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.
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-
ture, 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.
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: