Salinity
gradient energy is a sustainable, renewable, and clean
energy source. When waters with different salinities are mixed, the
change in Gibbs free energy can be harvested as energy and only brackish
water remains. Reverse electrodialysis is one of the technologies
that can harvest this sustainable energy source. High power densities
have been obtained in small lab scale systems, but translation to
large industrial scale stacks is essential for commercialization of
the technology. Moreover, power density is an important parameter,
and efficiency, i.e., the amount of energy harvested compared to the
amount of energy available in the feed waters, is critical for commercial
processes. In this work, we systematically investigate the influence
of stack size and membrane type on power density, thermodynamic efficiency,
and energy efficiency. Results show that the residence time is an
excellent parameter for comparing differently sized stacks and translating
lab scale experimental results to larger pilot stacks. Also, the influence
of undesired water permeability and co-ion diffusion (as reflected
in permselectivity) is clearly visible when measuring the thermodynamic
efficiency. An averaged thermodynamic efficiency of 44.9% is measured
using Fujifilm Type 10 anion exchange and cation exchange membranes
that have low water permeability and high permselectivity. This value
comes close to the thermodynamic maximum of 50%.
The breathing cell is a new concept design that operates a reverse electrodialysis stack by varying in time the intermembrane distance. Reverse electrodialysis is used to harvest salinity gradient energy; a rather unknown renewable energy source from controlled mixing of river water and seawater. Traditionally, both river water and seawater compartments have a fixed intermembrane distance. Especially the river water compartment thickness contributes to a large extent to the resistance of the stack due to its low conductivity. In our cyclic approach, two stages define the principle of the breathing concept; the initial stage, where both compartments (seawater and river water) have the same thickness and the compressed stage, where river water compartments are compressed by expanding the seawater compartments. This movement at a tunable frequency allows reducing stack resistance by decreasing the thickness of the river water compartment without increasing permanently the pumping losses. The breathing stacks clearly benefit from the lower resistance values and low pumping power required, obtaining high net power densities over a much broader flow rate range. The high frequency breathing stack (15 cycles/min) shows a maximum net power density of 1.3 W/m. Although the maximum gross and net power density ever registered (2.9 W/m and 1.5 W/m, respectively) is achieved for a fixed 120 μm intermembrane distance stack (without movement of the membranes), it is only obtained at a very narrow flow rate range due to the high pressure drops at small intermembrane distance. The breathing cell concept offers a unique feature, namely physical movement of the membranes, and thus the ability to adapt to the operational conditions and water quality.
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