Thermodynamic Modeling of HVAC Plant Cooling Equipment for Quantification of Energy Savings Through Continuous Commissioning Measures

The Continuous Commissioning (CC) process is applied to existing buildings in order to reduce energy consumption by optimizing HVAC system operation and improving occupant comfort. The CC process consists of implementing energy saving measures for the air-side and plant-side of HVAC systems. Current development of a computer program (WinAM) by the Energy Systems Laboratory allows the expected energy savings from applying air-side CC measures to a given building to be estimated. However, there is no means for quantifying the potential energy savings from applying plant-side CC measures. The quasi-steady-state method and a regression of EnergyPlus library data were used for chiller modeling and the Merkel method was used for cooling tower modeling. Implementation of the models developed provides a means for quantifying the energy savings associated with plant cooling equipment CC measures.

Chiller models have been developed for the following, with capacity range, average error, and standard deviation in parenthesis: air-cooled scroll chillers (15-168 tons, 8.07%, 9.13%), air-cooled screw chillers (69-513 tons, 7.38%, 6.13%), water-cooled scroll chillers (20-200 tons, 8.16%, 9.72%), water-cooled reciprocating chillers (20-364 tons, 10.30%, 7.81%), water-cooled screw chillers (194-498 tons, 9.87%, 3.65%), and water-cooled centrifugal chillers with inlet guide vane capacity control (233-677 tons, 12.07%, 5.96%) and with VSD capacity control (210-677 tons, 12.18%, 4.61%). From the chiller models developed, energy consumed by the chiller can be calculated as building cooling loads and fluid operating temperatures vary.

Cooling tower models have been developed to predict cooling tower energy consumption as building cooling loads, added load from chillers, fluid operating temperatures, and ambient air temperatures vary. The models developed provide for predicting energy consumption when fan operation is by single-speed, two-speed, variable-speed with modulating outlet dampers, or variable-speed with VFD control.

Implementation of the chiller and cooling tower models developed will allow WinAM users the ability to quantify the potential energy savings associated with changing plant cooling equipment operation.