Introduction to Ground Source Heat Pumps

GSHP

Ground Source Heat Pumps absorb heat from the ground by circulating fluid in buried pipes. The fluid passes through a heat exchanger into a heat pump. A compression cycle raise this low grade heat to a higher temperature capable of heating water for property heating and hot water circuits.

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FACTOID

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. 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.

In small properties with heat losses exceeding 10kW, GSHP’s should be used in favour of ASHP’s as their performance does not suffer reduction in cold extremes.

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 25W/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:

TYPE OF SOIL/GROUND
TYPICAL HEAT EXTRACTION RATE - W/m
Sand (dry)
    <20
Sand (saturated)
    55-65
Clay (moist)
    30-40
Consolidated rock (hard)
    70
Unconsolidated rock (saturated)
    45-50
Unconsolidated (dry)
    <25

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.

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

View the homemicro.co.uk article on thermal store buffer vessels

INFORMATION & RELATED SITES

Information on laying Thermal Collector Pipework
Efficiency, Cost & Carbon Comparison - Homemicro article on operating cost and carbon emissions
Comprehensive B&ES article on GSHP
For thermal conductivity testing Loopmastereurope or Earth Energy
Energy Saving Trust article on GSHPs

GSHP - last updated 13th March, 2016 by Corny