Modeling the dynamic simulation and control of a single effect LiBr–H2O absorption chiller

Abstract

In this study, a dynamic model of a single-effect LiBr–H2O absorption chiller with improved accuracy is built to explore the dynamic performance and the control strategy. Holman correlation is applied to the calculation of heat transfer coefficients of the evaporator, condenser, solution heat exchanger, and generator. The heat and mass transfer in the absorber, chiefly controlled by the mass transfer resistance on the liquid side, are described according to a lumped but accurate physical model based on Nusselt’s film theory. The dynamic performance of the absorption chiller is evaluated under the disturbance of the cooling water inlet temperature and heat source temperature. The effect of the thermal mass of the main components on the dynamic performance is explored. Two control strategies are implemented in the model of the absorption chiller: one is setting the chilled water outlet temperature as the manipulated variable, and the other is setting the generator solution temperature as the manipulated variable. The control performances of the two strategies are compared in detail. Results show that either increasing the heat source temperature or decreasing the cooling water inlet temperature increases the risk of crystallization in the dynamic process. The time to reach steady state is highly dependent on the thermal mass of the generator, rather than the thermal masses of the condenser, evaporator, and absorber. The single-effect chiller has better control performance in off-design condition when the generator solution temperature rather than the chilled water outlet temperature is selected as the manipulated variable.