Cost Comparison Between Ground Source & Air Source Heat Pumps
When comparing ground source heat pump costs with air source heat pump costs it quickly becomes apparent that the former requires significantly greater capital investment than the latter. However, an objective analysis is not as simple as that. Given that the realistic life expectancy of a heat pump system should be in the order of 15 years, the lifetime cost should be considered and when doing so, the following variables come into play.
- Average efficiency otherwise known as coefficient of performance (COP)
- External space
- Noise limitations
NB. COP is the ratio of power output to power absorbed
When making a comparison based on efficiency only, it is important to view the data as an average because the COP of a ground source heat pump is generally about 2.5 – 3, whereas the COP for an air source heat pump will vary between about 1.5 and 6. The instantaneous COP in each instance will be dependent upon the low grade energy source temperature and thus it is realistic to say that whilst the ground source heat pump COP remains consistently stable, the air source heat pump COP is at its least efficient when air temperature is at its lowest and heating demand is greatest.
Therefore, based on like for like heating demand, it is reasonable to conclude that the ground source heat pump will use less electrical energy over the same seasonal period. To gain specific data, a more in depth study can be conducted using predictive software in order to quantify the actual running cost difference and hence the payback against the additional capital expenditure.
Both arrangements require external space and may be a determining factor when selecting the type of heat pump system best suited to a particular application. In the case of ground source heat pump systems there are two methods of absorbing heat and neither require plant to be installed permanently in an outdoor location.
- A bore hole or holes allowing heat exchange pipes to be buried deep in the ground. This method occupies the minimum footprint, renders a slightly higher stable COP, requires a ground survey and is comparatively more expensive.
- A ground loop is a shallow trench (approx. 1.0m deep) in which heat exchange pipes are buried in the ground. This method requires excavation over a fairly large area, renders a less stable COP due to the proximity to the surface but can be cost effective when an abundance of space is temporarily available and if other works are in hand at the same time.
Air source heat pumps on the other hand require a permanently installed outdoor unit/s. These outdoor units have undergone many design improvements over recent years, to optimise performance, minimise noise and minimise footprint. Furthermore, they have the versatility to be able to be installed in locations not suited to ground source heat pumps such as roof tops and where the ground is too small or not suitable. Combine this with comparatively lower capital expenditure, it is little wonder that air source heat pumps are generally the system of choice for medium sized commercial applications, such as hotels and small to medium sized office buildings.
Noise produced by outdoor plant is a factor that should not be ignored. Ground source heat pump systems may be regarded as silent; however, air source heat pump outdoor units do make a limited amount of noise. Data is available from manufacturers and should be considered when locating such plant, especially in residential areas. It is worth bearing in mind that the environmental health officer has the power to deny use or insist upon attenuation if the sound emitted is considered to be a nuisance.
Having been responsible for the design and application of many heat pump systems, we offer our experience through services from feasibility to handover.