Optimization of the performance of absorption systems during transient operations such as start-up and shut-down to minimize lifecycle costs is particularly important for small-capacity chillers and heat pumps. Dynamic models in the literature have been used to study responses to step changes in single parameters, but more complex transient processes, such as system start-up, have not been studied in detail. A robust system-level model that simulates the transient behavior of an absorption chiller is developed here.
Individual heat and mass exchangers are modeled using detailed segmental models. Thermal masses of the heat exchangers and energy storage in the heat exchanging fluids are accounted for to achieve realistic transient simulation of the heat transfer processes in the chiller. Mass and species storage in the cycle are modeled using storage devices. A rapid iterative solver is developed for quick estimation of unknown vapor and liquid outflow states. Other components such as the rectifier, expansion valves, and solution pump are modeled as quasi-steady devices.
System parameters are representative of a 1 RT (3.5 kW) cooling absorption chiller currently under development. System start-up is simulated from ambient conditions, and the system attains steady-state in approximately 450 s. The evaporator cooling duty and COP of the chiller during steady-state are observed to be 3.42 kW and 0.60, respectively. Several control responses are also investigated using this dynamic simulation model. System responses to step changes in the desorber coupling fluid temperature and flow rate, solution pumping rate, and valve setting are used to study the effects of several control strategies on system behavior. This analysis can be used to minimize start-up times and also enhance steady state performance. The model can also be used for devising and testing control strategies in commercial applications.