Air-cooled LiBr–water absorption chillers for solar air conditioning in extremely hot weathers

A low temperature-driven absorption cycle is theoretically investigated for the development of an air-cooled LiBr–water absorption chiller to be combined with low-cost flat solar collectors for solar air conditioning in hot and dry regions. The cycle works with dilute LiBr–water solutions so that risk of LiBr crystallization is less than for commercially available water-cooled LiBr–water absorption chillers even in extremely hot ambient conditions. Two-phase heat exchangers in the system were modelled taking account of the heat and mass transfer resistances in falling film flows by applying the film theory in thermal and concentration boundary layers. Both directly and indirectly air-cooled chillers were modelled by properly combining component models and boundary conditions in a matrix system and solved with an algebraic equation solver. Simulation results predict that the chillers would deliver chilled water around 7.0 °C with a COP of 0.37 from 90 °C hot water under 35 °C ambient condition. At 50 °C ambient temperature, the chillers retained about 36% of their cooling power at 35 °C ambient. Compared with the directly air-cooled chiller, the indirectly air-cooled chiller presented a cooling power performance reduction of about 30%.