Not being contrary Jonti - I used billingsgate as an example of how the ground temperature increased from frozen because heat flowed three dimensionally to the coldest point (greatest delta T) and that heat flowed perhaps some considerable distance if we believe that no heat (or little heat) is gained if we concrete over the surface.

As for extracting heat at a rate faster than it's replaced then i guess we need to consider system size etc and diurnal conditions - it's very rare to have a constant heating load - and if it were true then at city scale we would be producing enough heat to modify the local environment anyway (heat island effect ?)

Given that we receive all of the ground temperature via solar energy, is it credible that the total heating demand of a conurbation could outstrip the total solar gain falling on and around that conurbation - personally, i don't think it is.

For sure we may see a local and short term dip in ground temperature at the point of extraction, but as that temperature declines, the compensating effect takes over and ground temperature stabilises at the initial condition.

Regards

Barney

That's the point i was making Jonti - in reverse though. It was only the energy added to make ice that was able to diminish ground temperature locally - once that energy input was removed, equilibrium was maintained. At a wider scale, the ice had no effect on the ground temperature - it just extended a temperature gradient.

So effectively yes, Jonti - I am saying that you can remove as much heat from the ground as that ground receives in solar radiation - for all practical purposes, the amount of heat removed will only ever be a small fraction of the total heat received from solar irradiation. So over a suitable time frame, with appropriate system sizing it would be quite feasible to deploy city wide GSHP without dropping the ground temperature by any discernible amount - and that discernible amount would be a function of the area you measured over.

It's no different a concept to say using solar thermal - you can remove the solar energy up to the rate at which it is received.

Regards

Barney

Okay, finally some harmony in the house

This linked report includes a claim that wind –power reduces carbon burden by a minimum of 350kg/MWh but equates to 1284 kg of CO2 which appears a gross overestimate when aware gas combustion is scheduled to create 385 kg of CO2/MWh (105kg of carbon).
http://www.guardian.co.uk/business/2012/aug/29/wind-power-study-claims-unfounded
Surely we should prioritise low GHG production which highlights the need to embrace impact of all sources . A current renewable combustion project details levels of oxides of nitrogen created will equate to a CO2 burden of 1511kg/MWh. With scheduled water content 55% this type of process brings additional impact consequences.
Electricity produced by clean sustainable processes offers a very low carbon energy source with easy highly efficient conversion into heat plus it is relatively silent and easily transmitted and controlled.

http://www.guardian.co.uk/business/2012/aug/29/wind-power-study-claims-unfounded

says:

“The economic model GL Garrad Hassan adopted showed that every megawatt-hour of electricity wind power produced led to carbon savings of a minimum of 350kg.”

Actually, the IPPR report says the saving is 350 kg of CO₂, not 350 kg of carbon. The Guardian's reporting is ambiguous to the point of being misleading and really not very helpful at all.

Still, this is such a common error that I'm a bit surprised that Brianwilson didn't spot it.

(PS, just tweeted at the author over this: https://twitter.com/ed_davies/status/243307127534399488)