Model-based feedback control of an ammonia-water absorption chiller

In this study, potential control strategies for a small-scale, 3.5-kW ammonia-water absorption chiller are developed and investigated numerically using a dynamic simulation model. The dynamic model is employed to study the response of the system under varying ambient temperatures and cooling load demands. Two strategies are investigated for feedback control of the system. The first control loop maintains the evaporator temperature glide set-point (T-evap,T-out - T-evap,T-in), while the second control loop regulates desorption temperature to provide the desired cooling duty and maintain the delivered coolant temperature at a set-point. At design operating conditions, the proposed control for evaporator temperature glide requires similar to 250 s to reach the set-point of 3 degrees C, starting from ambient conditions. With the proposed implementation of desorption temperature control, the system can respond to changes in ambient temperature and cooling load demand, and maintain the delivered coolant temperature within +/- 0.75 degrees C of the set-point. At part-load operation, the proposed techniques can increase the system coefficient of performance by 8% or greater.