This. Is. Awesome. We could grow our food underneath our cities!
To figure out whether this is feasible/more efficient than above ground farming, we need a few numbers: The percent of solar energy plants convert (photosynthetic efficiency), the percent of solar energy solar panels absorb, the efficiency of LED bulbs, and the efficiency of plants in absorbing LED light.
*Numbers pulled out of ass warning*
If plants absorb 10% of solar energy, solar panels get 20%, LED efficiency is 90%, and plants absorb 90% of LED light, the ratio would be:
10% solar vs. 16.2% (20%*90%*90%) artificial.
So, If you have a building with two floors and a solar panel on top you could fill 1.62 of those floors with plants, running it entirely off solar power, and yield a net gain in production versus open field farming.
But anyway, the question is, what are the real numbers? Without anyone quoting that it's hard to say this won't work. And even if the numbers do
say it won't - there's nothing to say that solar panel efficiency and so on can't be improved to the point where it is feasible. Let's see what I can find:
Wikipedia says photosynthesis is about 3-6% efficient: http://en.wikipedia.org/wiki/Photosynthetic_efficiency
The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants and algae. Photosynthesis can be described by the simplified chemical reaction
H2O + CO2 + energy --> CH2O + O2,
where CH2O represents carbohydrates such as sugars, cellulose, and lignin. The value of the photosynthetic efficiency is dependent on how light energy is defined. On a molecular level, the theoretical limit in efficiency is 25 percent for photosynthetically active radiation (wavelengths from 400 to 700 nanometer). However, photosynthesis is now known to occur up to 720 nm wavelengths (see Chlorophyll). For actual sunlight, where only 45 percent of the light is photosynthetically active, the theoretical maximum efficiency of solar energy conversion is approximately 11 percent. In actuality, however, plants do not absorb all incoming sunlight (due to reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels) and do not convert all harvested energy into biomass, which results in an overall photosynthetic efficiency of 3 to 6 percent of total solar radiation. If photosynthesis is inefficient, excess light energy must be dissipated to avoid damaging the photosynthetic apparatus. Energy can be dissipated as heat (non-photochemical quenching), or emitted as chlorophyll fluorescence.
There's a new film that improves solar panel efficiency by 300%, but I don't know if the wikipedia numbers for solar efficiency are from before or after this stuff is applied: http://blogs.forbes.com/eco-nomics/2011 ... 0-percent/
Solar panel efficiencies vary from 6 to 42.8%: http://en.wikipedia.org/wiki/Solar_cell_efficiency
Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40.7% with multiple-junction research lab cells and 42.8% with multiple dies assembled into a hybrid package. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-19%. The highest efficiency cells have not always been the most economical — for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power.
There's some efficiency numbers for LEDs: http://en.wikipedia.org/wiki/Led#Effici ... parameters
Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts [mW] of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt [W]. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die.
One of the key advantages of LED-based lighting is its high efficacy,[dubious – discuss] as measured by its light output per unit power input. White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with a luminous efficacy of 18–22 lumens per watt [lm/W]. For comparison, a conventional 60–100 W incandescent light bulb emits around 15 lm/W, and standard fluorescent lights emit up to 100 lm/W. A recurring problem is that efficacy falls sharply with rising current. This effect is known as droop and effectively limits the light output of a given LED, raising heating more than light output for higher current.
In September 2003, a new type of blue LED was demonstrated by the company Cree Inc. to provide 24 mW at 20 milliamperes [mA]. This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Also, Seoul Semiconductor plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be nearing an order of magnitude improvement over standard incandescents and better than even standard fluorescents. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA.
Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.
Note that these efficiencies are for the LED chip only, held at low temperature in a lab. Lighting works at higher temperature and with drive circuit losses, so efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W).
Cree issued a press release on February 3, 2010 about a laboratory prototype LED achieving 208 lumens per watt at room temperature. The correlated color temperature was reported to be 4579 K.
But since I'm not sure on the percentage of electric energy converted to light energy, let's look that up:http://en.wikipedia.org/wiki/Luminous_efficacy
In some systems of units, luminous flux has the same units as radiant flux. The luminous efficacy of radiation is then dimensionless. In this case, it is often instead called the luminous efficiency or luminous coefficient and may be expressed as a percentage. A common choice is to choose units such that the maximum possible efficacy, 683 lm/W, corresponds to an efficiency of 100%. The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.
So, going by the best numbers, 145 lm/W is 145/683 = 21.2%
For the percent of LED light the plants absorb... can't find that, but assuming if it were 100% and the best numbers for the other stuff, we'd have:
Natural light: 3-6%
Artificial: 42.8% panel efficiency * 21.2% LED efficiency * 100% absorption = 9.08 % efficiency, or better than natural light.
Well, obviously we won't get 100% absorption, so let's say half - Converting solar energy to artificial light would be 4.54%, or on par with using straight natural light. Anyone have any idea what the real numbers are?
I think we're going to have to see some technical improvements before we start seeing farming skyscrapers run on 100% solar power, but I'd say it'll happen eventually. In the meantime this new LED tech is still useful stuff.