24 June 2014

IMPROVING THE ASHRAE METHOD FOR VERTICAL GEOTHERMAL BOREFIELD DESIGN


The design of a system of Borehole Heat Exchangers (BHE) coupled to a
geothermal heat pumps have to be done with a suitable dynamic model able to cope with the intrinsic time varying behaviour of the ground mass and building heat load profile. Among the models based on the solution of transient conduction equation, the Ashrae method (Kavanaugh and Rafferty) is probably the fastest algorithm for calculating the overall length of ground heat exchangers starting from the knowledge of the building thermal energy requirements and ground properties. This method employs Infinite Source (IS) solutions for describing the ground response to a series of three heat pulses, representing the building thermal history from the short to the long period. Since IS solutions cannot describe 2D and 3D effects in the ground temperature field, a correction parameter is introduced. This parameter is named Temperature Penalty Tp, which also accounts for the thermal interactions of neighbouring boreholes in the long term period.

In this paper a new method is proposed for the calculation of the Tp parameter. The new method is conceived for maintaining the simplicity of the original Ashrae model while
enabling a more accurate design of the BHE field. The validation of the proposed procedure and the estimation of the constants related to the new method is made by assuming the Tp
values as inferred from FLS generated g-functions, able to describe the ground response to a large number of BHE configurations, including square, rectangular, in-line, L-shaped, open
rectangles. With reference to the present set of BHE configurations (240), it is demonstrated that the Ashrae Tp values are typically underestimating the “correct” value counterparts (average deviation more than 40%), thus leading to an underestimation of the BHE field
overall extension. The proposed method, based on the calculation of a set of constants to be applied to specific geometries (square, rectangular, in line arrangements) is able to provide Tp values well centered around the benchmark line and with an average deviation of less than 10%, with estimated BHE overall lengths (ground heat extraction mode) very close (1.4%) to reference FLS values.