Simulation of an ammonia-water absorption cycle using exchanger effectiveness

Abstract

• NH 3 /H 2 O absorption chiller modeling based on exchanger effectiveness. • Tuning of the numerical model on experimental results. • Comparison of simplified and effectiveness-based modeling. • Characterization of performance for changing operating conditions. • Parametric study changing the size of components. This paper presents the numerical simulation of an efficient ammonia-water single-stage absorption chiller integrating a new combined desorber, able to produce and purify the refrigerant vapor. An experimental campaign was conducted on the pilot plant by varying the main operating parameters, namely the temperatures of external sources and the mass flow rate of the working fluid. Direct experimental measures were analyzed and indirect calculation of other physical quantities was used for tuning of the numerical models. First, a simplified model of the cycle was developed, based on fixed effectiveness and pinch temperatures. Absolute values and tendencies outside nominal working conditions were not sufficiently accurate to predict the performance of the cycle even at the small scale, so more accurate modeling was undertaken. Accordingly, components were characterized by effectiveness, modeled using three operating parameters: the Jakob number ( Ja ), the number of transfer units ( NTU ) and the energetic ratio ( R en ). The small average errors for calculated effectiveness confirm that these parameters are well suited for the application studied. Global cycle model results showed errors compared to experimental results below 6% for the COP and 15% for the cooling power output. The tuned model was used to perform a parametric analysis on the dimension of the components, highlighting the performance improvements/reductions obtainable by increasing/decreasing their size. The use of dimensionless parameters makes this approach well suited for analysis at larger scales of industrial interest, the development of more complex or combined cycles as well as to perform techno-economic or exergo-economic analysis.