[Space] Gliese 581

[gjc8]

Originally Posted by lwcamp

Thermodynamics does impose an ultimate limit on what fraction of the energy can be harvested from the sunlight, because any physical process can be driven in reverse, and the absorption mechanisms can be thermally activated to cause emission. The ultimate efficiency depends on the temperature of the star's surface and the temperature at which your photopigment operates. For life on earth, operating at about 300 K and harvesting light from a 6000 K surface, the maximum theoretical efficiency is (6000 K - 300 K)/(6000 K) = 95%. You can see that life on earth has a long way to go before the physical limit is reached. For life around a red dwarf sun, with a temperature of 3000 K, the maximum efficiency drops to (3000 K - 300 K)/(3000 K) = 90%. Again, this limit is probably well above the efficiency of any actual photopigments that would have evolved.

Ahh, right. But as far as I can tell, photosynthesis is fairly far from this theoretical limit even at the peak limits. If I recall, efficiency is closer to 30% for energy absorbed by the chlorophyll. Now, I expect you'll see some loss of efficiency (just as you do for heat engines operating with lower temperature differences), but I don't think that's likely to be a problem. The difference is small, compared to other inefficiencies.