Each of the filled dots shows actual average performances of CHP systems
in the UK, grouped by type. The hollow dots marked “CT” show
the performances of ideal CHP systems quoted by the Carbon Trust; the
hollow dots marked “Nimbus” are from a manufacturer’s product specifi-
cations. The dots marked “ct” are the performances quoted by the Carbon
Trust for two real systems (at Freeman Hospital and Elizabeth House).

The main thing to notice in this diagram is that the electrical efficiencies
of the CHP systems are significantly smaller than the 49% efficiency
delivered by single-minded electricity-only gas power stations. So the heat
is not a “free by-product.” Increasing the heat production hurts the electricity
production.

It’s common practice to lump together the two numbers (the efficiency
of electricity production and heat production) into a single “total efficiency”
– for example, the back pressure steam turbines delivering 10%
electricity and 66% heat would be called “76% efficient,” but I think this
is a misleading summary of performance. After all, by this measure, the
90%-efficient condensing boiler is “more efficient” than all the CHP systems!
The fact is, electrical energy is more valuable than heat.

Many of the CHP points in this figure are superior to the “old standard
way of doing things” (getting electricity from coal and heat from
standard boilers). And the ideal CHP systems are slightly superior to the
“new standard way of doing things” (getting electricity from gas and heat
from condensing boilers). But we must bear in mind that this slight superiority
comes with some drawbacks – a CHP system delivers heat only
to the places it’s connected to, whereas condensing boilers can be planted
anywhere with a gas main; and compared to the standard way of doing
things, CHP systems are not so flexible in the mix of electricity and heat