Optimal Control Method With Variable Differential Pressure Setpoint In Air Conditioning Chilled Water System

As the critical object of building energy efficiency investigation during the eleventh five-year plan in China, large scale building has great energy saving potential. Optimal regulation and control of air-conditioning water system has led an important role in energy saving implementation.Most unfavorable water loop has been adopted as referenced loop during operation and control process in air-conditioning water system. Most unfavorable water loop actually concerns about the hydraulic characteristics in the conventional conception, which can be defined as most unfavorable hydraulic loop (MUHL). This paper firstly analyzes that application of MUHL in variable flow regulation is unreasonable, then points out that MUHL only presents the distributing status of system hydraulic characteristics under design condition, variable flow control strategy should be seleced according to the load demend of each user. Therefore, this paper defines a new type of most unfavorable water loop, namely most unfovarable thermodynamic loop(MUTL), then presents MUTL identification method in typical air-conditioning water system with continuous flow regulation and on-off flow regulation, respectively. With respect to network stability issues that led by variable water flow regulation, based on conventional stability definition, this paper presents the conception of thermodynamic stability, which is to evaulate effect of flow change in active branches on cooling capacity of air-conditioning terminal in passive branches.This paper also presents the evaluation index of thermodynamic stability using the control parameters attached in terminal control loops as referenced variable. Based on the definition of MUTL and thermodynamic stability, by means of simulation and experiment, this paper develops and investigates variable setpoint control strategy in differential pressure control loop in variable flow chilled water system.Firstly, according to MUTL and its identification method, this paper presents a novel variable differential pressure setpoint control strategy, which includes optimal reset strategy based on valve position domain in maximum valve position loop and optimal allocation strategy on speed ratio and operational number in parallel pump system. Using the simulated model of air-conditioning chilled water system built based on practical project, this paper investigates the feasibility of presented strategy, disscusses the application effect of three strategies involving non differential pressure control, constant differential pressure setpoint control and variable differential pressure setpoint control by simulated cases adopting thermodynamic stability as evaluation parameter. Simulation results show that variable differential pressure setpoint control strategy has great pump energy saving potential while effectively decreases or avoids the occurring probability of MUTL. However, the adjustment degree of differential pressure setpoint needs to be selected with reference of thermodynamic stability.Secondly, this paper presents two reset algorithms of differential pressure setpoint including linear reset algorithm and Mamdani fuzzy model based adaptive reset algorithm, investigates the parameter tuning, adaptability and energy saving potential of each algorithm by simulation. Results show that the adaptive reset algorithm performs better on adaptability than linear reset algorithm and attains 15.1% pump energy saving compared with linear reset algorithm. Integrated with presented strategy and algorithm, optimal control method with variable differential pressure setpoint is developed.Finally, this paper conducts a series of experiment to investigate the practical effect of optimal control method with variable differential pressure setpoint, test work include parameter tuning test in supply air temperature control loop and differential pressure control loop, online MUTL identification test, reset algorithm test and comparative test on application effect of four differntial pressure control methods. Experimental results show that optimal control method with variable differential pressure setpoint presented in this paper has preferable effect on MUTL elimination and supply temperture control.Optimal control method with adaptive reset algorithm could attain 10.6% to 45% pump energy saving than other three control method.