a sunnier spot such as Australia. (An energy yield ratio bigger than one
means that a system is A Good Thing, energy-wise.) Wind turbines with a
lifetime of 20 years have an energy yield ratio of 80.

Aren’t photovoltaic panels going to get more and more efficient
as technology improves?

I am sure that photovoltaic panels will become ever cheaper; I’m also
sure that solar panels will become ever less energy-intensive to manufac-
, so their energy yield ratio will improve. But this chapter’s photo-
voltaic estimates weren’t constrained by the economic cost of the panels,
nor by the energy cost of their manufacture. This chapter was concerned
with the maximum conceivable power delivered. Photovoltaic panels with
20% efficiency are already close to the theoretical limit (see this chapter’s
endnotes). I’ll be surprised if this chapter’s estimate for roof-based photo-
voltaics ever needs a significant upward revision.

Solar biomass

All of a sudden, you know, we may be in the energy business by being able to grow grass on the ranch! And have it harvested and converted into energy. That’s what’s close to happening. George W. Bush, February 2006

All available bioenergy solutions involve first growing green stuff, and
then doing something with the green stuff. How big could the energy
collected by the green stuff possibly be? There are four main routes to get
energy from solar-powered biological systems:

  1. We can grow specially-chosen plants and burn them in a power sta-
    tion that produces electricity or heat or both. We’ll call this “coal
  2. We can grow specially-chosen plants (oil-seed rape, sugar cane, or
    corn, say), turn them into ethanol or biodiesel, and shove that into
    cars, trains, planes or other places where such chemicals are useful.
    Or we might cultivate genetically-engineered bacteria, cyanobacteria,
    or algae that directly produce hydrogen, ethanol, or butanol, or even
    electricity. We’ll call all such approaches “petroleum substitution.”
  3. We can take by-products from other agricultural activities and burn
    them in a power station. The by-products might range from straw (a
    by-product ofWeetabix) to chicken poo (a by-product ofMcNuggets).
    Burning by-products is coal substitution again, but using ordinary
    plants, not the best high-energy plants. A power station that burns
    agricultural by-products won’t deliver as much power per unit area
    of farmland as an optimized biomass-growing facility, but it has the
Figure 6.9. Solar photovoltaics: a 10 m2 array of building-mounted south-facing panels with 20% efficiency can deliver about 5 kWh per day of electrical energy. If 5% of the country were coated with 10%-efficient solar panels (200 m2 of panels per person) they would deliver 50 kWh/day/person.