The reliable and safe provision of fresh water is becoming an increasingly important issue world-wide either due to its scarcity (quantity problem) or because of its low quality (pollution or salt problems). The historical approach of developing new water sources to meet the rapidly increasing demand has reached its limits since inexpensive resources have already been developed and new ones are prohibitively expensive to exploit. In many areas, Desalination could be an alternative option for fresh water supply. Desalination is an energy intensive process and Renewable Energy Sources could provide an environmentally sustainable prospect to this problem.

The main objective of the present action is the development, installation testing and performance evaluation of an innovative stand-alone, solar desalination system. The system consists of vacuum tube solar collectors and produces mechanical work through the application of a Low Temperature Organic Rankine Cycle (SORC). The generated mechanical work drives a Reverse Osmosis (RO) desalination unit.

The system performance depends on the availability of solar source and the target potable water cost to be achieved. The latter depends strongly on the competitive price of alternative water supplies. It is known that in arid regions, where the quantity problems are predominant, water prices are high and even a low efficient desalination process (which is based on a low-capital cost plant) can be competitive in the real market meaning of the term. In non-arid regions the water problems are frequently related to quality problems. Especially for the first case SORC for RO desalination can provide a cheap water supply to a more complicated water producing technology.

The efficiency of RES-powered desalination systems depends strongly on the continuous energy supply. In case of applying solely a solar desalination technology, the efficiency is low because of the night cease of operation of the thermal unit. This can be acceptable for arid regions where the competitive fresh water supply may be rather expensive. However, solar Rankine cycle is characterised by the highest possible efficiency since approaches the efficiency of Carnot cycle. Its application for RO desalination can be ideal solution. Additionally, the proposed technology is flexible enough to be integrated to other thermal sources of continuous heat generation such as industrial thermal wastes, geothermal energy or even energy from biomass or Municipal Solid Wastes (MSW). The optimum design technique to be applied will assure a 24-hours per day water supply taking into consideration the adequate water storage. Water desalinisation has become increasingly important in many isolated areas and coastal regions where the natural inlet of rain water is sustainable below of the population demands either for the natural comfort or for the demands of new activities such as tourism.

Technologies for water desalinisation have moved from heat and vapour solutions (very costly in terms of energy use) to more efficient membrane separation technologies (RO in acronym form) that albeit continue to use energy (in electricity form) are much more energy efficient. Nevertheless all these technologies require a stable and substantial electricity supply that in some cases is not easily ready or affordable in economic terms.

For that reason, in parallel with membrane technologies based on grid electricity, new solutions have been developed that combine the use of renewable technologies (mainly wind energy or in some cases trough the combustion of biomass or residues) with that rO technologies to produce desalinised water in a smaller more decentralised scale.

Nevertheless, all these solutions have a technological drawback, which is the need to transform the firstly produced mechanical energy (in the form of wind force or combustion steam) into electricity (with an energy transformation loss of around 60%).

In this context, is where it steps this new technological approach that use the mechanical energy derived from solar collectors to run the mechanical parts associated with the
reverse osmosis technologies, so producing a very important leap forward in the energy efficiency of the whole system.

The benefits of the utilisation of SORC as energy carriers are the following:

• Solar energy is a sustainable and renewable resource, particularly in those countries, which have very little or no reserves of good quality fossil fuels as in all Southern EU countries.

• The application of SORC is much less harmful for the environment compared to conventional fuels.

• The integrated system is cost effective since SORC is a cheap technology (compared to other solar thermal technologies) with very low maintenance cost.

There is also a strong interest on EU policies to guarantee the viability of natural resources. For the years ahead, the water resources are going to be gradually declined. So, there is a requirement to regenerate this resource in an environmentally friendly way. The most safe and promising route to achieve this is the application of renewables. The exploitation of a natural resource (solar energy) to produce another natural resource (water) is an ideal solution.