Thermal modelling and evaluation of borehole heat transfer.

Doktorsavhandling, 2012

The use of ground source heat pump systems to provide heating and cooling in buildings has increased greatly in the last decade or so. These systems have a high potential for energy efficiency, which has environmental and economic advantages. Moreover, the energy efficiency of the ground source heat pump systems can be further enhanced by optimizing the performance of the system. However, a key obstacle to the performance optimization of ground source heat pump systems is the scarcity of mathematical models that can rapidly, yet accurately, simulate the dynamic thermal response of the borehole system. This study aims to develop analytical models and methods that can simulate the thermal response of a borehole system in time scales from minutes to years. An analytical solution to model the short-time response of the borehole system is presented. The solution studies the heat transfer problem in the Laplace domain and provides an exact solution to the radial heat transfer problem in the borehole. A finite-length line-source solution to determine the long-term response of the borehole system is also presented. The line-source solution can be used for modelling both single and multiple borehole systems. The analytical and finitelength line-source solutions were combined to obtain step-response functions for various configurations of borehole systems. The step-response functions are valid from short (hours) to long (years) periods. A load aggregation method is also presented to speed up the simulations of the borehole systems. All the proposed models and methods can be easily implemented in any building energy simulation software to optimize the overall performance of ground source heat pump systems. The study also analyzes various aspects of the thermal response testing and evaluation of borehole systems. A ground source heat pump test facility with nine boreholes was used for the experimental investigations. Several thermal response tests were conducted for issues that include random variations between tests, sensitivity of system design to uncertainties in test results, convective heat transport in boreholes, and recovery times after a test. The evaluations of multiinjection rate tests on groundwater-filled boreholes were also extensively studied. Recommendations regarding each of these issues are suggested to improve the testing and evaluation procedure of borehole systems.