak a bird's wings, but we have built aircraft with completely different designs that fly many times faster than any bird.

A solar cell collects energy over a range of frequencies, while a plant (if I recall) uses only two. You can think of it this way. Wheat is say 1% efficient at solar collection, while even crappy solar cells are like 8%.

Of course you need to transmit electricity and re-emit the light efficiently. You will still end up with acres of solar panels, as I said. Before you jump on someone for getting their science wrong, make sure YOURS IS RIGHT!

"a plant (if I recall) uses only two [frequencies]"

You mis-recall. It has two peaks in the absorption spectra but they are not narrow. In other words, a plant is green. There's no way to absorb only two frequencies and still be green.

"Wheat is say 1% efficient at solar collection"

Using http://en.wikipedia.org/wiki/Photosynthetic_efficiency as a guide, about 28% of sunlight energy is collected by chlorophyl. Of that, much is used to keep the plant alive (sugar production, respiration, growing roots). About 1% is left for human use.

It's the 28% that's relevant here, not the 1%.

In other words, 28% for plants beats "crappy solar cells" at 8%. Now, 47% of the energy is outside the 400-700nm range that chlorophyll can process so you might be able to double the energy use if you can find a good way to upconvert/downconvert light into that range. That would at best double what you can grow.

Which is still nowhere close to getting viable farming in an urban center.

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