Air Source Heat Pumps (ASHP)
What are they?
Essentially very large refrigerators in reverse ie. the hot coils on the back are for heating your house and the cold from the inside is taken away by the air outside the house.
What is the trick?
For every kilowatt hour (kWh) of electricity you use to drive the ASHP you can get more kWhs of heat, up to 3 times more, for your house. This mark-up is known as the COP (coefficient of performance). This is a much better idea than a fan heater where 1kWh of electricity gives you 1kWh of heat.
Isn’t electricity expensive?
It is, but not if your alternative fuel is half the cost of electricity per kWh of heat4. So for the cost of 1 kWh of electricity, you could buy 2 kWhs of heat from your normal fuel but 2.8 kWhs of heat from your ASHP if it is working at a COP of 2.8.
How are they fitted into my house
The ASHP can be regarded as a substitute for your boiler with a few differences. The main unit is designed to sit outside the house, preferably near to where your old boiler was located.
Inside the house is a cylindrical heat exchanger which transfers the heat into the hot water of the radiator system and to heat your hot water.
Any drawbacks?
One or two.
- The hotter you run your radiators, the lower will be the COP. The ideal temperature is 300C to 350C which means that your existing radiators will be too small as they are designed to work at about 650C. The solution is to install under floor heating or increase the area of your radiators. The first way is expensive and probably only worth doing if you are renovating a house. The second way is limited by the amount of wall area you have available although modern double panelled and finned radiators go a little way towards the solution.
- The colder the outside air, the lower will be the COP. Studies by the Energy Saving Trust1 & 2 give a range of 1.3 to 3.0 with a seasonal average of about 2.2
- An ASHP unit specification is usually quoted at a COP of 2.8. In other words, a 14 kWh unit (for example the Mitsubishi Ecodan HW140) will consume 5kWh of electricity. When the outside air temperature drops and the lower COP of 1.3 is encountered, the heat output is 5 x 1.3 = 6.5 kWh. So you will need a back-up heating source. You will also need an immersion heater as back-up.
So are they worth having?
The proposed Renewable Heat Incentive (RHI) is due to take effect in October 2012 but will apply retrospectively to installations from 15th July 2009. The RHI only applies to houses not on the gas grid. Under the RHI it is expected that3 a tariff of 7.5 p per kWh of heat produced (not electricity consumed) will be paid for a period of 18 years. This will be very advantageous as the net cost of electricity (at a COP of 2, say) will be zero4. Depending on the cost of installation, a payback time of about 10 years can be expected for a typical project but could be less. However, don't jump the gun as it is only a proposal at the moment and the legislation won't be known until April 2012.
Will my carbon footprint improve?
Not very much. Grid electricity has gone up from 0.42 kg of CO2 per kWh to 0.55 and is forecast by the Government to rise to 0.59. With an overall COP of 2.2, this would give an ASHP a footprint of 0.27 which compares with a condensing boiler of about 0.22. However, in time it is expected that the electricity grid will be decarbonised as more renewable sources come on stream and old coal fired power stations are phased out or converted to carbon capture and storage (CCS).
Is it possible to improve the COP?
There are two ways. The first is to go for a ground source heat pump where the source temperature does not vary like the outside air temperature. These are more expensive to install but can achieve COPs up to 3.6. (see: Ground Source Heat Pumps)
The second way is under development and involves pre-heating the air with a conventional fuel. This may seem a little counter-productive but the ASHP is providing a top-up service at very efficient levels with an efficient condensing boiler doing the donkey work.
References
1. Energy Saving Trust – Heat pump field trial
2. Report on the Scottish Renewable Heating Pilot (overview):
http://www.dimplex.co.uk/assets/Downloads_Documents/EST_Scottish_Renewable_Heating_Pilot.pdf
3. For the RHI expected deal:
http://www.rhincentive.co.uk/eligible/levels
4. For comparative energy prices:
http://www.biomassenergycentre.org.uk/portal/page?_pageid=75,59188&_dad=portal
Other Renewable energy sources: