Implementation of the characteristic equation method in quasi-dynamic simulation of absorption chillers: Modeling, validation and first results

Abstract

This work presents a quasi-dynamic approach to an absorption refrigeration system that uses NH3/LiNO3 as the working fluid. The goal is linked to the development and validation of a mathematical model, based on the characteristic equation method, to simulate the quasi-dynamic behavior of absorption chillers. The mathematical model was developed using the first law of thermodynamics through the conservation of mass and energy and the integration of the characteristic equation method, considering external parameters such as temperatures and flow rates of hot, cold, and chilled water circuits as the global heat transfer coefficients. The computational model was built using the F-Chart EES® software, the initial part, to facilitate the resolution of the equations (characteristic equation method). Finally, MATLAB software was used to perform the simulation with the characteristic equation and the transient model. The comparison between the experimental results found in the literature and those obtained by the developed model showed good agreement with relative errors lower than 5% within the values found in the measurement uncertainties. A sensitivity analysis was performed to verify the dynamic behavior of the absorption chiller, considering the activation, cooling, and thermal load temperatures and the products of the global heat transfer coefficients, checking the temperature, pressure, profiles, heat fluxes, and the COP of the chiller. The results showed a coherent behavior when the system was disturbed by varying activation, cooling, and chilled water temperature.

Keywords

• Absorption chiller
• Transient analysis
• Ammonia/Lithium Nitrate
• COP