Also, it's possible to use LEDs tuned to the absorbtion spectra of chlorophyll, as you say.
If my back of the envelope calculations above are wrong by more than an order of magnitude, it might in fact be a commercially viable option for farming.
Is this [1] the Singaporean project you were refering to? They probably don't need heating, but nonetheless their quoted energy needs of 3$/month/structure are astonishingly low.
[1] http://permaculturenews.org/2014/07/25/vertical-farming-sing...
This isn't true. The majority of light from an HPS light is green, yet huge crops are grown using that light all the time. Plants utilize almost every visible wavelength (and some invisible wavelengths) to some extent. LEDs generally just focus on the wavelengths that have been determined to be efficiently utilized. It is all however still 100% crop-dependent. Some plants simply hate LED lighting because of the missing spectrum (like my orchids.)
Except for the lamp, it is just like growing plants in pots outside, except there are 90% less insects and no hungry critters to snack on your crops.
I did some experiments with adjusting the lamp timer and long days didn't keep my beans from producing. Some plants want "seasons" but mine didn't seem to mind.
Now I have a wife, and a basement again, and we are both interested in starting a small "farm." LEDs are viable now and I want to look into this.
I've been looking for a change from outdoor gardening and a high density 1 m^2 indoor plot with 100W red/blue LED + 100W panel + automatic watering system sounds like a fun combination. Plus I can finally try growing tropical fruit and vegetables.
This is interesting - do you have more information on it?
* 45% of sunlight is in the photosynthetically active wavelength range
* Solar cells are 20% efficient
* Red LEDs are 39% efficient and blue LEDs are 35% efficient
So no, I was kind of off base there. Straight sunlight is still 6x more energy for plants. But it is close! 40% efficient panels exist and my LEDs efficiency numbers were from 2012 so the gap might be smaller. If you do this calculation yourself, remember to ignore lumens. They are a human perceptual unit and are completely meaningless for plants.
Now for human-visible light it is a little different. With the most efficient solar cells and white LEDs we can do better than the sun. (Straight sunlight is 93 lumens per watt. A 40% solar cell and 250 lumen/watt LED outperforms the sun.) Maybe this is what I was thinking of.
But 12kWh/m^2 is for direct sunlight which cabbage etc. probably do not need, as they will have had some thoughts into that, also other hydroponic farms like in Singapore have the same problem. Also the light is tinted more violet and thus more efficiently usable for the plants. And I would not count with 20ct/kWh, as they probably pay industrial rates (though I have no idea how much that is over there).