Thermodynamic modelling of a two-stage absorption chiller driven at two-temperature levels

The thermodynamic model we develop in this paper considers (i) the external irreversibilities of the endoreversible models; (ii) the irreversibilities due to heat losses; and (iii) the generation of internal entropy due to pressure drops and the temperature and concentration gradients. We considered: (i) external heat losses between the generators of high and intermediate pressures and the ambient and between the ambient and the evaporator; and (ii) internal heat losses from the generators towards the condensers and from the absorber towards the evaporator. This simple but precise model faithfully represents the trend towards efficiency variation at partial loads. We have used the model to analyse the behaviour of a water-LiBr double-stage absorption chiller with 200 kW of cooling power. This machine can operate in summer as a double-stage chiller driven by heat at 170 degrees C from natural gas, as a single-stage chiller driven by heat at 90 degrees C from solar energy, or simultaneously in combined mode at both temperatures. It can also operate in winter in ''double-lift'' mode for heating with a driving heat at 170 degrees C from natural gas. We studied the efficiency of the machine at partial loads for several solar fractions and the distribution of the heat transfer areas between the various components of the chiller. (C) 2007 Elsevier Ltd. All rights reserved.