Description of the Power Cycle — Reverse Osmosis Desalination Plant
The system consists of three main subsystems: the Rankine cycle, RO and solar plants. The Rankine cycle subsystem is a thermodynamic power cycle that uses water or an organic working fluid. The working fluid is heated to boiling, and the expanding vapour is used to drive a turbine and more generally any expander (Figure 1). This expander provides all the mechanical energy required to drive the high-pressure RO pump and the high pressure ORC and solar plant circulation pumps. In this process, after its expansion, the working fluid vapour is condensed back into the liquid state using the raw water feed for the desalination subsystem as a coolant. This water is in turn preheated to increase the performance of the RO. The condensed working fluid is recycled back through the system to again produce work. A regenerator is included to increase energy efficiency in the case of using an organic fluid. The heat required to preheat, evaporate and superheat the working fluid is obtained in this case from a thermal solar plant using thermal oil as a heat transfer fluid.
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The TRNSYS simulation environment for thermal systems [15] was selected to model the thermal solar plant and calculate the desalted water production based on its completeness regarding meteorological and solar plant component libraries. In this study the steam Rankine cycle is modelled using the same Trnsys environment. However, it is very difficult to implement rigorous models to predict the thermodynamic properties of other working fluids. To solve this problem in the literature has been proposed to use transfer functions to import and export variables between Trnsys and EES [17, 18]. In the present study the strategy selected was to connect directly both programs. This option is more time consuming than the use of correlated functions but if the analysed time period is not too long the running time is reasonable.
