Ground source heat pumps

Ground source heat pumps are also known as brine to water heat pumps and they extract heat from the ground via pipes buried in it, either horizontally or vertically. 

The liquid in these pipes is actually water mixed with anti freeze, it is often called brine, and the wetter the ground in which the pipes are buried, the better the heat pump works.  If you have room, then laying the ground source pipes in a horizontal grid is cheaper, but if the land available is limited, then the pipes can be fitted in vertical bore holes.

The holes are up to 100 metres deep and we use specialised contractors to bore the holes and install the pipes, known as probes.  The liquid in the ground loop is a mixture of water and ethylene glycol, which is an antifreeze solution.  The loop is normally protected against freezing down to about –15° Celsius.  It is quite normal in winter for the ground loop to run at a constant temperature close to or below 0° Celsius.  This is particularly true of horizontal collector ground loops, as they rely on heat in the ground which comes from the sun.  This is built up over the year by rainfall soaking into the ground, carrying solar energy with it.  In a horizontal collector the pipes are normally buried approximately 1.2 meters deep, as the temperature at this depth hardly varies throughout the year. 

It is very important that the ground loop is correctly sized, as if it is too small, then the heat pump, which is really just a large refrigeration unit, can freeze the ground, and ultimately freeze the liquid in the ground loop.  Before it gets to that stage, though, the efficiency of the unit will drop off dramatically, because in the same way that the higher the source temperature the more efficiently the heat pump can run, then it follows, that the lower the source temperature, the less efficient the heat pump will become.  Also, permafrost in the garden is a bit of a problem, as it takes a long time to thaw out!  

Vertically fitted ground loops are a better option, but obviously cost more.  The benefits are that because they normally go at least 50 metres deep, they usually pass through a water table.  This means that because of the movement of the water in the earth, there is more heat available for extraction by the heat pump.  This usually means that the ground loop is physically smaller if you fit vertical bore holes as opposed to a horizontal collector.  The other benefit of the vertical bore hole installation is that if the correct heating equipment is installed in the property to be heated, then the ground source can be used for passive cooling in the summer, if required. 

The most important rule is; If you are spending the money to fit a ground loop for a heat pump, it might as well be done right first time.  We can calculate the required size of the ground collector, once we know the size of the heat pump installation.  As a rough ‘rule of thumb’ a 5 kW heat pump requires one 100 metre deep bore hole.

For a case study on an ground source heat pump installation click on Hanson Hall - Ground source heat pump using bore holes