Ocean thermal power generation

4) China is also rich in ocean temperature difference energy, but the research work started late. In 1980 Taiwan Power Company planned to use waste heat from nuclear plants and ocean temperature differences to generate electricity. In 1985, Guangzhou Institute of Energy Research, Chinese Academy of Sciences began to study the method of "droplet lifting cycle" in temperature difference utilization. This method uses the temperature drop between surface and deep water to increase the potential energy of seawater.

Efficiency improvement

Marine thermal energy is a low-grade energy. Compared with existing biochemical energy and nuclear energy, the main reason why it cannot be applied commercially on a large scale is the low cycle thermal efficiency. The most effective way to improve the circulating thermal efficiency of OTEC system is to increase the temperature difference between cold and warm seawater, and the temperature difference between warm and cold seawater should be at least 20℃ to achieve ocean temperature difference power generation. According to the average temperature of the sea surface of 25 ° C, the cold sea water of about 5 ° C is generally taken from the depth of the ocean of about kilometers, if you want to continue to expand the temperature difference, the depth will be deeper. In this way, not only will the investment be greater, but the available sea area will be greatly reduced. Build a "buoy-type" solar pool on the sea surface, use natural sunlight to "boil" a pool of seawater, and then use a pump to draw out the warm seawater from the sea surface, and flow through the pipe to the bottom of the heated pool. In this way, the high temperature at the bottom of the pool can heat the warm water to 32 ° C, and the temperature difference between the water and the cold water at the bottom of the ocean can be increased to 27 ° C. In this way, after heating the solar pool, the efficiency of ocean temperature difference power generation can be increased by 10%, reaching about 12%, and the cost performance is greatly improved.

The research results of NoboruYamada et al. [28] show that the use of 5000m2 solar collector can increase the warm sea water by 20K ~ 40K, and the net efficiency of Rankine cycle of the ocean thermal power system (SOTEC) after the use of solar collector is increased from 2.3% to 6.3%-9.5%. The average annual thermal efficiency is 1.5 times higher than the net efficiency of conventional OTEC circulation systems. The technology can be used to increase the temperature of warm seawater, that is, the warm seawater extracted by the warm water pump is first sent to the solar collector for heating, and then enters the evaporator to heat the circulating working medium after the temperature rises. It can also be used to increase the temperature of the working medium at the inlet of the steam turbine, that is, the working medium out of the evaporator is sent to the solar collector for reheating, and then sent to the steam turbine to do work. The efficiency of Rankine cycle is improved by increasing the temperature of the working medium at the inlet of the steam turbine, no matter heating the warm seawater or the working medium with the solar collector. In this way, under the premise that the installed capacity of the unit is unchanged at 100kW, the improvement of the Rankine cycle efficiency of the SOTEC system reduces the mass flow rate of cold sea water, resulting in the power consumption of the cold sea pump is reduced by about 30% compared with that of OTEC, and the power consumption of the warm sea pump and circulating working medium pump is also reduced accordingly. Therefore, the net output work of SOTEC is higher than that of OTEC systems.

Technical problem

There are some technical problems in ocean temperature difference power generation, which is the bottleneck restricting the development of technology.

1) The surface of the heat exchanger is easy to attach microorganisms, which reduces the surface heat transfer coefficient, which has a great impact on the economy of the entire system. The research results of BergerLR et al show that when 25-50μm microorganisms are attached in the heat exchanger pipe, the heat transfer rate is reduced by 40-50%. The Argonne Laboratory in the United States found that intermittent chlorination for 1 hour a day can effectively control the attachment of organisms. However, this method has a certain impact on the environment, so it is still necessary to find a more suitable method. Scientists in a simulated heat exchanger experiment in 1977, after ten weeks of heat exchanger operation, despite the thin surface attachment of the heat exchanger, the heat transfer of the system is still significantly reduced. An experimental study in Hawaii in 1985 confirmed that although regular cleaning of microorganisms can remove most of the attached microorganisms, there is still a hard adhesion layer on the surface of the heat exchanger after long-term use that cannot be removed by simple cleaning. Another study showed that the use of sea rubber containing additives can effectively remove microorganisms attached to the system, but this will cause the microorganisms to attach and grow faster, and the cleaning will be more frequent.