Hybrid Ground Source Heat Pumps: Life Cycle Costs Compared to Non-Hybrids (ST-16-C025)

Simulations were conducted in order to make assessments regarding life cycle costs of hybrid ground source heat pump systems with different pumping configurations on an elementary school design.  The long term simulations evaluated the systems’ performance over a 20 year period. The basecase in this study is an all-ground heat exchanger (all-GHX) configuration.  A closed circuit cooling tower (CCCT) and dry fluid cooler (DFC) were separately modeled to provide heat rejection in the hybrid systems.  Life cycle costs of the all hybrid systems presented herein for an example building are estimated between 35-40% less than an all-GHX configuration.  The basecase life cycle cost is estimated at $894,000 while the hybrid options ranged from $565,000 to $578,000. These likely do not represent the lowest life cycle cost designs which would balance the sizing of the system components including the ground source heat exchanger and supplement heat rejection device with the associated energy costs’ present value.  In many instances, there are space constraints on sizing individual components, most notably the ground heat exchanger.  As such, the alternate hybrid designs arbitrarily utilize 140 bores which is one-half the size of the base design at 280 bores, with the addition of a supplement heat rejection device which is sized to provide acceptable borefield temperatures. The control scheme utilized across all hybrid systems considered common in industry (Thornton 2014),  inherently allows loop temperatures to elevate and forms a solid basis upon which to make valid conclusions regarding life cycle comparisons between an all-GHX design and Hybrid systems.  The heat pump entering water loop temperature (EWT) target was designated as 95⁰F, but iterations to reduce the sizing of the supplemental heat rejection device and allow higher temperatures of no more than 100⁰F were acceptable if resultant life cycle costs were favorable.  DFC physical size and acoustical concerns were peripheral considerations. Hybrid systems are designated as Case One thru Case Four. Additional simulations with further reduced borefields are briefly explored which indicated possible life cycle costs savings of over 40% with total life cycle costs of around $500,000, and presented as Case 1A and 4A.  Pumping configurations included the use of dual individual circulator pumps, single circulator pumps w/central variable speed pump, and a single central variable speed pump.  The basecase which utilized dual circulator pumps used the most pump energy, while a central variable speed pump achieved 60% pump energy savings.