NH 3/H 2OLiBr as working fluid for the compression/absorption cycle

Abstract

In the search for working fluids which are better than ammonia/water for the compression/absorption cycle, the ternary working mixture NH 3/H 2OLiBr was evaluated. NH 3/H 2OLiBr was expected to have thermodynamic properties which were advantageous for the cycle but the choice was also determined by the fact that some experimental data were available for this mixture. The general behaviour and the performance of the cycle are calculated for the ternary mixture with a range of salt concentrations up to 60% by mass. Thermodynamic properties are estimated where they are not directly available from measurements. Comparisons of the cycle performance are made with the binary mixture NH 3/H 2O. At 60% salt solution, the ternary mixture gives an up to 10% better cycle performance than the binary mixture. When optimizing the internal operating conditions for the cycle, it is found that the best operating point for the ternary mixture is located at a larger internal temperature gradient (i.e. a larger difference in concentration between the solution streams) than is the case for the binary mixture. For small values of the internal temperature gradient, the COP of the binary mixture exceeds that of the ternary mixture. The difference in behaviour of the cycle performance for the two fluids is mainly explained by the differences in the latent heat of vaporization, the specific heat of the solution and the vapour pressure. The thermodynamic advantages of the ternary mixture, such as a larger heat of vaporization, compensate the disadvantages only at large internal temperature gradients. For small values of the internal temperature gradient, the influence of the size of the solution recirculation flow dominates and the binary mixture with the smallest solution flow is the better one. Results for ternary mixtures with salt concentrations between 0 and 60% indicate that the best mixture would be a solution with a salt content of about 40–50% by mass. These calculations, however, are rather uncertain since the properties of such mixtures are to a large extent estimated from the properties for NH 3/H 2O and NH 3/H 2O60% LiBr and are not experimentally verified. Nevertheless, the results show that the highest salt concentration (i.e. the largest vapour pressure decrease) might not be the most advantageous for the compression/absorption cycle. This is opposite to what is found for the absorption cycle, where the highest possible salt concentration is preferred.