cu-arpa.dietz@Cornell.ARPA (05/22/84)
Photosynthesis is very inefficient in converting light to usable chemical energy (efficiency ~ 1%). One problem is the thermal nature of sunlight. Much of the energy arrives at wavelengths where chlorophyll absorbs inefficiently. The massive part of a space farm is likely to be the pressure vessel and soil in which the plants are grown -- reflectors for sunlight have low mass. Too much sunlight focused on the plants causes heat dissipation problems. One can differentially reflect different wavelengths of sunlight with plastic diffraction grating mirrors. Focusing only those wavelengths where chlorophyll absorbs efficiently could greatly increase crop yields, without too much waste heat frying the plants. Ultimately, one would want to breed plants of algae to efficiently absrob light in a narrow frequency range, then illuminate with a laser beam of that frequency. If the laser was chosen to radiate at a frequency where the human body was fairly transparent, perhaps the algae could be encouraged to grow in the human body, eliminating the need for breathing. So: how efficient is chlorophyll at various wavelengths? Has anyone ever tried growing algae by laser light?
REM%MIT-MC@sri-unix.UUCP (05/24/84)
From: Robert Elton Maas <REM @ MIT-MC> Using laser light to grow crops means you have to first convert sunlight to electricity then run lasers, with loss of efficiency and cost of all that equipment. Your idea of diffraction grating to selectively reflect desired wavelengths of sunlight sounds like a better idea but why not just use multi-layer thin-film mirrors? The principle of enhancement of wavefronts is the same as diffraction grating, but you can reflect just the wavelengths you want whereas a diffraction grating reflects all wavelengths in different directions. This would reduce the light pressure (a la solar sailing) on the mirror, and thus reduce the need for support for the mirror. It would also avoid other wavelengths focusing in other places near or in the greenhouse where they might do harm. It would also allow a second mirror behind the selctive mirror to collect the rest of the sunlight for other less wavelength-selective purposes such as heating something to generate electricity by turbine or thermocouple etc.