Hybridizing a geothermal heat pump system with solar thermal collectors is advantageous in realizing smaller, lower cost borehole heat exchanger arrays, in addition to achieving a more sustainable geothermal system over the long term. However, solar-geothermal heat pump system operation involves multiple, simultaneous physical processes, including building load dynamics, heat pump dynamics, heat transfer in the Earth, and solar thermal processes. Each of these processes occurs over various time scales on the order of minutes up to many decades. Lack of accurate design tools to accurately capture these effects leads to lack of confidence in further deployment of these systems. In this paper, we describe the development and use of a new, novel simulator for hybrid solar-geothermal heat pump systems. The simulator was developed using TRNSYS, a transient systems simulation environment with a modular structure, where the thermal performance of system components is described in the FORTRAN computing language. A controls and multi-variable optimization strategy has been implemented in to the simulator, where the result is the optimal depth and number of borehole heat exchangers and solar thermal collectors to achieve balanced thermal loads on the Earth over the annual cycle.

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