Hybrid Geothermal Heat Pumps

Why Geothermal Heat Pumps?

 The advantages of Geothermal Heat Pumps over conventional alternatives make them a very attractive choice for space conditioning and water heating for The advantages of GHPs over conventional alternatives make them a very attractive choice for space conditioning and water heating for both residential and commercial/institutional buildings. However, Geothermal Heat Pumps 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 Geothermal Heat Pump 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 Geothermal Heat Pump system will vary with location and climate, but it must be at least large enough to handle the building heating requirements. Hybrid Geothermal Heat Pumps can also be used or 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.A number of recent reports and research papers have been published that deal with both design of hybrid GHPs and operating experience with a few installations. ASHRAE (1995) and Kavanaugh and Rafferty (1997) both discuss advantages of hybrid Geothermal Heat Pumps and present design procedures. The former sizes the auxiliary heat rejecter based on the difference between monthly average heating and cooling needs of the building and offers general guidelines for integration of the heat rejecter into the
system piping. The latter bases heat rejecter sizing on peak loads at design conditions
and the difference between required ground heat exchanger borehole lengths for heating and cooling.


Based on the results from the two case studies analyzed for this review, the following observations are made.
• Hybrid GHP systems can significantly reduce system first costs even when a tower needs to be purchased. Costs can be reduced by more than 50% for very highly cooling dominated applications such as the small office building in Houston (cooling-to-heating load ratio of 24:1). For applications where a suitable tower already exists (as at the Oceana study site), a hybrid system can result in system cost reductions of more than 50% even when the building is not overly cooling load dominated.
• For heavily cooling dominated sites, hybrid GHPs can result in heat pump and system energy savings compared to full GHPs when the supplementary heat rejecter is operated enough hours to reduce the average heat pump entering fluid temperature during the cooling season.
• The authors of both case studies point out that none of the hybrid system designs they examined have been optimized. A design optimization method is needed to balance GHX size, supplemental heat rejecter size and type, control strategy, and electric rate structure to achieve lowest life-cycle or first cost designs for a given location.