Most heat pumps in the UK use the ground as the source of heat. A typical Coefficient of Performance (CoP) quoted for such units is 4. This would be for an output temperature of 35C, for a more useful output temperature of 45C it falls to less than 2.8 (lets be optimistic and use a value of 3). So, a CoP of 3 would deliver 3 kWh of heat for every 1 kWh of electricity used to extract it.
So as to have something to compare to we will use one of the most common space heating solutions in the UK - the gas boiler. A modern boiler will have a CoP of 0.9 or more (SEDBUK A) - so, for every 1 kWh of gas 0.9 kWh of heat is output.
Let us say that the annual space heating requirement for this example is 10,000 kWh. This would be typical for an existing 3 bedroom semi-detached house (100m2) which has been upgraded to a reasonable thermal performance or a larger new energy efficient house.
To supply this amount for heat we need to look at how the energy is generated from the primary fuel and transported, not just the efficiencies once it arrives at the doorstep.
As heatpumps are electrical devices we need to check how efficient the power generation is as well as the transmission involved in getting it to its end destination. Efficiencies of power stations vary of course by fuel type and age of generating technology but we can probably use an average efficiency of 40% with reasonable confidence. So, for every 1 kWh of fuel burnt we can expect 0.4 kWh of electricity. Transmission losses are equally variable depending on where you are located in relation to the power grid and the condition of the local network but we can probably use an efficiency of 93% as a typical value. So, for every 0.4 kWh of electricity generated 0.372 kWh of electricity is delivered to the user. For a heatpump with a CoP of 3 to deliver 10,000 kWh of heat you would therefore require 3,333 kWh of electricity (about £400). To generate this electricity would take 8,961 kWh of primary fuel (say gas). A direct CoP of primary fuel to heat of 1.12.
The same example for a gas condensing boiler works out like this. The gas distribution system has an energy cost of about 2.1%. So for every 1 kWh of gas held in store 0.979 kWh of gas are delivered to the doorstep. With a SEDBUK A rated boiler having a minimum efficiency of 90% we end up with a figure of 0.881 kWh of heat generated for every 1 kWh of primary gas. For this boiler to produce 10,000 kWh of heat would require 11,350 kWh of primary gas. A direct CoP of primary fuel to heat of 0.88.
It looks like there is a 27% benefit, in energy terms, of using a heatpump. Taking fuel costs into account balances the equation slightly differently. Prices vary but lets use 12p/kWh for electricity and 5p/kWh for gas. The annual cost of running the heat pump would be about £400 and the annual cost of running the gas boiler would be about £550. If we take the hardware cost into account by simply saying the lifespan of both options would be 15 years: a heatpump plus coil costing £8000 and a gas boiler costing £1500 we get to an annual cost of £533 & £100 respectively (for simplicity we will assume other installation costs and maintenance to be equal). This brings the actual annual cost of running a heatpump to £933 and £650 for the gas boiler. With sustainable technology, as many things, it is best to spend your money where you get the biggest return first. A solar hot water system can, for example, be installed for around £3000 and yeild a contribution to the annaul water heating load of 2000kWh. This amounts to an annual raw energy saving of about 650 kWh per £1000 investment. In comparison a heatpump gives a 1039 kWh saving on raw energy per year for an £8000 investment which equates to 130 kWh per £1000 investment.
Another point worth considering is the fact that we are using an ever increasing amount of electricity in our domestic lives and the infrastructure to supply this electricity is struggling to keep up. An average household in the UK will use about 3000 kWh of electricity a year for lighting and appliances (ie no cooking or space/water heating). By installing a heatpump for space heating this figure would be doubled to about 6000 kWh per year. If 1% of the UK's 25 million households install heatpumps, the grid would need to supply an additional 750GWh of electricity per year. To put this into perspective the total electricity consumed by the UK domestic sector in 2003 was 118,754 GWh. 750GWh represents the output of a 100MW power station (small/medium sized gas power station). Alternatively this increase could be met with 200 wind turbines rated at 500kW each. It can be argued that if the electricity used to power a heatpump is generated using renewable energy technologies then it becomes a clearer choice. In the UK I wonder if this is true as every kWh of renewable electricity used to provide heat is one less kWh to offset against fossil fuel generation? With this in mind, should we be using high grade electrical energy to deliver low grade heat energy?
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