Influence of groundwater flow on cost minimization of ground coupled heat pump systems

This paper introduces a new sizing methodology for ground coupled heat pump (GCHP) systems which takes into account groundwater flow (by using G-functions based on analytical models) in order to achieve an economic optimization of the total cost of the project. The procedure includes the calculation of the initial and the annual operational costs. Optimal design variables (boreholes depth, distance between consecutive boreholes, etc.) and borefield layouts (number of boreholes in the x-direction) are presented for different values of the thermal conductivity of the ground. In addition, a parametric study is done to measure the impact of the groundwater flow velocity and angle with respect to the borefield on the economics of the project. It is shown that the effects of the groundwater flow velocity on total cost become apparent only for high velocities, i.e., Pe>∼10−2. On the other hand, the groundwater flow angle is less impactful regardless of the groundwater flow velocity, i.e., the net economic gain that can be obtained by choosing the optimal orientation is much smaller compared to total cost of the system (less than 2%). A simultaneous comparison of the initial and operational costs shows that for all Pe values, higher initial costs usually result in lower operational costs. Finally, optimized designs are tested under off-design operating conditions. It can be observed that the economic consequences of operating under off-design conditions are far worst if the groundwater flow velocity is overestimated, which can lead to an increase of the operational costs of as high as 5.8%. A wrong estimation of the flow angle in the design phase, however, only leads to an increase of the operational costs of at most 2.4% for the cases considered in this paper.