Comparative evaluation of two membrane distillation modules

Abstract

Freshwater shortage difficulties make it necessary to find new sources of supply. Nowadays desalination is the solution adopted in many countries to solve this problem. All around the planet, regions with lack of freshwater match up with those with large amounts of available solar radiation. Therefore, solar desalination can be a suitable and sustainable option to tackle the water scarcity problems in those particular areas, especially in the coastal ones. Membrane distillation (MD) is a thermal membrane technology developed since late 60’s which uses low exergy heat to drive a separation process in aqueous solutions. One of its applications is desalination where thanks to its separation principle, very high distillate quality can be obtained. MD is a thermally driven process that differs from other membrane technologies in that its driving force, rather than the total pressure, is the difference in water vapour pressure across the membrane, caused in turn by a temperature difference between the cold and the hot side of it. In comparison with other membrane-based desalination processes like reverse osmosis (RO), MD shows very high rejection rates and much lower operational pressures, also the nature of MD membranes (larger pore sizes than RO) makes them much less sensitive to fouling. Compared to conventional thermal desalination processes like MSF or MED, MD is less demanding regarding vapor space and building material’s quality [1] leading to potential lower construction costs. Amongst its advantages, its low operating temperatures (ranging between 60–90°C [2]) make possible the use of low-grade heat, the kind of energy delivered by static solar collectors, as the only thermal supply. This, jointly with its low operational pressure and small footprint, make solar membrane distillation (SMD) in principle, a promising technology. Despite these advantages, SMD has been developed to a lesser extent, compared with other solar desalination technologies like PV-driven RO or solar stills, and although many encouraging laboratory experiences can be found in literature, large-scaling and module design is still an issue. It is precisely because of this preliminary state MD is in, that very preliminary, low energy efficiency and not commercial available MD prototypes are still found. In MD there is still a trade-off between efficiency (heat consumption) and production (distillate per square meter of membrane), as a result very high specific distillate fluxes can be attained (up to 80 kg h–1 m–2 of membrane [3]) but heat losses (mainly trough the membrane by conduction) are still substantial. Under the framework of an European project (MEDESOL: Seawater Desalination by Innovative Solar Powered Membrane Distillation) which main objective was to develop a stand-alone desalination system based on multi stage MD to supply decentralized rural areas [4], the status and future possibilities of currently developed MD have been evaluated. This paper presents the results obtained from the experiments realized with two different pre-commercial MD modules, coupled to a solar field comprised of static collectors. Both modules were tested in the same facility under the same conditions, in order to make a reliable comparison between them. Data on energy efficiency, production ratios and operational issues will be shown.