Double cascade organic Rankine cycle for solar-driven reverse osmosis desalination

This paper deals with the application of parabolic trough solar collectors to a double cascade of organic Rankine cycles for reverse osmosis desalination. Two different solar collectors are operated either with thermal oil or with the organic fluid of the top cycle. The solar thermal system drives a seawater reverse osmosis system with around 1000 m(3)/d of fresh water production if a thermal energy backup is available. Isopentane is selected for the low temperature cycle from a set of working fluids preselected among fluids proposed in the literature for organic Rankine cycles with similar top temperature range. This cycle is coupled to a top temperature cycle operated with toluene or hexametildisiloxane. Results show that the solar system required to directly drive the high pressure pump of the reverse osmosis unit with the top cycle is also able to provide the electricity requirement of auxiliaries by the bottom cycle.