CH-18-008 -- Analysis of Whole-Building HVAC System Energy Efficiency ✓ Most Recent

This paper defines a whole-building cooling and heatingefficiency measure, called the building systems load/energyratio (LER), as a comprehensive HVAC system efficiencymeasure. The LER for cooling--or separately, heating--isthe ratio of the total cooling/heating loads on the building'sheating and cooling systems to the total energy inputprovided by all the heating and cooling system componentsto meet the loads. Hence, it is basically a system coefficientof performance (COP) or "efficiency" that includes all of theheating and cooling system energy consumed instead of justthe boiler or chiller energy. Specifically, the building systemstotal load is composed of the envelope load, the load frominternal gains, and the ventilation load on the secondarysystems. The total energy input includes inputs to chillers,boilers, pumps, and fans on both the air side and water side.Hourly LER values for both heating and cooling are calculatedseparately when there are both heating and coolingloads in a building during the same hour. The LER may bedefined in terms of the cost of different energy sources, interms of site energy consumption, or in terms of source energyconsumption. Two buildings on the Texas A&M Universitycampus that are served by a central utility plant are used ascase studies to illustrate the methodology. The annual coolingcost LERs for the two case study buildings are 2.28 and2.46 (1.54kW/ton and 1.43kW/ton), respectively--about halfof the chiller plant's annual average cooling load/energyratio of 4.6. The annual heating cost LER for these buildingsis 0.043 for one building and 0.46 for the other building, whilethe boiler plant average heating efficiency is 0.79. The siteenergy cooling LER is 19% to 37% lower than the coolingcost LER for the two case study buildings, while the sourceenergy cooling LER is about one-third of the cooling cost LERin both cases. The site energy and source energy heating LERvalues range up to 28% larger than the corresponding heatingcost LERs. The current minimum seasonal energy efficiencyratio (SEER) value for a split-system air-conditionerin Texas is 14, which corresponds to a COP of 4.1. It is noteworthythat to the extent that SEER represents annual performance,the annual cooling cost LER values of the case studybuildings are 60%, or less than the minimum COP value fora split system, even for a variable-air-volume (VAV) systemwith very little reheat.