On the evaluation of design parameters effects on the heat transfer efficiency of energy piles

In this paper a methodology for comparative evaluation of design alternatives of cast-in-place energy piles (EPs) is presented. The methodology proposes the comparison to be based on the difference between the temperature of fluid in the pipes and the temperature at the axis of the pile, under a constant heat injection rate in a homogeneous infinite concrete – ground solid, for a period that the thermal influence between all the pipes has been developed, while the conductive heat transfer along the pile is not significant. It is proved that the temperature at the axis of an EP does not depend on the number of the pipes, thus behaving as a reference temperature at a given time for an EP of a given diameter. The difference between the mean fluid temperature in the pipes and the temperature at the axis of the pile is analysed in four components. It is proved that only one of these components depends on time, and it is common in all EPs of the same diameter, the other three depending on the basic design parameters of the EPs, i.e. the number of pipes, the type and dimensions of pipes and the heat/flow characteristics in the pipes. EPs with different design parameters can be compared, the comparison based on the, easily calculated, constant in time temperature difference between the fluid and the axis of the pile.