Nuclear CHP: why settle for less?

An intriguing title for a blog I hope you’ll agree. Firstly let me say that I’m not a fan of nuclear power. I’m concerned about the safety, the cost, the waste disposal issues and the fact that it’s not renewable. But I do recognise that the government seems to be absolutely determined to get the industry to build more nuclear power stations. For evidence of this see its willingness to provide a fixed price for nuclear power of double the wholesale price of electricity for 35 years with 65% of the capital costs covered by treasury backed loan guarantees. This is in addition to the  repeated mantra that we hear from government ministers that we need a “balanced” portfolio of electricity generation including nuclear, renewables and fossil fuels with (they hope ) carbon capture as we move towards our low carbon future.

This emphasis on nuclear being part of the generation mix is combined with a general desire to see the electrification of both heating and transport. This means that without a very significant reduction in demand “the balanced portfolio” will need to add up to a larger total than we currently have. Without any demand reduction at all nuclear power capacity would need to be around 3x its current level in order to maintain its contribution to the current mix. I recognise that these are very approximate figures but I’m talking in very ballpark terms here. If you think of our energy demand cake as roughly a third power, a third heat and a third transport, if you then move the heat and the transport into the power sector then you have 3 times our current power demand.

Now here I hope you can see where nuclear CHP comes in. If, rather than the electricification of heating, we build district heating systems and connect those to the waste heat from nuclear power stations then we can avoid having to build one third (very approximately) of those new nuclear power stations. We only have to double our capacity (to supply the newly electrified transport sector) not triple it.

Now I can imagine your objections already and I’ll try and deal with them in turn.

  1. Consumers will never accept heat from nuclear power stations in their homes.
  2. Water in district heating systems isn’t like sea water or ground water which we commonly hear associated with radioactivity concerns. It’s different because it’s treated to remove impurities. Pure treated water can’t carry radioactivity.

    Furthermore and rather obviously the water in the district heating network is not the same water as that heated directly by nuclear fission. This water is turned into steam by the heat given off by fission. This steam drives a steam turbine and then is re-condensed into water in the power stations condensers.  Generally sea water is used to condense the steam in nuclear power stations in the UK.  So the first (heat exchange) barrier is at the condensers.  Potentially you could have more heat exchangers in the system though those these come at a cost in that they increase the return temperature of the water back to the power station which increases the loss. We already have an infrastructure that connects our homes to nuclear power stations. It’s the electricity network. Arguably contamination is about as likely as contamination being transmitted through the wires that connect power stations to our homes.

    You also need to think through the scenario in which this district heating future would take place.  The first step would not be the construction of the pipes between cities and power stations. We’d most likely build up district heating within cities with local gas fired CHP (William Orchard’s Hub Concept). As we move towards 2050 we’d see more interconnection and in many cases nuclear would be one contributory heat source to those heat networks.

  3. Nuclear power stations are a long way from conurbations where heat is actually needed and heat unlike power can’t be cost effectively transmitted long distances.
  4. It is true that nuclear power stations are not ideally placed to provide heat to cities but it is entirely practicable to transport large quantities of heat long distances. William Orchard examined this in an article for the CIBSE Journal using Sizewell B (a nuclear power station on the Suffolk coast) and a 128kM 2meter diameter heat main to London as an example.  The water would actually arrive in London 0.1 degree hotter than when it left the power station. That’s because the energy inputs from pumping the water are lost into the water, increasing its temperature.  Additionally very large heat mains lose proportionately less heat than small heat mains because the surface area through which heat is lost (the outer surface of the pipe) is smaller as a proportion of the heat transmitted. Obviously the pipes are insulated as well. The costs of pumping water are less as well, proportionately, with larger diameter pipes – less friction per unit of heat transmitted.

    In Finland the Lovisa 3 nuclear plant was put forward as a CHP plant providing heat to Helsinki’s district heating system. It‘s around 100km from Helsinki. The proposed plant has been rejected by the government but not, as far as I can discern, because of the CHP element.

    If you look at a map of current and planned nuclear power stations then it’s easy to imagine how many of major conurbations could be supplied e.g. London from Bradbury and Sizewell, Greater Manchester from Heysham, Newcastle, Sunderland etc from Hartlepool, Birmingham from Oldbury and Bristol from Hinckley Point. Ok all of those would involve huge civil works but what route to 2050 carbon reduction doesn’t and how does building a few hundred miles of connecting pipes compare to building the double the number of nuclear power stations?

  5. Building district heating is too costly. You’d be digging up streets from now until 2050.
  6. Well we already are digging up streets. I’ve recently, with a mixture of frustration and sadness, watched as the gas network in the town where I live was systematically dug up and replaced with more gas network. And if we persist down a route where we electrify both heating and transport then we will need to dig up the electricity network and replace that to accommodate the increased electricity requirements. If you don’t believe this then take a look at the current activities of UK Power Networks who are responsible for the distribution networks in London, South East and the East of England. They are very keenly looking around for alternatives to digging up London because they envisage the problems. One of their alternatives is small-scale CHP embedded in the network. This has 2 functions – it provides an alternative to electrification of heat and because electricity is generated locally it can provide an alternative to new larger wires in the ground.

  7. Doing this locks us into nuclear power for a very long time. What if a future nuclear accident somewhere in the world causes a future government to shut down. We’ll then be faced with a double whammy – loss of a significant electricity supply and heat.
  8. The situation is not really different to the scenario which we are avoiding i.e. one where we build more nuclear in order to supply heating that is then generated electrically. In our district heating future, nuclear power won’t be the only heat provider to our networks and we’ll have large thermal stores with electricity as a back up so we won’t be dependent on nuclear as the heat supply. It will cause a serious problem if nuclear goes but only in the same way that it would have done had we gone down the electric heating with triple the nuclear capacity route.  Furthermore we decrease the chances of a nuclear accident as we have to build less nuclear plant so there is less nuclear around to go wrong in the first place.

    If, as we build up district heating, it turns out that, happily, we’re not going to go down the nuclear route then district heating is flexible enough to be able to take heat from a variety of sources whether that be large scale solar thermal (as is common in Denmark), geothermal  or surplus renewable electricity or hydrogen. If we go down the route of switching our heating systems to electricity and digging up the streets to put larger cables in then we won’t have that flexibility.

  9. Waste heat from power stations isn’t really free because when you start using the waste heat from a power station you sacrifice some electricity production.
  10. This is true but the amount of electricity you sacrifice depends on the design of the district heating system. Specifically it depends on the temperature at which you reject heat compared to the temperature you were rejecting heat before you made the connection. So let’s say your nuclear power station was rejecting heat at sea water temperature – as most do. You then connect up to your district heating system and the temperature of the water returning from the city’s radiators and hot water cylinders is the temperature at which you are now rejecting heat from the power station. In a conventionally designed system the “Z factor” i.e. the heat used divided by the electricity lost might be as poor as 4. However by optimising the design of the district heating to achieve a low return temperature then you can significantly reduce the electricity loss and increase the Z factor.  That’s why the various innovations put forward in William Orchard’s Hub concept are so important. If you can get the return water temperature down to approaching sea water temperature then you minimise this electricity loss. A further factor is the design of the power station’s turbine and the way heat is extracted from it (again William Orchard and Robert Hyde have come up with innovations here). This will be the subject of a separate blog so I won’t dwell on it here but taking all these Hub innovations together means that the electricity production lost can be as low as 1/14 of the heat supplied to district heating. If you thought of CHP as a sort of heat pump then that would be a Co-efficient of Performance (COP) of 14 – rather better than the COP of 3 we rather optimistically think of heat pumps achieving today.

It’s strange to write a blog proposing something you don’t really want but life is full of compromises. The advantage of going down the route where we allow for the possibility of nuclear CHP is clear: either nuclear does get built and less nuclear capacity is needed or it doesn’t and we have district heating that can draw its heat from a variety of other sources.

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