Friday, April 1, 2016

In praise of CHP

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:  

And for a global view: www.iea-dhc.org/home.html

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