NUMERICAL ANALYSIS OF EFFECT OF GROUNDWATER VELOCITY ON EFFICIENCY OF GROUND SOURCE HEAT EXCHANGER

Abstract

Geothermal heat pumps are used for various purposes as a green energy source. These pumps are used as an alternative of conventional heating/cooling devices in heat ventilation and air conditioning (HVAC) systems and in binary cycle steam power plants. In this study, the effect of groundwater movement on the heat exchanging efficiency of the heat pump is investigated by numerical experiments. A two-dimensional model representing heat transport in porous media is used to study the heat transfer mechanism of geothermal heat pumps in subsurface system. A single borehole heat pump is considered for the heat transfer at a constant heat flux of 10W/m to the subsurface for a simulation period of 200 days under different groundwater velocities. The region inside the borehole is studied considering a steady state condition by taking a constant heat flux at borehole wall. Temperature condition of the region outside the borehole is taken transient state throughout the simulation period. The results show that a temperature drop of 0.71-2.7% is achieved in mean fluid temperature at outlet by changing the groundwater velocity ranging from 1-10m/year. With increasing groundwater velocity the thermal affected zone is shifted to downstream direction meaning faster travel of thermal plume which ultimately increases the heat exchange between bore well and subsurface. This phenomenon of heat transfer is governed by the Peclet number which is defined as ratio of thermal fluxes by advection to the diffusion in the same direction. Therefore a parameter variation test is done to see the most influencing parameters amongst those constituting the Peclet number. The parameters are varied from -20% to +20% for getting the set of isothermal contours. The results show that variation in thermal conductivity of groundwater does not affect the thermal affected zone. While variation in porosity of the subsurface and thermal conductivity of soil, has a little impact on the thermal affected zone. The variation in specific heat of soil has comparatively more impact on thermal affected zone while variation in groundwater velocity and specific heat of groundwater has highest impact on thermal affected zone. The mean fluid temperature is also insensitive to the variation in groundwater thermal conductivity. Since there are various hydrogeological sites having different characteristics, therefore, a better estimation of subsurface water can help in reduced dimensions of borehole and finally the cost of project while designing the ground source heat exchanger. Thus, a precise range of Peclet number for a certain temperature drop in mean fluid temperature can help in precise estimation of groundwater flow effect in better designing of heat exchangers.