Assessment of Hybrid Geothermal Heat Pump Systems - Technology Installation Review

No portion of this publication may be altered in any form without prior written consent from the U.S. Department of Energy and the authoring national laboratory. both residential and commercial/institutional buildings. However, GHPs often have higher first costs than conventional systems making short-term economics unattractive. This disadvantage can be magnified in commercial buildings, many of which have much larger cooling needs than heating needs, especially for buildings located in climates typical of the southern United States. For GHP systems using closed-loop vertical ground heat exchangers, this load imbalance can result in a ground temperature increase over time causing system performance deterioration. Increasing the size of the ground heat exchanger or increasing the distance between adjacent heat exchanger boreholes can postpone the temperature increase problem but will also result in higher system cost. An alternative, lower cost approach for such applications can be use of a hybrid GHP design. In hybrid GHPs, the ground heat exchanger size is reduced and an auxiliary heat rejecter (e.g., a cooling tower or some other option) is used to handle the excess heat rejection loads during building cooling operation. The extent to which the ground heat exchanger size can be reduced in a hybrid GHP system will vary with location and climate, but it must be at least large enough to handle the building heating requirements. Hybrid GHPs can also be used for sites where the geological conditions or the available ground surface will not allow a ground heat exchanger large enough for the building cooling loads to be installed.