Calculating the g-Function of Bore Fields with Series- and Parallel-Connected Boreholes

From eSociety, November 2019

Thermal response factors used in the design and simulation of ground-source heat pump systems have been extended to fields with mixed parallel and series connections between the boreholes. They previously were only applicable to fields of parallel-connected geothermal boreholes. 

In a recent article from Science and Technology for the Built Environment, Massimo Cimmino, Associate Member ASHRAE, with Polytechnique Montréal, presents a semi-analytical method for the calculation of g-functions of bore fields with mixed arrangements of series- and parallel-connected boreholes.

Cimmino discusses his research. 

 1. What is the significance of this research?

Thermal response factors used in the design and simulation of ground-source heat pump systems were previously only applicable to fields of parallel-connected geothermal boreholes. This research extends their application to fields with mixed parallel and series connections between the boreholes.

2. Why is it important to explore this topic now?

With the growing popularity of ground-source heat pumps and borehole thermal energy storage systems, new tools are needed to not only assess the consequences of the borehole positions within the borehole field but also the impacts of the piping connections between the boreholes.

3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

The research introduces the concept of the effective borehole field thermal resistance, which gives a relation between the total heat extraction rate from the borehole field, the mean fluid temperature within the borehole field and the effective borehole wall temperature. 

It is seen that that series connections between boreholes increase the borehole field thermal resistance when compared to strictly parallel connections between boreholes. This borehole field thermal resistance is then needed for accurate predictions of ground-source heat pump efficiency and an accurate design of a borehole field.

4. How can this research further the industry's knowledge on this topic?

The presented methodology enables the analysis of borehole fields with both parallel and series connections between the boreholes. It will prove useful in the design and simulation of ground-source heat pump systems as well as the interpretation of thermal response tests.