24 June 2014

FULLY ANALYTICAL FINITE LINE SOURCE SOLUTION FOR FAST CALCULATION OF TEMPERATURE RESPONSE FACTORS IN GEOTHERMAL HEAT PUMP BOREFIELD DESIGN


Ground-coupled heat pump (GCHP) systems very often employ vertical borehole heat exchangers (BHE) which are in charge of extracting or injecting heat from and to the ground by exploiting the conductive properties of the soil. The ground-to-BHE-field thermal interactions are a complex transient phenomenon that requires the knowledge of the building
energy demand in time and a suitable engineering model for predicting the ground temperature variations in the short and long term. A computationally efficient way to tackle this problem is the recursive calculation of a basic thermal response factor for given different heat pulses representing the building energy demand. In this paper a review of the existing response factor models for BHE analysis is performed and the Finite Line Source (FLS)
model is employed to develop and refine new fully analytical and explicit FLS solutions suitable for fast spatial and temporal superposition. The calculations of the temperature
response functions are made for a great number of BHE configurations, including different layouts, from compact geometries (e.g. rectangular matrixes) to in-line and open
arrangements. Comparisons are finally made among the reference and approximated solutions, also in terms of calculation time. This analysis shows that present analytical expressions can reduce the computation time of large borefield g-function generation down
to 1% while maintaining an acceptable average error of about 3% with respect to literature analytical FLS solutions.