Boosting Capacitive Blue-Energy and Desalination Devices with Waste Heat

Abstract

We show that sustainably harvesting "blue" energy from the spontaneous mixing process of fresh and salty water can be boosted by varying the water temperature during a capacitive mixing process. Our modified Poisson-Boltzmann calculations predict a strong temperature dependence of the electrostatic potential of a charged electrode in contact with an adjacent aqueous 1∶1 electrolyte. We propose to exploit this dependence to boost the efficiency of capacitive blue engines, which are based on cyclically charging and discharging nanoporous supercapacitors immersed in salty and fresh water, respectively [D. Brogioli, Phys. Rev. Lett. 103, 058501 (2009)]. We show that the energy output of blue engines can be increased by a factor of order 2 if warm (waste-heated) fresh water is mixed with cold sea water. Moreover, the underlying physics can also be used to optimize the reverse process of capacitive desalination of water. DOI: 10.1103/PhysRevLett.113.268501 PACS numbers: 92.05.Jn, 82.47.Uv, 84.60.Rb, 92.40.Qk In river mouths an enormous free-energy dissipation of the order of 2 kJ takes place for every liter of fresh river water that mixes with an excess of salty sea water. This so-called "blue energy" can nowadays be harvested due to newly available nanostructured materials such as selective membranes [1,2] and nanoporous electrodes [3,4], as also used in supercapacitors [5]. In fact, several test factories have been built based either on pressure-retarded osmosis using waterpermeable membranes [6,7] or on reverse-electrode dialysis using ion-selective membranes [8], and recently a lot of progress has been made with capacitive mixing processes that involve highly porous carbide derived electrodes [9][10][11]. In all these cases, the spontaneous and irreversible ionic mixing process is intercepted and converted to a voltage difference by an enginelike device, very much in the spirit of a Stirling or Carnot engine that intercepts the spontaneous heat flow between a hot and a cold heat bath and converts it (partially) to the mechanical energy of a rotating flywheel [12]. However, the temperature of the river and sea water has been assumed constant throughout the cycle in all devices considered so far. Given the intrinsic scientific and societal interest in combined chemical-heat engines and heat-to-power converters, the ongoing development and upscaling of blue-energy devices and test factories, and the availability of a lot of waste heat in the form of "warmish" water in the industrial world, it is timely to consider temperature tuning in blue-energy devices. In this Letter we predict that the work extracted from capacitive mixing devices can be boosted by a factor of order 2 if warm fresh water is mixed with cold salty water. Moreover, we show how varying the temperature during a desalination cycle reduces the required energy input substantially, and we predict on the basis of a general thermodynamic argument that adiabatic (dis)charging processes lead to significant temperature changes on the order of several deg...

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