Ocean thermal power generation

Power generation principle

The ocean thermal power generation system uses the temperature difference between shallow and deep layers of seawater and different heat sources of temperature and cold to generate electricity through heat exchangers and turbines. In the existing Marine thermal power generation system, the source of heat energy is the warm sea water on the ocean surface, and there are basically two methods for power generation: one is to use warm sea water to evaporate the low boiling point working fluid in the closed circulation system; The other is that the warm water itself boils in a vacuum chamber. Both methods produce steam, which then drives a turbine to generate electricity. The steam after power generation can be cooled by cold sea water at a very low temperature and turned back into a fluid to form a cycle. Cold sea water is generally drawn from a depth of 600 to 1000m below sea level. Generally, the temperature difference between warm sea water and cold sea water is above 20 ° C, which can generate net electricity.

Cold seawater extracted from the deep sea, not only low temperature (generally 4, 5 ° C), sterile and nutrient rich, has a variety of uses, such as the production of fresh water, freezing, air conditioning, aquaculture, pharmaceutical, etc., can improve the economic value of ocean temperature difference power generation, this application is called deep sea water utilization (DOWA).

Systematic classification

closed

Closed circulation system uses low boiling point working fluid as working medium. Its main components include evaporators, condensers, turbines, working fluid pumps, and warm and cold sea pumps. Because the workflow system circulates in a closed system, it is called a closed cycle system. When the warm sea water pump pumps up the warm sea water and conducts its heat source to the working fluid in the evaporator, it evaporates. The evaporated working fluid expands adiabatically inside the turbine and pushes the turbine blades to generate electricity. The generated working fluid is transferred to the condenser and its heat is transferred to the cold seawater pumped from the deep layer, which is cooled and restored to a liquid, and then pumped through the circulating pump to the evaporator, forming a cycle. The working fluid can be recycled repeatedly, and its types are ammonia, butane, chlorofluorocarbons and other gas coolants with high density and high vapor pressure. Ammonia and HCFC-22 were the most likely working fluids. The energy conversion efficiency of closed cycle system is 3.3% ~ 3.5%. If the energy consumption of the pump is deducted, the net efficiency is between 2.1% and 2.3%.

open

The open circulation system does not use the working fluid as the working medium, but directly uses warm seawater. First of all, warm sea water is imported into a vacuum evaporator, so that it is partially evaporated, and its vapor pressure is about 3kPa (25 ° C), equivalent to 0.03 atmospheric pressure. The water vapor is adiabatically expanded in the low-pressure turbine, and after the work is done, it is introduced to the condenser, where it is cooled into a liquid by cold seawater. There are two methods of condensation: one is that water vapor is directly mixed into cold seawater, called direct contact condensation; The other is to use a surface condenser, where the water steam is not directly in contact with the cold seawater. The latter is the incidental method of preparing fresh water. Although the energy conversion efficiency of the open system is higher than that of the closed system, due to the uncertainty of the efficiency of the low pressure turbine, and the low density and pressure of the water vapor, the power generation device has a small capacity and is not suitable for large capacity power generation.

hybrid

The hybrid circulation system is somewhat similar to the closed circulation system, the only difference is the evaporator part. The warm water of the hybrid system is first passed through a flashevaporator (a device that rapidly compresses a fluid and then rapidly decompresses it to produce boiling evaporation), which converts some of the warm water into water vapor. The steam is then channelled into a second evaporator (a combination of evaporator and condenser). Here the water vapor is cooled and released to its potential; This potential in turn evaporates the working fluid at a low boiling point. The working fluid circulates through this to form a closed system. The purpose of the hybrid power generation system is to avoid the biological attachment of warm seawater to the heat exchanger. The system can also produce fresh water byproducts in a second evaporator. At the same time, the low capacity of the open power generation system can be improved.

There are not many countries in the world that develop Marine temperature difference technology, and Japan, France, Belgium and other countries have built a number of Marine temperature difference energy power stations, with power ranging from 100kW to 5MW. Japan has invested heavily in the research and development of Marine thermal energy, and is ahead of the United States in Marine thermal power generation systems and heat exchanger technology, and has built a total of three Marine thermal test power stations, all of which are shore-based. It is expected that by 2010, there will be 1,030 Marine thermal power stations in the world.

Development course

The idea of thermoelectric power generation was proposed as early as 1880 by the French D 'Arsonde (1851 ~ 1940), and in 1929 his student Claude (G. Clude) built a 22-kilowatt seawater thermoelectric power test plant on the coast of Cuba. The Claude pilot plant's power system uses an opencycle (it is worth mentioning that one of the main advantages of this cycle is that fresh water can be obtained from it). Claude's ocean thermoelectric power plant ended in failure, but it proved the feasibility of ocean thermoelectric power from the experiment. In order to avoid the problems encountered in the ocean thermal power station built by Claude, in 1965, the Anderson and his son in the United States proposed a power generation method using propane as the working medium.

In 1979 the United States first developed the ocean temperature difference generator (Oceanthermalenergyconversion OTEC) system, the capacity is only 50 kw. In 1981, it was planned to develop a large 40MW plant and put its 1MW intermediate unit into trial. The United States 50kWMINI-OTEC seawater thermal power generation ship, converted from a barge, the generator emits 50kW of electricity, most of which is used for pumping water, with a net output of 12-15kW. This is a historic development in the use of ocean heat energy. Due to the small temperature difference of the OTEC system, the net efficiency of the Rankine cycle is only 3%-5%.

In the "Sunshine Plan" of the Japanese Institute of Industry and Technology, the low-temperature differential power Generation Committee has planned a floating power station with a power generation capacity of 100,000 kilowatts, and the Rankine cycle efficiency of the power station is 3.44% and the net efficiency is 2.04%. The Peruvian seawater thermal power plant, part of Japan's "Sunshine Project", uses freon HCFC22 instead of ammonia as a working medium. Since the 1980s, Japan has developed power generation equipment with different capacities such as 50kW, 75kW, and 100kW, and in 1996, it also verified the use of NH3/ water mixed working medium cycle test equipment, as well as power generation equipment set on the surface of the ocean. The power station is built on shore and has a maximum generating capacity of 120kW and a net output of 31.5kW.

The Indian government will develop Marine thermal energy as one of the important energy sources in the future. In 1997, the National Institute of Ocean Technology of India and the University of Japan signed an agreement to jointly develop the Marine thermal energy of the Indian Ocean, jointly develop 1MW power generation equipment, and develop 25-50MW large-scale commercial equipment after the verification and evaluation of the simulator. We intend to invest in the establishment of a commercial OTEC system in India. In 1999, in the southeast sea of India, the world's first set of 1MW Marine thermal power generation experiment equipment was successfully operated.

In 1989, Taiwan proposed to the Pacific International Technology Research Center (PICHTR) to implement the OTEC commercialization strategic plan in Taiwan, preparing to build a 5MW small-scale OTEC pilot power plant on the eastern coast of Taiwan Island. Taiwan Hongchai seawater thermal power plant plans to use the 36-38℃ of waste hot water discharged from the Maanshan nuclear power plant and 300m deep cold seawater (about 12℃) temperature difference to generate electricity. The cold water pipe, with an inner diameter of 3m and a length of about 3200m, extends to a trench of about 300m depth in the Taiwan Strait. It is expected that the power generation of the power plant will be 14.25MW, and the net power generation will be about 8.74MW after deducting the power consumption such as pumps.

Key technology

So far, the Marine thermoelectric power generation technology has made great progress in the research of thermal power cycle mode, efficient compact heat exchanger, working medium selection and Marine engineering technology, and many technologies have gradually matured.

1) Heat exchanger is the key equipment of ocean thermal power generation system. Titanium heat transfer and corrosion resistance is good, but the price is too expensive. Researchers at Argonne National Laboratory in the United States have found that the life of the improved brazed aluminum heat exchanger can reach more than 30 years in the corrosive warm seawater environment. Plate heat exchanger has small volume, good heat transfer effect and low cost, and is suitable for use in closed cycle.

2) The latest Lorentz cycle organic liquid turbine can work at 20-22℃ temperature difference, suitable for closed cycle equipment. Lorentz cycle is characterized by high thermal efficiency and close to the actual cycle, and its turbine uses more than two freon mixtures as working medium, and is matched with a suitable heat exchanger.

3) There are two types of ocean thermal power generation: shore-based and offshore. The shore-based type locates the power generation device on the shore and extends the pump to 500-1000m or deeper in the deep sea. The offshore type is to lift the suction pump from the ship, the generator set is installed on the ship, and the electricity is transmitted through the submarine cable. In 1979, the United States built a mini-OTCE power generation device on the western coast of Hawaii, which was the first time in the world to obtain practical electricity from ocean temperature differences. The Pacific High Technology International Research Center (PICHTR) has also developed subsidiary industries using cold seawater for air conditioning, refrigeration and mariculture, which show good market prospects in tropical islands.

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.

2) Cold water pipe is a great challenge for the future development of OTEC technology. The cold water pipe must be strong enough to guarantee a 30-year service life. The thermal insulation performance of the cold water pipe is also better, so as not to affect the thermal efficiency of the cold sea temperature rise. These problems have not been fully resolved.

3) In order to achieve the commercial scale utilization of ocean temperature difference energy and achieve industrialization, in addition to solving the technical problems, there are other factors that need to be considered. Such as natural conditions and geographical location, only in the sea area near the equator within a certain range, the surface water temperature reaches more than 25 ° C, it is suitable for ocean temperature difference power generation. If the power generation location is too far away from the load center, the transmission cost is bound to increase; Wind speed, sea wave, ocean current and other factors affecting the stability of surface temperature have a direct impact on the overall efficiency of the device.