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    • #30992
      Nick Grant

      A friend just sent me this link.

      Sustainable Energy – without the hot air

      A popular book by David J.C. MacKay

      Professor of Natural Philosophy
      Department of Physics
      University of Cambridge

      To be published on paper, December 2008. [This book will always be available for free on the internet.]


      Just started reading it but so far it is a nice example of doing the numbers and presenting them at a personal scale.

      There is a podcast talk with some slides as well.


    • #35506
      Tom Foster

      Seems overwhelmingly about the supply-side of energy, with only a nod to (limited) opportunity for major demand reduction. The whole thrust of the book is about satisfying our barely-abated current demand, and so gets it fundamentally wrong, i feel, though as a quarry of info it's terrific.

    • #35507

      I agree that the most interesting parts are largely about the supply side. But he does say quite a bit about transport and heating etc. In his plans (p204), he says that transport energy will be halved and heating will be cut by a quarter so I don't think you can say he is ignoring demand reduction.

      I don't know whether he says it in the book but in a presentation of his, he said he doesn't want to consider either population reduction or lifestyle changes because they are political and difficult and so outside his scope. The worthy subject of somebody else's book, I think.

      His discussion about buildings barely mentions Passivhaus though, implying it's an aim rather than mainstream reality. His comparisons between for example, his own house with the thermostat turned down and a Passivhaus run at 20 C are also misleading. Clearly it's possible to turn down the thermostat in a Passivhaus and it will achieve better results too.

      The issue that has galvanised me though is his attitude to heat pumps; he believes they should be used now in preference even to gas boilers. p151: “Let me spell this out. 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”. He claims most opponents' views are based on out-of-date performance figures. I'd be interested to know what the AECB thinks?

      BTW, I just came across a press release from 2008-12-04

      Cheers, Dave

    • #35508
      David Olivier

      Yes, the reported COPs are quite good. Much better than the few figures measured in the UK and slightly better than a large number of older figures measured in Switzerland.

      Unfortunately, despite putting himself forward as a energy expert McKay doesn't seem to be very familiar with combined heat and power. Here are some empirical figures from Germany and Denmark respectively:

      (1) The above ground source heat pump study (source: Fraunhofer)
      To produce 1 kWh of heat, 0.8 kWh of fuel must be burned at the power station (on the basis of the UK electricity generating system, which has an overall efficiency of 0.36 and includes a mixture of gas- and coal-fired plants)

      (2) Today's CHP system in Copenhagen (source: Danish Board of District Heating)
      To produce 1 kWh of heat, 0.12-0.13 kWh of fuel must be burned at the power station (it's modified to reject heat at 70 or 80 degC instead of sending cooling water to the sea or river at 25 or 30 degC; the electricity output sacrificed is taken into account in working out the fuel burned per unit of heat).

      So once we go beyond gas boilers, for buildings in towns CHP would appear to be more energy-efficient than heat pumps. It's able to use other heat sources after gas is no longer available, such as solar and geothermal.

      Also, too many ground source coils in towns could cool the ground too much.

      I e-mailed McKay on this subject but didn't receive a reply.


    • #35509
      David Olivier

      Too far-reaching I suspect!

      Government reports in 1979/80, Energy Papers 20 and 35 I think, concluded that CHP and district heating would be a more economic option than electric heat pumps in the future when piped natural gas was no longer freely available. The other option they studied was synthetic gas from coal (this was before climate change was on everyone's mind.)

      Very detailed engineering studies were done of where the pipes might go in large cities, much more detailed than any study today of heat pumps. The only omission was that they didn't use standard Danish technology such as direct connection and supply temperatures of max. 70 or 80degC which reduce piping costs and make it economic to connect smaller loads and less dense areas to heat mains.

      Empirically I suggest that the Danish figures empirically refute McKay's suggestion to optimise plant for electricity generation alone and operate heat pumps(s) off it. Also if air source heat pumps had to be used in cities their COP would be lower than 3.7.

      I recommend reading a good text on themodynamics, perhaps starting with Wikipedia which explains what determines the performance of heat pumps and heat engines. CHP can be analysed in terms of a virtual heat pump; e.g. see http://www.energypolicy.co.uk/energy%20solutions/Energy%20Solutions%20Figures.pdf

      The difference between real and theoretical performance of CHP systems and heat pumps is down to factors such as scale effects (large machines are more efficient than small ones) and the number and design of heat exchangers (each one creates a temperature difference and reduces overall efficiency).

      I calculated the electric grid overall efficiency when I was drawing up the CLP Energy Standards. Valid for period 2004-06. Needs to be reviewed if the use of coal-fired plants continues to grow compared to gas.


    • #35510

      I honestly don't think my problem in understanding is down to my admittedly poor knowledge of thermodynamics. My first degree was in Mathematics so I'd be the first to say that others who read physics, engineering or even chemistry are likely to have a better understanding. But I do have various textbooks to refresh my aching brain when necessary and yes, Wikipedia has some good content.

      My problem, I think, is finding reliable and preferably peer-reviewed sources of figures about performance in practice.

      For example, I know of the Danish success with district heating. But I don't know the author, title and year (or URL 🙂 ) of a paper or other document that contains the detailed numbers for quantity and efficiency of electrical generation achieved and quantity of heat delivered to buildings. For example, their web statistics page http://www.dbdh.dk/artikel.asp?id=480&mid=24 concentrates on heat production rather than consumption and on sources of fuel rather than efficiency. So I would be very grateful for some guidance to the appropriate results.

      I think you're right that the numbers of the energy papers were 20 & 35, and possibly also 53 is relevant as well. I haven't been able to find them online though and I suspect that their efficiency projections may now look dated. If you're sure they're relevant, I'll request paper copies.

      In regard to operating air source heat pumps in cities, I expect you're considering locations where they provide heating? Clearly they're already very extensively used for cooling in cities. I don't know if anybody has modelled the decrease in temperature and/or increase in wind speed that would result from large-scale use of ASHP-heaters. I suppose the Japanese might be approaching a real-life experiment now.

    • #35511

      On reflection, I don't think there's a problem with ASHPs for heating in cities, is there?

      The heat pumped into a house will be equal (a bit less than) the heat lost by the house, mainly to the air. The heat taken from the air will be less than the heat pumped into the house by the power used by the compressor etc. So the net effect should be to heat the air somewhat, but less than the house would do if heated by say a gas boiler. A similar change to the effect of adding more insulation. There's no depletion effect as there is with a GSHP.

      Or did I miss something?

    • #35512
      Nick Grant


      I think David was referring to GSHP cooling the ground.

      David, I did a couple of years of thermo at Uni but would not claim that anything more than the basics has rubbed off on me.

      Could you explain this wrt CHP:
      'To produce 1 kWh of heat, 0.12-0.13 kWh of fuel must be burned at the power station'

      Glad the book has sparked off discussion. What I liked was the approach plus I warmed to sentences such as:

      'Roof-mounted micro-wind turbines areanutter waste of re-


    • #35513


      I don't pretend to fully understand it but I think the 0.12 kWh is calculated using the virtual heat pump idea David mentioned, in so far as I can deduce what that is from the diagram he referenced. The way I understand it and using the diagram on p150 of MacKay's book as an example, consider the green square point “30%-efficient electricity, 80%-efficient heat” and the blue circle on the axis “Best Gas”. They can be interpreted as the results of burning a set amount of fuel in two different ways. The green square produces less electricity, so we could burn extra fuel to make it produce the same quantity of electricity. It's basically that extra quantity of fuel that is being discussed, but first it is normalised so that we consider a total heat production of 1 kWh.

      So it's an alternative figure of merit to the COP. It looks complicated the way I've written it, so I'm sure there's a better way to express it. But my main concern is that there's more than one candidate for the pure electricity generator baseline. The rational one to my mind is the best available generating plant, as I chose. But that would of course change over time as plant designs improve. Another possible choice would be the CHP plant itself, if it were capable of being operated to produce just electricity. But it might not be a very efficient plant operated in that way. A third possibility would be that there is some kind of standard used to make such measurements.

      The original source of the data would hopefully include sufficient details to be able to work out exactly what the 0.12 kWh means, if we could find it.

      My posting about ASHPs was responding to David's comment that “if air source heat pumps had to be used in cities their COP would be lower than 3.7”. I think the possibility of depletion problems if GSHPs are widely used in cities is one thing David Olivier, David MacKay and I all agree on. But I don't know what problems ASHPs have that limit their COPs only when operated in cities.

      Cheers, Dave

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