The efficiency of a heat pump is dependent upon the temperature to which the building hot water is heated, and with air source heat pumps, the performance falls with falling external temperatures. As the efficiency or co-efficient of performance (COP) decreases, the economic advantage of running a heat pump diminishes. Furthermore with a reducing COP the carbon emissions increase to achieve the same energy output.
In considering different fuels it is important to be aware of the carbon emission differences. For example gas has 0.194kg of CO2 related to every kWh used, whereas electricity has 0.422kg per kWh, this is 218% more.
These systems can be highly efficient delivering up to 250% seasonal efficiency. However, as the external temperature falls the operating efficiency falls too.
The significant drawback with ASHP is that you must consider what fuel it is replacing. If it is in place of a conventional gas system it could actually be worse since the change from gas to electricity as a primary fuel source means an increase of CO2 concentration per kWh. Gas consumption emits 0.194kg of CO2 for every kWh used, whereas electricity generation emits 0.43kg per kWh, more than 200% increase! The system improvements have to counter this before any savings can be made.
Running Cost
The break even cost point can be determined by the following formula. It indicates that heat pump plant should have a COP greater than 3.01 for the operating cost to be more favorable than gas.
- Electrical cost p/kWh / Gas cost p/kWh x boiler efficiency = COP for energy equality
- 11.41p / 3.52p x 0.93 = 3.01 (COP)
- Typical energy figures August 2009.
Carbon Comparison
To determine the break even point for carbon emissions use the following formula. It indicates that heat pump plant should have a COP greater than 2.02 for the carbon emissions to be more favorable than gas.
- CO2/kWh of electricity / CO2/kWh of gas x boiler efficiency = COP for carbon equality
- 0.422kg CO2/kwh / 0.194kg CO2/kwh x 0.93 = 2.02 (COP)
- CO2 figures from Part L 2006.
Homemicro article: Sizing a Buffer Vessel for use with an ASHP
View energy prices statistics at Department of Energy and Climate Change details here
Greenhouse gas conversion factors for company reporting can be found on the DEFRA website
BSEE magazine article on ASHP - ASHP's compared to condensing boilers
Energy Saving Trust article Generate your own energy with Air Source Heat Pumps
