Thursday, 23 February 2012

Do Heat Pumps Deliver?

Do heat pumps actually deliver the energy and carbon savings that housing professionals and energy assessors predict?  A study by the Energy Saving Trust (EST) appears to show that they don't.

The measure of the performance of a heat pump is the coefficient of performance (COP).  This is the ratio of the heat energy that it delivers to the electrical energy that the unit consumes to do so.  An electric fan heater or storage heater would have a coefficient of performance of 1.0 - every unit of electricity consumed in the device ends up as a unit of heat energy in the building.  Heat pumps use electricity more intelligently than this, driving a condensation cycle which cools the ground or air outside a building and delivers the energy as higher temperature heat inside.  The technology is similar to a refrigerator, which transfers heat from inside the fridge to the room outside using the same principle.

For a heat pump to produce a carbon saving relative to a gas-fired boiler in the UK, it needs to be operating at a COP higher than 2.35.  This is because grid electricity has a high carbon-intensity.  When you consume a unit (kWh) of electricity, 0.52kg of carbon dioxide are emitted at a power station.  When you use a gas boiler to produce a unit of heat for your home, around 0.22kg of carbon dioxide are emitted from its flue.

70% of homes would have had lower carbon emissions with a gas boiler

The chart above shows the range of COPs measured in the EST trials (white) compared to the range of COPs claimed by manufacturers (orange) and used to calculate household energy for building regulations.

Fully 70% of households in the EST field trial would have lower carbon emissions from heating their properties if they had instead selected much cheaper gas-boilers.  It also appears that building regulations calculations are over-stating the efficiency of heat pumps by around 60% compared to the real-life examples in the trial.

Can any professionals from heat pump companies offer an explanation for the EST trial results, or highlight errors in this analysis?  Please leave your comments below.  Check workings here if you like.


Note Added 28/03/2012:

Thanks for all the comments to the original post.  The explanation appear to be that  heat pump performance is strongly diminished if it is:
  • attached to inappropriately sized heat emitters (for example too small radiators)
  • sized wrongly for the heat load
  • not used correctly (for example run hard for short periods)

Following on from the EST trial, a group of trade associations created a document to help with the first two issues.  The heat emitter guide helps installers of heat pumps ensure that they achieve more optimal system design and achieve heat pump performance that approaches the laboratory test conditions.

8 comments:

  1. I'm told by a prospective heat pump supplier that COP varies with ambient temperature at the heat exchanger (mainly a factor with ashp, less so for gshp) so maybe the 'real world' data reflects the fact that units mostly operate when air is cold & the cycle is less efficient than at test conditions. Newer ashp equipment is supposed to maintain most of its nameplate COP down to about -20C.

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  2. The problem is typically not the COP of the heat pump itself, but the poor working of the entire system. Many good heat pumps are installed in a way that the efficiency of the system over the whole year (often called "seasonal performance factor") remains very low, thus using more primary energy than a good (condensing) gas boiler.

    EST explains the "system efficiency" in its study:
    The amount of heat the heat pump produces
    compared to the amount of electricity needed to
    run the entire heating system (including domestic
    hot water; supplementary heating; and pumps).
    This report’s conclusions and recommendations
    are based on the measured system efficiency.

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  3. Whilst the EST results appearing to be damming of heatpump technologies, when the results were analysed they have clearly shown that almost exclusively the biggest cause for the reduction on the COP was due to the auxillary heater(s) kicking in. These heaters are mainly electrically driven and therefore have diluted the COP with a significant degree of direct electric heating from these heaters.

    Why?

    Overall it was proven to be down to a lack of space heating and hot water design knowledge, ultimately leading to heatpumps being installed where either the building was not suitable, or had a higher heatloss demand than the heatpump could satisfy without using its auxiliary heater(s)

    What has been done?

    The heatpump standard in the UK used by most installers is MIS 3005 for the MCS scheme, this standard has been extensively rewritten (now at Vesion 3.1a) and is nearing the end of its phasing in period so all installers should be operating to this very shortly.

    The updates give specific requirements to ensure that wherever possible the auxiliary heater does not get used for the space heating, unless the outside ambient air temperature is below that of the external design temperature which is also specified within the standard.

    More information and the standard can be obtained here: http://www.microgenerationcertification.org/installers/installers

    I hope that this provides a useful insight into the disparity between test COP and installed COP for these units

    You can find out more about this topic during my seminar on heatpumps at Ecobuild on Wednesday 21st march - Sustainable energy and business opportunities from heat pumps: win-win situation?

    Cheers

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  4. Field trials on the continent have shown better results in terms of the Seasonal Performance Factor (SPF) achieved, see
    http://www.ise.fraunhofer.de/en/press-and-media/press-releases/press-releases-2011/electric-heat-pump-passes-long-time-field-tests-fraunhofer-ise-presents-final-reports-and-commences-new-long-time-tests

    But it's important to keep in mind that you need a CoP of around 3 to just "break" even with mains gas in terms of money and CoP.

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  5. Excellent and useful article! Thanks for taking the time to post this.

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  6. HEAT PUMPS - HEATING AND COOLING BUILDINGS
    www.geotermiaitalia.it/catalogo/prodotti.html

    Geothermal heat pumps;
    Air-water heat pumps;
    Exhaust air heat pumps.

    They are energy efficient complete systems for heating and cooling which is both economic and environmentally sustainable .
    This technology is available both for residential single / multi-family and industrial and commercial buildings.

    The geothermal heat pump uses as a heat source the subsoil or ground water and allows a cost savings for the heating up to 80%.
    You do not need to reside in a thermal area to enjoy the benefits of underground heat: from 10 m depth the soil temperature remains almost constant throughout the year, and around 100 meters the temperatures are around the 10-12 °C constituting a heat source for the optimum operation of the heat pump.

    The air-water heat pump uses as a source of heat the outside air and allows a cost savings for the heating up to 50-60%.
    It can produce hot water for heating or sanitary uses up to temperatures of 65 °C with only the compressor and work up to temperatures of -25 °C!

    The exhaust air heat pump uses as a heat source stale air present in the house, allows a cost savings for the heating up to 50%.

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  7. Heat Pumps are important!

    ReplyDelete