Combined Heat and Power versus Heat Pumps?
Over Christmas, I read about two thirds of the "Sustainable Energy" book by Prof David MacKay. It's an excellent book; lots of great info and written in a friendly, witty style.
To cut to the chase... I'm writing to flag up (just in case you haven't already read it) MacKay's argument that decentralised energy production isn't necessarily greener (or more robust) than centralised production. In particular, MacKay makes the argument that gas-powered CHP (of the variety that Southwark Council are keen on) isn't the most efficient way to produce heat and electricity. The bottom line of MacKay's argument is that it produces less CO2 to produce electricity in large, centralised gas-fired power plants and to heat houses using a ground-source or air-source heat pumps than it does to produce both heat and electricity in district-sized gas-fired CHP plants. This is due to two factors: 1) Heat pumps have efficiencies of between 300-400% (i.e. for every kilowatt hour of electrical power sent to the heat pump, you get 3-4 kilowatt hours of heat out) and 2) District-sized CHP plants produce electricity less efficiently than large gas-fired power stations. I really haven't done his arguments justice. The section about CHP versus heat pumps starts on p144 (in section 2) (which corresponds to p156 in the whole-book PDF available for free download here: http://www.withouthotair.com/download.html ). The section concludes:
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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.
Heat pumps are superior in efficiency to condensing 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% efficient,” 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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My conclusion: can we reduce the energy we consume for heating? Yes. Can we get off fossil fuels at the same time? Yes. Not forgetting the low-hanging fruit – building-insulation and thermostat shenanigans – we should replace all our fossil-fuel heaters with electric-powered heat pumps; we can reduce the energy required to 25% of today’s levels. Of course this plan for electrification would require more electricity. But even if the extra electricity came from gas-fired power stations, that would still be a much better way to get heating than what we do today, simply setting fire to the gas. Heat pumps are future-proof, allowing us to heat buildings efficiently with electricity from any source.
Nay-sayers object that the coefficient of performance of air-source heat pumps is lousy – just 2 or 3. But their information is out of date. If we are careful to buy top-of-the-line heat pumps, we can do much better. The Japanese government legislated a decade-long efficiency drive that has greatly improved the performance of air-conditioners; thanks to this drive, there are now air-source heat pumps with a coefficient of performance of 4.9; these heat pumps can make hot water as well as hot air.
Another objection to heat pumps is “oh, we can’t approve of people fitting efficient air-source heaters, because they might use them for airconditioning in the summer.” Come on – I hate gratuitous air-conditioning as much as anyone, but these heat pumps are four times more efficient than any other winter heating method! Show me a better choice. Wood pellets? Sure, a few wood-scavengers can burn wood. But there is not enough wood for everyone to do so. For forest-dwellers, there’s wood. For everyone else, there’s heat pumps.
I'm sure you're already aware of this argument but I just wanted to forward this info to you just in case it's useful.
Warmest regards,
Jack
#1
That's really helpful Jack [no I wasn't aware of these arguments].
This puts into context the Foresight report I mentioned yesterday - it was informed by over 30 "state of the science" reports, and concluded that since it isn't known how any of these developments will actually pan out, the wisest course of action is to use the full range of technologies for low-carbon energy generation.
Will start reading Prof MacKay's book tonight [it only arrived yesterday!] Sounds like it will really help inform our understanding of what's best to do.
Bw
Anna
#2
mmm...but when you consider the loss of electricity in transmission you might get a different story. I think there are counter views to McKay's (aren't there always!) - George Monbiot for example has written on this.
This exchange flags up the need for a blog. This discussion is limited to us, I think it needs to be wider. And I think a facilitator of the discussion would be useful too.
carl
#3
Yes, I definitely agree that a blog / public forum would be good. It's coming, I promise!
MacKay takes the transmission losses into account (and the losses associated with turning the gas into electricity).
Thanks,
Jack
#4
I've been having an email conversation with Mr Bob Fiddik (Sustainability Manager, Southwark Council) and Mr Fiddik has very kindly allowed me to reproduce his reply here on the website in case anyone's interested...
#5
Hi Jack....
Belated Happy New Year! And, yes I did write Southwark's climate change strategy.... and its always a pleasant surprise when someone's read it, let alone been complementary....so many thanks.
I heard about the Prof's book last year on BBC R4, but only started reading it following your suggestion....so thanks. It's a great read...but what I really like is his combination of insistence on real numbers and a totally open approach - free download, encourages feedback and debate.
When it comes to the section on heat, I'm afraid I think the Prof makes some pretty misleading comparisons. Firstly, it is not clear to me whether he's mainly refering to micro-CHP, or large scale neighbourhood or city wide CHP. While he likes to point to best available and future performance of heat pumps, he doesn't consider the same for CHP....which covers a very wide range of technologies.
He states that CHP locks us into gas - while heat pumps are more future proof.
Simply not true, unless you really are just talking about micro-CHP which is either a micro-gas turbine or gas Stirling engine sitting in your house. I agree with the Prof. that such technology can't achieve the system efficiencies of large scale CHP because the demand patterns for heat and power in a single dwelling are so different and seasonal. On an economic basis, you also have an expensive bit of kit doing nothing when you're out of your house. And of course they still use gas.
Medium to large scale CHP can be fuelled by anything that can heat up water to make steam to go through a turbine - you could turn a nuclear power station into CHP if you want. Nuclear stations are at best around 33% efficient (heat output to electricity) because all they do is boil water. But you can also take one of the Profs. "best available" combined cycle gas turbines and adapt this for CHP. In both cases, the Prof is of course right that you lose electrical generation...but I'll get back to this. The issue is that you can burn coal, gas, wood, straw, waste or nuclear fuel to make steam (or in the case of gas go through a combined cycle) and all of these can be operated as CHP. Fuel cells can also be considered as a form of CHP, taking chemical energy and converting this directly into electricity and heat (without the usual thermodynamic losses) – but at the moment still very much non-commercial.
But CHP shouldn't be considered in isolation of the energy networks that make it work. The best long-term energy networks are those that don't lock you into using a specific fuel. So, the Prof is right, electricity is a good choice, electricity is an energy "carrier" not a fuel. Gas is bad, for obvious reasons, it is a limited fuel and long term future of prices is upward... but this is how the majority of the UK now gets its heat and hot water.
CHP (beyond single buildings) needs a hot water network to work, and this is also an ideal energy carrier – it is what is actually used in buildings for space heating, and hot water itself – and is not locked into any particular fuel. The restriction on district heating is that you can’t transport it really long distances because of pumping costs. But in Denmark where they have the good sense to site medium scale power stations near to cities, it is viable to pump the waste heat several kilometers.
Key argument is that best gas generation + best heat pumps easily beats all CHP configurations in efficiency
The Prof's argument with the efficiency diagrams comparing the best fossil case - gas CCGT generating electricity supplying heat pumps - is also pretty misleading. For one, after saying that CHP locks you into gas, his best fossil fuel solution does the same. You can't have the high CCGT efficiencies (and hence, low CO2) without gas turbines. This is the first problem, we're never going to have a centralised electricity generating fleet comprising only the most efficient gas CCGT. We’re currently seeing new applications for coal plant, like Kingsnorth which will be at best 45% efficient, with a fuel with high CO2 content.
His overall view, like Sir David King’s is that we should move to an “all electric UK”. This inevitably requires massive expansion of nuclear and investment in the network. I don’t have any ideological problems with nuclear, but the economics are just so opaque. I don’t know of any nuclear power programme in the world that isn’t primarily financed by the public sector (e.g. state owned EdF buys British Energy which UK government had a majority stake).
The laws of thermodynamics constrain both heat pumps and generation turbines. For heat pumps the smaller the temperature difference between the heat source and internal building the higher the efficiency or COP. Hence a ground source heat pump feeding underfloor heating has a much higher COP than an air-source pump feeding standard radiators. For generating turbines you’re trying to turn heat into work and this process is most efficient when the temperature drop is greatest. When you turn a normal power station (whether CCGT or not) into CHP you have to take the cooling water off at a higher temperature (around 100 – 120 degC for district heating) and this decreases the efficiency and electricity output from the plant.
However, the fair figure for comparison with heat pumps is - what heat output do you get from turning pure electricity generation to CHP for the sacrifice made in electricity output? The answer is at least 10 times – i.e. a COP of 10….much better than heat pumps.
Out of interest I looked at the figures for the SELCHP waste to energy plant in Lewisham which was originally intended to be CHP, but is currently optimized for electricity generation. If you take the water out at 110 deg C for district heating you lose 19,685 MWh/year of electricity, but gain 196,852 MWh/year of heat….that’s a COP of 10 (from a plant over 15 years old, with not the most sophisticated design).
Finally, all technologies have to demonstrate their promise in real applications. It’s hard to find case studies of actual heat pump installations – e.g. post installation monitoring over at least a year to see what the real seasonal COP is. I believe the EST has just embarked on such a trial….so I look forward to seeing the results. Don’t get me wrong, I think heat pumps are a fantastic low carbon technology – but not for urban areas. And until the CO2 of grid electricity is low enough, we should be prioritising ground source heat pumps and low temperature systems (like underfloor) to ensure maximum COPs (i.e. new build in non-urban areas).
Sorry for the essay...but felt I needed to do Prof. MacKay's arguments justice.
Regards
Bob
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