I was interested to examine the relative running costs associated air source and ground source heat pumps. To look at this question I started with a rather simple two season model.
The broad implications may be best described by considering the two extreme situations:
Where summer and winter temperatures are constant (but less than the desired indoor temperature) there would be no advantage to be gained from the additional costs of a ground based system. (Intuitively, ground source systems utilise the stabilising effect of the ground as a thermal sink. Where air temperature is constant, there is no need for an additional stabiliser, so that money spent on it is wasted.)
Where summer and winter temperatures differ widely, the ground source heat pump is likely to be more economical, particularly if summer temperatures are high and a combined air conditioning unit is required.
In practice, these extremes are not to be found in the UK, but situations approaching the extremes, may tip the balance in favour of either air source or ground source.
The rather crude modelling that I have done suggests, for example, that the North-East coastal areas of Scotland, with its climate moderated by the North Sea and ‘enjoying’ cool summers (mean July temp 58 degrees F) and ‘relatively’ mild winters (mean January temp 38 degrees F), shifts the balance towards air source heat pumps. The model suggests that they are only slightly more expensive to operate, so that the additional capital costs of ground source heat pumps may not be justified. Other parameters included in the simple two season model are as follows:
Proportion of year as Summer 0.5
Desired indoor temp 70
COP in Winter of an air source heat pump 2.3
COP in Summer of an air source heat pump 3.3
COP in Winter & Summer of a ground source heat pump 2.8

Using these parameters, I calculate the ratio of air source energy use to ground source energy use to be: 1.1

I’d be grateful for comments, particularly on the COP parameters that I’ve used. Getting reliable data on this crucial variable seems to be somewhat problematic!!

Ken

Posted By: DantenzASHP's inherantly, have more parasitic losses than a GSHP so, even if the source temperature was the same for both, an ASHP would always come off slightly worse.

I don't think there is anything in this. An ASHP has the outdoor unit (with a fan) and the GSHP has a circulation pump. Both probably use about the same power.

The UK is an ideal climate for an ASHP since it is almost always above 0C. In this case, an ASHP would not have to enter defrost mode (where it loses efficiency).

Since ASHPs have a much larger market than GSHPs, they are always likely to be more efficient for a given operating temperature - and cheaper too. However, most of that (worldwide) market is for reversible units - something which the UK seems to be averse to - probably because A/C is rarely required.

The main disadvantage of an ASHP is that the outdoor unit makes noise, whereas as GSHP is quieter because it doesn't have a fan for the equivalent of the outdoor unit.

Paul in Montreal.

Anybody have any thoughts on or experience with using solar-air-heating panels on the input side to an ASHP?

It's the other way round, you only want space heating in the winter. So, the average of 45'C would is what the GSHP would operate off and 30'C the ASHP.

Also, you still want hot water in summer.

Posted By: DantenzThe refrigeration cycle always works with sub-zero tempeartures within the closed refrigerant loop. Refrigerant, by nature ,is cold. This is why the ground collector heat transfer fluid on A GSHP (loop, slinky, borehole etc) is filled with an anti-freeze. It has nothing to do with possible freezing temperatures in the ground ( unlikey with a borehole drill depth of 180 m) it is all to do with temperatures within the fridge circuit.

This is just not true. Refrigerant is not "cold". It simply has the capacity to absorb and release heat through phase change and compression. The refrigerant can never be colder than the coldest part of the evaporator. In the case of a water source (i.e. GSHP) heat pump, this is determined by the input temperature of the working fluid and the flow rate. I know that in my case in Montreal, the input temperature can be as cold as 2C towards the end of winter and then the return temperature is around -3C at the flow rate we have - so the refrigerant is also at -3C at that point too. Places with higher ground temperatures do not need antifreeze in the circulating fluid in the ground. My heat pump has jumpers to select whether the ground loop has antifreeze or not as there is a temperature sensor which shuts down the heatpump in case the ground loop temperature is too low (i.e. would freeze) - the jumper is configurable for -12C and -1C.

In Ken's case, an ASHP would be more efficient in summer - but then the heat load is lower (since there's less differential between inside and outside) but this would be offset by the ASHP being less efficient in winter and with a lower output capacity. So if his ASHP was sized for winter heating, it would be oversized for summer (of course). A GSHP would end up being less oversized for summer operation and would be more efficient in winter. All that said, an ASHP would be much cheaper to install than a GSHP for his climate and probably be the better solution. As for internal gains being sufficient in summer, that depends on how well the house is insulated. It could be that internal gains are sufficient, but a Scottish cool and damp 60F can be pretty dismal And, as it has been pointed out, hot water is still needed.

Paul in Montreal

You didn't read what I wrote. The refrigerant temperature is the post evaporator temp. In my GSHP, there is a 7F differential between the input and output water temps. This, multiplied by the mass flow rate determines the amount of heat absorbed by the refrigerant. There is no temperature change to go from liquid to gas if the latent heat of evaporation is supplied. I didn't say the source temp = refrigerant temp - it's the post heat-extraction temp of the now chilled source. All your rules are correct - rule number 1 isn't relevant for rule number 2 as the boiling point of the refrigerant is much lower than the source (evaporator) temperature - around -41C for R22 and -26C for R410a - both much lower than the input to the evaporator of a GSHP or and ASHP.

Paul in Montreal.