Dynamics and control of ammonia-water absorption heat pumps

With the development of advanced concepts for miniaturized heat and mass exchangers, absorption heat pumps can now be implemented in small-scale applications for waste-heat recovery or thermally driven cooling and heating. Such systems operate under varying conditions of load and ambient temperature. This necessitates a well-designed control system to maintain optimal performance of these heat pumps. The first part of this study focuses on the development of a computationally efficient, reduced-order transient model to predict the response of absorption systems to time-varying inputs. The transient model is utilized to develop feedback control methodologies and evaluate them for realistic operating scenarios. The control algorithms are based on the feedback of key variables, such as evaporator temperature glide, desorption temperature, ambient conditions, and cooling load requirement. These algorithms require only inexpensive temperature measurements at key locations in the system and account for the time-delay associated with the thermal capacitance of different components. The controller simulations indicate potential for enhancement in system efficiency at part-load operation and off-design ambient temperatures. A small-capacity (2.7 kW cooling) experimental ammonia-water absorption system is fabricated to evaluate the performance of these algorithms. A multivariable feedback control algorithm is implemented to control the capacity of the system by adjusting the solution flow rate, and heat source temperature and flow rate. The resulting system demonstrates fast and stable response to changes in cooling capacity down to 50% of the design value, and at off-design ambient conditions while maintaining high operating efficiency. The multivariable feedback control developed in this study furthers the advancement of thermally-driven cooling and heating systems for small-scale residential and mobile applications.Ph.D