Ocean energy utilization-text uses certain methods, equipment and devices to convert various ocean energy into electrical energy or other usable forms of energy. It is an important aspect of human utilization of natural energy.
Types of ocean energy Ocean energy is renewable energy generated during the movement of sea water, which mainly includes temperature difference energy, tidal energy, wave energy, tidal energy, ocean current energy, salinity difference energy, etc. Tidal energy and current energy come from the gravity of the moon, the sun and other planets, and other ocean energy comes from solar radiation.
Seawater temperature difference energy is a kind of thermal energy. The sea surface water temperature at low latitudes is higher, forming a temperature difference with the deep water, which can produce heat exchange. Its energy is proportional to the temperature difference and the amount of heat exchange water. Tidal energy, tidal current energy, ocean current energy, and wave energy are all mechanical energy. The energy of tides is proportional to the tidal range and tidal volume. The energy of a wave is proportional to the square of the wave height and the area of ??the waving water. There is also seawater salt difference energy (also known as seawater chemical energy) in estuary waters. There is a salinity difference between the fresh water flowing into the sea and the ocean salt water. If separated by a semipermeable membrane, the freshwater will penetrate to the seawater side, which can generate osmotic pressure. The energy is directly proportional to the pressure difference and the penetration energy.
Characteristics of ocean energy ① It has large reserves and can be regenerated endlessly. It is estimated that the available power of seawater temperature difference energy on the earth is on the order of 1010 kilowatts; renewable power such as tidal energy, wave energy, ocean current energy, and seawater salinity difference energy are all on the order of 109 kilowatts. ②The energy flow is unevenly distributed and has low density. The largest temperature difference between the surface layer of the ocean and the depth of 500 to 1,000 meters is only about 20°C. The largest tidal range along the coast is about 7 to 10 meters, and the speed of large tidal currents and ocean currents in the offshore is only 4 to 7 knots. ③Energy is changeable and unstable. Among them, seawater temperature difference energy, ocean current energy and salinity difference energy change relatively slowly, while tide and tidal current energy change in a short-term periodic pattern, and wave energy has significant randomness.
Technologies and facilities for ocean energy utilization The key link in ocean energy utilization is energy conversion. Different forms of ocean energy have different conversion technology principles and devices.
The utilization of seawater temperature difference energy is to convert thermal energy into mechanical energy and then into electrical energy. The thermodynamic cycle method is used to convert thermal energy into mechanical energy. There are two common processes (Figure 1): ① Closed circuit cycle (also known as the intermediate medium method), which uses a system composed of an evaporator, a turbine generator, a condenser and a working fluid pump , the evaporator passes through the ocean surface hot water, and the condenser passes through the ocean deep cold water. The working fluid pump pumps liquid ammonia or other working fluids from the condenser into the evaporator as an intermediate medium. The liquid ammonia becomes high-pressure ammonia gas due to the action of hot water. , drives the steam turbine to generate electricity; and the low-pressure gaseous ammonia coming out of the steam turbine returns to the condenser and is cooled again into liquid oxygen, thus forming a closed loop. ② Open cycle (also called flash evaporation method or expansion method), evaporates hot seawater into steam in a partial vacuum evaporator (flash evaporator), and drives the steam turbine to generate electricity; the used low-pressure steam then enters the condenser for cooling, and the condensed Desalinated water is either recycled or discharged into the ocean. Early experimental devices mostly used open-circuit circulation processes. Since the equipment was susceptible to corrosion, closed-circuit circulation processes were switched to after the 1960s. The actual thermal efficiency of seawater temperature difference power generation is very low, often only about 2%. The amount of cold and hot water processed is large, so the corresponding various components are very large in size. The long cold water pipes extending to the deep water layer of the seabed are technically difficult. big.
The utilization of tide, wave, tidal current and ocean current energy only requires the conversion of mechanical energy into electrical energy, which is generally divided into three steps: the first step is to receive energy, such as building a tidal reservoir to receive and store tides Energy: Use a runner (water wheel) to absorb the kinetic energy of ocean currents and tidal currents; use water columns-air chambers, floats that rise, fall or swing with the waves, compressible air bags, etc. to accept wave energy. The second step is transmission, usually using mechanical, hydraulic, pneumatic and other methods. The transmission terminal is usually equipped with a hydraulic turbine or a gas turbine. Tidal power stations use bulb tubular turbine units or full tubular turbine units that adapt to low water levels (Figure 2); in recent years, symmetrical airfoil air turbines have been used for wave energy transmission, which can rotate in one direction under the action of waves. The third step is to convert it into electricity or other power. Usually converted into electricity via a generator. Since ocean energy is unstable, energy storage facilities such as reservoirs, gas tanks, batteries and flywheels are generally required throughout the conversion process.
Research on conversion methods for utilizing seawater salinity difference energy has only begun in recent years. One idea is to build two dams where the river mouth enters the sea, with a buffer reservoir in the middle, and a semipermeable membrane in the channel between the buffer reservoir and the open sea.
The fresh water in the buffer reservoir seeps out through the semipermeable membrane, and its osmotic pressure causes the water level in the buffer reservoir to decrease. The water level difference between the buffer reservoir and the river can be used to generate electricity. This method requires a large scale of engineering due to the large amount of water entering and exiting it.
Engineering facilities that utilize ocean energy are generally divided into two categories: seaside type and offshore type according to their location. The former is based on coastal land or shallow sea waters, while the latter is a floating structure set up in deep water waters. Seaside and offshore facilities close to the shore can use submarine cables or pressure pipelines to transmit power ashore; offshore facilities far offshore can only use power locally, such as ammonia production or seawater chemical products.
Economic benefits of ocean energy utilization The utilization of ocean energy is still very expensive. Taking the Lens tidal power station in France as an example, the investment per kilowatt of installed capacity is US$1,500 (1980 price), which is higher than that of conventional tidal power stations. power station. But in the process of utilizing ocean energy, other comprehensive benefits can also be obtained. For example, the reservoir of a tidal power station can take into account aquaculture and transportation; the nutrient-rich deep seawater obtained by the ocean thermal energy conversion device can be used to develop fisheries; the open-circuit circulation system can desalinate seawater and extract brine containing useful elements; large-scale wave power generation devices It can simultaneously suppress waves and protect seaports, coasts, offshore buildings and aquaculture farms. At present, in coastal areas (including islands) that are severely lacking in energy, it is still advisable to use ocean energy as a supplementary energy source.
Development overview The utilization of ocean energy first started with the use of tidal energy. Tidal mills appeared in the 11th century. In 1966, France built the Lens tidal power station with an installed capacity of 240,000 kilowatts. It is currently the largest tidal energy power station in the world (see color map). In 1981, the first 500-kilowatt unit of China's Jiangxia Tidal Experimental Power Station (see color picture) was officially put into operation. The world's first patent for a wave energy conversion device was obtained in France in 1779. In 1965, Japan successfully developed a wave power generation device for navigation lights. At present, countries such as Japan, the United Kingdom, Norway and China are conducting various wave power generation test studies. The larger one is the "Haiming" wave power generation ship tested in the Sea of ??Japan by Japan and other five countries. The first phase of the test will generate electricity in 2018. It measured 190,000 degrees and initially successfully transmitted electricity to the shore. Japan has also established a shore-type wave power test station. China has developed a new wave power generation device using symmetrical airfoil air turbines and installed it on a floating beacon light in the South China Sea for trial use (Figure 3). In 1881, the French first proposed the principle of utilizing the temperature difference of sea water. Since the 1970s, the United States has ranked first in the world in terms of funding for research on ocean thermal energy conversion. In 1979, the United States conducted a seawater temperature difference power generation test with an installed capacity of 50 kilowatts on a barge off Hawaii Island. Later, Japan established a shore-type experimental seawater temperature difference power station on Nauru Island with an installed capacity of 100 kilowatts.
With the increasing world energy demand and the improvement of ocean energy utilization technology, it is expected that in the 20th century, it will be possible to build 100,000 to 1,000,000 kilowatt-level tidal power stations on the coasts of estuaries with large tidal ranges. ; And there will be medium and small practical wave power generation devices and experimental seawater temperature difference power generation devices. In the long run, the utilization of ocean energy will become an important aspect of the world's new energy sources.