Osmotic power plant

An osmosis power plant (salt gradient power plant ) is a power plant that uses the difference in salt content between fresh water and sea ​​water to generate energy and electricity. Proposals for a power plant that technically utilizes osmosis energy (salt gradient energy) were first published in the 1970s. There have been specific research and development projects since the second half of the 1990s. As the world's first prototype of an osmotic power plant, a micro power plant was put into operation on November 24, 2009 in Tofte, Norway on the Oslofjord .

Operating principles

Working principle

The source of energy for an osmotic power plant is the difference in salinity (the salt gradient) of two solutions, which tend to equalize their concentrations. In conventional hydropower plants, the energy from the location (as with storage power plants ) or the combination of kinetic and location energy from large water masses (like in run-of-river power plants ) is used. In contrast, the osmotic power plant uses the hydration energy of the ions of the salts, their hydration shell enlarges.

If fresh and salt water are in contact with one another via a semipermeable membrane , pure water diffuses through the membrane to the salt water side (osmosis). With a salt content of 3.5% in seawater, an osmotic pressure of around 28 bar results at a temperature of 10 ° C compared to fresh water .

The technical realization requires special membranes that efficiently retain salts, but at the same time are well permeable to water. Due to the lack of suitable membranes, the principle could not be implemented in the 1970s. Since the mid-1990s, there have been new approaches to developing suitable membranes from polymers .

Underground power plant

Underground osmotic power plant

The functional principle of an underground osmotic power plant is relatively easy to understand. First, the energy of the fresh water is used. At the lower end of a downpipe, a turbine generates electrical energy from it. This corresponds to the energy of the sunken water:, where is the mass of the water, the gravitational constant and the height of fall. The water at the turbine outlet must then be directed further into the sea. This can be done without using energy, theoretically as long as the pressure of the sea water column is lower than the osmotic pressure. At 28 bar osmotic pressure this corresponds to almost 280 m. ${\displaystyle E=m\cdot g\cdot h}$ ${\displaystyle m}$ ${\displaystyle g}$ ${\displaystyle h}$

In such a structure, however, the diffusion of fresh water through the semipermeable membrane would be very slow. The flow through the membrane is roughly proportional to the pressure drop across the membrane. When building a power plant on a river, the goal would be to optimize the power gained, i.e. the energy per time. In the literature, a depth of 100-130 meters is given.

In the diagram, the pressure of the fresh water column , the pressure of the sea water and the pressure between the turbine and the membrane are shown at the designated points . Where and is the density of fresh and sea water, the pressure drop across the turbine and the pressure drop across the membrane. ${\displaystyle p_{1}=\rho _{S}gh}$ ${\displaystyle p_{2}=\rho _{M}gh-p_{\textrm {Osmose}}}$ ${\displaystyle p_{3}=p_{1}-\Delta p_{\textrm {Turbine}}=p_{2}+\Delta p_{\mathrm {Membran} }}$ ${\displaystyle \rho _{S}}$ ${\displaystyle \rho _{M}}$ ${\displaystyle \Delta p_{\textrm {Turbine}}}$ ${\displaystyle \Delta p_{\mathrm {Membran} }}$

Celestial power plant

Osmosis power plant with pressure exchanger

Salt water is filtered and put under pressure ( pressure exchanger ) before it mixes with fresh water in the membrane modules. Exactly as much water flows through the turbine as diffuses through the membrane. In order to maintain the difference in concentration, roughly twice the amount of salt water is passed through the system.

A pressure builds up on the side of the seawater, which can be used to drive a turbine to generate electricity . However, the osmotic pressure decreases within the system and decreases as a result of the dilution that occurs. The maximum performance is achieved when the static pressure difference is half of the osmotic pressure and the other half is available to overcome the membrane resistance .

Reverse electrodialysis

Reverse electrodialysis

Another method is reverse electrodialysis , (English RED, reverse electrodialysis ). Separated by ion-selective membranes, salt and fresh water are passed each other. The ions that diffuse through the respective membrane lead to a (small) electrical voltage that can be calculated using the Nernst equation . By connecting many of these arrangements in series, the voltage can be increased accordingly.

When the voltage is tapped in order to use the energy, a current flows which balances the separated charges and the voltage decreases. In order to keep this as low as possible, the membranes are arranged in close succession with a small spacing (<1 mm). The smaller the membrane distances, the greater the flow resistance that has to be overcome for the two types of water to flow through. The optimum distance between the diaphragms can be selected so that the power expenditure of the pumps to maintain the flow is as great as the gain in power by reducing the ohmic resistance for the charge carriers.

A pilot system has been in operation on the Dutch final dike since the end of 2014 . The membrane area is 400 m², the throughput is 220,000 m³ per hour of fresh and sea water. This generates an output of 1.3 W / m², and economical operation is expected from 2–3 W / m².

Potential for energy generation

Possible locations for osmotic power plants can be found at river mouths into the sea. In addition, all locations are conceivable as locations where two watercourses with different salt contents occur, for example also direct discharges of highly saline wastewater into rivers. The energy gain that can be achieved is greater, the higher the flow rate and the greater the difference in salt content.

When considering the energy potential of osmotic power plants, it should be noted that full use of an entire river in osmotic power plants is not feasible in practice - for technical reasons as well as out of consideration for shipping and the ecology of the rivers. For these reasons, it makes sense to consider ecological potentials, which, in addition to the technically-related conversion losses, also include the limitation of the maximum permissible water withdrawal amount. Assuming that 10% of the global runoff is used, the technical potential of osmotic power plants is 1300 TWh per year, which corresponds to about half the electricity production in the EU.

The most potential location on German soil is where the Elbe flows into the North Sea . The ecological potential of the use of all German rivers that flow into the North Sea and Baltic Sea is given as approx. 42 MW and approx. 330 GWh / a. This would correspond to a little more than 0.5% of Germany's electricity demand. Another study, on the other hand, assumes only around 0.05% of Germany's electricity demand. The discharge volumes from the Rhine and Danube are not included, as they flow outside Germany.

On a global level, the ecological potential in 2012 was estimated at around 65 GW or around 520 TWh / a. A study published in 2016 puts the actually usable potential somewhat higher at approx. 625 TWh; this corresponds to around 3% of global electricity generation. The distribution of the potential across the continents and regions is analogous to the distribution of the discharge values.

In relation to the operating volume flow, higher specific power plant outputs could be achieved on bodies of water that have a higher salinity than the North and Baltic Seas, especially on the Mediterranean Sea and especially on salt lakes such as the Dead Sea or the Great Salt Lake in Utah, USA. The Heidelberg physicist Florian Dinger estimates the potential at Kara-Bogas-Gol east of the Caspian Sea at more than five gigawatts .

Osmosis power plants use renewable energy, it is ultimately supplied by the sun: As solar energy leads to the evaporation of water from the sea, it enables the separation of (remaining in the sea) salt water and (evaporated) fresh water. The evaporated water flows back into the sea via cloud formation, precipitation and rivers, where, when mixed again, the energy that was originally generated by the sun can be partially recovered in an osmosis power plant. The osmosis energy is "refilled" by the sun. It is therefore a form of renewable energies , which was officially recognized by its mention in the German Renewable Energy Sources Act (under the name of salt gradient energy, see § 3) even before its technical implementation.

implementation

View of Statkraft's prototype of an osmotic power plant near Hurum in Norway

The foundations of a membrane that is sufficiently stable for large-scale use have been created since 2004 in an EU-funded research program. System partners are Statkraft SF (Norway), Instituto de Ciencia e Tecnologia de Polimeros (Portugal); Norwegian Institute of Technology SINTEF (Norway); Helsinki University of Technology (Finland) and the Helmholtz Center Geesthacht (Germany). An electrical output of three watts per square meter of membrane can currently be achieved.

In autumn 2007 the Norwegian state-owned company Statkraft announced the world's first construction of such a power plant near Hurum , at an estuary in the southern foothills of the Oslofjord . The world's first prototype went into operation on November 24, 2009. For this purpose, membranes were used that can generate 3 watts instead of the previous 0.2 watts of electrical power per square meter. The next goal planned for 2015 was a 25 megawatt power plant with a membrane area of ​​5 million square meters. Statkraft estimates that Norway can obtain 10% of its electrical energy from osmotic power plants in the long term. However, Statkraft stopped further investments in the research of Osmosekaft at the end of 2013 because the goal of competitive energy generation could not be achieved.

A project group at the Helmholtz-Zentrum Geesthacht worked on the development of membranes with higher performance until the beginning of the 2010s as part of an EU- funded project. Their project manager Peinemann named in 2006 an output of five watts per square meter as a prerequisite for the economical operation of an osmotic power plant. This performance has not yet been achieved for economic applications (as of 2019). Membranes with a high power density of up to 3 W / m² are complex to assemble and insufficient in continuous use (e.g. due to contamination ).

See also

• Tidal power plant
• Wave power plant
• Ocean current power plant
• Marine thermal power plant
• Ocean energy

literature

• Loeb, Sidney (1975): Osmotic Power Plants . Science 189, 654-655.
• Loeb, Sidney (1998): Energy Production at the Dead Sea by Pressure-Retarded Osmosis: Challenge or Chimera? Desalination 120, 247-262.
• Norman, Richard S. (1974): Water Salination: A Source of Energy. Science 186, 350-352.
• Stenzel, Peter (2012): Potentials of osmosis for the generation and storage of electricity . LIT Verlag, ISBN 978-3-643-11271-2 .

Web links

Commons : Osmotic Energy  - Collection of Images, Videos, and Audio Files

Wiktionary: Osmosekraftwerk  - explanations of meanings, word origins, synonyms, translations

• Osmosis as a power source. In: Hamburger Abendblatt , March 2005
• Osmotic power plant (salinity gradient) - brief overview in the 'Book of Synergy'
• Power from the osmosis power plant Article in the journal "Unter Uns" of the research center GKSS Geesthacht, April 2005 (PDF; 3.3 MB)

Individual evidence

1. ↑ The world's first osmotic power plant has opened. Statkraft : Press release of November 24, 2009
2. ^ Osmotic power plant , Max Planck Institute IPP
3. ↑ David A. Vermaas, Enver Guler, Michel Saakes, Kitty Nijmeijer: Theoretical power density from salinity gradients using reverse electrodialysis . In: Energy Procedia . Volume 20, 2012, pp. 170–184 DOI: 10.1016 / j.egypro.2012.03.018 ( Open Access )
4. ↑ UT levert grote bijdrage aan eerste blauwe-energiecentrale (Dutch)
5. ↑ Fernanda Helfer, Charles Lemckert: The power of salinity gradients: An Australian example . In: Renewable and Sustainable Energy Reviews 50, (2015), 1–16, p. 2, doi : 10.1016 / j.rser.2015.04.188 .
6. ↑ ^{a b} Peter Stenzel: Potentials of osmosis for the generation and storage of electricity. LIT Verlag, 2012.
7. ↑ Thomas Isenburg: Osmosis power plants: Potential analysis for a future technology. Ruhr University Bochum, May 2, 2010, accessed on September 21, 2011 . 
8. ↑ Alvarez-Silva et al .: Practical global salinity gradient energy potential . In: Renewable and Sustainable Energy Reviews . tape 60 , 2016, p. 1387–1395 , doi : 10.1016 / j.rser.2016.03.021 .
9. ↑ ^{a b} Holger Dambeck: Osmotic power plant: Green electricity from sweet water. In: spiegel.de , March 30, 2012
10. ↑ The salinity power project. ( Memento of the original from September 5, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Final report of the research program, October 2004 (PDF; 314 kB) 
11. ↑ Max Planck Institute for Plasma Physics Olivia Meyer: Osmotic power plant: It's all in the mix. In: Energy-Persketiven. Newsletter of the Max Planck Institute for Plasma Physics . Edition 03/2005
12. ↑ ^{a b} Sebastian Balzter: Music of the future from the double garage. In: Frankfurter Allgemeine Zeitung . November 20, 2008, p. 20
13. ↑ Norwegians build the world's first salt power plant. In: ORF . October 13, 2007
14. ↑ Statkraft: Energy generation through osmosis: world's first prototype goes into operation. Press release from November 24, 2009
15. ↑ Alexander Budde : Electricity from salt: The world's first prototype of an osmotic power plant goes into operation in Norway. In: Deutschlandradio . November 23, 2009
16. ↑ Statkraft halts osmotic power investments. Press release from December 20, 2013
17. ↑ Anna-Lena Gehrmann: Estuaries: Generating clean electricity with osmosis. In: Spiegel Online . April 2, 2006
18. ↑ Xin et al .: High-performance silk-based hybrid membranes employed for osmotic energy conversion in Nature communications, 2019