Technology

Electrical energy can be generated when mixing salt and fresh water. The energy release when mixing 1m3 of fresh water in seawater (0.74 kWh) compares to hydropower of 1m3 falling 270 meters. This energy is released because of the increase in entropy when mixing solutions with different salinities, and is captured by means of ion-exchange membranes in reverse electrodialysis (RED). This technology is intensively studied and developed in the last decade. In RED electrical energy is harvested from mixing low and high salinity water resulting in brackish water as effluent. The opposite process, in which electrical energy is used to desalinate brackish water to produce fresh and saltwater, is known as electrodialysis (ED). As electrical energy can be converted back and forth to water streams with different salinities, and these streams can be stored in separate reservoirs, the combination of RED and ED creates in fact an electrical energy storage system. The concentration gradient flow battery or ‘Blue Battery’ charges by converting electrical energy into salinity gradients, when salt ions (Na+, Cl-) migrate through ion-exchange membranes from a low to high salinity solution (resulting in salt water and fresh water). During discharge, electrical energy can be recovered from the salinity gradient. The water solutions are stored in tanks and are recycled over the power conversion device (ED/RED membrane stack). In this way energy storage (i.e. solutions) and power generation capacity (i.e. membranes) are decoupled, which allows to tailor-make the battery for a specific customer.

Blue Battery: charged using electrodialysis (ED) and discharged using reverse electrodialysis (RED)

This reversible salinity gradient conversion is now possible to realize, as an efficient and practical operation of the conversion technologies has been scientifically developed by the applicants of this proposal. AquaB, the company commercializing this concentration gradient flow battery under the brand Blue Battery, has revealed that the obtainable energy density is ~1 kWh/m3 in practice. The energy density increases when increasing the salt content of the system. In practice, however, state-of-the-art polymeric ion-exchange membranes are becoming less selective when exposed to salt concentrations above 1 mol/L, resulting in a lower conversion efficiency. With the transport of ions, associated water in the primary hydration sphere will also be transported through the membranes. Hence, this intrinsic (electro)osmotic water transport limits the energy density that can be obtained with salinity gradients only.

Given this limitation, the challenge is to increase the energy density with using the same salt concentrations (1 mol/L). For this reason, we introduce in this project the BAoBaB in which energy is stored both in salinity gradients and in pH gradients.
With this approach, water is dissociated into protons and hydroxyde ions.
As a result, this pH difference creates an electromotive force (EMF) , which is 0.83V per membrane for 1M acid and base. As comparison, this voltage is an order of magnitude larger than the voltage due to salinity gradients only. Because the obtainable energy scales linearly with the voltage, and the maximum power is even proportional to the voltage squared, the obtainable energy and power density are boosted when introducing pH gradients in the BAoBaB system.
When the polarity of the electrodes is reversed, the system discharges and produces electrical power from the pH and concentration gradients (Fig. 1.3c). The H+ and OH- migrate back to associate to water and the counter-ions Na+ and Cl- migrate back to the salt solution compartment. Consequently, the acid and base solutions become more and more diluted, even back to neutral pH again, which completes the charge cycle.

Blue Acid/Base Battery (BAoBaB)

The BAoBaB system offers huge potential compared to existing electricity storage systems. A total volume of 3L (1L in each reservoir) with a gradient of DpH=14 over the BPM and a concentration gradient of Dc=1 mol/L over the IEMs, stores approximately 75 kJ of free energy. Hence, the theoretical energy density is 25kJ/L (7kWh/m3). This is equivalent to an imaginary 2.5km high hydropower dam. In other words: the energy density is an order of magnitude larger than for pumped hydropower, and also than for the recently proposed concentration battery that stores energy in a salt concentration gradient only (Dc=1 mol/L NaCl). To cover the electricity fluctuations of an average household over the day-night regime, an energy storage capacity in the range of 10 kWh would be required. This storage capacity can be achieved with a volume of 1-2 m3, which in practice can be placed under a house.

Energy density of non-faradaic EES systems