Heat carrier fluid
The heat transfer fluid and its concentration have a significant impact on the flow regime and heat transfer of the GHE, as well as on the required pumping energy. GHE Analysis allows you to quickly compare the impact of the heat transfer fluid and its concentration on the thermal conductivity (kf), dynamic viscosity (μf), density (ρf) and specific capacity (cp), and thus to calculate the convective resistance (Rf) of the borehole heat exchanger as a function of the entering fluid temperature (EFT).
Heat carrier fluid - Thermo-physical properties of five fluids are integrated into GHE Analysis, making it easy to compare the impact of the fluid on system performance. The available fluids are pure water, ethylene glycol, propylene glycol, methyl alcohol and ethyl alcohol; their properties are computed from the Melinder correlations over the full temperature range.
Antifreeze mass concentration - This input box allows you to specify the mass concentration of antifreeze assuming that the solvent is pure water. The units of mass concentration are kg of antifreeze per kg of solution, reported as %.
Summary & charts: thermo-physical properties and convective resistance versus temperature
The page contains a summary showing the melting temperature of the fluid, the volume of fluid contained in the pipes, the different thermo-physical properties at TLim and the convective resistance of the geothermal well (Rf). The convective resistance Rf should be as small as possible in order to maximize the convective heat transfer in the pipes. The charts show the thermo-physical properties as a function of EFT while the vertical lines correspond to TLim for the cooling (blue) and heating (red) modes.
GHE in practice: antifreeze concentration trade-offs, fluid selection and pumping costs
Antifreeze allows the heat pump to operate in heating mode at lower temperatures and thus extend its operating time. However, a high antifreeze concentration requires higher pumping energy and leads to a greater convective resistance Rf. This generally results in a larger and more expensive geothermal field, as well as higher pumping costs. Special care must be paid to the selection of antifreeze and its concentration, both during the construction phase and during the useful life of the GSHP system.
As a rule of thumb, the freeze (or burst) point of the fluid should sit a few degrees below the lowest temperature the loop will ever experience — and no lower. Over-concentrating "for safety" is a common and costly habit: it thickens the fluid, pushes the flow toward the laminar regime, raises Rf and pumping energy, and ultimately enlarges the field. The choice between propylene glycol, ethylene glycol and the alcohols is also governed by toxicity, local code and material compatibility, which often weigh as heavily as the thermal properties themselves.