ESTIMATION OF DISPERSIVITY VALUES AND EFFECT OF PARAMETERS ON THERMAL AFFECTED ZONE (TAZ) EVOLUTION IN THE OPEN-LOOP HEAT PUMP SYSTEM

Thermal and hydrodynamic dispersivities affect the transport of heat as well as solute in porous media. To predict the environmental effects of the open-loop heat pump system, estimations of dispersivity values along with other hydraulic and thermal parameters are necessary. In this study, two push-pull tests were performed using bromide and heat as tracers in an alluvial aquifer in Yangpyeung, Korea. FEFLOW model was used to simulate the push-pull tests. From the best-fit which makes the root mean square error (RMSE) minimum, dispersivity values were estimated.

In order to estimate the effect of thermal properties on the thermal plume evolution in the aquifer, numerical simulations were performed. In the simulations, the effects of thermal conductivity, volumetric heat capacity, and thermal dispersivity on heat transport were evaluated. Even if thermal conductivity is a major parameter in the closed-loop heat pump system, temperatures in the observation points were weakly sensitive to the thermal conductivity. The volumetric heat capacity has a tendency to reduce the extent of thermal plume. The thermal dispersivity can influence the arrival time and the extent of the thermal plume. The extent of the plume increases as the thermal dispersivity increases. However, the observed temperatures at the points are inversely propotional to the thermal dispersivity.

Operation mode is another important factor affecting the aquifer thermal regime. In many studies, the operation of heat pump is regarded as continuous. In reality, the operation is intermittent rather than continuous. Such assumption might fail to estimate the thermal interference exactly. In this research, 12-hour operation per day was simulated and compared with the continuous operation model. Observed temperatures in the continuous operation were higher than those in the discontinuous operation. This result shows that the assumption of continuous operation can overestimate the thermal interference. In the discontinuous operation, the thermal affected zone (TAZ) recovers when heat pump stops. Since the recovery period for 12 hours is not enough to recover, thermal plume still extends with time.