  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 two 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.
