Modeling of vertical ground loop heat exchangers for ground source heat pump systems

Scope and method of study. The purpose of this study was to examine the short time step behavior of vertical ground loop heat exchangers for ground source heat pump systems. A transient, two-dimensional finite volume model of the ground heat exchanger is developed for two purposes: (1) Non-dimensional temperature response factors for various ground heat exchanger borehole configurations are developed for hourly or less time steps. Such response curves provide a computationally efficient method of determining borefield thermal responses for building energy simulation. The short time step model is cast as a TRNSYS component model and is validated using actual field data from two example buildings. (2) The numerical model of the ground loop heat exchanger is used to develop a procedure to predict the effective thermal conductivity of ground formation surrounding the borehole. The procedure utilizes the downhill simplex parameter estimation technique of Nelder and Mead and is based on the inverse heat transfer approach. The objective function for the parameter estimation is defined to minimize the experimental and predicted borehole thermal responses. The effective thermal conductivity of the ground is found by minimizing the objective function.; Findings and conclusions. The transient, two-dimensional finite volume model of the ground heat exchanger borehole is a significant improvement over the currently available analytical (line source or cylinder source models) and numerical modeling approaches. It provides a significantly more accurate representation of the ground heat exchanger borehole by separately modeling the individual borehole components. The non-dimensional, short time-step response factors are a very useful extension of the long time-step response factors. The short time step response factors allow for an hour-by-hour or shorter time-step evaluation of system energy consumption and electrical demand. A more accurate and detailed assessment of the short-term behavior of ground-coupled heat pump systems can thus be made for the design of ground loop heat exchangers, energy analysis of ground source heat pump systems, and design and dynamic behavior of hybrid ground source systems.