Optimal design of a hybrid ground source heat pump for an official building with thermal load imbalance and limited space for the ground heat exchanger

Abstract

This work presents the optimal design of a hybrid ground source heat pump (GSHP), taking into account thermal imbalance and space limitation for the ground heat exchanger field (GHE), applied to an official building. Once the building loads are calculated and devices selected, experiments carried out from a single vertical borehole obtain the ground thermal characteristics, including a local short-term period function (STGF). From them, the Finite Line-Source (FLS) model simulates the GHE behavior, from decomposing the ground thermal loads in hourly linear steps for 50 years. A set of input variables, such as geometric configuration data of boreholes field, and additional terms associated with this hybrid operation, must be provided to the model. For optimization purposes, a design of experiments (DoE) considers the thermal ground characteristics and input factors, providing both energy savings and the internal rate of return as outputs (objective functions). Pareto's optimal solutions method provides the selected case, considering a compromise between economic and environmental benefits. It has been established for 18 boreholes (rectangular disposition) of 120 m deep, providing a 33.12% energy saving and an internal rate of return of 3.9%, also showing 89% of the total building load supported by the GHE.