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.