Osmotic power or
salinity gradient
power is the energy retrieved from the difference in the
salt concentration between
seawater and
river water. Two practical methods for
this are
reverse
electrodialysis
(RED) and
pressure retarded
osmosis
(PRO).
Both processes rely on
osmosis with ion
specific
membrane. The key waste
product is
brackish water. This
byproduct is the result of natural forces that are being harnessed:
the flow of fresh water into seas that are made up of salt
water.
The technologies have been confirmed in laboratory conditions. They
are being developed into commercial use in the Netherlands (RED)
and Norway (PRO). The cost of the membrane has been an obstacle. A
new, cheap membrane, based on an electrically modified
polyethylene plastic, made it fit for potential
commercial use .
Other methods have been proposed and are currently under
development. Among them, a method based on
electric double-layer
capacitortechnology and a method based on
vapor pressure difference .
The
world's first osmotic plant with capacity of 4 kW was opened
by Statkraft
on 24 November 2009 in Tofte,
Norway.
Basics of salinity gradient power
Pressure-retarded osmosis
Salinity gradient power is a specific renewable energy alternative
that creates renewable and sustainable power by using naturally
occurring processes. This practice does not contaminate or release
CO2 emissions (vapor pressure methods will release dissolved air
containing CO2 at low pressures—these non-condensable gases can be
re-dissolved of course, but with an energy penalty). Also as stated
by Jones and Finley within their article “Recent Development in
Salinity Gradient Power”, there is basically no fuel cost.
Salinity gradient energy is based on using the resources of
“osmotic pressure difference between fresh water and sea water.”
All energy that is proposed to use salinity gradient technology
relies on the evaporation to separate water from salt. Osmotic
pressure is the "chemical potential of concentrated and dilute
solutions of salt". When looking at relations between high osmotic
pressure and low, solutions with higher concentrations of salt have
higher pressure.
Differing salinity gradient power generations exist but one of the
most commonly discussed is Pressure Retarded Osmosis (PRO). Within
PRO seawater is pumped into a pressure chamber where the pressure
is lower than the difference between fresh and salt water pressure.
Fresh water moves in a semipermeable membrane and increases its
volume in the chamber. As the pressure in the chamber is
compensated a turbine spins to generate electricity. In Braun's
article he states that this process is easy to understand in a more
broken down manner. Two solutions, A being salt water and B being
fresh water are separated by a membrane. He states "only water
molecules can pass the semipermeable membrane. As a result of the
osmotic pressure difference between both solutions, the water from
solution B thus will diffuse through the membrane in order to
dilute the solution". The pressure drives the turbines and power
the generator that produces the electrical energy.
Methods
While the mechanics and concepts of salinity gradient power are
still being studied, the power source has been implemented in
several different locations. Most of these are experimental, but
thus far they have been predominantly successful. The various
companies that have utilized this power have also done so in many
different ways as there are several concepts and processes that
harness the power from salinity gradient.
At the Eddy Potash Mine in New Mexico, the technology of a salinity
gradient solar pond (SGSP) is being utilized to provide the energy
needed by the mine. The pond collects and stores thermal energy due
to density differences between the three layers that make up the
pond. The upper convection zone is the uppermost zone, followed by
the stable gradient zone, then the bottom thermal zone. The stable
gradient zone is the most important. Water in this layer can not
rise to the higher zone because the water above has lower salinity
and is therefore lighter and it can not sink to the lower level
because this water is denser. This middle zone, the stable gradient
zone, becomes an insulator for the bottom layer. This water from
the lower layer, the storage zone, is pumped out and the heat is
used to produce energy, usually by turbine.
Another method to utilize salinity gradient is called
pressure-retarded osmosis. In this method, seawater is pumped into
a pressure chamber that is at a pressure lower than the difference
between the pressures of saline water and fresh water. Freshwater
is also pumped into the pressure chamber through a membrane, which
increase both the volume and pressure of the chamber. As the
pressure differences are compensated, a turbine is spun creating
energy.
This method is being specifically studied by
the Norwegian
utility Statkraft, which has calculated that up to 25TWh/yr would be
available from this process in Norway. Statkraft has built
the world's first prototype osmotic power plant on the Oslo fiord
which was opened by Her Royal Highness Crown Princess Mette-Marit
of Norway on November 24, 2009. It aims to produce enough
electricity to light and heat a small town within five years by
osmosis. At first it will produce a miniscule 4 kilowatts - enough
to heat a large electric kettle, but by 2015 the target is 25
megawatts - the same as a small wind farm.
A third method being developed and studied is reversed
electrodialysis or reverse dialysis, which is essentially the
creation of a salt battery. This method was described by Weinstein
and Leitz as “an array of alternating anion and cation exchange
membranes can be used to generate electric power from the free
energy of river and sea water.”
The technology related to this type of power is still very much in
its infant stages, even though it was suggested over 30 years ago.
Standards and a complete understanding of all the ways salinity
gradients can be utilized are important goals to strive for in
order make this clean energy source more viable in the
future.
Possible negative environmental impact
The impact of the brackish water waste on the local marine and
river environment could cause harm to the environment.
Marine and river environments have obvious differences in water
quality, namely salinity. Each species of aquatic plant and animal
is adapted to survive in either marine, brackish, or freshwater
environments. There are species that can tolerate both, but these
species usually thrive best in a specific water environment. The
main waste product of salinity gradient technology is brackish
water. The discharge of brackish water into the surrounding waters,
if done in large quantities and with any regularity, may alter the
aquatic environment significantly. Fluctuations in salinity will
result in changes in the community of animals and plants living in
that location. However, while some variation in salinity is usual,
particularly where fresh water (rivers) empties into an ocean or
sea anyway, these variations become less important for both bodies
of water with the addition of brackish waste waters. Extreme
salinity changes in an aquatic environment may result in findings
of low densities of both animals and plants due to intolerance of
sudden severe salinity drops or spikes. The disappearance or
multiplication of one or more aquatic organisms as a result of an
influx of brackish water has the potential to cause ecosystem
imbalance. According to the prevailing environmentalist opinions,
the possibility of these negative effects should be considered by
the operators of future large blue energy establishments.
References
- Power generation by reverse
electrodialysis
- How does PRO work?
- History of osmotic power
- D. Brogioli, Extracting renewable energy from a salinity
difference using a capacitor, Phys. Rev. Lett.
103 058501-1-4 (2009).
- M. Olsson, G. L. Wick and J. D. Isaacs, Salinity Gradient Power: utilizing vapour pressure
differences, Science 206 452--454
(1979)
- (Jones, A.T., W. Finley. “Recent developments in salinity
gradient power”. Oceans. 2003. 2284-2287.)
- (Brauns, E. “Toward a worldwide sustainable and simultaneous
large-scale production of renewable energy and potable water trough
salinity gradient power by combining reversed electrodialysis and
solar power?” Environmental Process and Technology. Jan 2007.
312-323.)
- (Brauns, E. “Toward a worldwide sustainable and simultaneous
large-scale production of renewable energy and potable water
through salinity gradient power by combining reversed
electrodialysis and solar power?.” Environmental Process and
Technology. Jan 2007. 312-323.)
- Salinity Gradient Solar Pond Technology Applied to
Potash Solution Mining
- Salinity-gradient power: Evaluation of
pressure-retarded osmosis and reverse electrodialysis
- Recent Developments in Salinity Gradient Power
- [1]
- http://news.bbc.co.uk/1/hi/world/europe/8377186.stm BBC News
Norway's Statkraft opens first osmotic power plant
- Montague, C., Ley, J. A Possible Effect of Salinity Fluctuation
on Abundance of Benthic Vegetation and Associated Fauna in
Northeastern Florida Bay. Estuaries and Coasts. 1993. Springer New
York. Vol.15 No. 4. Pg. 703-717
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