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.
• 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.