The application of renewable energies such as solar energy to produce fresh water is receiving increased interest due to the need for solving the water shortage problems in various areas of the world at the same time as conventional energy sources used for obtaining water in different scenarios become depleted.

The world desalination installed capacity has increased from a bit more than 10 million cubic meter per day in 1986 to more than 42 million in 2006. Only in Spain the capacity of newly commissioned plants in 2006 was higher than 400 000 m3/day [1]. So it is not surprising that the use of renewable energy sources in water desalination is of high interest, especially for remote areas where a conventional energy supply is not easily available. This application is, however, still not well developed and it has only been tested in pilot plants and at a few demonstration sites. Numerous examples of research work on renewable energy desalination systems can be found in [2-5] as well as in other reviews in the literature. There are various desalination methods available on the market that use mainly thermal or mechanical energy in their fundamental separation processes. Among them,

Reverse Osmosis (RO) is quite suitable for small to medium capacity systems and also has good perspectives for cost reduction and improvement in efficiency in the near future [6]. In RO, pressure applied to the saline solution forces pure water through a semipermeable membrane. The membrane is selective and allows the passage of water but is impermeable to other substances.

The energy to produce the required pressure for RO can be generated with renewable energy sources such as wind energy, dish-Stirling systems, solar thermoelectrical plants or photovoltaic solar electrical generation. Solar thermal energy coupled to a power cycle by using direct mechanical power can also be employed.

Water is commonly used in Rankine power cycles, although other types of inorganic (ammonia, ammonia/water, …) and organic fluids (hydrocarbons, fluorocarbons, siloxanes, …) can be used. The main advantage of organic working fluids in Rankine cycles (ORC) is that they can be driven at lower temperatures than similar cycles using water and also in many cases superheating is not necessary.

The research on ORC has been focused in the production of electricity, mainly related to recovery of low temperature waste heat, geothermal heat, biomass, or solar energy. Many references to these applications are available in the literature. A couple of facilities with ORC plants using solar thermal energy were constructed in 1978 in Cadarache, France and in 1981 at El Hamrawin, Egypt, but unfortunately there has been little information published about them [7]. A commercial parabolic trough ORC power plant (Saguaro plant) completed in 2006 in Arizona is of particular interest. It is a 1 MWe plant using n-pentane as the working fluid for the ORC, and is based on plants used in geothermal applications having 10340 m2 of parabolic trough collectors [8]. Studies on ORC applications for desalination are very scarce although a few projects exist and some studies are available. Burgess and Lovegrove [9] discussed the application of solar thermal powered desalination using membrane and distillation technologies. One of their conclusions was that more detailed analyses of solar driven RO are required to determine its costs and applicability. In [10] it is proposed a solar ORC system using R-134a and evacuated tube collectors. The system efficiency is low (7%) but the authors considered it comparable to equivalent photovoltaic desalination systems. The first laboratory test simulating the heat provided by solar collectors has been given by Manolakos et al [11]. An screening and performance assessment of working fluids for RO solar thermal desalination can be found in Delgado [12]. In Bruno et al [13] it is presented a model optimising the solar field/ORC global efficiency using a process simulator and calculating the required solar field area for several types of collectors and selecting the most appropriate fluid for each one. Also a technical and economical comparison with a photovoltaic/RO system is presented. The main conclusion was that solar driven systems specially those using medium-high temperature collectors can compete favourably in terms of specific annual cost €/m3 with the PV/RO system.

Using medium-high temperature solar collectors e. g. trough collectors, the use of water or organic fluids is possible, e. g. the Solar-Thermal power plant Andasol (Spain) using water or the Saguaro solar plant (USA) using n-pentane. In summary, existing research on solar ORC for desalination is very limited, and few efforts have been reported on determining the most useful working fluids for this application and their optimal operation conditions.

In this paper it is presented a rigorous model developed in Trnsys/Trnopt [14] linked with EES [15] for the optimisation of the operating temperature of solar Rankine cycles connected to RO desalination

plants to maximise the desalted water production. Two cases are presented and compared to provide the mechanical energy required for the RO system: a steam Rankine cycle and an organic Rankine cycle using n-pentane. The selected thermal solar field consist of trough solar collectors because in previous preliminary studies made by the authors [13] it was concluded that they provided the higher global efficiency. The complete solar field/organic Rankine cycle is modelled using Trnsys in the case of steam and Trnsys for the solar field and an EES model for the ORC system linked with Trnsys in the case of the organic fluid. The performance of the RO system is calculated using the ROSA software [16] and the modelling parameters used in [13].