Ground heat exchanger performance with variable speed ground-source heat pumps

Abstract

The objective of this paper is to compare the required length and performance of ground heat exchangers as well as heat pump energy consumption for fixed and variable speed ground-source heat pumps. In the first part of the paper, a physics-based model of a water-to-water heat pump is briefly presented. This model is incorporated in TRNSYS simulations using a performance map where variable speed operation is handled through a linear relationship linking the COP to the percentage of the full capacity being used. The ground heat exchanger is modeled using a thermal resistance and capacitance approach to account for borehole thermal capacity. Simulations are performed on a typical residential building located in a cold climate (Montreal, Canada) and equipped with either a fixed or variable speed ground-source heat pump. Results are obtained for eight cases with: variable or fixed speed operation (VSC or FSC), with or without consideration of borehole thermal capacity (TC or NTC), and with annual heating needs covered at 90% or 100% by the heat pump. The differences in the required borehole length between the TC and NTC cases are relatively small. The smallest required borehole length is for the FSC-90%-TC case (180 m) and the longest is for the VSC-100%-TC case (250 m). The VSC-100% case has the largest seasonal Performance factor (SPF) at 4.14 and the FSC-90% case has the lowest at 3.11.