| Summary: | Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015. === This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. === Cataloged from student-submitted PDF version of thesis. === Includes bibliographical references (pages 93-96). === Studies of the future competitiveness of reverse electrodialysis (RED) with other energy technologies show that the projected levelized cost of electricity realized through current stack designs is prohibitively high. Despite these high projected costs, RED maintains other advantages - namely, the harnessing of a renewable and non-intermittent energy resource, the emissions-free operation, the direct conversion of chemical potential energy to electrical energy, and other environmental merits. Motivated by these advantages, system-level design enhancements and strategies are proposed and analyzed with the primary objective of reducing the projected levelized cost of electricity produced by an RED stack. The combined recommendations presented in this work result in projected cost reductions of over 40%. A major source of the cost reductions arises through the implementation of a new reverse electrodialysis stack design strategy which prioritizes the minimization of the levelized cost of electricity as opposed to the maximization of stack power density. The shift in strategy not only results in the definition of an optimal stack length which accounts for the trade-off between stack costs and pretreatment costs, but also the identification of an optimal load resistance which accounts for streamwise salinity variations and an optimal feed velocity which accounts for the trade-off between concentration polarization losses and pumping power losses. For all three design parameters, the identified optimum is smaller than what is currently prescribed in the literature. Further costs reductions are realized by blending the incoming river water feed with a higher salinity stream to reduce the largest source of irreversible losses in the stack - the electrical resistance of the diluate channel. The increase in diluate salinity, however, sacrifices some of the chemical potential between the inlet feeds. This trade-off is analyzed and the optimal inlet diluate salinity is identified for three different stack configurations. In all cases, the optimal salinity is higher than most local river water salinities at promising RED sites, rendering blending beneficial. Furthermore, the development of back-end blending, in which the diluate exiting the stack is used as the high salinity stream for blending, results in additional pretreatment cost reductions, as the already-pretreated diluate is recycled. === by Adam Michael Weiner. === S.M.
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