Numerical investigation on semi-GAX NH3–H2O absorption cycles

• Two versions of double-lift semi-GAX cycle are numerically modelled.
• The model results are compared with available data, finding good agreement.
• The cycle performances are calculated at conditions suitable for solar cooling.
• Air temperature and split ratio influence internal heat recoveries and cycle COP.
• The cycles COP can be optimized at any working condition adjusting the split ratio.

Double-lift absorption cycles represent a suitable solution for air-cooled thermally driven cooling applications. Among the several existing double-lift configurations, semi-GAX cycles are known as the most promising in terms of efficiency. These cycles incorporate the GAX effect in a pressure staged cycle, by means of a split on the solution leaving the low pressure absorber. Two configurations of the semi-GAX cycle have been proposed in the past, the semi-GAX 1 and the semi-GAX 2. The former achieves the GAX effect between the intermediate and the high pressure levels, the latter between the low and the intermediate. Within this paper, the semi-GAX cycles are numerically investigated at operating conditions suitable for a low temperature driven (e.g., by flat plate solar collectors) air conditioning application. The peculiarities of the two cycles are described and the factors affecting their performances are underlined. The COP resulted to be strongly influenced by the split ratio, which determines the intermediate pressure and the possibility to achieve the GAX effect. If the split ratio is optimized to achieve the maximum COP, the COP is higher for semi-GAX 2 for air temperatures below 27 °C and for semi-GAX 1 above. In both cases, the maximum air temperature which allows a circulation ratio below 15 is 40 °C, with chilled water at 7/12 °C and driving temperature of 90 °C.