16   Geothermal

Geothermal energy comes from two sources: from radioactive decay in the
crust of the earth, and from heat trickling through the mantle from the
earth’s core. The heat in the core is there because the earth used to be
red-hot, and it’s still cooling down and solidifying; the heat in the core is
also being topped up by tidal friction: the earth flexes in response to the
gravitational fields of the moon and sun, in the same way that an orange
changes shape if you squeeze it and roll it between your hands.

Geothermal is an attractive renewable because it is “always on,” inde-
pendent of the weather; if we make geothermal power stations, we can
switch them on and off so as to follow demand.

But how much geothermal power is available? We could estimate
geothermal power of two types: the power available at an ordinary location
on the earth’s crust; and the power available in special hot spots
like Iceland (figure 16.3). While the right place to first develop geothermal
technology is definitely the special hot spots, I’m going to assume that the
greater total resource comes from the ordinary locations, since ordinary
locations are so much more numerous.

The difficulty with making sustainable geothermal power is that the
speed at which heat travels through solid rock limits the rate at which heat
can be sustainably sucked out of the red-hot interior of the earth. It’s like
trying to drink a crushed-ice drink through a straw. You stick in the straw,
and suck, and you get a nice mouthful of cold liquid. But after a little
more sucking, you find you’re sucking air. You’ve extracted all the liquid
from the ice around the tip of the straw. Your initial rate of sucking wasn’t

If you stick a straw down a 15-km hole in the earth, you’ll find it’s nice
and hot there, easily hot enough to boil water. So, you could stick two
straws down, and pump cold water down one straw and suck from the
other. You’ll be sucking up steam, and you can run a power station. Limit-
less power? No. After a while, your sucking of heat out of the rock will
have reduced the temperature of the rock. You weren’t sucking sustainably.
You now have a long wait before the rock at the tip of your straws
warms up again. A possible attitude to this problem is to treat geothermal
heat the same way we currently treat fossil fuels: as a resource to be mined
rather than collected sustainably. Living off geothermal heat in this way
might be better for the planet than living unsustainably off fossil fuels; but
perhaps it would only be another stop-gap giving us another 100 years of
unsustainable living? In this book I’m most interested in sustainable energy,
as the title hinted. Let’s do the sums.

Figure 16.1. An earth in section.
Figure 16.2. Some granite.