Considering groundwater flow in BHEs design through an evolution of the ASHRAE sizing method
According to literature, the presence of groundwater flow enhances Borehole Heat Exchangers operation, since an advection mechanism is added to conduction. Therefore, if the effect of groundwater flow was included in the design phase, it is expected that it would result in smaller borefields and lower excavation costs. However, simple BHE sizing methods accounting for groundwater flow are presently lacking. In this paper we propose to modify the well-known sizing method from the ASHRAE standard to improve accuracy and include the medium and long-term groundwater impact. The procedure exploits the reformulation of the ASHRAE sizing equation and of the so-called Temperature Penalty in terms of g-functions. To identify the most suitable analytical solutions to calculate the medium and the long-term ground thermal response, depending on the relevant distances and heat transfer mechanism, the behavior of the main analytical solutions for heat sources is first discussed. Then the Finite Line Source and the Moving Finite Line Source g-functions are implemented in the sizing equation for the case without and with groundwater flow, respectively. To illustrate the new methodology, a series of case studies is generated by varying the ground loads, the Darcy flow velocity and direction, as well as the borefield characteristics. The borefield sizings resulting from the new method are then compared with those obtained by applying the original ASHRAE method. The comparison showed that Darcy velocity equal to 10-7 m/s can be considered the minimum threshold above which advection effect cannot be neglected. The beneficial influence of the groundwater flow on the BHEs sizing is more pronounced for small borehole spacing and unbalanced heating and cooling loads in the ground.