1. advantage that it doesn’t monopolize the land. Burning methane gas
    from landfill sites is a similar way of getting energy, but it’s sustain-
    able only as long as we have a sustainable source of junk to keep
    putting into the landfill sites. (Most of the landfill methane comes
    from wasted food; people in Britain throw away about 300 g of food
    per day per person.) Incinerating household waste is another slightly
    less roundabout way of getting power from solar biomass.
  2. 4.We can grow plants and feed them directly to energy-requiring hu-
    mans or other animals.

For all of these processes, the first staging post for the energy is in a chem-
ical molecule such as a carbohydrate in a green plant. We can therefore
estimate the power obtainable from any and all of these processes by es-
timating how much power could pass through that first staging post. All
subsequent steps involving tractors, animals, chemical facilities, land-
fill sites, or power stations can only lose energy. So the power at the first
staging post is an upper bound on the power available from all plant-based
power solutions.

So, let’s simply estimate the power at the first staging post. (In Chapter D
we’ll go into more detail, estimating the maximum contribution of each
process.) The average harvestable power of sunlight in Britain is 100 W/m2.
The most efficient plants in Europe are about 2%-efficient at turning solar
energy into carbohydrates, which would suggest that plants might deliver
2 W/m2; however, their efficiency drops at higher light levels, and the best
performance of any energy crops in Europe is closer to 0.5 W/m2. Let’s
cover 75% of the country with quality green stuff. That’s 3000 m2 per
person devoted to bio-energy. This is the same as the British land area

Figure 6.10. Some Miscanthus grass enjoying the company of Dr Emily Heaton, who is 5′4″ (163 cm) tall. In Britain, Miscanthus achieves a power per unit area of 0.75 W/m2. Photo provided by the University of Illinois.
Figure 6.11.E Power production, per unit area, achieved by various plants.E For sources, see the end-notes. These power densities vary depending on irrigation and fertilization; ranges are indicated for some crops, for example wood has a range from 0.095–0.254 W/m2. The bottom three power densities are for crops grown in tropical locations. The last power density (tropical plantations*) assumes genetic modification, fertilizer application, and irrigation. In the text, I use 0.5 W/m2 as a summary figure for the best energy crops in NW Europe.