Theoretical analysis of exergy destruction and exergy flow in direct contact process between humid air and water/liquid desiccant solution

Abstract

Direct contact between humid air and water/liquid desiccant (LD) solution is common in air-conditioning systems, where transfer (heat/mass) and conversion (evaporation/condensation) processes occur. This work analyzes these processes using exergy theory and a two-film model. The exergy flow and exergy destruction of heat transfer and mass transfer can be expressed using a unified exergetic expression and depicted by a psychrometric chart. Results show that the equivalent air film of the water/solution is a medium for exergy flow and is thus used to determine the exergy change of the water/solution. Direct evaporative cooling and condensation dehumidification are two representative cases between humid air and water. In the former, the thermal exergy and humid exergy both flow from the humid air to water, where exergy destruction and conversion of the humid exergy into thermal exergy are inevitable. In the latter, thermal and humid exergy flows are all reversed. Dehumidification and regeneration are two crucial and reversed processes between the humid air and solution. The concentration exergy of the solution is involved in the thermal and humid exergy flows. The solution exports its concentration exergy to complete the exergy flows during dehumidification, and it obtains the concentration exergy from exergy flows during regeneration.