Design of an autonomous low-temperature solar Rankine cycle system for reverse osmosis desalination

Abstract

The present paper regards the design outline, of a low temperature solar organic Rankine cycle system for reverse osmosis (RO) desalination. The thermal processing taking place is described briefly below: Thermal energy produced from the solar collectors array evaporates the working fluid (HFC-134a) in the evaporator surface, changing the fluid state from sub-liquid to super heated vapour. The super-heated vapour is then driven to the expanders where the generated mechanical work produced by the processing drives the high pressure pump of the RO unit, circulation pumps of the Rankine cycle (HFC-134a, cooling water pump), and the circulator of the collectors. The saturated vapour at the expanders' outlet is directed to the condenser and condensates. HFC-134a condensation is necessary in the Rankine process. On the condenser' s surface, seawater is pre-heated and directed to the seawater reservoir. Seawater pre-heating is applied to increase the fresh water recovery ratio. The saturated liquid at the condenser outlet is then pressurised by the HFC-134a pump. Specific innovations of the system are low temperature thermal sources can be exploited efficiently for fresh water production; solar energy is used indirectly and does not heat seawater; the RO unit is driven by directly mechanical work produced from the process; the system condenser acts as sea water pre-heater and this serves a double purpose; (1) increase of feed water temperature implies higher fresh water production, (2) decrease of temperature of “low temperature reservoir” of Rankine cycle implies higher cycle efficiencies. Such a system can be an alternative to PV-RO systems, while low temperature energy sources like thermal wastes may be used for RO desalination. Based on the design results, a prototype installation is going to be realised.