Page 166 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

154 Fundamentals of Ocean Renewable Energy

membrane towards the sea water, increasing the pressure inside the sea water
chamber. This pressure, which can reach a theoretical pressure equivalent to a
water column that is over 100 m in height, is used to spin a turbine and hence
turn a generator. A 10 kW PRO power plant in Tofte (Norway) was opened in
2
2009, consisting of 2000 m of membrane.
Reversed Electro Dialysis
In conventional electrodialysis, salt ions are transported from one solution,
through ion-exchange membranes, to another solution under the influence of an
applied electric potential difference. As the name suggests, in RED this process
is reversed, because the end product is the electric potential difference that can
be converted into electricity. Salt water and freshwater flow along alternating
cation and anion-exchange membranes. Due to the concentration difference,
salt ions permeate through these membranes from salt to fresh water: negatively
charged chlorine through one membrane, and positively charged sodium through
another. The resulting electric potential can then be converted into electricity.
Hundreds or thousands of membranes can be stacked between two electrodes
to increase the electricity output. A 50 kW RED pilot plant, opened in 2013,
has been constructed on a causeway in the Netherlands (the Afsluitdijk) that
separates fresh water from sea water in the Wadden Sea.

6.5 TECHNOLOGICAL CHALLENGES

One of the challenges of salinity gradient power plants is the vast surface area of
membrane that is required to generate meaningful levels of electricity. For exam-
6
ple, a 2 MW plant using current technologies would require at least 1 × 10 m 2
3
of membrane surface area, and this membrane would require maintenance and
replacement over a time period of around 5 years [17]. The maximum power
2
of membranes is currently less than 3 W/m , but recent laboratory experiments
2
have achieved power densities in excess of 14 W/m [17]. Such improvements
in membrane efficiencies applied to larger scales are required before the concept
of salinity gradient can become commercially viable. Other issues include bio-
fouling, pretreatment of water, and the development of the modules that house
the membranes.

REFERENCES
[1] T.S. Garrison, Oceanography: An Invitation to Marine Science, Nelson Education, Toronto,
ON, 2015.
[2] S. Imawaki, A.S. Bower, L. Beal, B. Qiu, Western boundary currents, in: Ocean Circulation
and Climate: A 21st Century Perspective, 2013.
[3] J.C. Larsen, T.B. Sanford, Florida Current volume transports from voltage measurements,
Science 227 (4684) (1985) 302–304.

3. Or less if efficiencies improve.