The high energetic potential of hydraulic fracturing wastewaters with both salinity and temperature gradients for electricity generation using a reverse electrodialysis stack

Hydraulic fracturing (HF) wastewaters have high salinities relative to feed waters and can be an economic and environmental burden. However, salinity differences between these two solutions offer the opportunity for electricity generation using reverse electrodialysis (RED) stacks. HF wastewaters are much hotter than feed waters, but the relative impact of temperature for RED has been relatively unexplored. We investigated power generation using HF fluids in RED stacks, focusing on the impact of temperature differences between feed and wastewater. Power from synthetic solutions (mixtures of NaCl, Na₂SO₄, and CaCl₂) produced up to 0.96 W/cp m² when both solutions were 25 °C and increased to 1.51 W/cp m² when both solutions were at 60 °C. With an actual HF wastewater power production was 1.27 W/cp m² at 25 °C with no temperature difference, and 3.74 W/cp m² with a 25 °C feed solution and 60 °C HF wastewater. Comparing synthetic and HF wastewaters showed that the presence of sulfate and calcium ions reduced permselectivity and increased the electrical resistance of the membranes. Increasing the temperature, however, reduced electrical resistances. A thermodynamic model using Gibbs energies, and short- and long-range interactions of ions/ion-water was used to assess energy efficiency. The overall energy efficiency of the RED stack, based on ion concentrations in the inlet/outlet streams, was 24 % using NaCl at 50 °C. These results show that electricity can be effectively generated using salinity differences of HF streams and highlight the additional benefits of using solutions with different temperatures.