earth energy - capability
Our professional team has a proven track record of designing and installing ground source heat pump systems in a wide variety of building types from houses and village halls to new schools and hospitals. Design Engineers create a computer model of the proposed ground loop heat exchanger for the exact location of the site ensuring feasible solutions are presented even at the earliest stages of a project.
UK Geology
The diversity of UK geology is unique, ranging from hard granite in Cornwall and Scotland to stiff clays and sands in the South East. The physical and thermal properties of the ground vary significantly and must be properly considered in the design and construction of the ground loop.
Ground Temperature
Observed equilibrium temperature data for the UK indicate that some areas have stable ground temperatures of 15 °C at depths of 100m. Conversely other regions show stable temperatures at 100m depth of only 7 °C.
EarthEnergy makes an estimate of the local geothermal gradient and rest temperature for use in thermal modelling of each ground loop heat exchanger using the estimated thermal conductivity of the bedrock at the site.
Thermal conductivity
Thermal conductivity varies widely (0.2 - 6.6 W/m/K) for the range of rocks encountered in the UK. Superficial deposits and soils are complex aggregates of mineral and organic particles so exhibit a wide range of thermal characteristics.
EarthEnergy has been installing systems in the UK since 1994 and our extensive experience and library of geological records enables us to make an estimate of the appropriate thermal conductivity to be used.
If required, EarthEnergy can attend site and undertake an in-situ thermal conductivity test to confirm the actual site conditions. This may be beneficial for a large scheme, where the number of boreholes may be reduced through refinement of the design parameters.
Groundwater
The thermal conductivity of rocks is greatly affected by the degree of saturation, which is controlled by the porosity of the rock and the location of the water table and its fluctuations.
Most GSHP design techniques are based on the assumption that the heat will be dissipated by conduction. If heat advection due to groundwater flow is significant at a site it is likely that this will have a beneficial effect. However, given the uncertainty of ground water table levels and flows over the 50-year design life of the system, EarthEnergy bases our ground loop heat exchanger design on heat transfer through ground conduction only.
Thermal Modelling
In the design of plants for the thermal use of the ground, two cases must be differentiated:
- Short-term influence (operation of the plant with maximum output)
- Long-term influence (long-term operation of the plant with variable output)
In both cases, EarthEnergy considers the temperatures prescribed by the plant (e.g. minimum evaporation temperature of a heat pump) and the temperature limits determined by the ground to ensure a sustainable design. Under-dimensioning of the ground loop heat exchanger can lead to very low heat source temperatures in the short term and falling ground temperatures from heating season to heating season in the long term.
EarthEnergy uses the industry standard software GHLEPRO to model behaviour of the ground loop heat exchanger for its design life using the following design limits for heat pump entering water temperature, for satisfactory operation.
Maximum allowable temperature +35°C
Minimum allowable temperature +0 °C
NB: A reduced borehole array size would extend these temperatures and lead to a decrease in heat pump efficiency.
Hydraulic modeling
EarthEnergy undertakes iterative design to ensure the number, depth and layout of boreholes and diameters of U-tubes and interconnecting pipework meet the following hydraulic requirements:
- Turbulent flow (i.e. Reynolds Number > 2100) to be achieved under maximum demand conditions in the borehole U-tubes.
- Hydraulic pressure drop in total ground loop array (i.e. as seen by heat pump) to be <50kPa.
This ensures efficient heat transfer in the ground whilst limiting the hydraulic head loss through the system and thus minimising the size of circulation pump required.
