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ABSTRACT
A detailed borehole heat exchanger model is presented, cast as a TRNSYS component model, for use in ground source heat pump system simulations with optimization of system subcomponents. The proposed borehole heat exchanger model is based on non-dimensionalized short time step response factors, and includes a time-dependent borehole thermal resistance that is due to transient responses from the surrounding ground and short time-step thermal storage effects of the borehole grout and heat carrier fluid. These effects have been accounted for by developing a finite element model of a borehole heat exchanger where the fully transient borehole thermal response is modeled and coupled to a short time step ground response factor model. Furthermore, each response factor function (g-function), describing the thermal response of a particular borehole field to a unit heat pulse is developed to allow for varying borehole spacing-to-depth ratios so that borehole spacing is independent of the borehole depth. The model is developed with the specific objective use in optimization problems for hybrid ground source heat pumps systems. A detailed model validation and sensitivity analyses are presented and discussed.
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INTRODUCTION AND BACKGROUND
In the United States, ground source heat pump systems that utilize vertical U-tube ground heat exchangers in closed loop configurations have, over the last decade, experienced market growth in space air-conditioning for residential, commercial, and institutional buildings. The upward trend in market growth is relatively steady and similar advances are expected in European and Asian markets (Lund et al. 2005 and Energy Information Agency 2008). The market gains are primarily due to the fact that ground source heat pump systems offer significant advantages over their conventional alternatives with respect to energy savings due to higher coefficients of performance and associated improvements in system life cycle and operating costs, reduced greenhouse gas emissions, and improvements in building thermal load profiles. In the past, many such systems were designed and installed with a "seat-of-the-pants" approach, and system sizes were mostly justified based on the experience of the design engineers, resulting mostly in oversized designs. With the development of more accurate and reliable system design and simulation tools that have been available to field engineers and the engineering design community, confidence of building owners in ground source heat...