37 that can help reduce the peak electricity demand in the winter. Armstrong noted that while GSHP systems are expensive, proper sizing of the ground loop, better coupling to ground mass, dual-source heat pumps (i.e. both air and ground can be used as a heat source), variable speed pumps, and optimizations in the condenser and evaporator systems can dramatically reduce first and operating costs and ultimately deliver better performance for the occupants. Armstrong also discussed an engineering model he has developed to better size ground source heat pumps using building loads and weather data as well as a 3D finite element model developed to determine the spatial and seasonal variation in ground temperatures. Discussion The discussion after the presentation focused on the importance of balancing heating and cooling loads for the ground heat exchanger, otherwise the system will inadvertently increase or decrease ground temperatures dramatically reducing the long-term efficiency of the system. The discussion also noted that, while simply increasing borehole depth is a great way to increase system efficiency, optimization is the most effective path to a least life-cycle cost solution. Photo: Justin Knight Fig 4.3 Prof. Norford presenting to the group
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