AN ENERGY DELIVERY EFFICIENCY INDEX TO EVALUATE SIMULTANEOUS HEATING AND COOLING EFFECTS IN LARGE COMMERCIAL BUILDINGS

HVAC systems of large commercial buildings consume energy in excess of the sum total of the building loads. This excess energy use is due to the fact that a single air-handler unit in a HVAC system, having to p rovide conditioned air at different supply temperatures to different z ones in the building, can do so only by resorting to either (a) a cert ain amount of mixing of cold and hot air streams as in dual-duct syste ms or (b) to terminal reheating in single-duct systems. This mixing of cold and hot air streams or terminal reheating result in an energy pe nalty which can be minimized by say, converting a constant air volume (CAV) system to a variable air volume (VAV) system, but it cannot be e ntirely eliminated. This paper proposes an index, called the Energy De livery Efficiency (EDE), which characterizes this penalty and rates th e energy performance of HVAC systems on an absolute scale. We develop the mathematical basis of the EDE approach for both one-zone and two-z one buildings, that allows determining the variation of the ideal EDE with outdoor temperature for a specific building. Year-long measured w hole-building cooling and heating energy use data from two retrofitted buildings are finally used to illustrate differences between actual E DE plots of CAY and VAV systems, how they compare with the ideal EDE o f a two-zone building, and how the approach can provide diagnostic ins ights into HVAC system efficiency in specific buildings.