Comparison of the energy use for different heat transfer fluids in geothermal systems

Abstract

Geothermal systems that operate under 0°C must use antifreeze mixtures instead of water to avoid operational problems. This paper examines the energy consumption of the circulating pump and heat pump for various heat transfer fluids used in a residential geothermal system. Propylene glycol, ethanol and methanol solutions at different concentrations are compared. Effects of fluid temperature and viscosity on head losses, borehole thermal resistance and heat pump operation are reviewed. Efficiency curves for currently available circulators are proposed. Annual energy simulations are then performed on a residential GCHP system. Energy consumption (pump and heat pump) is evaluated subhourly based on fluid temperature and properties prevailing during each time step. Results show, as expected, that higher mixture concentrations and higher flow rates lead to higher energy consumption. Methanol with a concentration of 15% and a 1.5 gpm/ton flow rate provides the best energy performances while ethanol at 30% with 3 gpm/ton is the worst choice, requiring 16% more energy and 525% more pumping power than for the methanol case. Laminar flow in boreholes appears to be favorable when compared to turbulent flow which leads to relatively high pumping energy consumption. Shorter boreholes piped in parallel decrease energy consumption as well.