Blue energy, i.e. the Gibbs energy from mixing seawater and river water, is a promising source of clean energy. Pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) are by far the most successful methods for extracting such energy, although their performance is limited by challenging issues associated with the required membranes.
Researchers at the University of Hawai’i have developed a method for direct electricity generation, in which carbon electrodes with asymmetric distributions of nanopores are used to generate electrical power from salinity gradients.
The researchers demonstrated that two electrodes made with carbon, e.g. carbon nanotubes, respond diﬀerently to a salinity gradient if their pore distributions are dramatically diﬀerent. Such diﬀerent responses lead to a discrepancy in the capacity change of the electrical double layers (EDLs) that spontaneously establish at the solid-electrolyte interface, which can create a signiﬁcant potential diﬀerence between the electrodes, and thus, generate electrical power. When the electrodes are successively immersed in solutions of various concentrations, a concentration gradient is created between the solution captured in the pore and that in the ambient. The Gibbs free energy from mixing of the two solutions can be harvested through the potential diﬀerence generated.
Using NaCl solutions and electrodes fabricated with commercially available carbon materials, the researchers observed an energy density of 69 mJ/g and a power density of 1.720 kW/m3, higher than that of current membrane-based methods (1.1 kW/m3). In the seawater stage, the power density reached 7.4 kW/m3.
This technology allows for direct electricity generation without the need of a membrane or a turbine. It does not require an external charge source as in traditional capacitive methods. It can be scaled up for grid-level applications. Because there is no need for expensive membranes, maintenance cost can be significantly lower. It is possible to use this method to recycle energy with normal seawater and waste brine from a water desalination process, which would help alleviate issues with desalination as well as increase the desalination efficiency by recycling the energy consumed.
Benefits and Advantages
• Easy-to-use: No need for membranes or turbines; easy to integrate and thus can provide high power density
• Economical: Activated carbon is readily available and can be used to achieve the same performance
• Environmentally friendly