Technology
Ground Source Heat Pumps
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The extension and the existing lounge are to have underfloor heating installed.
We have decided to power this with a Ground Source Heat Pump (GSHP).
This system is now widely used in Scandinavia and Switzerland. It involves using the same principles as a domestic refrigerator to withdraw heat from outside the building and use it to warm the water that is circulated within the floors.
A GSHP installation at the National Energy Foundation. This pump is rather larger than we should need, it will be around the size of a domestic fridge.
We think (and hope) that it will provide the best use of energy for the house since we have come to the conclusion that solar heating panels would combine inefficiency (to far North and partiallt shaded) with significant complexity.
Ground sources of heat can be tapped in three ways:
Firstly (if you live in Iceland) you simply harness the hot water that flows out from the ground in volcanic areas. Not too likely in the UK!
Bore holes can be used, but have a high installation cost and are more generally used for large commercial buildings.
A surface heat exchanger will suit us better as the installation costs are much lower and we have sufficient land area to provide for our needs.
The "Slinky" is a significant length of tubing arranged in coils in a ditch around 1.5 to 2 metres below ground level. A water/glycol mix will be pumped through this system.
We anticipate two heat exchangers each comprising 50 metres of trench and containing several hundred metres of tubing.
The cooling of this water by around 10 degrees will heat the water in the underfloor heat exchanger to around 35 degrees.
This is not a way of avoiding the use of gas or electricity. Rather it is a way of making more efficient use of (electrical) power. It is hoped that our 10kWatt (output) installation will consume around 2.5 kW of electrical power.
GSHPs are at their most efficient when raising water to around 35 to 40 degrees. They lose efficiency when made to heat water to 60 degrees, that is to the temperature required for domestic hot water. We therefore intend to use the GSHP to heat the house and also to heat the domestic water to 40 degrees or so with an electric immersion heater to take it to 60 degrees.
We do not intend to have any other installed heat source for the extension, (except for an AGA cooker).
We did wonder about using the system to actively cool the pantry (an underfloor pipe connected to the ground loop side of the pump, but currently we aim to achieve this by a thermal barrie rin the concrete floor rather than using active cooling.
Finally there are meant to be government grants available towards the installation of these systems. We shall see, meanwhile all the pigs are fed and ready to fly!
We are interested in installing water heating panels since we have a large South-facing roof.
Until now we have been put off by out favoured system being incompatible with our pressurised hot water tank. This system does not use a conventional circulating water/antifreeze mix which is used to preheat the water tank feed. This is usually done either by using s separate small heat exchanger tank or installing a new water tank with two heater coils - one for the boiler and one for the solar heated water. Instead it takes water from the cold feed and returns it to the outlet pipe from the tank. Consequently the pipes must be of "food quality" since the water that goes through the panel may end up in your coffee!!
Commercially this system employs silicone rubber tubing which will not apparently withstand the 3 to 4 bar tank pressure.
I have therefore been trying to source silicone tubing with a view to pressure testing some samples. If I can find some what will take (say) 5 bar then it seems worthwhile to invest in 25 metres or so and make up a panel using an other double glazed window and some of the roof/wall insulation that litters the site at the moment. At this stage I could use a cheap pump or a gravity feed and see what sort of flow rates and temperatures I can achieve with the panel sitting in the garden. If it doesn't work, then its a relatively small loss. If it works then we can go to good quality glass panels and a photovoltaic driven pump which I have already sourced - though at a cost. I have two samples to hand at the moment and plan to test them in the next couple of weeks.
If the concept flies, then I have no doubt that I can improve on the £2500 +VAT + installation costs for a commercial 4 by 8 foot panel. We would in any case probably need three such panels to get a useful contribution.
Written 2007-04-01
Links:
Further information GSHPs can be found here:
The National Energy Foundation
and specifically on domestic installations:
http://www.nef.org.uk/gshp/documents/CE82-DomesticGroundSourceHeatPumps.pdf