NAMS 2002 Workshop - ICOM 2008

Hybrid and Novel Processes I – 5
Monday July 14, 4:30 PM-5:00 PM, Kaua’i
Reverse Electrodialysis: Energy Recovery from Controlled Mixing Salt and
Fresh Water
J. Post (Speaker), Wageningen University, Wetsus, The Netherlands
H. Hamelers, Wageningen University, Wetsus, The Netherlands, bert.hamelers@wur.nl
C. Buisman, Wageningen University, Wetsus, The Netherlands
The global potential to obtain clean energy from mixing river water with sea water
is considerable. The gross power potential of this unconventional energy source
was estimated to be 2.4-2.6 TW [1, 2] when the average discharges of all rivers
were used. It was assumed [1, 3] that from each cubic meter of river water that
flows into the sea, 2.3 MJ of work could be made available. A main question is
how much of this salinity-gradient energy can be converted into sustainable
electricity. Recently, we reviewed literature on two membrane-based techniques
that can be used for this conversion [4] , namely pressure-retarded osmosis and
reverse electrodialysis, and found that actually hardly attention was paid to the
energetic efficiency. In the papers concerning reverse electrodialysis, for
instance, we descried more-or-less founded estimates for the obtainable energy
recovery ranging from 0.35 MJ per m 3 of river water [5] to 0.6 MJ per m 3 of river
water [6] . These are not quite attractive numbers, especially not when the costs of
pre-treatment are taken into account. From this point of view, the absence of
experimental investigations regarding the obtainable energy recovery is a
peculiar gap in the field of reverse electrodialysis. The aim of our study [7] ,
therefore, was to investigate the energy recovery that can be obtained.
In our experimental setup, two batches of salt solutions with same volumes (550
mL each) were recycled over a reverse electrodialysis stack, namely 0.5 M NaCl
(‘sea water’) and 0.005 M NaCl (‘river water’). The available work from mixing is
then 0.80 kJ (i.e. 1.36 MJ per m 3 of river water, which is considerably lower but
more realistic then the mentioned 2.3 MJ). The mixing process was carried out at
different current densities (5, 10&25 A/m 2 ). During the mixing process, the stack
voltage was measured. From this measurement, the energy yield can be
calculated. For a reverse electrodialysis stack with 0.5 mm inter-membrane
distance which was operated with a current density of 5 A/m 2 , the energy yield
after complete mixing was 0.65 kJ (an energy recovery of 83%). Obviously, the
energy recovery was lower at higher current densities.
Theoretically, the internal losses could be minimized by reducing the intermembrane
distance, especially from the compartments filled with the lowconducting
river water. It was found, however, that a reduction of the
compartment thickness from 0.5 mm to 0.2 mm resulted in an almost equal