Multivariable feedback control of small-capacity ammonia-water absorption systems

•Multivariable control algorithm for absorption heat pumps developed.•All degrees of freedom in the system utilized in feedback control loops.•Fast and efficient response of the controller at part-load operation.•Stable and repeatable performance of control system at off-design conditions. This paper presents the development and experimental evaluation of a multivariable feedback controller for small-capacity ammonia-water absorption systems. Experiments were designed to identify the pairings between various controlled variables and the degrees of freedom available in the system. A small-scale experimental system was fabricated with access to all degrees of freedom in the system as tunable parameters. Transient analysis of variations in the degrees of freedom assisted in tuning the parameters of the control system to achieve fast, stable and repeatable performance. A methodology for controlling the cooling capacity of the system in response to changes in the load or operating conditions by adjusting the concentrated solution flow rate and the heat source temperature and mass flow rate is developed. In addition, the performances of the evaporator and the desorber are optimized by using flow control valves. Case studies using the developed controller demonstrated fast and stable performance of the system in achieving part-load performance as low as 50% of the baseline cooling capacity, and at off-design ambient temperature. Moreover, the system maintained high, near-design COP (0.55–0.65) values during these test cases.