In the World Energy Assessment published by the UNDP, Rogner (2000)
writes: “Assuming a 45% conversion efficiency to electricity and yields of
15 oven dry tons per hectare per year, 2 km2 of plantation would be needed
per megawatt of electricity of installed capacity running 4,000 hours a year.”
That is a power per unit area of 0.23 W(e)/m2. (1 W(e) means 1 watt of
electrical power.)
Energy for Sustainable Development Ltd (2003) estimates that short-rotation
coppices can deliver over 10 tons of dry wood per hectare per year, which
corresponds to a power density of 0.57 W/m2. (Dry wood has a calorific
value of 5 kWh per kg.)
According to Archer and Barber (2004), the instantaneous efficiency of a
healthy leaf in optimal conditions can approach 5%, but the long-term energy-
storage efficiency of modern crops is 0.5–1%. Archer and Barber suggest that
by genetic modification, it might be possible to improve the storage efficiency
of plants, especially C4 plants, which have already naturally evolved a more
efficient photosynthetic pathway. C4 plants are mainly found in the trop-
ics and thrive in high temperatures; they don’t grow at temperatures below
10°C. Some examples of C4 plants are sugarcane, maize, sorghum, finger
millet, and switchgrass. Zhu et al. (2008) calculate that the theoretical limit
for the conversion efficiency of solar energy to biomass is 4.6% for C3 photo-
synthesis at 30°C and today’s 380 ppm atmospheric CO2 concentration, and
6% for C4 photosynthesis. They say that the highest solar energy conversion
efficiencies reported for C3 and C4 crops are 2.4% and 3.7% respectively;
and, citing Boyer (1982), that the average conversion efficiencies of major
crops in the US are 3 or 4 times lower than those record efficiencies (that
is, about 1% efficient). One reason that plants don’t achieve the theoretical
limit is that they have insufficient capacity to use all the incoming radiation
of bright sunlight. Both these papers (Zhu et al., 2008; Boyer, 1982) discuss
prospects for genetic engineering of more-efficient plants.

43Figure 6.11. The numbers in this figure are drawn from Rogner (2000) (net
energy yields of wood, rape, sugarcane, and tropical plantations); Bayer
Crop Science (2003) (rape to biodiesel); Francis et al. (2005) and Asselbergs
et al. (2006) (jatropha); Mabee et al. (2006) (sugarcane, Brazil); Schmer et al.
(2008) (switchgrass, marginal cropland in USA); Shapouri et al. (1995) (corn
to ethanol); Royal Commission on Environmental Pollution (2004); Royal So-
ciety working group on biofuels (2008); Energy for Sustainable Development
Ltd (2003); Archer and Barber (2004); Boyer (1982); Monteith (1977).

44Even just setting fire to dried wood in a good wood boiler loses 20% of the
heat up the chimney
. Sources: Royal Society working group on biofuels
(2008); Royal Commission on Environmental Pollution (2004).