Performance and economic analysis for optimal length of borehole heat exchanger considering effects of groundwater

Abstract

This study introduces a method for designing borehole heat exchangers (BHEs) that considers the system performance and cost according to the design parameters. A cost-performance factor for the BHE length (CPL) is also proposed to determine the optimal BHE length that balances the initial and operating costs. The long-term performance of ground source heat pump (GSHP) systems was analyzed in a single-family house in Seoul, South Korea. The ground thermal properties applied to this study are determined based on the analysis results of the thermal response test. The groundwater velocity is estimated by the apparent effective thermal conductivity which is overestimated in the TRT analysis. By varying the borehole length and design parameters, the changes in the ground temperature and system performance were calculated along with the initial and operating costs. As a result, applying the minimum CPL in each case, the optimal BHE lengths were determined to be 150 m for Case 1 and 140 m for Cases 2 and 3. It could reduce the −645.7, −757.5 and −807.9 USD with maintaining the high system performance, compared with ASHP for 20 years of the operating system.