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DOE, Alliant Energy, and Madison Gas & Electric co-funded the study, which analyzed operating data from three working hybrid GSHP systems. The Energy Center of Wisconsin, with assistance from the University of Wisconsin Solar Energy Laboratory, observed two hybrid systems in Las Vegas and one in Madison, Wis., for one year to examine hybrid GSHP in climates with different heating and cooling needs. In all three cases the hybrid design was effective in comparison to other HVAC system designs. The study validated hybrid systems as a cost-effective alternative to traditional ground-source heat pump systems.
In standard GSHP systems, also known as geothermal heat pumps, a large underground heat exchanger made from a loop of plastic pipes is buried or drilled into the ground. A fluid circulates through the pipes, to move heat between the in-ground heat exchanger and a heat pump, which regulates building temperature. The upfront installation cost for this underground loop is significant, up to half the cost of the entire system, but necessary to achieve the desired temperature levels.
With the hybrid system, the size of the underground loop is significantly smaller, making drilling less expensive and less invasive. To compensate for the smaller loop system, conventional technology (such as cooling towers or boilers) is used to meet a portion of the peak heating or cooling needs. Balancing the load of the GSHP with traditional HVAC equipment lowers up-front costs, while still providing the energy efficiency of a GSHP. This study showed that due to typical building load fluctuations, the standard equipment runs so infrequently it doesn’t substantially reduce the energy savings.
With DOE support, the Energy Center developed documents and tools for HVAC system designers to use, including free modeling software that can assess the benefits of applying the hybrid GSHP approach on building projects. The software can be used to:
• Determine how much money a building owner can save by choosing a hybrid geothermal system.
• Select optimal sizes for the equipment in a hybrid system.
• Compare different hybrid geothermal installation approaches to understand the financial, energy, and environmental impact for a given building project.
• Analyze the effects of different control strategies for a geothermal or hybrid system, to keep maintenance costs and energy bills as low as possible.
For more information, visit www.ecw.org/project.php?workid=1&resultid=464.
Publication date: 11/14/2011