Exergetic analysis on the two-stage reverse osmosis seawater desalination system

Cardona et al. proposed a two-stage RO system. As reported, this two-stage RO system presents about 20% energy saving compared with conventional single-stage RO system when pressure exchanger is excluded from consideration. Since two mixing steps occur in this two-stage RO system, will it still show advantages from the point of the second law of thermodynamics? This paper makes an exergetic discussion on this two-stage RO system and a conventional single-stage RO system. It is found that input pressure exergy accounts for a large proportion of total input exergy in RO systems. Two-stage RO system presents about 42% of rise in input pressure exergy and 33% of rise in total exergy destruction compared with single-stage system. The blending exergy destruction is much smaller than the total exergy loss. Therefore, the key points that improve the exergetic efficiency of this two-stage RO system lie in other ways. Increasing the product flow by heating feed is an effective method. Because of the higher investment, solar collector is not appropriate for application. Solar pond can also provide heat for feed water. Because the salt for the solar pond can be supplied by the brine exiting the first stage in this two-stage RO system, the main investment comes only from the construction of the solar pond. It is possible that within the life range of the solar pond, the increased investment can be offset by the increased product.