A new thermodynamic cycle is proposed named mass engine cycle. In the proposed cycle, mass is transferred from a high mass concentration reservoir to the cycle, mass is rejected to a low mass concentration reservoir, and a net positive work is generated. This is similar to heat engine cycles where heat is transferred from a high temperature thermal reservoir (heat source) to the cycle; heat is rejected to a low temperature thermal reservoir (heat sink), and a net positive work is generated. The heat engine cycle uses heat exchangers to transfer heat between the cycle and the thermal reservoirs, while the mass engine cycle uses membrane mass exchangers which transfer mass between the cycle and the mass reservoir. These membrane mass exchangers transfer water through a semi-permeable membrane and reject other substances. The driving force for the mass transfer is the hydrostatic and osmotic pressure differences. Similar to Carnot limit of the thermal efficiency of the heat engine cycle, a theoretical limit is obtained for the proposed mass engine cycle under reversible thermodynamic conditions.

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