## Geothermal power that would be sustainable forever

First imagine using geothermal energy sustainably by sticking down straws
to an appropriate depth, and sucking gently. Sucking at such a rate that
the rocks at the end of the our straws don’t get colder and colder. This
means sucking at the natural rate at which heat is already flowing out of
the earth.

As I said before, 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. In a typical continent, the heat flow from
the centre coming through the mantle is about 10 mW/m2. The heat flow
at the surface is 50 mW/m2. So the radioactive decay has added an extra
40 mW/m2 to the heat flow from the centre.

So at a typical location, the maximum power we can get per unit area
is 50 mW/m2. But that power is not high-grade power, it’s low-grade heat
that’s trickling through at the ambient temperature up here. We presum-
ably want to make electricity, and that’s why we must drill down. Heat
is useful only if it comes from a source at a higher temperature than the
ambient temperature. The temperature increases with depth as shown in
figure 16.4, reaching a temperature of about 500 °C at a depth of 40 km.
Between depths of 0 km where the heat flow is biggest but the rock tem-
perature is too low, and 40 km, where the rocks are hottest but the heat
flow is 5 times smaller (because we’re missing out on all the heat generated
from radioactive decay) there is an optimal depth at which we should
suck. The exact optimal depth depends on what sort of sucking and power-
station machinery we use. We can bound the maximum sustainable power

Figure 16.3. Geothermal power in Iceland. Average geothermal electricity generation in Iceland (population, 300 000) in 2006 was 300 MW (24 kWh/d per person). More than half of Iceland’s electricity is used for aluminium production. Photo by Gretar Ívarsson.
one milliwatt (1 mW) is 0.001 W.
Figure 16.4. Temperature profile in a typical continent.
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