Desalination 203 (2007) 346–365 Desalination by using alternative energy: Review and state-of-the-art E. Mathioulakis*, V. Belessiotis, E. Delyannis National Center for Scientific Research (NCSR) “Demokritos”, Aghia Paraskevi, 153-10, Athens, Greece Tel. +30 (210) 6503817; Fax +30 (210) 6544592; email: [email protected] Received 6 February 2006; accepted 15 March 2006 Abstract Energy is a critical parameter for economic and of vital importance in social and industrial development, as it is also quality water. Numerous low-density population areas lack not only fresh water availability, but in most of the cases electrical grid connection or any other energy source as well, except for renewable energy sources, mostly referring to solar radiation. For these regions desalination is a moderate solution for their needs. In using RE desalination there are two separate and different technologies involved: energy conversion and desalination systems. The real problem in these technologies is the optimum economic design and evaluation of the combined plants in order to be economically viable for remote or arid regions. Conversion of renewable energies, including solar, requires high investment cost and though the intensive R&D effort technology is not yet enough mature to be exploited through large-scale applications. This paper presents a review of the highlights that have been achieved during the recent years and the state-of-the-art for most important efforts in the field of desalination by renewable energies, with emphasis on solar energy applications. Keywords: Desalination; Alternative energy; Review 1. Introduction velopment agenda. The social and economic health of the modern world depends on sustainable Water and energy are two of the most important supply of both energy and water. These two critical topics on the international environment and de- resources are inextricably and reciprocally linked: the production of energy requires large volumes *Corresponding author. Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved E. Mathioulakis et al. / Desalination 203 (2007) 346–365 347 of water while the treatment and distribution of resources, the high land requirements and invest- water is equally dependent upon readily available, ment costs of renewable energy facilities. low-cost energy. The ability of nations to provide Apart from the referred reasonable arguments both clean, affordable energy and water is being for the use of RES towards the emerging and seriously challenged by a number of emerging stressing energy problems, there is a number of issues. reasons related to the more specific issue, the one Through the natural water cycle, one may have of the suitability of RES for seawater desalination: the wrong impression that fresh water is a renew- • Arid regions are often remote, coastal areas or able item, and in some way it is, even though the little islands where renewable energy sources availability of quality fresh water resources is are available and conventional energy supply decreasing dramatically due to the population in- is not always possible or at least easy to imple- crease, the irrational waste and especially to severe ment. In these cases RES represent the best contamination of the existing resources. There is energy supply option for autonomous desalina- a world-wide crisis concerning availability of tion systems. good quality water, and this goes to local, regional • Climatic reasons lead to remarkable agreement and national levels, despite the huge amount of on a time-basis, between the availability of water covering the earth’s surface. This situation RES, especially when referring to solar energy, leads to the application of cleaning methods or to and the intensive demand of water. Further- desalination of brackish and seawater. more, often freshwater demand increases due In most of the arid, semi-arid and remote re- to tourism, which is normally concentrated at gions fresh water is very scarce. This is especially times when the renewable energy availability true for the Mediterranean basin and the Middle is high. East regions where many big cities and small • Both renewable energy systems and desalina- villages suffer from lack of quality fresh water tion refer to self-sufficiency and local support. but, at the same time, they are blessed with abun- The operation and maintenance of related sys- dant salt-water sources. In seawater desalination tems in remote areas are often easier than con- the trend refers mainly to large centralized or dual- ventional energy ones. Furthermore, the imple- purpose desalination plants, as being more econo- mentation of RES-driven desalination systems mical and suitable for large density population enforces sustainable socioeconomic develop- areas, ignoring through this practice small poor ment by using local resources. communities. However, numerous low-density • Renewable energies allow diversification of population areas lack not only fresh water but, in energy resources and help to avoid external most the cases, electrical power grid connections dependence on energy supply. This has to be as well. For these regions renewable energy desali- seen through the prospect of the least develop- nation is the only solution. ed countries, where major water shortage Since ancient times humans have used renew- problems exist, considering as well the fact that able energy sources (RES), but the remarkable seawater desalination processes are highly development of renewable systems has taken place energy-consuming methods. after the petroleum crisis of 1973. At present, the increasing preoccupation for the environment has However, present situation does not reflect the consequently increased the interest for the use of obvious advantages of RES and desalination con- renewable energies. Naturally, one has to consider junction. More specifically, RES-driven desalina- the existing main limitations, related to the tem- tion systems are scarce, presenting usually limited poral and space-dependent character of these capacity. Accordingly, they only represent about 348 E. Mathioulakis et al. / Desalination 203 (2007) 346–365 0.02% of the total desalination capacity [1]. The problems arising. The criteria of analysis refer reasons for this are related to various, often corre- amongst others to RES availability, technological lated, aspects: maturity, simplicity, availability of local resources • Technology: The use in desalination systems for handling and maintenance, possibility to en- of alternative energy sources imposes the con- sure a given level of fresh water production, suit- junction of two separate and different techno- ability of the system to the characteristics of the logies: the energy conversion and the desalina- location, efficiency, etc. tion systems. Both are considered mature to a lesser or greater degree, even though there are 2. Matching renewable energies with desalina- still significant margins regarding the effici- tion units ency increase, as well as volume and costs de- crease. A real challenge for these technologies Renewable energies and desalination plants are would be the optimum technological design two different technologies, which can be com- of combined plants through a system-oriented bined in various ways. The interface between the approach. renewable energy system and the desalination sys- • Cost: Exploitation of RES and development tem is met at the place/subsystem where the energy of desalination plants represent capital-inten- generated by the RE system is promoted to the sive installations. By this time, renewable ener- desalination plant. This energy can be in different gy technologies are not considered totally ma- forms such as thermal energy, electricity or shaft ture and the various system components are power. Fig. 1 shows the possible combinations. still expensive. Even though prices decrease One has to note that in this figure, there have been continuously, still in many cases they are pro- introduced some changes regarding relevant hibiting for commercialization. references [2]. These changes concern the inser- • Availability: Renewable energies are unlimit- tion of direct solar distillation (SD), humidifica- ed, being though transient, thus presenting tion–dehumidification (HD) and membrane dis- intermittent character, leading to limitations tillation (MD) systems. concerning the maximum exploitation capaci- Renewable energy-driven desalination systems ties per time unit. Furthermore, the geographi- fall into two main categories: thermal processes cal distribution of RES potential does not al- and electromechanical processes. As regards the ways comply with the water stress intensity at energy source, a desalination plant powered by a local level. renewable energy is likely to be a stand-alone sys- • Sustainability: In most of the cases, the matu- tem at a location which has no electricity grid. rity of the associated technologies does not Stand-alone systems are often hybrid systems, match the low level of infrastructures which combining more than one type of renewable ener- often characterizes places with severe water gy sources, for instance, wind and solar energy or stress. Experience has