The rapid increases in global population and economic growth have resulted in serious water scarcity in many countries. One of the solutions is to harvest water via dehumidification from ambient air by condensation. However, similar to the traditional air conditioning system, it takes a lot of energy in the condensation process to cool down air to the dew point temperature for dehumidification. Therefore, we propose a combined hollow fiber membrane dehumidification & humidity harvesting system in this study, which not only dehumidifies air but also harvests humidity to generate clean water simultaneously. The driving force for water vapor permeation comes from a dry sweep air combining with a small vacuum pressure of 0.17 bars. The calculation results show that the newly proposed membrane system not only produces fresh cool air but also condenses clean water with minimum energy. It produces 1 m(3) fresh cool air at merely 8.0 kJ while the traditional air conditioning system needs 10.55 kJ, which means an energy saving up to 24.3%. Moreover, it also produces by-product clean water. The calculation shows that it consumes only 687.4 kJ to produce 1 kg clean water while it needs 1125 kJ to do the same from the atmospheric air directly. Therefore, the newly proposed membrane system saves 38.9% energy on humidity harvesting.