Marine thermal power plant

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A marine thermal power plant converts the temperature difference between warm and cold water masses at different depths of the oceans into electrical energy . Used internationally is the abbreviation OTEC ( English Ocean Thermal Energy Conversion ), also, the designation ocean other premix gradient power plant are used. Jacques-Arsène d'Arsonval provided the theoretical basis for this type of energy conversion in 1881; the first test facility with an output of 22 kW was built in 1930 by Georges Claude , a student of d'Arsonval, in Matanzas , Cuba . However, this type of power plant could not prevail. With the exception of a few smaller pilot plants, there are or have not been any marine thermal power plants, and this type of power plant has so far had no practical significance for energy generation. A study by the French government assumes a global potential of 150 gigawatts, which, however, cannot be economically exploited given the current state of the art.

General

Temperature differences between water layers

The water on the surface of the oceans has a higher temperature than the water in deeper layers. This thermal gradient (thermal gradient ) makes the Ocean thermal energy conversion to Use. If the difference between the upper (0–50 m) and the lower layers (from 600–1000 m) of the water is more than 20 ° C, a cycle can be set in motion that is able to generate energy, for example a generator to deliver.

It is worth noting that, compared to other alternative power generators, a marine thermal power plant can produce this continuously and does not depend on the time of day or other variable factors. Real efficiencies are in the order of three percent, whereby the energy source - warm seawater - is usually available in excess and free of charge and is constantly being renewed by solar radiation. At a water temperature of 6 and 26 ° C, an efficiency of 6.7% can theoretically be achieved. However, the technical implementation is always fraught with efficiency losses.

The practical performance of these power plants is determined by the amount of water that is used by the cycle. An output of 100 megawatts (MW) for the closed circuit and around 2.5 MW for the open circuit is seen as the upper technically sensible limit. With a 100 MW power plant, around 200 cubic meters of water per second would be conveyed to the power plant through a pipeline with a nominal diameter of around 11 meters. In addition, there is another 400 m³ of warm surface water per second. This corresponds to about 1/5 of the Nile current into the Mediterranean. In an open circuit, the size of the turbine is the limiting element.

The currently largest cost factor (up to 75%) for systems of this size is the pipeline in which the deep water is brought to the surface. It would be made of fiberglass-reinforced plastic or reinforced concrete. If the pumps are attached to the lower end of the line, a hose line made of more flexible, cheaper plastic could also be used.

The complexity and the enormous size of the technical systems in relation to the energy yield is the main reason that has prevented commercial use or a wider spread of this type of power plant so far.

Operating principles

Marine thermal power plants, which also include ice power plants, work according to the physical principle of a low enthalpy - Clausius-Rankine cycle . The function of a marine thermal power plant is possible in two different circulatory systems. Both systems can also be combined.

Closed circle

Scheme of a closed OTEC circuit

In a marine thermal power plant with a closed circuit, warm surface water is pumped in an Organic Rankine Cycle , which causes a working medium that boils at a low temperature to evaporate in a heat exchanger . The evaporated working medium is passed through a turbine connected to a generator , in which part of the heat is converted into kinetic energy. Then the working medium with the cold water pumped from the depths is liquefied again in a condenser in a further heat exchanger and can be fed into the evaporator again.

The working principle corresponds to that of a steam power plant , only water vapor is not used as the working medium. Various substances are conceivable as a working medium for a marine thermal power plant, the use of which, however, has both advantages and disadvantages.

Open circuit

Scheme of an open OTEC circuit

A marine thermal power plant with an open circuit uses the warm surface water as a working medium, which is evaporated under vacuum . The steam generated drives a turbine to generate electricity. The steam, which loses its initial pressure in the turbine, is then liquefied again in the condenser with the help of cold deep water. If a heat exchanger is used for this and direct contact is avoided, the result is desalinated fresh water that can be used as drinking water . In a vacuum chamber, the air dissolved in the water is constantly sucked out by a vacuum pump . This ensures that the water boils and evaporates without the need for additional heat.

Hybrid cycle

Scheme of a hybrid OTEC circuit

Both of the aforementioned systems are combined in a hybrid marine thermal power plant. The warm surface water is used to evaporate the working medium in the closed turbine circuit. After the working medium has passed the turbine, it is again condensed by cold deep water and reintroduced into the circuit.

The still warm surface water is evaporated in a vacuum chamber after the heat has been given off to the turbine circuit. This water vapor is condensed with the help of the cooling water, whereby fresh water can be obtained.

Another arrangement first provides for the vacuum evaporation of the warm sea water. This warm water vapor is then used to evaporate the working medium of the turbine circuit. During this process, the water vapor is again condensed into fresh water. The cold deep water is only used to condense the working medium of the turbine.

Other combinations are also possible. A favored design has not yet established itself in the few test systems.

Barjot ice power station

The physicist Dr. At the beginning of the 20th century, Barjot developed a concept to use the temperature difference between the air at the poles of a maximum of −22 ° C and the water below the ice cover , which is up to over 3 ° C depending on the suction depth. He suggests butane (boiling point: −0.5 ° C) as the operating medium. The heating / cooling circuit in this technology - also known as the polar power plant - works inversely to the better-known OTEC in tropical hot water regions. A heat exchanger that protrudes into the cold polar air is responsible for the condensation of the working medium, the water pumped up from under the ice cover for the evaporation. Since butane is practically insoluble in water, the working medium and deep water can be mixed directly in the evaporator. Calculations show that with a theoretical efficiency of only 4%, the same amount of energy can be obtained from one cubic meter of water at a temperature of +2 ° C and an air temperature of −22 ° C as from the fall of this cubic meter from a height of 1,200 m .

History / test facilities

In 1881, the French engineer Jacques-Arsène d'Arsonval devised a closed-circuit marine thermal power plant. However, it was never tested by him.

In 1930, a small open circuit system was installed on the north coast of Cuba , but it ceased operations after just a few weeks. It was designed by the French Georges Claude , a friend and student of Jacques Arsene d'Arsonval and inventor of the neon tube . He had the principle of the open circuit patented . The pumps required more power than the 22 kW than the power delivered by the generator. The reasons for this were the poorly chosen location and problems with algae . Claude's next project, an OTEC floating power plant off Brazil , was ended by a storm that damaged a pipeline. The hapless inventor died practically bankrupt from his OTEC attempts.

In the 1970s, the US government funded research into the marine thermal power plant with $ 260 million. However, after the 1980 elections, government support was severely cut.

In 1979, on board a US Navy barge off the coast of Hawaii, an experiment, the so-called " Mini-OTEC ", was successfully carried out with a closed circuit with the participation of the State of Hawaii and an industrial partner. It took about three months. The generator output was around 50 kW, and the grid feed-in output was around 10-17 kW. About 40 kW were required to operate the pumps, which carry the 5.5 ° C cold water with a delivery rate of 10.2 cubic meters per minute from 670 m depth in a 61 cm diameter polyethylene pipe and the 26 ° C warm surface water as well conveyed to the system with a delivery rate of 10.2 m³ / min.

In 1980, components of a closed circuit under the project name " OTEC-1 " were tested on board a converted marine tanker, which was anchored off Kawaihae on the Kona Coast (Hawaii) . The aim was to examine the environmental impact of a power plant anchored in the sea. The plant could not generate electricity.

In 1981 a small marine thermal power plant was in operation on the island of Nauru for a few months , which had been built by a Japanese consortium for demonstration purposes. Of the 100 kW generator power, around 90 kW were required by the pumps. The total operating time was 1,230 hours.

Pilot plant in Hawaii

As early as 1983, a 40 MW OTEC test power plant was planned on an artificial island at Kahe Point off the coast of Oahu (Hawaii). After the construction work was completed in 1984, however, no funds could be obtained for the construction, as the OTEC power plant could not compare to cheaper fossil-fuel power plants. However, after further research, especially on the evaporators and condensers, it was hoped that the costs of an OTEC power plant with a closed cycle would be greatly reduced.

From 1993 to 1998, an experimental open-cycle marine thermal power plant was successfully operated by the Natural Energy Laboratory of Hawaii Authority at Keahole Point, Hawaii . The generator output was 210 kW, with a surface water temperature of 26 ° C and a deep water temperature of 6 ° C. In late summer at very high temperatures, the generator could deliver up to 250 kW. Around 200 kW were used by the pumps to deliver the water. Around 24,600 cubic meters of cold water were pumped through a 1 meter diameter pipe from a depth of around 825 meters and 36,300 cubic meters of warm surface water on land. A small part of the steam generated was used to obtain desalinated water (approx. 20 l / min). The tests showed that a ratio of around 0.7 of generator power to grid feed-in power could be achieved in commercial power plants.

Individual evidence

  1. ↑ James Chiles: The Other Renewable Energy . In: Invention and Technology . 23, No. 4, 2009, pp. 24-35.
  2. ↑ a b Popular Mechanics Magazine . Vol. 54, No. 6. , 1930, pp. 881-883 ( online ).
  3. ^ Ocean Energy Europe via OTEC
  4. ↑ http://www.energyprofi.com/jo/index2.php?option=com_content&task=view&id=104&pop=1&page=0&Itemid=160
  5. ^ John Daly: Hawaii About to Crack Ocean Thermal Energy Conversion Roadblocks? In: OilPrice.com. December 5, 2011, accessed March 28, 2013 .
  6. ↑ L. Meyer, D. Cooper, R. Varley: Are We There Yet? A Developer's Roadmap to OTEC Commercialization. (PDF; 2.0 MB) In: Hawaii National Marine Renewable Energy Center. Retrieved March 28, 2013 .
  7. ^ Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State. Energy Information Administration, September 2007, accessed September 30, 2013 .
  8. ^ William H. Avery, Chih Wu: Renewable Energy from the Ocean A Guide to OTEC . Oxford University Press, 1994, ISBN 0-19-507199-9 .

literature

  • Patrick Takahashi, Andrew Trenka: Ocean Thermal Energy Conversion . John Wiley & Sons, 1996, ISBN 0-471-96009-8 .

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