Design of ground source heat pump systems. Thermal modelling and evaluation of boreholes.

Licentiate thesis, 2010

Ground source heat pump systems are fast becoming state-of-the-art technology to meet the heating and cooling requirements of the buildings. These systems have high energy efficiency potential which results in environmental and economical advantages. The energy efficiency of the ground source heat pump systems can be further enhanced by optimized design of the borehole system. In this thesis, various aspects of designing a borehole system are studied comprehensively. A detailed literature review, to determine the current status of analytical solutions to model the heat transfer in the borehole system, indicated a shortage of analytical solutions to model the short-term borehole response and the long-term response of the multiple borehole systems. To address the modelling issue of long-term response of multiple boreholes, new methods based on existing analytical solutions are presented. To model the short-term response of a borehole system, new analytical and numerical solutions have been developed. The new analytical method studies the heat transfer problem in the Laplace domain and provides an exact solution to the radial heat transfer problem in boreholes. The new numerical solution studies the one dimensional heat transfer problem using a coordinate transformation technique. The new solution can be easily implemented in any building energy simulation software to optimize the overall performance of ground source heat pump systems. Another significant aspect analyzed in this thesis is the uncertainty of input parameters when studying the thermal response of a borehole system. These parameters are often determined using in-situ thermal response tests. This issue has been investigated by conducting thermal response tests on nine boreholes of a newly developed ground source heat pump laboratory. The data from thermal response tests have been used to evaluate undisturbed ground temperature, ground thermal conductivity and borehole thermal resistance values for all nine boreholes. The sensitivity analysis of estimated parameters suggested that the short duration of the test causes the largest uncertainty in the ground thermal conductivity estimations. For tests longer than 48 hours the ground thermal conductivity estimations for nine boreholes vary within ±7 % of the mean value. The effects of variations in the estimated parameters on the design of a borehole system are examined for single as well as multiple borehole applications. The results indicate that the variations in the estimated parameters do not significantly affect the borehole field design.