British Geological Survey for ground source heat pumps

[vivienz]

I'm still considering a ground source heat pump for the new build and came across this document in the course of my research.  It may be helpful/interesting to someone if they're struggling with insomnia!

gshp_jet.pdf

[ProDave]

I discounted a GSHP when I found the cost of the pipe you have to bury, and the cost of the antifreeze to put in it (which is supposed to be replaced every 10 years, where are you going to dispose of it?) Even having my own digger so the install cost would have been minimal.
 

[Jeremy Harris]

That BGS/NERC study was done a long time ago, now (2005), but was what our hydrogeologist used when producing the report for us when we were looking at using a borehole system for a GSHP.  The second borehole would have been cheap for us (relatively speaking) as we had to have a borehole for water, anyway.  Even with this much-reduced collector cost, we still still couldn't justify a GSHP, as it was still never going to come close to paying back the saving in running cost, in fact the periodic antifreeze replacement was going to cost more than the energy saving over an ASHP.  The ASHP only uses about £20 worth of antifreeze, whereas the GSP was going to need more than £250 worth of the stuff, plus the waste disposal cost.

 

Given the poorer thermal conductivity and heat capacity of antifreeze/water mix when compared to just water, I did look at whether it would be possible to limit the return temperature to ensure it was always above freezing point, so that corrosion-inhibited water could be used in the primary circuit, as there's about a 10 to 20% performance improvement by doing this.  I also looked at using a standing column primary side system, using the same borehole as used for the water supply, as used in Canada and the US.  Sadly the sums didn't workout for this option, unless the heat pump is massively over-sized, which just pushes the capital investment up even higher.

 

The other thing that hits overall GSHP efficiency, and which I found wasn't always included in the COP calculations, is the circulating pump power on the primary, collector, side.  Often a fairly sizeable pump is needed, especially with a standing column collector, and this effectively reduces the COP a bit, unless it's already been accounted for by the manufacturer.