william mook wrote:

The absorption maxima of chlorophyll a are 430 and 662 nm, that of
chlorophyll b are at 453 and 642 nm.


http://www.biologie.uni-hamburg.de/b-online/e24/3.htm

So, solid state LEDs that emit around these frequencies would be very
efficient at growing plants.

Looking at the spectrum of the sun ...

http://zebu.uoregon.edu/~js/glossary/planck_curve.html

(there are 10 angrstoms per nm)

and multiplying the chlorophyl response curve by the solar spectrum,
we can see that its possible to grow plants with the right colors of
light at about 5% of the energy of full spectrum sunlight.

This suggests that dyes, or high-efficiency solar cells powering high
efficiency LEDs of the right colors might be capable of growing more
food per unit area than natural sunlight.

It would appear that using PV cells getting ~ 15 - 20% of energy from
sunlight would allow one to build 3-5 high greenhouses that were much
more space efficent (which is not a trivial concern) than ones directly
using sunlight.

So to solve teh worlds food supply problems for a long time one would then
just need a cheap way to de-orbit food.

A concentrating mirror can focus sunlight onto an appropriate
apparatus that simultaneously produces electricity and increases
growing efficiency and area underneath the mirrors. The electricity
can be used in part to take nitrogen and water vapor in the air and
combine it to produce ammonia based fertilizers from sunlight and air.

But there are not just plants that have nitrogen fixing symbiotic bacteria
(which includes soybeans, and other beans / peas which are likely to be
in the crops list) and also many free-living bacteria do this. So is a
separate process other than reprocessing results of biological processes.

Also, many photosynthesising cyanobacteria are also nitrifying.

--
Sander

+++ Out of cheese error +++