Computational simulation and optimization methodology of an ammonia–water GAX absorption cooling system

Abstract

This paper presents a methodology for the steady-state simulation and optimization of a water–ammonia GAX cycle cooling system for residential and commercial air-conditioning applications. The study is based on an experimental GAX absorber unit described in the literature designed with a cooling capacity of 7.1 kW and uses ammonia–water mixture as the working fluid. The model uses thermodynamic relations of species conservation and energy conservation and general heat transfer expressions in terms of the product of the global heat transfer coefficient by the heat transfer area, UA. Thermodynamic state relations are analytically derived from two equations representing the Gibbs free energy in terms of pressure, temperature and concentration; vapor–liquid equilibrium of the ammonia–water mixture is computed in subroutines adopting the Newton–Raphson method with analytical computation of the Jacobian matrix. The resulting system of nonlinear equations was solved by the Substitution-Newton–Raphson method with a numerical estimate of the Jacobian matrix. The model was first used to simulate the performance of the experimental system, calibrating the necessary component parameters. After that, an optimization study was carried out from the experimental prototype based on the evaluation of the effect of varying the UA product of each GAX cycle component on the refrigeration effect, adopting the criterion of maintaining the overall system size represented by the sum of the UA products. Optimization results show an increased fridge effect and COP of 10.75% and 31%, respectively, together with a simplification of the systems.