No fossils in MY house...part 1 - heat pumps.

Wouldn't it be great if we could just take ourselves off the electricity and gas grids?  Provide our own sources of heat and electricity for running all our gadgets and household objects?  Some people have done it, but it's no easy undertaking for the majority of us.

I have looked at some options for our house.  We live in a city, in a reasonably large, but old house.  We have some flat roof space and we have a reasonably large garden (a bit over a hundred square metres.)  It should be possible to do something useful with that.

Given the garden, I was initially attracted to ground source heat pumps.  These are brilliant devices that work like fridges running in reverse.  They pump heat from the ground into your house using electricity to run the pump to move the refrigerant through a heat exchanger.  In hot weather it's possible to run them in reverse to cool the house down and heat the ground up again.  You simply bury a long length of pipe in your garden, fill it with "refrigerant" and install a heat exchanger.

The key parameter in a heat pump is something called the coefficient of performance - which is basically the ratio of how much energy you can extract from the ground divided by how much electrical power you need to put into the device to pump.  Numbers on the order of 4 or just under are good for a ground source heat pump.

There are however some challenges.  To get a good coefficient of performance one needs as small a temperature difference between the ground and the target as possible.  The ground temperature in the UK is about 11 degrees centigrade.  A target of about 40 degrees centigrade can be achieved at a fairly good coefficient of performance.  As you try to raise the temperature higher the coefficient of performance falls - and you also risk over cooling the ground unless you have a large enough capture area.

To get good use from such pumps therefore requires very good insulation and ideally switching from radiators to underfloor heating.  That pretty much blows away the idea for our house and indeed for anyone who isn't moving into a house built under the newest regulations for insulation.  We could just about rip up the floors to install underfloor heating - no easy undertaking - but the insulation problem is I'm afraid insurmountable.

There is a very useful site for anyone interested in such devices here.  I originally looked at the Iceenergy site - I am not endorsing their products - although plenty of high profile names are, including Claire Short, however it looks like a good place to start.  That said, they provide a product and some engineering, but you're expected to do quite a bit of the work towards an installation yourself.

If one doesn't have space for a ground source heat pump one could always go for an air source pump.  The principle is exactly the same - except the energy to heat the house comes from the outside air.  There are some obvious benefits since you don't need a garden to dig up.  Coefficients of performance as high as 6 can be achieved.  You still have the challenge of insulation to deal with - and you are more at the mercy of the outside temperature.  Whereas the ground temperature is fairly constant, air temperature varies - depending on where you live by perhaps a large amount.  The colder it is the higher the difference in temperature likely being desired and the less efficient the heat pump will be.

Of course, heat alone isn't enough - you need electricity to run the pumps.  More on that in a future blog...