Combined Heat and Power (CHP) is about making use of some of the waste
heat produced by power stations. The CHP idea became popular in the UK in the
1970s. In theory, with CHP, the overall energy conversion efficiency could be
raised from the 30-35% or so typical of the then dominant large coal fired
plants, to 70-80%. Large CHP plants, feeding city-wide district heating (DH)
networks, were enthusiastically promoted as an idea in the 1970s by some
radical local councils (notable Newcastle and Sheffield) and by trade union and
labour movement groups like SERA- and indeed by the Open University: if you did
an OU energy course then you could hardly miss it. Even the then chief scientist, Walter
Marshall, was a fan- he chaired the definitive study on CHP, but, as head of
the CEGB, he backed nuclear (US PWRs) even more. In the event, after an attempt
to push a big PWR programme (only one was build - at Sizewell), Maggie Thatcher
opted instead for privatization, which put nuclear and coal-fired CHP mostly out
of the running, with the dash for gas being one result: North Sea gas was
plentiful and cheap and combined cycle gas turbines were quick and cheap to
build, and could use gas reasonably efficiently, with conversion efficiencies
of up to 50% being claimed. Although that still means half the heat energy is
wasted.
Some smaller
CHP plants have been developed for industrial complexes, where there were large
on-site heat demands, but unlike in central and northern Europe, where its use
is widespread, CHP/DH was hard to promote for large-scale urban use in the UK,
given that it always seemed to have cheap energy sources (coal and then North Sea
gas), so upgrading the efficiency of its uses didn’t seem that urgent. And ‘socialistic’
community heat provision didn’t tie in well with privatisation and
liberalisation. Now, with fuel sources
less available and climate issues mounting, it’s back on the agenda- this time also
pushed from the political right!
One idea
(borrowed from Russia) is to use nuclear plants to provide the heat. For
example, the old Leningrad Nuclear plant, on the outskirts of (what is now
called) St Petersburg, supplies heat to the city. Like all conventional single-cycle stream
raising plants, nuclear plants currently waste about two thirds of the heat they
produce. In theory some of that could be
used for heating cites. And if mini nukes (Small Modular Reactors) were
developed, they could be in or near cites, feeding heat to local users. That
seems pretty fanciful and risky. Gas is the obvious interim option, and many
CHP/DH systems on the continent use biomass (straw) and some community DH
networks use solar, with interseasonal heat stores, and these flexible systems are
seen as one way to balance variable wind. Denmark aims to get 40% of its
extensive DH network fed with solar heat by 2050. By contrast, do we really
want mini-nukes in or near cites?
Building
CHP/DH systems isn’t cheap. They only make sense where there are big local heat
loads and supplying them in existing cities means digging up the streets to lay
heat mains. But once you have installed the DH infrastructure, then you can
feed in heat from whatever is the best current source. It’s a very good, very
flexible, long-term investment. And
energy costs can then be competitive, as was demonstrated in Woking, which has
installed medium sized gas-fired CHP plants feeding its local heat grid and
‘private wire’ power networks.
Taking heat
out of a gas turbine system, usually at slightly earlier stage than in normal
CCGT operation, so as to get higher temperatures, does decrease the efficiency
of electricity production slightly, but the ratio of heat to power can be
adjusted to match demand. That can help CHP/DH with heat stores to balance
variable renewables. When there is surplus electricity on the grid, e.g. from wind
or PV, the CHP power output can be reduced and heat output can be increased and
stored if necessary, until needed. When there
is a shortage of renewable electricity, the proportion of CHP power output can
be increased and if more heat is also needed it can be drawn from the store.
Small CHP
plants, and domestic scaled micro CHP units especially, tend to be less
efficient than large community scaled units, since the latter can use large
efficient heat stores and can service the averaged-out heat and power demands
from many users, rather then the much more variable demands of individual
consumers. Collective heat provision does of course present some problems. Some
early DH systems were unreliable, sometimes offering a poor service, especially
for users at the far end of the network. In which case collective billing
became an issue- and providing heat meters is expensive. Regular maintenance is essential. Lack of
that is one reason why Russia’s old centralized heating systems have got a bad
reputation. Modern DH systems are
better, using lower temperature heat in larger pipes, rather than high
temperature steam.
DH grids can
be extensive, with heat running through miles of pipes, and the heat can be
supplied from plants a long distance away without significant energy loses. Oslo’s district heating network is fed via a 12.3 km
pipe from a waste burning plant in the city outskirts. In Denmark there is a
17km link from a CHP plant to the city of Aarhus. Helsinki’s scheme is part of
a1150 MWe and 3600 MWth CHP/DH system, supplying over 93% of Helsinki’s heat,
including a plant linked in via a 30km pipe in a tunnel, while in the Czech
Republic heat is delivered cross-country by a 200 MW capacity heat main to
Prague from a power station 65 km away. Whether that would make sense, or be
acceptable, in densely populated countries like the UK is unclear, though, if
inner city mini nukes were not acceptable, there might be no alternative if the
nuclear plant option was taken… But if that is avoided, inner city CHP/DH,
increasingly using green energy sources like biogas from wastes, could be come
a major option. That is certainly the main focus of the Thousand Flowers 2050
UK scenario produced recently by a group of UK academics, which has 44GW of
mainly smaller scale community based CHP:
For a recent exhaustive EU
report see: http://setis.ec.europa.eu/system/files/JRCDistrictheatingandcooling.pdf
