Performance of a mixing entropy battery alternately flushed with wastewater effluent and seawater for recovery of salinity-gradient energy

Salinity-gradient energy, also referred to as blue energy, is a largely untapped source of renewable energy. Coastal wastewater treatment plants discharge a continuous stream of low salinity effluent to the ocean and are thus attractive locations for recovery of blue energy. One method of tapping this gradient is a “mixing entropy battery” (MEB), a battery equipped with anionic and cationic electrodes that charges when flushed with freshwater and discharges when flushed with seawater. We constructed a plate-shape MEB, where the anionic electrode was Ag/AgCl, and the cationic electrode was Na4Mn9O18 (NMO). Over a single cycle with a single cell, the net energy recovery was 0.11 kW h per m3 of wastewater effluent. When twelve cells were connected in series, the net energy recovery (energy produced after subtracting energy invested) was 0.44 kW h per m3 of wastewater effluent. This is 68% of the theoretical recoverable energy of 0.65 kW h per m3 of wastewater effluent. We conclude that (1) wastewater effluent can be effectively used for charging of a MEB, (2) cells in series are needed to optimize net energy recovery efficiency, (3) there is a trade-off between net energy recovery efficiency and capital investment, (4) there is a trade-off between net energy recovery efficiency and power output, and (5) new electrode materials are needed to increase capacity, decrease cost, and to avoid release of Ag to seawater.