Ground source, or air source? How do I choose?

Heat pump specialist Bob Long considers what scenarios are best for ground source and air source heat pumps

This is probably the question I am asked the most frequently and, indeed, the most difficult to answer because there are many factors to consider, and some not always obvious.

The choice does not entirely depend upon how much ground you have available, how much money you want to spend, or even how much you get back through the RHI.

Although these are all factors to consider, there are only two basic essentials to ensure.

Minimum cost of operation and maximum carbon reduction

Assuming there is enough land available to consider a ground source heat pump (GSHP), the first step should be to complete a survey assessing the potential energy yield of the available ground. The survey should also indicate the best choice of energy collector i.e. bore holes or trenches.

Results will vary geographically, and it’s essential for the potential energy requirement never to exceed the capability of the land.

I recently visited two GSHP installations where the economics of operation are currently being questioned.

In both instances the installations are 3-4 years old, and each system had performed adequately in years 1 &2, with a noticeable fall in efficiency by the third year.

In the first example, the flow and return temperature of the working fluid was -1 and +3, and in the second example, the working fluid temperatures were -4 and -1, indicating that the ground surrounding the collector is probably frozen.

Rehabilitation of frozen ground is a lengthy process, as latent heat is absorbed into the ice surrounding the collectors. The remedial process can absorb vast amounts of energy, while showing no improvement in ground temperature.

A well designed ground source collector should be capable of delivering return glycol temperatures of +5˚ C or higher under continuous load conditions, and if this is not possible, more ground must be allocated, together with a greater length of collector.

Glycol temperatures, returning from the ground collector below 5˚C will compromise the efficiency of the GSHP, and probably produce a COP lower than an air source heat pump (ASHP) at say +7˚C ambient air temperature.

A proposed installation site that cannot support all of the above needs should not proceed with the installation of a ground source system.

ASHPs are a viable alternative, only when annual climatic conditions in the intended location are suitable, and must be a primary consideration.

In certain parts of the UK, predictably severe winter conditions could make an ASHP the wrong choice, although a hybrid system with an alternative fuel source could provide the most economical mix.

The correct choice is made easier if we create a monitory value for each kW/hr. of energy, delivered by a range of sources.

The following values are rough illustrations of cost comparison: 0.12p/kWh for electric, 0.08p oil, 0.07p LPG and 0.05p natural gas.

When the ambient temperature is +7˚C and higher, a reasonably good heat pump will deliver a COP of about 3:1.

At this COP, the cost of energy per kW would be 0.4p/kWh making it the cheapest method of heating available.

However, when the outdoor temperature plummets to -15˚C, the heat pump COP is going to drop to perhaps 1.2:1 or even lower, increasing the energy cost to at least 0.10p/kWh.

Based upon fuel cost, climatic condition and COP, a hybrid heating arrangement should produce the most economical result for certain areas of UK.

Parts of the country that experience fairly mild winter conditions, would not notice much financial difference, although for off-gas areas, a cheap bivalent energy source such as LPG should always be considered, before using electrical bivalent support from immersion heaters @ 0.12p/kWh.