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Abstract
Air-dehumidification systems using liquid desiccant and a desiccant wheel are common approaches using desiccant for air humidity control. This article focuses on the performance of air-dehumidification systems using liquid desiccant and a desiccant wheel. Based on basic air-handling processes using desiccants, the characteristics of the driving forces are investigated with the help of an unmatched coefficient ξ and exergy analysis. The performace of liquid desiccant and desiccant wheel systems is investigated, and approaches for improvement are proposed. It is indicated that the temperature difference ΔT or humidity ratio difference Δω in the liquid desiccant process is influenced by the inlet state of the air and solution. The liquid desiccant process heating solution, rather than air, for regeneration is optimal from the perspective of lowering ξ and reducing unmatched destruction. The ξ is approaching 1 between air and solid desiccant, i.e., the driving force is relatively uniform with the same dehumidification and regeneration regions. A multi-stage process is supposed to be effective for optimizing the desiccant wheel system, helping to reduce the exergy destruction arising from the unmatched coefficient. The present study is beneficial to cast light on the relationship between air-dehumidification systems using liquid desiccant and a desiccant wheel.
Nomenclature
| cp | = | specific heat capacity (kJ/kg·°C) |
| ΔEde | = | exergy destruction (kW) |
| h | = | enthalpy (kJ/kg) |
| hv | = | latent heat of vaporization (kJ/kg) |
| L | = | thickness of the desiccant wheel (m) |
| m | = | mass flow rate (kg/s) |
| NTU | = | number of transfer unit (dimensionless) |
| Q | = | heat exchange (kW) |
| Ra | = | gas constant for air (kJ/mol K) |
| T | = | absolute temperature (K) |
| T0 | = | reference temperature (K) |
| t | = | temperature (°C) |
| ΔT | = | temperature difference (°C) |
| W | = | water content (kg water/kg dry desiccant) |
| X | = | concentration (mass ratio of desiccant to solution) (%) |
| Z | = | wheel thickness direction (m) |
Subscripts
| a | = | air |
| dew | = | dew point |
| h | = | heat transfer process |
| in | = | inlet state |
| m | = | mass transfer process |
| s | = | solution |
Greek symbols
| α | = | convective heat transfer coefficient (W·K−1·m−2) |
| ξ | = | unmatched coefficient (dimensionless) |
| φ | = | relative humidity ratio (%) |
| ω | = | humidity ratio (kg/kg) |
| Δω | = | humidity ratio difference (kg/kg) |
Funding
This research was supported by National Natural Science Foundation of China (No. 51422808, No. 51521005), the National Science and Technology Pillar Program during the 12th Five-year Plan Period (2014BAJ02B01), the Key Project of Chongqing Application and Development Program (cstc2014yykfB30003), and the support from the Tsinghua University Initiative Scientific Research Program (20131089189).