Heat pump technology: will ground source energy displace air source?

Ground Source Heat Pump technology (GSHP) is receiving a lot of publicity at the moment, due to its assumed energy efficient credentials. Here we look at GSHP in comparison to Air Source Heat Pump technology (ASHP), and ask whether there will come a time when it displaces air-source entirely, and if this is a necessary conclusion for energy efficiency.
To begin with, in terms of creating an energy efficient HVAC system, the source of the energy is only one part of the process. Making one part less energy intensive does not mean that the system is better overall.
ASHP can be both good and bad, and when GSHP is compared with an inefficient ASHP system, of course it is going to look favourable. However, an efficient ASHP system can be almost as good as a GSHP system, and a lot less expensive. The figures in table 1 are derived from real projects Colt has worked on:
Table 1:

A GSHP system costs considerably more to supply and install than an ASHP system – on average six times more – yet the savings, at roughly 20 -25%, are fairly marginal compared to a good ASHP system. Consequently, the pay back is more than 20 to 30 years and this price is unlikely to reduce through economies of scale and competition.
Largely due to these financial constraints, it is unlikely that ground source will ever completely replace air source. Despite this, we advocate GSHP as part of an approach to making energy conscious choices driven by a genuine desire to improve carbon emission levels, not just for financial gain, or because legislation dictates it. 
Wherever possible, it should be the first priority to select the most energy efficient HVAC solution, where the energy efficiency level of the complete system is measured and not just that of the heat source. Of course, any system which negates or drastically minimises the use of fossil fuels will eventually be beneficial, and will also reduce running costs.
We advocate a mixed-mode approach to HVAC technology. By combining ground source, air source and an energy efficient air-conditioning system, such as Water and Refrigerant Flow (WRF), it is possible to create an optimum combination of capital and fuel running costs.