say what? who said plants under hid are a dull green?
it depends on the color temp and cri of the bulb as well. The pigmentation and refraction values of the surface, and the wavelengths of absorbed and reflected light. This is a little off topic tho.
As for thermodynamics
each time a energy type is converted a certain amount of that is given off as waste (heat) until there is either no energy left or it is stored (potential energy)- the chemical energy you are ranting about.
we can find out how much chemical energy is stored by releasing it by combustion or decomposition.
Lets break it down for you in terms of heating values.(Btu/hour)
Potential chemical energy in 1 pound plant material (wood for this example) = 6400 btu
(based on total release of energy in one hour)
Total heat energy from 1000 watt bulb = 3413 btu/hour x 12 hours x 50 days 12/12= 2047800 btu.
So there it is... the plant will offset by storing as potential energy approximately 6400/2000000 or 0.32 percent of heat.
The other 99.68 percent of the energy used by the light becomes waste heat.
Please don't try to "explain" thermodynamics to me. I've taught it several times.
I find that plant material of this type of plant is approximately 24,000 J/g (Journal of Biogeography, Vol. 12, No. 2 (Mar., 1985), pp. 121-134). Converting to the units you used gives about 53,000 btu per pound, making it more dense, energetically, than wood.
Next, you use a 1000W watt bulb for many plants, not just one. A target for growers is 1g/watt of dried plant material. If we assume that the roots, stems, and leaves that support the bud material are 3x the dried weight, that gives 4g/W of total plant material.
So:
1000W = 1000 J/s * 3600 s/hr * 12 hr/day * 60 days = 2,592,200,000 J
1000W * 4 g/W * 24,000 J/g = 96,000,000 J
This gives about 4%. This is in the ballpark of the literature-cited photosynthetic efficiency at one sun of about 10-13% (New Phytologist, 131(3), 291, 2006).
So yes, the numbers do suggest that direct conversion to plant matter under the equivalent of one sun via photosynthesis is not going to make an enormous difference.
That said, there are other things to consider:
1. Much of the plant's derived energy is used on processes that are not manifested in additional plant matter, such as nutrient transport and cell-wall repair. These are cyclic processes that occur during light and dark, with losses in the form of irreversibilities (entropy) and yes, heat.
2. At lower light levels, photosynthetic efficiency can be near 100% quantum efficiency. Less light, more efficient processing.
3. The plant has an effective way of reducing temperatures - evaporative cooling via transpiration. The plant is cooled by the energy input required to vaporize water, which is then presumably taken from the system via ventilation.
If a plant on average drinks about 500ml of water per day (obviously this varies with size of the plant), and you have about 20 under a 1000W HID, the energy expended by vaporization is:
500 ml/day * 1 g/ml * 2270 J/g * 60 days * 20 plants = 1,362,000,000 J
This is about 50% of the energy input to the system, so yes, this would make a difference.
