Now, here is a good paper -- which I hope is
technically accurate -- that explains a lot about
photosynthesis and plant efficiency:
http://www.pnas.org/cgi/content/full/103/14/5251
David A. LaVan and Jennifer N. Cha, "Approaches
for biological and biomimetic energy conversion," PNAS
QUOTES:
"The average solar radiation available for a
flat-plate collector in the U.S. is 5 kW·h/m^2
per day (1 kW·h = 3.6 x 10^6 J). Conservatively,
100 million residences, each with an available
roof area of 90 m^2, receive 5 x 10^19 J of solar
energy, which is equal to half of the annual
energy consumption in the U.S. Typical
commercially available PV cells offer nominal
efficiencies of 15%, with higher levels
attainable up to a theoretical efficiency for
silicon PV cells of 32%; however, a significant
fraction of the installation costs are related to
infrastructure, such as supporting framework,
wiring, power inverters, and grid connections.
For example, in a study published in 2003 of a
35-kW PV array (2), the total reported cost was
$239,945 ($6.86/W), with infrastructure
comprising 3540% of the total amount. This
system saved $2,678 per year in energy costs
compared with the preinstallation expenditures.
If, hypothetically, the same installation could
be made with cells at 1/10 the current price and
32% efficiency but the same infrastructure costs,
the system would cost $100,000 and save $8,000
per year. Based on these values, it is apparent
that improving efficiency and reducing device
costs is vital to using PV technologies but that
addressing infrastructure costs will also be necessary. . . ."
". . . For comparison, the high-energy photons in
the UV region have energies in the range of
3.104.28 eV. The difference in energy between
the absorbed photon and the electron donor is
lost as heat, just as the excess energy is lost
when a high-energy photon is absorbed by a
silicon PV cell. Most plants are not able to deal
with the excess energy of UV photons and often do
not absorb them at all. If these high-energy
photons were absorbed, they might reduce the
genetic stability of the plant's genome (6),
destroy the light-harvesting complex (7), or
induce apoptotic-like (programmed cell death)
changes (8). Photons of lower energies are also
not used for photosynthesis. It is possible to
calculate the energy efficiency of green plants,
given the spectral distribution of light at the
Earth's surface; the net energy has been shown to
have an upper bound of 9.2% (9). Because some of
this energy is consumed by the plant, a maximum
net efficiency of 5% is commonly reported (911)."
I have seen estimates of 1% to 2% for plants in
natural, year round conditions. This is not far
from the 5% to 9% for ideal conditions. The 1% is
for plants that grow in deserts and other harsh
environments. My estimate of 15% for the Japanese
food factory was with PAR light only, not natural
light. I assume this 9% applies to natural light.
This paper does not mention green sulfur bacteria.
- Jed
