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

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 (φin); and Type II: on different iso-relative humidity lines (φin). 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 ((NTUm)F = (NTUm)R) and matched flow rates (mRcp.e = mFcp.e = mscp.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 (ηω). For certain heat and mass transfer abilities (NTUm), the single-stage counter-flow LDTHR device exhibits the highest enthalpy efficiency (ηh). 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.