Energy Center of Wisconsin | Hybrid ground-source heat pump installations: experiences, improvements and tools
  Energy Center of Wisconsin  

Sunday, Apr 20, 2014




Hybrid geothermal systems: less is more, presented by Scott Hackel. Originally broadcast Sept 29, 2011.


Scott Hackel
Scott Hackel
PE, LEED AP, Senior Energy Engineer

what we do :: BUILDINGS

Hybrid ground-source heat pump installations: experiences, improvements and tools

Install a hybrid geothermal heat pump system and reduce the cost of implementing geothermal heating and cooling on your building project. A hybrid system reduces the peak capacity of your ground loop, letting you install a smaller, less expensive ground heat exchanger.

diagram of hybrid geothermal heat pump systemWe collected a year of operating data on three working hybrid system installations to analyze the economic and environmental impact of the hybrid approach and to compare it to other HVAC system designs. We used this data to investigate what contributes to an effective hybrid design and to validate energy models of these systems.

The Energy Center of Wisconsin with assistance from the University of Wisconsin Solar Energy Laboratory studied three working hybrid system installations to compare their economic and environmental impacts with other HVAC systems. The study was funded by the United States Department of Energy, Alliant Energy, and Madison Gas and Electric. As a result of this project we developed documents and tools for HVAC system designers to use, including a freeware version of our model, to assess the benefits of applying the geothermal hybrid approach on their building projects.

Other resources

Our work on this project complements other resources available on hybrid ground-source design, operation, and economics.


  • For basic design information, see: A Design Method for Hybrid Ground-Source Heat Pumps by Kavanaugh (ASHRAE Transactions 1998); other work by Kavanaugh and Rafferty. Available for purchase at the ASHRAE Bookstore.
  • Oklahoma State University has completed in-depth research on hybrid ground-source systems. Recent research focuses on an array of different hybrid control options and is summarized in a thesis by X. Xu PDF.
  • Hackel's previous work at the University of Wisconsin looked at optimal size and control of hybrids, including a few sample control sequences.


  • Ground Loop Design 2010 or GLHEPro can be used to test out the viability of some hybrid designs (its hybrid functionality is relatively basic).
  • TRNSYS and EnergyPlus (well-tested full-building simulation software packages) have the capability to complete detailed sub-hourly models of hybrid ground-source heat pumps systems.