Preliminary design of a solar thermal-powered seawater reverse osmosis system

This paper presents a preliminary design for a solar thermal-powered reverse osmosis desalination system. The high pressure pump in the seawater reverse osmosis unit requires a 95-kW input, while the reject energy is recovered by way of pressure exchangers. The unit's specific energy consumption is 6.48 MJ/m3 (1.8 kWh/m3) for a recovery rate close to 50% and a feed pressure of 5.53 MPa. The unit was coupled to a solar power cycle based on a Rankine cycle with toluene, hexamethyldisiloxane (MM) and octamethylcyclotetrasiloxane (D4) as working fluids and two different models of parabolic trough collectors. In addition, configurations using both direct vapor generation as well as a heat transfer fluid are proposed. The coupling was done assuming all the mechanical energy produced by the cycle was consumed by the high pressure pump in the reverse osmosis unit. A subsequent assumption was that all the cycle's rejected thermal energy went to preheat the seawater feed flow. This latter aspect does not result in a significant increase in the desalinated water output. The results obtained indicate that with the system proposed it is possible to produce, for a solar direct irradiance of 850 W/m2, 0.11 m3/h with toluene as the working fluid in the cycle and 0.088–0.094 m3/h with D4 or MM of desalinated water per square meter of LS3s parabolic trough collector aperture area. Likewise, for the IND300 collector, it is possible to obtain 0.078–0.085 m3/h with toluene and 0.07–0.077 m3/h with D4 or MM.