formance of 3 or 4, assuming that the extra electricity for the heat pumps
is generated by an average gas power station or by a top-of-the-line gas
power station, and allowing for 8% loss in the national electricity network
between the power station and the building where the heat pumps pump
heat. The top-of-the-line gas power station’s efficiency is 53%, assuming
it’s running optimally. (I imagine the Carbon Trust and Nimbus made a
similar assumption when providing the numbers used in this diagram for
CHP systems.) In the future, heat pumps will probably get even better
than I assumed here. In Japan, thanks to strong legislation favouring effi-
ciency improvements, heat pumps are now available with a coefficient of
performance of 4.9.

Notice that heat pumps offer a system that can be “better than 100%-
efficient.” For example the “best gas” power station, feeding electricity to
heat pumps can deliver a combination of 30%-efficient electricity and 80%-
efficient heat, a “total efficiency” of 110%. No plain CHP system could
ever match this performance.

Let me spell this out. Heat pumps are superior in efficiency to condens-
ing boilers, even if the heat pumps are powered by electricity from a
power station burning natural gas. If you want to heat lots of buildings
using natural gas, you could install condensing boilers, which are “90% ef-
ficient,” or you could send the same gas to a new gas power station making
electricity and install electricity-powered heat pumps in all the buildings;
the second solution’s efficiency would be somewhere between 140% and
185%. It’s not necessary to dig big holes in the garden and install underfloor
heating to get the benefits of heat pumps; the best air-source heat
pumps (which require just a small external box, like an air-conditioner’s)
can deliver hot water to normal radiators with a coefficient of performance
above 3. The air-source heat pump in figure 21.11 (p147) directly delivers
warm air to an office.

I thus conclude that combined heat and power, even though it sounds
a good idea, is probably not the best way to heat buildings and make
electricity using natural gas, assuming that air-source or ground-source
heat pumps can be installed in the buildings. The heat-pump solution has
further advantages that should be emphasized: heat pumps can be located
in any buildings where there is an electricity supply; they can be driven by
any electricity source, so they keep on working when the gas runs out or
the gas price goes through the roof; and heat pumps are flexible: they can
be turned on and off to suit the demand of the building occupants.

I emphasize that this critical comparison does not mean that CHP is
always a bad idea. What I’m comparing here are methods for heating
ordinary buildings, which requires only very low-grade heat. CHP can
also be used to deliver higher-grade heat to industrial users (at 200 °C, for
example). In such industrial settings, heat pumps are unlikely to compete
so well because their coefficient of performance would be lower.

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