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Primary chilled water system control optimization integrated with secondary system linerization

Lixia Wu, University of Nebraska - Lincoln


As a large energy consumer, it is well-known that building heating, ventilation, and air-conditioning (HVAC) systems are highly nonlinear and dynamic. The primary system is designed to provide cooling or heating to the secondary system, such as the valve-coil system, to maintain the supply or zone temperature set point. Such complex systems may be arranged in a variety of configurations, and operated under different operating conditions in various environments. Proportional and integral (PI) control loop, which is designed for linear and time-invariant system, may cause valve hunting issues and temperature fluctuation when applied for HVAC system. One of the most significant challenges of this control problem is the presence of significant nonlinearities and dynamics in the response from the control input. Therefore, superior control is required at the primary system level to link the control of the primary system with the nonlinearities of the secondary system.^ Therefore, the objective of this study is to develop an integrated primary system optimal control method which can create a linear environment for the secondary system. First, a thorough investigation of the variable primary flow chilled water system was conducted on the pump head and water loop differential pressure under differing load distributions. Characterization of the secondary system was conducted through theoretical modeling and simulation to reveal the key control characteristics and the impacts of such characteristics on the control and energy performance of the integrated system. This study concluded that the differential pressure in the water loop substantially impacts the control performance and stability of the secondary system.^ The traditional primary system control method is constant differential pressure (DP) set point control. An innovative linearization algorithm was developed and integrated with the secondary system flow resistance to ensure that constant-gain linear PI control design can achieve desired system performance and conserve energy.^ Both traditional and optimal control algorithm were implemented and compared in two full-scale real building experiments. Compared to the prior art, this innovative optimal algorithm created a system that was more energy efficient, given the desired control performance, and resulted in increased control stability. The linearization algorithm is advantageous in that it results in lower pump head and improves pump efficiency, better coil-valve control stability, and significantly reduces the frequency of control valve repositioning.^

Subject Area

Engineering, Architectural|Engineering, General

Recommended Citation

Wu, Lixia, "Primary chilled water system control optimization integrated with secondary system linerization" (2010). ETD collection for University of Nebraska - Lincoln. AAI3432317.