Variable Speed Chilled Water System Modeling & Optimization
Abstract
The emergence of increasingly affordable variable speed drive technology has changed the approach for how chilled water systems equipped with variable speed drives should be controlled. The purpose of this study was to estimate the potential energy savings that can be achieved through optimization of a single chiller system equipped with variable frequency drives on all pieces of equipment in the condenser water system. Data for a case study was collected from a local museum’s chilled water system. The chilled water system was already equipped with variable frequency drives on the condenser water pump, cooling tower fan and the centrifugal chiller, but the building automation system did not possess appropriate control logic for controlling equipment speed to reduce the system’s energy consumption. To accomplish the objective, physical component models of the centrifugal chiller, cooling tower and condenser water pump were established with the goal of incorporating the system’s condenser water flow rate and cooling tower fan speeds as optimization variables. Furthermore, a simple cooling load prediction algorithm was developed using a multiple non-linear regression model in order to approximate the buildings cooling load subject to a range of environmental conditions. The inputs and outputs of the individual component models were linked to estimate how adjusting the cooling tower fan and condenser water pump speed would influence the system’s overall performance. The overall system model was then optimized using a generalized reduced gradient optimization algorithm to determine the potential energy savings through speed control with VFDs and ascertain a simple control logic strategy for the building automation system to operate the system. After running the optimization algorithm it was discovered that optimizing the cooling tower fan speed could save approximately 12- 15% of the system’s energy consumption. Alternatively, optimizing both the cooling tower fan speed and the condenser pump power demand offered almost no energy saving potential over optimizing the cooling tower fan alone. The control strategies investigated were to control the cooling tower fan speed directly based on the ambient wet-bulb temperature and indirectly control the fan to achieve an optimal tower approach based on the ambient wet-bulb temperature. Based on the results of the optimization process, the correlation between the optimal fan speed and the wet-bulb temperature was substandard while the correlation between optimal tower approach and the ambient wet-bulb temperature was superior.
Degree
M.Sc.
Advisors
Razban, Purdue University.
Subject Area
Atmospheric sciences|Energy|Thermodynamics
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