Ground Source and other forms of Heat Pump

This is one of the most exciting developments in "alternative" energy supply at the moment - probably number 2 in the top ten (if there are 10) of renewables for getting a return on your investment.  I know from speaking to prospective clients that there is a great deal of confusion about what exactly can be achieved with a heat pump and indeed about what they are and how they work.  Hopefully I can clear up some of the grey areas and give you a positive insight into this valuable technology.

So what is the big idea?  First things first - What is a heat pump?

Well, very simply, (so I can get my head around it too!), heat pumps move heat energy from one place to another, and convert it from a lower to a higher temperature.  Your household fridge is the best example of a heat pump, with heat being removed from the contents and discharged via the evaporator (the metal grill thingy on the back!).  They can be used for either heating or cooling.  In heating applications, heat is removed from ambient air, water, soil or bedrock for example, and delivered to a treated space as either warm water or air.  In cooling applications the process is simply reversed.

Heat pumps are very energy effective since they require only a small energy input (usually electric to power a pump) to absorb heat, upgrade it and move it to where it is required.  The system also needs a refrigerant to enable upgrade of low grade heat to useful temperatures.  For every unit of electricity used to drive the heat pump, several units of heat can be delivered.  The efficiency with which a particular heat pump carries out this conversion is referred to as the COP (coefficient of performance), so, for example, a COP of 5 implies that for 1kW of electricity to drive the pump, 5kW of heat can be delivered.

Ok so the buzz is all about Ground Source heat pumps - why?  I guess because it is fairly easy to understand.

The average ground temperature just below the surface in the UK for example is between 8-13^oC and this remains constant all year round.  If you circulate a fluid, lets say cold water or anti-freeze, through plastic pipes buried in the ground, the fluid picks up low grade heat from the surrounding ground and returns to source warmer than it started - this is referred to as a "ground loop".  The fluid in the ground loop is then passed through a heat exchanger within the heat pump to heat refrigerant which is then further energised in a compressor.  This upgrades the heat to a useable temperature for either space heating or domestic hot water.  There are limitations to just how much the heat can be upgraded and it is worth knowing that the water output from a heat pump is typically 35-45^oC which is of course much lower than is achieved by a domestic boiler.  You can get slightly higher output temperatures (up to 55^oC) but maintaining a lower output temperature ensures the overall efficiency of the system. 

What this means is that you can't just hook up a ground source heat pump (GSHP) to your standard radiators and expect to be comfortable, nor can you rig up a water to air heat exchanger and expect masses of hot air.  Most central heating systems need water at 70-85^oC and warm air systems can require water at 90-95^oC so clearly you need some kind of additional energy input to reach these temperatures.  Most often this is in the form of an electric immersion heater that boosts the water to the required final temperature for whatever your chosen application.  This is an additional energy burden that must be considered when you are evaluating the cost effectiveness of a heat pump system.

Now it is not all doom and gloom - heat output at 35-45^o is ideal for underfloor heating and also for those systems with big old cast iron radiators operating at low temperatures.  This should be borne in mind when it comes to specifying a GSHP as it will materially affect your heating system design.  GSHP's are not appropriate for every building but if they can be applied to your property then you can make some very valuable energy savings over more traditional heating / cooling systems.  Another major consideration is of course cost.  I have just specified a system for a client who needs just 40kW which in the great scheme of things is a fairly small demand.  But he needs ten 100m deep boreholes (costing about £3,000 each) or 500 square metres of trench laid slinky's - even more costly!

In my experience a typical 8kW system (using slinky's) - which really augments a boiler but doesn't replace it - costs from £4,500-£6,000 plus the actual heating system itself. 

A 300kW+ system installed recently with 6 sets of slinky pipes sunk into a Scottish loch cost just over £50,000 so if you have a body of water handy it can be a relatively cheap option.

There is funding and advice available to help with the cost of installing GSHP's. 

In Scotland, householders can access capital funding of 30% of the installed cost, up to £4,000 from the SCHRI (Scottish Community & Householder Renewables Initiative).  Tel: 0800 138 8858.

Elsewhere in the UK you should contact the Clear Skies organisation (Tel: 08702 430 930).

But of course ground source is not the beginning and end.  I just mentioned a Water Source Heat Pump and I am now looking at an Air Source Heat Pump for the guy who needed 10 boreholes.  Watch this space