MEDESOL PROJECT: SEAWATER DESALINATION BY SOLAR. DRIVEN MEMBRANE DISTILLATION
E. Guillen1* , W. Gernjak1, D. Alarcon1 and J. Blanco1
1 CIEMAT-Plataforma Solar de Almeria, Ctra. de Senes s/n, 04280 Tabernas, Almeria, Spain.
Corresponding Author, slena. guillen@psa. es
Abstract
Freshwater shortage difficulties make it necessary to find new sources of supply. Nowadays desalination seems to be the solution adopted in many countries to solve this problem. All around the planet, regions with a lack of freshwater match up with those with large amounts of available solar radiation. Therefore, solar desalination can be a suitable option to tackle these water scarcity problems in those particular areas, especially in the coastal ones. This paper will describe the first results of MEDESOL Project (Seawater Desalination by Innovative Solar - Powered Membrane Distillation System) funded by the European Commission within the 6th Programme Framework. Main objective of the project is to develop an environmentally friendly cost-improved desalination technology to supply fresh water in arid and semi-arid regions in EU and developing countries based on Solar Membrane Distillation (SMD). The system to be developed is intended to be a stand-alone technically simple to operate arrangement, and to have a capacity between 0.5 to 50 m3/day. SMD is a solar thermally driven process differs from other membrane technologies in that its driving force is the difference in water vapour pressure across the membrane, caused in turn by the temperature difference between the cold and hot side of it, rather than the total pressure. Feed water is heated by solar energy and comes into direct contact with the hydrophobic membrane, which allows only the vapour to cross it. Despite its advantages SMD has been developed to a lesser extent, compared with other solar technologies like Solar PV-driven RO or Solar Distillation. Amongst these advantages, its low operating temperatures which make it possible to use low-grade heat as the only thermal supply (that is the case of energy delivered by static solar collectors) and its low operational pressure and footprint, make SMD a promising technology. There are several configurations to create the vapour pressure difference and to try to deal with the main issues of this technology which are the heat losses and the mass transfer concerns. In the case of MEDESOL project, the configuration chosen has been Air Gap Membrane Distillation (AGMD) in order to lessen heat losses. The project introduces a novel scheme based on multistage-concept in order to achieve maximum heat recovery, minimum membrane area required and substantially reduce brine generation. Along the lines of that concept, several configurations will be assessed. The heat supply will come from an innovative compound parabolic solar concentrator, specifically developed for the intended range of temperatures (90° C) the same way an advanced non-fouling surface coating for the seawater heat exchanger will be devised and checked. As said before, this paper will show the results obtained so far.
Keywords: Solar membrane distillation, AGMD, desalination.
The confluence of fresh water shortage difficulties and solar radiation in isolated regions constitutes the perfect framework to apply simple technologies like solar membrane distillation (SMD) to supply the shortfalls. Nowadays renewable energy driven desalination technologies are considered suitable for decentralized systems where water demand is lower than 20 m3/d (water demand in shortage conditions is normally of about 55 l/inhabitant/day). The lack of a centralized energy supply and established infrastructures and the low water demand that characterizes these kinds of regions make it worthless to scale-down larger desalination plants technologies, speaking for the use of solar or whichever renewable energy driven process available, for water desalination systems. Moreover, the selection of an appropriate desalination system must take into account the special requirements of these regions that are basically:
• Hard climate and environmental conditions of these regions make it necessary to have a robust technology which could stand for example all types of raw water and be practically maintenance-free.
• Lack of technicians, thus sophisticated technologies that imply complex pre-treatments or intricate maintenance procedures are contraindicated. Stand-alone technologies are therefore required.
• Necessity of high quality water production, as the water is for human consumption.
The solutions given to tackle this problematic are basically two, photovoltaic energy coupled with reverse osmosis systems (PV-RO), and solar thermally driven distillation systems (STD). There are difficulties when operating small scale PV-driven stand alone systems [1] and it is also well-know that RO systems are easily affected by raw water conditions (in many cases pre-treatment is required) and energy input varies as a function of salt concentration. Moreover, RO units have the requirements of a continuous process [2] and solar energy is intrinsically discontinuous. Amongst thermally driven systems, solar stills are the most common, because their practically non-existing operation and maintenance requirements. But their main drawbacks are a low yield (the production capacity of a simple type still is in the range of 2-5 l/m2/day [3]) low thermal efficiency and large specific collector area per cubic meter (Energy efficiency is a crucial factor when designing solar-powered systems, since the largest investments costs come from the collector area needed)[1]. Solar Membrane Distillation complies with all the requirements specified above and combines the advantages of a membrane-based technology, smaller installation areas, and those of the thermal-driven processes, low operational and maintenance costs [4] and has also a low environmental impact as it uses solar energy instead of conventional fossil fuels.
In another vein, the low temperature heat demand of membrane distillation technology make it also possible to use other energy sources such as waste heat to drive the process and thus use it for larger applications.
