28 Nov 2007

Micro CHP

Combined Heat and Power (CHP) is a low carbon technology which produces both hot water and electricity. It’s been used mostly at the commercial level for some time but there is a small domestic version, known as Micro CHP.

To date, it’s not been staggeringly successful. The proposition isn’t that appealing, at least as far as homeowners are concerned. You shell out about two or three times as much as your would for a gas boiler, and you get some of your electricity requirement thrown in with your hot water. Whilst large and medium sized CHP plants are doing rather well, down at the micro level things have come becalmed, especially as the only one commercially available, the gas-fired Whispergen, pictured here, has temporarily ceased production.

Thanks to the boys over at Carbon Limited who have flagged up a report from The Carbon Trust on the micro CHP. It has drawn some interesting conclusions in comparing the performance of 87 Micro CHP units with 27 condensing boilers over a four-year period.

• Key finding is that in order to operate effectively, micro CHP has to be in a situation where it can run for long periods uninterrupted. If the system cycles on and off frequently, it ends up using more electricity than it generates.

• Essentially this means that they are suitable for sites such as residential care homes and leisure centres, where there is a reasonably large and consistent hot water demand, In these instances, there can be significant savings — the report quotes 15% to 20% — relative to using condensing boilers

• But in housing, micro-CHP advantages are marginal at best. The demand has to be significant to make any savings, so it may be a runner if the house is very large or very old and uninsulated. Think listed manor houses – that sort of thing. The cut-off point identified by the report is a heat demand of more than 20,000kWh/annum, which would apply to almost all 20th century housing over 200m2 internal floor area (say five bedrooms).

• One of the main issues with today’s generation of micro CHP is that they only produce 1 unit of electrical power for every 10 units of hot water. This doesn’t match general domestic use, which is more like 1:3. Not until new technology kicks in (fuel cells anyone?) will a micro CHP plant start producing a better balance.

• Having said that, there is a good match time-wise. Peak electricity and hot water demand tend to occur at the same time (think dark winter nights), unlike technologies such as roof mounted PV arrays. So there is every chance that the electricity you produce, you will actually consume rather than having to export it to the grid at knockdown prices.

• The report also has some factoids about condensing boilers. It concludes that they achieve efficiencies about 5% less than their SEDBUK rating would suggest. And also that the electrical controls use large amounts of power to run the pumps, fans and control systems. Some designs are worse than others and the difference is significant. In some instances, the electrical consumption associated with condensing boilers may account for 15% of the household electricity bill.


  1. "Key finding is that in order to operate effectively, micro CHP has to be in a situation where it can run for long periods uninterrupted. If the system cycles on and off frequently, it ends up using more electricity than it generates."

    In that case, it's just badly designed. There's nothing inherent about this. Also: big hot water cylinders?

    According to this:


    "82 per cent of energy used in households is for space or water heating."

    So actually, that's pretty favourable.

    That said, I don't actually understand the argument that the proportions are relevant. Provided that the sale price for electricity is the same as the purchase price, then (for a grid-tied system, and it amazes me they don't make off-grid versions) all that matters is whether the saving (or profit) from running the thing justifies the capital cost. It's a very simple economic calculation.

    Clearly the higher the gas usage, the more profit it will make (since output is obviously governed by the hot water demand), so that part does make sense.

    I think the real reason these haven't caught on is that they're too expensive and not very good. Get someone in China to make them for £25 and you're on to something.

    As you probably know, there was a company called Microgen making a song and dance about these a while ago, but unfortunately they forgot the golden rule of manufacturing (make something), and appear to have disappeared. Do you know anything about that?

    passivhausfan (from greenbuildingforum)

  2. Passivhausfan,

    I am aware that others are ploughing this furrow but I don't know enough about the field to know who is doing what, and when products will become commercially available. I get the impression that single house scale CHP is proving to be problematic - a lot of engineering for minimal savings.