I just went back to look at Fig. 7 of the PDF in:
The Photosynthetic Action Spectrum of the Bean Plant -- Balegh and Biddulph 46 (1): 1 -- PLANT PHYSIOLOGY
The action curve mostly ranges from 72 to 113 molecules of CO2 utilized
per 1,000 photons.
Typical red LED light with peak wavelength of 635 nm appears to me,
according to that curve, to achieve utilization of 108 molecules of CO2
per 1,000 photons incident upon the leaves.
These photons have average eergy close enough to 1239.7/635 electron-
volts each, which is 1.952 electron-volts.
Multiply 1.952 by 1,000/108, and so far it looks like 18.075
electron-volts of energy in the form of red LED light is required to
convert one CO2 molecule and one water molecule into one O2 molecule and
carbohydrate.
To convert this to joules per mole, multiply by Faraday's Number, which
is 96,485 coulombs per mole. This indicates that 1.744 megajoules of red
light from LEDs having peak wavelength of 635 nm is required to remove one
mole (44 grams) of CO2 from the air.
Good LEDs of this peak wavelength are around 25% efficient. This means
about 7 megajoules, or about 1.94 kilowatt-hours, of electric energy must
be delivered to such red LEDs to remove 1 mole (44 grams) of CO2 from the
air.
Keep in mind that these red LEDs usually have their dominant wavelength
mentioned more than their peak wavelength. The dominant wavelength is a
color specification and is in the mid 620's of nm for these red LEDs.
This usually appears as a slightly orangish shade of red.
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I am repeating these calculations for two other LED wavelengths used for
growing plants:
660 nm: My eyeball estimate is that that one's spectrum achieves 109
molecules CO2 utilized per 1,000 incident photons, rather than 108. A 660
nm photon has 1.878 electron-volts of energy rather than 1.952. So light
energy requirement to remove 1 mole of CO2 from the air is 1.744
megajoules * (108/109) * (1.878/1.952), which is 1.663 megajoules.
I know of one manufacturer of such LEDs achieving about 25% efficiency
(LEDEngin). That indicates about 6.7 megajoules (about 1.85
kilowatt-hours) of electrical energy must be delivered to these 660 nm
LEDs to remove 1 mole (44 grams) of CO2 from the air.
Since plants have some requirement for blue light, I am repeating these
calculations again for 450 nm, from a Philips-Lumileds "Luxeon Rebel" LED
of "Royal Blue" color. My "eyeball estimate" of photosynthetic action by
that LED's spectrum is 92 molecules of CO2 per 1,000 photons. The average
energy per photon here is 2.75 electron-volts.
So, 1.744 * (108/92) * (1.952 / 2.75) means about 2.88 megajoules of
such blue LED light are required to remove 1 mole (44 grams) of CO2 from
the air. It appears to me nowadays that a top rank royal blue Luxeon
Rebel is about 30% efficient at a conservative current of 350 milliamps.
That means 9.6 megajoules, or 2.67 KWH, of electrical energy must be
delivered to these LEDs to remove 1 mole of CO2 from the air.
Red LEDs need conservative amounts of current and excellent heatsinking
to achieve 25% efficiency. LEDEngin 660 nm red LEDs probably achieve 25%
efficiency at 350 milliamps, possibly at 700 milliamps. The shorter
wavelength orangish red ones usually achieve 25% efficiency at 500 mA at
the most, preferably 350 mA.