Gas engine-driven heat pumps (GHPs) are an emerging technology offering a cost-effective option to reduce peak electric demand, electricity consumption, and lifecycle costs as compared to conventional space conditioning equipment. The objective of this project was to evaluate the annual performance of a GHP system in a side-by-side comparison with an electric cold climate heat pump (CCHP) relative to the baseline performance of existing conventional HVAC at Naval Station Great Lakes (NSGL) in North Chicago, Illinois. Both heat pumps selected for this demonstration had variable refrigerant flow (VRF) configurations to provide multi-zone heating and cooling. The demonstration evaluated the energy and economic benefits of each technology for cold climate Department of Defense applications based on measured performance data.
The GHP design is similar to an electric vapor compression heat pump, but in place of the electric motor, GHPs use an advanced natural gas engine with a demonstrated long life (30,000 hours). During cooling, GHPs consume natural gas in place of electricity, significantly reducing peak electric use. GHP rated efficiency is 50% higher than standard gas furnaces or boilers. Heat recovered from the engine is used to supplement the GHP output during heating mode to increase the overall system efficiency and maintain supply temperatures at low ambient conditions. In contrast, electric heat pumps often require inefficient resistance heating to supplement the heat pump output at cold temperatures.
Based on measured data, the GHP reduced peak electric demand by up to 59 kW (90%) compared to the VAV baseline and by up to 30 kW (82%) compared to the CCHP. The GHP also reduced annual energy costs by 71% relative to the baseline and by 41% relative to the CCHP/DOAS. While energy savings is an important measure of performance, a more meaningful metric is life-cycle cost. In this measure, GHP was 4% lower than CCHP, given the higher first-cost of GHP, and 37% lower than the baseline. The site-specific utility rates (gas and electric) will have a significant impact of the relative economic performance of GHP and CCHP and should be considered when evaluating options.
Both VRF systems improved comfort and provided significant energy savings, lower peak electric demand, and potential savings in life-cycle costs compared to conventional VAV systems. The demonstration also identified operational issues for both VRF technologies in this application. The electric CCHP was unable to meet the heating load for several days when ambient temperatures dropped below winter design conditions indicating a need for supplemental heating for this climate (American Society of Heating, Refrigeration, and Air Conditioning Engineers Zone 5). The demonstration also highlighted the low range of part-load operation for VRF heat pumps, even when sized appropriately. Very low part-load operation adversely impacted both heat pumps; however, this specific GHP model had lower than expected performance at part-load operation. While not inherent in this class of technology, this may be due to product-specific controls or equipment sizing. These results suggest additional development is needed to optimize GHP performance and to reduce installed costs in order to improve regional economics and support broader market adoption.