Ground Source Geothermal Heating Systems use solar energy that is absorbed in the ground throughout the year. In the UK the ground has a near constant temperature of around 10 - 12° C at a depth of 2 meters or more. This low grade thermal energy can be extracted and used to provide heating and hot water. Heat pumps will typically operate with an efficiency of between 300% to 400% or coefficient of performance (COP) of 3 to 4 . This means that for every kilowatt used 3 - 4 kilowatts of heat will be generated.
Ideally underfloor heating should be used as this requires water to be heated to a low temperature of about 35 - 40° C. Radiators can be used but need to be oversized compared to that which would be required for a gas or oil powered system. In practice radiators should be oversized by about 2.5 times. In a modern house built to an insulation standard of 0.35W/°C this is aesthetically feasible.
There are two main categories; closed loop and open loop systems.
Closed loop systems use pipes buried in the ground through which water mixed with antifreeze circulates extracting the stored heat. The pipes can be buried in trenches or vertically in bore holes.
Open loop systems use water that is pumped from the ground and directly circulated through the heat exchanger and then returned back to the ground. This water can be pumped from a well, borehole or bottom of a pond. Water pumped from a borehole will have a temperature of 7°-10°C. The water can be pumped through a plate heat exchanger where it releases its energy to the refrigerant. Flow rates are sufficient to prevent freezing occurring within the heat exchanger.
The diagram below shows an open loop heating system. Here water is pumped from one borehole through a heat exchanger and returns cooled to the ground through a second borehole.
Closed Loop Ground Source Heating System
Vertical boreholes can be used where there is insufficient space to dig trenches for a coiled pipe collector. For new construction projects it is sometimes possible to incorporate pipes in the concrete foundations of the property. This significantly reduces the amount of trench work required around the property