Performance analysis of an organic Rankine cycle for a reverse osmosis desalination system using zeotropic mixtures

The use of low-grade thermal energy to power desalination processes by coupling an organic Rankine cycle (ORC) with seawater reverse osmosis (RO) is a promising technology to reduce the cost and environmental impact associated with the use of fossil fuel sources. A low-enthalpy geothermal ORC for a RO desalination system with zeotropic mixtures is proposed. Zeotropic mixtures can improve the thermodynamic performance of ORC systems owing to their excellent temperature glide characteristics during evaporation and condensing processes. A case study with butane/pentane (R600/R601) and butane/isopentane (R600/R601a) is investigated, aiming to analyze the effect of seawater temperature increase on the cycle performance. In the temperature range investigated, the power profit first increases rapidly then decreases. For the mixture R600/R601 at a mole fraction of 0.9/0.1, the maximum power profit value of 29.3 kW occurred with the temperature rise of 26 K, and for the mixture R600/R601a at a mole fraction of 0.9/0.1, the maximum power profit value of 30.9 kW occurred with the temperature rise of 27 K. The results show that, it is necessary to consider the effects of seawater temperature increase in both the ORC and RO simultaneously to design the ORC-RO system.