another (its back panel). So one way to heat a building is to turn a refrig-
erator inside-out – put the inside of the refrigerator in the garden, thus
cooling the garden down; and leave the back panel of the refrigerator in
your kitchen, thus warming the house up. What isn’t obvious about this
whacky idea is that it is a really efficient way to warm your house. For
every kilowatt of power drawn from the electricity grid, the back-to-front
refrigerator can pump three kilowatts of heat from the garden, so that a
total of four kilowatts of heat gets into your house. So heat pumps are
roughly four times as efficient as a standard electrical bar-fire. Whereas
the bar-fire’s efficiency is 100%, the heat pump’s is 400%. The efficiency of
a heat pump is usually called its coefficient of performance or CoP. If the
efficiency is 400%, the coefficient of performance is 4.

Heat pumps can be configured in various ways (figure 21.10). A heat
pump can cool down the air in your garden using a heat-exchanger (typ-
ically a 1-metre tall white box, figure 21.11), in which case it’s called an
air-source heat pump. Alternatively, the pump may cool down the ground
using big loops of underground plumbing (many tens of metres long),
in which case it’s called a ground-source heat pump. Heat can also be
pumped from rivers and lakes.

Some heat pumps can pump heat in either direction. When an airsource
heat pump runs in reverse, it uses electricity to warm up the outside
air and cool down the air inside your building. This is called air-condition-
ing. Many air-conditioners are indeed heat-pumps working in
precisely this way. Ground-source heat pumps can also work as air-con-
ditioners. So a single piece of hardware can be used to provide winter
heating and summer cooling.

People sometimes say that ground-source heat pumps use “geothermal
energy,” but that’s not the right name. As we saw in Chapter 16,
geothermal energy offers only a tiny trickle of power per unit area (about
50 mW/m2), in most parts of the world; heat pumps have nothing to do
with this trickle, and they can be used both for heating and for cooling.
Heat pumps simply use the ground as a place to suck heat from, or to
dump heat into. When they steadily suck heat, that heat is actually being
replenished by warmth from the sun.

There’s two things left to do in this chapter. We need to compare heat
pumps with combined heat and power. Then we need to discuss what are
the limits to ground-source heat pumps.

Heat pumps, compared with combined heat and power

I used to think that combined heat and power was a no-brainer. “Obviously,
we should use the discarded heat from power stations to heat buildings
rather than just chucking it up a cooling tower!” However, looking
carefully at the numbers describing the performance of real CHP systems,
I’ve come to the conclusion that there are better ways of providing electric

Figure 21.11. The inner and outer bits of an air-source heat pump that has a coefficient of performance of 4. The inner bit is accompanied by a ball-point pen, for scale. One of these Fujitsu units can deliver 3.6 kW of heating when using just 0.845 kW of electricity. It can also run in reverse, delivering 2.6 kW of cooling when using 0.655 kW of electricity.