MULTI-EFFECT DISTILLATION (MED) IN WATER DESALINATION WITH SOLAR ENERGY
Desalination of the seawater is undoubtedly one of the most promising fields in which the solar thermal energy is currently applied due to the prevailing scarcity of fresh water, availability of seawater as well as reasonable levels of solar radiation (Micale, Cipollina and Rizzuti, 2009). Hence, the Multiple-effect distillation (MED) has been extensively used in the processes of seawater desalination due to its affordability and convenience. However, this method consists of multiple stages also referred to as “effects” whereby in each of these stages the feed water is usually heated by the steam that is in the tubes. This results to the evaporation of the water evaporates whereby the generated steam usually flow into the tubes that are consisted in the next stage, leading to more heating and evaporation of more water. Moreover, each of the stage in essence reuses the solar thermal energy obtained from the previous stage (Garcia-Rodriguez and Gomez-Camacho, 2001).
However, considering the initial processes that were used in multi-effect distillation of the seawater using solar energy has tremendously especially when the developments that have taken place in this method within the last ten years are considered. Hence, in the last ten years there has been significant developments in the energy efficiency process through the development as well as implementation of a heat pump that has double-effect absorption (Rizzuti, Ettouney and Cipollina, 2007). In addition, there has also been tremendous developments in the enhancing of environmental protection and economic efficiency. Thus, in the last ten years multiple-effect distillation (MED) method of seawater desalination has actually become the key solar thermal energy desalination option that has experienced tremendous development of new designs that have been very critical in ensuring that they allow lower operating temperatures, minimize scaling and corrosion, as well as increased efficiency (Micale, Cipollina and Rizzuti, 2009).
Initially the steam tubes were been submerged in the feed water but the recent developments have seen this trend change to the spraying of the feed water ( seawater) outside the tubes usually containing the steam resulting into a typical effect. Moreover, most of the today’s multiple-effect distillation (MED) designs includes a thermal vapour compressor (TVC) which plays a very crucial role in increasing the system efficiency (Rizzuti, Ettouney and Cipollina, 2007). This is mainly because the TVC recycles some of the produced vapour in the process of desalination thereby leading to the reduction of the total amount of steam that is actually needed to drive the process. This is due to the fact that the TVC is in real essence a steam ejector responsible of entraining the vapour at low-pressure from the downstream effect together with the motive steam thereby discharging the overall mixture into the first effect.
In addition, despite the widespread acceptance of the MED technology for the last twenty years, the recent developments have made the MED plants to be built in units consisting of approximately 100 m 3 per day up to 36,400 m 3 per day which is equivalent to 0.03 to 8 mgd, thereby permitting this design to be used in the applications of smaller volume (Garcia-Rodriguez and Gomez-Camacho, 2001). Also multiple units are likely to be combined together in a single MED plant further capacity increment. Nowadays, the MED system also produces very high-quality product water whose total dissolved solids (TDS) is of concentrations of 25 mg/l or sometimes less meaning that some minerals are added back to make the water suitable for human consumption.
Garcia-Rodriguez, L. and Gomez-Camacho, C. 2001. Perspectives of solar-assisted seawater distillation. Desalination, vol. 136, no. 3, pp. 213-218.
Micale, G., Cipollina, A. and Rizzuti, L. Eds. 2009. Seawater desalination: Conventional and renewable energy processes. Heidelberg: Springer-Verlag.
Rizzuti, L., Ettouney, H.M. and Cipollina, A. 2007. Solar desalination for the 21st century: a review of modern technologies and knowledge. Heidelberg: Springer-Verlag.