The driven heat and circulating pump in a closed absorption system replaces the compressor in a vapor compression heat pump system and circulates the working fluid while increasing its internal energy

thus saving the compression work from consuming high-grade electric power. Unlike in the closed absorption heat pump (refrigeration) system

in the open absorption system

the indirect heat transfer driven by the temperature difference is replaced with the mass transfer driven by the vapor pressure difference and the heat transfer driven by the temperature difference simultaneously. Heat and mass transfer with the environment can occur in different components (evaporator E

absorber A

generator D

and condenser C) in open absorption systems. For applications in different fields

the structure is simplified based on a closed absorption system

and the performance is optimized. This paper summarizes state-of-the-art open absorption systems in terms of three aspects: working fluid pairs

configurations

and components. Research on the working fluid pairs of open absorption systems has mainly focused on aqueous working fluid pairs with water as the refrigerant. Common salt solutions LiBr-H2O and LiCl-H2O have high performance

but they are corrosive and easy to crystallize. CaCl2-H2O is less corrosive but unsuitable for high-temperature applications. The performance of HCOOK-H2O is between those of LiBr/LiCl-H2O and CaCl2-H2O

resulting in low corrosion to the system. Further experimental studies are required for ionic liquids as potential alternative working fluids. A typical open absorption system has five basic configurations: D

AD

ADC

EAD

and EADC types. The D-type configuration can be used for direct evaporative cooling

refrigeration

and recovery of latent heat from moisture in air. When the evaporation temperature is low in the EAD-type

the evaporator can be used for cooling. In the EAD type

the absorption heat transfer absorber can be used for heating and the generator can be used for humidification. These can also be used for seawater desalination and purification. The ADC-type configuration can be used to recover latent heat from moisture and distillation. The EADC-type open absorption system is similar to the traditional closed absorption system and can be used for refrigeration

heat pumps

and distillation purification. Enhancing the heat and mass transfer between the environment and the system is the focus of the research on open absorption systems. For direct-contact heat and mass transfer components

packed towers are still the main form of adiabatic absorbers/generators. The use of indirect-contact absorption/generation heat exchangers based on membranes is required to fundamentally solve the carryover problem. However

the heat and mass transfer coefficients of the membrane absorption/generation heat exchanger still need to be improved compared with those of direct-contact heat exchangers.