We propose the use of electrodialysis to desalinate produced waters from shale formations in order to facilitate water reuse in subsequent hydraulic fracturing processes. We focus on establishing the energy and equipment size required for the desalination of feed waters containing total dissolved solids of up to 192,000 ppm, and we do this by experimentally replicating the performance of a 10-stage electrodialysis system. We find that energy requirements are similar to current vapour compression desalination processes for feedwaters ranging between roughly 40,000-90,000 TDS, but we project water costs to potentially be lower. We also find that the cost per unit salt removed is significantly lower when removed from a high salinity stream as opposed to a low salinity stream, pointing towards the potential of ED to operate as a partial desalination process for high salinity waters. We then develop a numerical model for the system, validate it against experimental results and use this model to minimise salt removal costs by optimising the stack voltage. We find that the higher the salinity of the water from which salt is removed the smaller should be the ratio of the electrical current to its limiting value. We conclude, on the basis of energy and equipment costs, that electrodialysis processes are potentially feasible for the desalination of high salinity waters but require further investigation of robustness to fouling under field conditions.
We increase the power density of a reverse electrodialysis (RED) stack by blending the low salinity feed with a higher salinity stream before the stack entrance. This lowers the capital cost of the system and the resulting levelized cost of electricity, enhancing the viability of RED renewable energy production. Blending increases the power density by decreasing the dominating electrical resistance in the diluate channel as well as the e↵ective resistance caused by concentration polarization, but not without sacrificing some driving potential. To quantify this trade-o↵ and to evaluate the power density improvement blending can provide, a one-dimensional RED stack model is employed, validated with experimental results from the literature. For a typical stack configured with a feed velocity of 1 cm/s, power density improvements of over 20% and levelized cost of energy reductions of over 40% are achievable, provided the salinity of the available river water is below 200 ppm. Additional cost reductions are realized through back-end blending, whereby the diluate exit stream is used as the higher salinity feed. Additionally, improvements from blending increase for higher feed velocities, shorter stack lengths, and larger channel heights.
We develop a framework for choosing the optimal load resistance, feed velocity, and residence time for a reverse electrodialysis stack based on minimizing the levelized cost of electricity. The optimal load resistance maximizes the gross stack power density and results from a trade-off between stack voltage and stack current. The primary trade-off governing the optimal feed velocity is between stack pumping power losses which reduce the net power density and concentration polarization losses which reduce the gross stack power density. Lastly, the primary trade-off governing the optimal residence time is between the capital costs of the stack and pretreatment system. Implementing our strategy, we show that a smaller load resistance and feed velocity as well as a larger residence time than what is currently proposed in the literature reduces costs by over 40%. Despite these reductions, reverse electrodialysis remains more costly than other renewable technologies.
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