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drhex
2005-Oct-22, 10:23 PM
I've read that the most efficient solar/photovoltaic cells can reach efficiencies of around 30%. How does that compare to what leaves of typical plants can do?
(leaves cannot be 100% efficient, or they'd be black)

JHotz
2005-Oct-23, 08:48 PM
I've read that the most efficient solar/photovoltaic cells can reach efficiencies of around 30%.What sort of efficiency is really important for solar panels? In space it is weight of panels and deployment apparatus verses power output. I recall the use of some lens arrays on the panels to increase the amount of light falling on them. This would not have been practical on earth because it is cheaper to add more panels. On earth I would say total power output for the life of the panel verses cost of alternatives. It is more efficient to directly heat water then use solar panel electricity.
How does that compare to what leaves of typical plants can do?
(leaves cannot be 100% efficient, or they'd be black)I do not know what percent of light falling on a leaf is used to make food and oxygen by the plant. There are other issues to consider though. The food is a more efficient energy source for the plant than electricity is for people. So how do you really compare the two? Plants us a great deal of the no visible spectrum do solar panels as well?

Jens
2005-Oct-25, 05:42 AM
Hopefully somebody with more expertize can help you, but for starters, I'm not sure if the efficiency of a solar cell and that of plants doing photosynthesis is directly comparable. I don't know if a leaf would have to be black to be efficiency, because it may be that the process is not the same, i.e. they are not simply using sunlight for it heating potential. It may be acting as a catalyst in the reaction. Also, it may be that the main constraining factor for photovoltaic cells is sunlight, but it's possible that for photosynthesis, there are other constraining factors that are more important, like need for carbon dioxide or water or whatever. So it may be that the efficiency wouldn't be measured in quite the same way.

Van Rijn
2005-Oct-25, 06:14 AM
According to this webpage: (http://www.fao.org/docrep/w7241e/w7241e05.htm)



Only light within the wavelength range of 400 to 700 nm (photosynthetically active radiation, PAR) can be utilized by plants, effectively allowing only 45 % of total solar energy to be utilized for photosynthesis. Furthermore, fixation of one CO2 molecule during photosynthesis, necessitates a quantum requirement of ten (or more), which results in a maximum utilization of only 25% of the PAR absorbed by the photosynthetic system. On the basis of these limitations, the theoretical maximum efficiency of solar energy conversion is approximately 11%. In practice, however, the magnitude of photosynthetic efficiency observed in the field, is further decreased by factors such as poor absorption of sunlight due to its reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels. The net result being an overall photosynthetic efficiency of between 3 and 6% of total solar radiation.

That's somewhat higher than numbers I remember reading ages ago. I'm sure it depends on what exactly you count, and this is certainly not my area of expertise, so I can't vouch for accuracy. But, yes, there are a number of steps in photosynthesis, just as there are a number of steps from PV to storing the energy chemically for applications such as running a car.