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Highlights • Comparison of absorption and desorption for absorption chillers & heat transformers• Absorption chiller: thin film leads to higher transfer rate during desorption • Heat transformer: favourable properties lead to higher transfer rate for absorption • Various assumptions have been evaluated to substantiate the effects Abstract A comparison of absorption and desorption is conducted using a detailed model describing heat and mass transfer. First, the influences of various assumptions have been evaluated. Second, typical conditions for both absorption chillers and heat transformers have been defined. The performance of absorption and desorption processes have been analysed for a flow length of 0.1m. In an absorption chiller, during desorption, the viscosity is lowered and the mass diffusivity is increased. These circumstances cause a 46% higher transfer rate as compared to absorption. Thus, the overall performance of the process is determined by the absorber component. In a heat transformer, during absorption at an elevated pressure and temperature level, the viscosity is lower and mass diffusivity is higher as compared to desorption. Therefore, the transfer rate of during absorption is 10% higher as compared to desorption. Hence, the desorber performance is somewhat more influential to the overall system performance. Nomenclature c pHeat capacityLongitudinal velocity (m s −1 )x Longitudinal coordinate, streamwise (m) y Transversal coordinate (m) Greek letters   Mass flow per unit length (kg m −1 s −1 ) sor h  Heat of sorption (J kg −1 ) δ Film thickness (m) λ Thermal conductivity (W m −1 K −1 ) μ Dynamic viscosity (Pa s) ξ Mass fraction (−) Page 2 of 25 ρ Density (kg m −3 ) Subscripts 0 Inlet condition abs Absorption avg Average A Absorbate e.g. water in liquid state des Desorption eq Equilibrium if Interface S Solution of absorbent e.g. LiBr − H 2 O V Vapour e.g. steam W Wall

This study describes a developing laminar falling film over a vertical plate while heat and mass transfer occurs. The model accounts for a developing film thickness and uni‐directional diffusion. The fluid properties are assumed to be constant, and latent heat, which is relatively large in comparison to the sensible heat, is set free at the liquid‐vapor interface. Temperature and concentration profiles in the film, the development of the interface velocity, as well as the absorbed mass fluxes are presented and two different boundary conditions have been applied to the inlet of the film.

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