Analysis and optimization of total heat recovery devices by using liquid desiccant as a coupling fluid in air-conditioning systems

Abstract Total heat recovery in an air-conditioning system can recycle both heat and mass from return air to preprocess fresh air. This study analyzes liquid desiccant total heat recovery (LDTHR) devices with a counter-flow path or a cross-flow path. The inlet states of fresh air and return air are divided into two types for analysis, namely Type I: on the same iso-relative humidity line ( φ in ); and Type II: on different iso-relative humidity lines ( φ in ). With respect to Type I, the single-stage LDTHR with a counter-flow path is investigated by using an analogy with a sensible heat recovery device in which the performance is optimized under matched heat and mass transfer ability (( NTU m ) F  = ( NTU m ) R ) and matched flow rates ( m R c p.e  =  m F c p.e  =  m s c p.s ). With respect to type II, the liquid desiccant solution of a single-stage device works at a fixed iso-relative humidity line, and this limits the mass transfer between air and solution. Increases in the stage number alleviates the problem and improves humidity efficiency ( η ω ). For certain heat and mass transfer abilities ( NTU m ), the single-stage counter-flow LDTHR device exhibits the highest enthalpy efficiency ( η h ). The application of the counter-flow device is restricted by the height and complex arrangement of air and solution flow. The enthalpy and humidity efficiency of a double-stage cross-flow device is close to that of the counter-flow device, and this is a practical method that is an alternative to the counter-flow device.