Since the Kyoto protocol and the report of the Intergovernmental Panel on Climate Change (IPCC)  on carbon dioxide capture and storage, there is an emerging need to reduce the emission of CO2 to the atmosphere. In principle, three possible routes can be envisioned focusing on (1) the reduction of the energy consumption, (2) the efficient use of energy sources (if desired combined with capture and storage of CO2), and (3) the use of alternative energy sources with reduced or no CO2 emission. In addition to
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Sustainability Science and Engineering, Volume 2 ISSN 1871-2711, DOI 10.1016/S1871-2711(09)00205-0
© 2010 Elsevier B.V., for Section 3.5, Statkraft. All rights reserved.
that, the limited amount of fossil fuels forces the developments in the direction of alternative energy sources.
Salinity gradient energy has a huge potential as alternative and sustainable energy source. It uses the Gibbs energy of mixing of two salt solutions with different concentrations to generate electrical energy. It is a nonpolluting (no emissions of CO2, SO2, or NOx), sustainable technology to generate energy by mixing water streams with different salinity. Salinity gradient power is available worldwide, everywhere where salt solutions of different salinity mix, for example, where fresh river water flows into the sea, or where industrial brine is discharged. The estimated global energy potential from estuaries alone is estimated to be 2.6TW , which is approximately 20% of the worldwide energy demand  and more than the global electricity consumption (2.0TW).
Pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) are the most frequently studied processes to extract the potential energy available from the mixing of freshwater and saltwater, although some other membrane-based processes are proposed as well. In PRO, two solutions of different salinities are brought into contact by a semipermeable membrane that only allows the transport of the solvent (water) and retains the solute (dissolved salts). In RED, a number of anion and cation exchange membranes (CEM) are stacked together in an alternating pattern between an anode and a cathode and allow the selective transport of salt ions only.
Although the potential of salinity gradient power was already recognized in the 1950s , until now, commercialization and industrial use are still limited; however, several initiatives are currently employed for pilot plant construction and upscaling of both technologies (see later in this chapter).
This chapter describes the process of salinity gradient energy and its potential. It first gives a thermodynamic overview of the theoretical amount of energy available from the mixing of a diluted and a concentrated salt solution, which in principle is independent of the used technology (PRO or RED). After that, the chapter continues with a section especially dedicated to PRO and a section only focusing on RED. Both sections describe the principle and theory of the specific technology and are followed by a detailed description of the literature and membranes used for PRO or RED. It also mentions the challenges for membrane development in this respect. After that, both sections address process design considerations. The last part of both sections is dedicated to the upscaling and commercialization of both processes. The chapter finally ends with some concluding remarks.
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