Ground Source Heat Pumps

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These systems work by taking low level retained heat from the ground and boosting it for heating a property and water for domestic use.

Working in a similar fashion to refrigerators these systems are best suited to provide a constant, lower level, of heating without sharp peaks in temperature such as is required by under floor heating systems.

Because of the way in which heat is extracted, normally through a network of coiled piping, ground area may be a factor in the ability to install these systems. These systems are highly efficient, delivering 300-400% efficiency against 86% typically seen with condensing gas boilers.

The coefficient of performance (CoP) will vary according to the amount of heat that is exchanged to the water circulated in a building heating system. The heat pump will have to work harder to provide a higher hot water temperature. See typical COP values for GSHPs details here... Always check the equipment data on CoP and cross check against other suppliers as there can considerable variation.

A GSHP system consists of a ground source heat exchanger, heat pump, and a heat distribution system. Water is pumped through the heat exchanger that consists of pipes buried either horizontally or vertically in the ground. The water temperature present in these pipes is lower than that in the surrounding ground. As a result, heat is transferred through the pipes warming the circulated water. The captured low grade heat is transferred to a heat pump, where it is used to heat up a refrigerant. The 'warmed' refrigerant is compressed increasing its temperature. The higher temperature refrigerant in turn heats a secondary water circuit to a higher temperature.

The result is an output temperature of 35°C-45°C. This is ideal for underfloor heating in a solid floor construction. In this case, for every 1kW of electricity you use to power the system, you get up to 3-4kW output. Heat pumps can achieve a temperature of 60°C but at a reduced efficiency output.

The installed cost of a GSHP, for a professional installation ranges from about £800-£1200 per kW of peak heat output (excluding the cost of the distribution system). A typical domestic system would require an 8-12kW GSHP at an installed cost of between £6,000 and £12,000 excluding the distribution system.

Heat Extraction Rate

The rate of heat transfer from the ground will be affected by insulation, ground temperature, geology, etc. However, a conservative guideline figure for heat absorption is 20W/m for a trench and 25 W/m for boreholes. So a typical 50 metre trench (100 metres of pipe) would provide 2kW and a 100 metre deep borehole would provide 2.5kW. These figures a very conservative. If the soil type is known, the following figures could be used:

Sand (dry)
Sand (saturated)
Clay (moist)
Consolidated rock (hard)
Unconsolidated rock (saturated)
Unconsolidated (dry)

Thermal conductivity testing is the only way to calculate exactly how much heat will be transferred for a given location. Visit these sites for more information on Thermal Conductivity Testing Loopmastereurope or Earth Energy

System Design

It is generally uneconomic to size a heat pump to the peak demand as the sole heat source (a monovalent system) so sizing and selection to match the system is important.

Most systems are bivalent - that is, heat is generated by two separate means. One bivalent design uses the two heat generators as alternative heat providers. The heat pump will satisfy the design load for much of the time. However, when the ambient temperature is too low or a fast heat-up is required, the heat pump is switched off and the alternative heat generator is used. This means that the alternative heat source must be sized for the peak load.

The alternative bivalent design runs the heat pump continuously and the second generator simultaneously when the heat pump cannot meet the total heating load. With this alternative design, the heat pump is usually selected to match the base load, ensuring near constant operation to maximise return on the investment costs. For an existing building, the base load can be accurately determined by charting energy consumption, ideally logged on an hourly or even half-hourly interval. If this information is not available, the heat pump should be sized to match the building heat loss above an outside air temperature of 4°C and the second generator sized to match the building heat loss between 4°C and the design outside air temperature.

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Bore Hole Pipework Distribution

The spacing of holes will depend upon ground conditions and the available energy in the ground. Typically boreholes should be placed at no less than 5 metre centres and at least 2 metres from a building, although 5 metres is better.

The distribution pipe (thermal collector pipe) between the building and the boreholes should be laid in a horizontal trench according to these rules:

  • Pipework must sit on 100mm of sand with 150mm minimum cover above the top of the pipe. Marker identification tape must be laid above the pipe.
  • Lay pipes at a minimum depth of 800mm; 1000mm is the ideal depth. If this depth cannot be achieved, then both the flow and return pipes will need to be thermally insulated.
  • The trench pipes will need to be thermally insulated. Provided the above trench depth is achieved, only the flow pipe needs to be thermally insulated along it´s entire length between the building and the boreholes. The return pipe should be insulated within the last 2 metres of any manifold location.
  • All thermal collector pipes must be thermally insulated within 1.5 metres of the building and wherever it crosses drainage pipes.
  • Where there is an external manifold all pipework and fittings within the manifold access cover should all be thermally insulated.
  • All pipes between manifolds and bore holes will need to be thermally insulated where pipes are in close proximity. Pipework with a separation distance less than 1000mm should be thermally insulated.
  • All pipes crossing water mains or drains must be thermally insulated 1000mm on each side of the crossing point.

Trench Pipework Distribution

To achieve optimum levels of heat absorption the following rules should be observed for pipes lay in trenches:

  • Lay pipes at a minimum depth of 1200mm; 1450mm is the ideal depth to account for sand fill.
  • Pipework must sit on 100mm of sand with 150mm minimum cover above the top of the pipe. Marker identification tape must be laid above the pipe.
  • Minimum trench width should be 600mm; 900mm is preferred width as the looped pipes should be 600mm apart.
  • Space trenches 1000mm apart. More distance is better.
  • Pipe loop length should not exceed 100 metres, that is, a 50 metre long trench. This can be increased if larger diameter pipe is used.
  • Trenches should be a minimum of 1000mm clear of all buildings (obviously the circuit will need to extend to the property).
  • The last 1000mm of pipe up to the property must be thermally insulated.
  • All pipes crossing water mains or drains must be thermally insulated 1000mm on each side of the crossing point.

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Buffer vessel sizing

The volume of a buffer vessel is dependent on system use and can be calculated from:
  V (intermittent use) (litre) = heating load (kW) x 25
  V (continuous use) (litre) = heating load (kW) x 80

Related Sites & Articles

Typical COP values for GSHPs details here...

For thermal conductivity testing Loopmastereurope This link opens a new window or Earth Energy This link opens a new window

Homemicro article on heat pump operating cost and carbon emissions - Efficiency, Cost & Carbon Comparison

  These links open in a new window

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