24 June 2014
Although geothermal heat pumps have been attracting more attention these days due to several energy efficiency and environmental benefits, they might be criticized over the fact
that the performance decreases significantly over seasonal operation due to the unbalance loading in the ground. However, in direct-expansion geothermal heat pumps using carbon
dioxide the performance drop is relatively small due to less sensitive operation to evaporating temperature, elimination of the secondary loop, efficient compression and relatively low pressure drop in the ground loop portion.
In this study, a theoretical quasi-transient model is developed to quantify the performance drop
of a CO2 direct-expansion geothermal heat pump over time. The system under study consists of a compressor, gas cooler, internal heat exchanger, expansion valve and a vertical geothermal borehole. Steady-state thermodynamic and heat transfer calculations of system components are
coupled with transient heat transfer model in the ground.
The model is used to simulate a relatively small heat pump over 48 hours. Results show that the
coefficient of performance decreases slightly by 4.9% over the first 12 hours and only by 1.8% over the next 36 hours due mainly to less sensitive performance to evaporating temperature. It is also shown that the evaporator temperature decreases only by about 2°C due to relatively low
pressure drop in the borehole attributed to significantly low kinematic viscosity and Surface tension of CO2.