What is geothermal energy and ocean energy

It is not comprehensive to talk about renewable energy instead of geothermal energy and ocean energy. These two types of energy are also valuable clean energy. However, their development is relatively difficult, and they will take time as large-scale, universal energy products. But in any case, it should be advocated to actively develop and utilize in places with superior conditions to make energy supply more colorful.

main content:

  1. 1 Deeply hidden geothermal energy
  2. 2 huge ocean energy


1 Deeply hidden geothermal energy

Deeply hidden geothermal energy

The so-called geothermal energy refers to the heat energy contained in the earth itself. How is geothermal energy regenerated? Due to the presence of radioactive elements such as radium (Ra), uranium (U), and thorium (Th) inside the earth, they generate heat during the decay process and accumulate to maintain the temperature of the earth. These radioactive elements have extremely long half-lives, and the time to generate energy is almost as long as the life of the earth. Even if the existing geothermal energy is used by humans, it will continue to be replenished. However, geothermal heat is deep in the earth's interior, and currently human beings use it only rarely.

The earth is a sphere with a half length of more than 6,300 kilometers, and its structure can be roughly divided into three layers: crust, mantle, and core. The core is a hot iron-nickel magma, which also contains a solid core. The mantle is about 280 kilometers thick and consists of iron-magnesium silicate. The inner part is a semi-solid asthenosphere, and the outer part is a rigid lithosphere. The lithosphere objectively acts as a barrier to prevent the heat from the earth’s core from leaking out, making the heat transfer from the earth’s interior to the surface extremely slow—it will take millions of years. The lithosphere almost acts as an “insulation”. Obviously, the lithosphere has also become a kind of heat storage area. However, the lithosphere also has weaknesses. Magma and soft fluids inside the earth will seep out from there, and it is easy to form volcanoes in the weak spots of the earth's crust. Outside the lithosphere of the mantle is the crust layer, which is about 30 kilometers thick, and is a thin rock layer composed of sodium (Na), potassium (K), aluminum (Al), and silicate. This crust is divided into several "plates", as if several "rafts" are placed on a rigid lithosphere. Due to the convection of the mantle asthenosphere, the lithosphere will drive the crustal "raft" to move in all directions, leading to the collision or separation of crustal plates. This is also the cause of geological activities such as volcanic eruptions, orogens, and earthquakes.

So, how to use geothermal energy? Too deep geothermal heat is currently beyond the reach of human beings. Take the range of about 10 kilometers from the surface of the earth’s crust as an example. There are already a lot of geothermal resources, equivalent to 4.6×1016 tons of standard coal, which is equivalent to 7000 times the mineable coal reserves in the world. It is really exciting. . However, after all, this geothermal resource is still very deep underground and extremely dispersed, which cannot be obtained by digging it down. It can only be exploited and utilized in favorable areas, such as the junctions of various crust plates and geological active areas (regions with rich geothermal resources). In these areas, the free water contained in the surface of the earth's crust will bring the heat in the heat storage area to the vicinity of the surface, or pass through the ground to form hot springs, so that geothermal heat is easy to be found and easy to use. The geothermal temperature there can be as high as hundreds of degrees Celsius, which not only forms hot springs, but can even generate high-temperature steam. From a global perspective, currently available geothermal resources are still very small, equivalent to about 1 billion tons of oil. However, in areas where conditions permit, great attention should be paid to the utilization of geothermal resources.

Humans have long started to use geothermal energy, use hot springs for bathing, medical treatment, heating, building crop greenhouses, aquaculture farms, and drying grains. However, the real understanding of geothermal resources and the large-scale development and utilization only began in the middle of the 20th century. The utilization of geothermal energy can be divided into two types: direct utilization and geothermal power generation. Power generation is the main method of geothermal utilization, and high-temperature geothermal fluid (steam or hot water) above 150°C should be used for power generation first. The geothermal heat generated by the steam forms a "steam field", which can be directly introduced into the steam turbine generator unit to generate electricity. However, this kind of steam field is a small number after all, and most of them are stored in deeper strata, making mining difficult. Hot water-type geothermal power generation has become a common method of geothermal power generation. It usually uses a low boiling point working medium to generate steam through a heat exchanger to generate electricity. The geothermal water that has completed the power generation work should be injected back into the stratum to supplement the water source for the "steam field" and prevent ground subsidence. Counting the numbers and calculating them, it is estimated that by 2010, the world's geothermal power generation capacity will reach 25 million kilowatts, and it will double every 10 years thereafter. Heating and heating are the most common ways of direct utilization of geothermal energy, especially in areas in high-cold areas, which are simple and economical. For example, Iceland built the world’s first geothermal supply system in the capital Reykjavík as early as 1928, pumping nearly 10,000 tons of geothermal water at 80°C per hour. In addition to supplying more than 100,000 residents in the city, it also uses Used for factory heating. Because there are no chimneys, the city is known as the "cleanest city in the world." The application of geothermal heat in agriculture and medical treatment has also been developing. Due to the medical function of hot springs and the special geology and landforms that accompany the hot springs, the hot spring areas often become tourist attractions, attracting a large number of healers and travelers. In Japan, there are more than 1,500 hot spring sanatoriums, attracting hundreds of millions of people to recuperate there every year.

China is located on the southeastern edge of the Eurasian plate of the Earth's crust and is connected to the Pacific and Indian Ocean plates. It is one of the countries rich in geothermal resources, with proven reserves equivalent to more than 3 billion tons of standard coal. Among them, high-temperature geothermal is mainly distributed in southern Tibet, western Yunnan, Fujian, Guangdong, Taiwan and other places; medium-low-temperature geothermal is spread all over the country, with thousands of natural outcrops. The discovered geothermal temperature in Yangbajing of Tibet exceeds 300℃, which is a rare high-temperature geothermal in the world. Now a geothermal power station has been built there with an installed capacity of nearly 30,000 kilowatts, making it one of the main power supply bases in Tibet.

2 huge ocean energy

huge ocean energy

Ocean energy is relatively unknown. People praised the magnificent waves of the sea with "benefit to all rivers, tolerance is great", and they may not imagine that there is energy available in it. The surface area of ​​the earth is about 510 million square kilometers, and the ocean area accounts for 70%; the average depth of the ocean is 3800 meters, containing 97.2% of the total water on the earth. The vast ocean not only provides the convenience of human navigation, abundant aquatic products and mineral deposits, but also contains huge energy, including tidal energy, wave energy, ocean current energy, sea temperature difference energy and salt difference energy. Among them, except for tidal energy from the forces of celestial bodies such as the moon and the sun, almost all other ocean energy comes directly or indirectly from solar energy.

First, let's solve the mystery of tidal energy. People living by the sea have obvious feelings about tides, which are caused by the movement of the earth and nearby celestial bodies and the interaction between them. The gravitational force of the moon on the earth is not felt by people on land; sea water is flowing, and it will move due to the combined force of the gravitational force of the moon and the centrifugal force of the earth's rotation. The combined force of the moon and the earth becomes the tidal-causing force for the rise and fall of the tides. The direction of the moon's gravity on the earth points to the center of the moon, and the gravitational force of seawater on the earth is different, and the magnitude of tidal fluctuations (called "tidal range") varies from place to place. In addition to the moon, the sun also has tidal force, because the sun is much farther from the earth than the moon. Although the sun is very massive, the tidal force is not as large as that of the moon, which is only half of the moon's tidal force. Other celestial bodies such as Earth’s sister planets are too small and far away from the Earth, so their tidal force is negligible. The energy manifested by tidal fluctuations belongs to potential energy and can be used. The theoretical reserves are also large, reaching 3 billion kilowatts worldwide, distributed on the coastline of land and islands. People can build tidal power stations to "capture" tidal energy. However, not all coastlines can be set up with tidal power stations. Only those places where the tidal range is large and the terrain can store a large amount of water for civil engineering. These are the two basic conditions for the establishment of tidal power stations. The principle of tidal power generation is similar to that of ordinary hydropower stations, that is, a dam is built at the estuary or bay to form a reservoir, and low water level and large flow power generation are implemented. In order to ensure the continuity of power generation, the “dual reservoir method” can be adopted, that is, two adjacent reservoirs with high and low water levels are built, and the hydro-turbine generators are assembled in the dams of the two reservoirs. When the water is discharged at low tide, there is always a difference in water level between the two reservoirs, so that electricity can be generated throughout the day.

Double reservoir tidal power station

Double reservoir tidal power station

Don't underestimate the tidal power station. In places with good conditions, the installed capacity is also very large. For example, Canada's Greater Cumberland Bay Tidal Power Station has an installed capacity of 4 million kilowatts, which is equivalent to a large nuclear power station. Many countries are actively preparing for the construction of tidal power stations with a large total installed capacity, such as Argentina (680,000 kilowatts), Canada (8.5 million kilowatts), India (7.9 million kilowatts), the United Kingdom (10.3 million kilowatts), and the United States (9.4 million kilowatts) , Russia (43.2 million kilowatts), etc., where the tidal range is above 5 meters, which is very beneficial to the construction of power stations. There are more than 10 tidal power stations built in China, all of which are small power stations with a low water level (about 3 meters) and have not yet formed a scale. Although the number is small, it is a good start.

The waves of the sea also have energy. Waves are caused by wind. As the saying goes, "No waves can't be made without wind." Wave energy is just the kinetic energy converted from wind energy. Therefore, the power of wave energy has a lot to do with the continuity of wind speed, direction, and blowing. Wave energy is unstable. With low-grade energy, the sea is sometimes calm and sometimes huge, which increases the difficulty of wave energy development. At present, power generation can only be developed on a small scale to be used for desalination and to supply fresh water to the islands. Ocean current energy is also a kind of ocean energy presented in the form of kinetic energy. The so-called ocean current refers to the horizontal movement of sea water, and a large amount of sea water flows from one sea area to another sea area over a long distance. Why does horizontal sea water flow? Ocean currents are not like "water flowing to lower places" on land, but have different reasons, mainly driven by wind, and secondly caused by differences in temperature and salinity in different sea areas. There are winds on the sea that have a constant wind direction throughout the year. For example, the southeast wind blows on the south side of the equator and the northeast wind blows on the north side. The wind blows the sea water, causing the surface water to move, and the "viscosity" of the water transmits the movement to the depths of the sea, driving the movement of the sea in the depths. Although the speed of the movement of the sea in the depths is reduced, the ocean is vast and has a great maneuver. There is room for allowing the slowly moving seawater to extend far away and form ocean currents, which is incomparable to the waters on land. In addition, the seawater temperature and salt content of different sea areas are often different, which will affect the density of the seawater. The difference in seawater density between two adjacent sea areas will also cause seawater circulation.

Famous ocean currents in the world include the Gulf Stream in the Atlantic Ocean, the North Atlantic Ocean Current, the Kuroshio Current in the Pacific Ocean, and the equatorial undercurrent. The Gulf Stream and the North Atlantic Current are the largest currents connected by the North Atlantic Ocean. They traverse the Atlantic Ocean at a speed of 1 to 2 nautical miles per hour, passing between Iceland and the British Isles, and finally into the Arctic Ocean. The warm Kuroshio current in the Pacific Ocean is about 100 kilometers wide, with an average depth of about 400 meters, and an average daily velocity of 30 to 80 nautical miles. The flow is equivalent to 20 times the total flow of all rivers in the world. The equatorial undercurrent is a deep-sea undercurrent with a total length of 8,000 nautical miles per mile, a width of 120 to 250 nautical miles, and a flow rate of 2 to 3 nautical miles an hour. The flow of seawater will generate huge energy, and the global ocean current can reach 5 billion kilowatts. Ocean current energy can be used to generate electricity, and the principle is similar to that of general hydroelectric power generation, which also uses hydroelectric generator sets. The "parachute type" transmission mode can be adopted. Like throwing a net to catch fish, the umbrella-shaped device transmits the power to the turbine generator set on the ship under the impact of the sea current, as long as the ship is fixed on the sea with an anchor. It can also be an underwater "windmill type" power generation device. Ocean current power generation is more reliable than land-based hydropower, and it is not affected by hydrological factors such as dry seasons and floods. At present, ocean current power generation has been used in coastal lights and navigation aids and other devices, and its development prospects are promising.

Schematic diagram of boat-towed ocean power generation

Schematic diagram of boat-towed ocean power generation

Underwater windmill type power generation device

Underwater windmill type power generation device

The temperature difference can also be used. The ocean is a huge solar collector and heat storage device. Solar energy radiates to the earth's surface and most of it is absorbed by seawater, which heats up the surface of the ocean. The surface water temperature can reach 20~30℃, or even higher, while at a depth of 500~1000 meters from the sea surface, the seawater temperature is only 3~6℃. This vertical temperature difference contains energy, which is huge, reaching 2 billion kilowatts. Therefore, the construction of thermoelectric power stations has also become a goal pursued by people. Thermoelectric power generation generally uses surface warm sea water under negative pressure to "flash" to produce steam to do work, and deep-layer cold sea water is used as condenser cooling water. However, ocean thermoelectric power generation, after all, has a very small temperature difference between cold and heat, so the efficiency is very low, only about 3%, much lower than ordinary thermal power plants, and there are disadvantages such as large heat exchange area and high construction cost, which limit the development of thermoelectric power generation. However, it is not a dream to build an ocean thermoelectric power station. Such power stations have appeared in many countries, including China. The largest power station has an installed capacity of 200,000 kilowatts.

There is also salt difference energy, which is ocean energy in the form of chemical energy. Sea water is salty water, which contains a lot of salt and various minerals. Sea water contains 36 million tons of salt per cubic kilometer. The water in rivers on land is fresh water, and the salinity difference (abbreviated as "salt difference") is formed in the water area at the junction of the estuary and the sea. The salt difference contains energy and can be used. The salt difference of the world's estuaries can reach 30 billion kilowatts, and there are as much as 2.6 billion kilowatts available. It can be developed using the principle of "osmotic pressure", that is, a stream of fresh water is drawn from the surface of the fresh water to mix with seawater tens of meters deep into the sea surface, and a large osmotic pressure difference will be generated at the mixing point. This pressure is used to push the water The generator set generates electricity. Generally, when seawater contains 3.5% salt, the osmotic pressure generated is equivalent to 25 standard atmospheres and a water level of about 250 meters. The greater the salt concentration of seawater, the greater the osmotic pressure. The salinity power generation is still in the research stage and has been increasingly recognized by people. The development of salinity energy has become the goal of people’s efforts. Looking back at ocean energy, there are tidal energy that appears as potential energy, ocean current energy and wave energy that appear as kinetic energy, and heat energy. The temperature difference energy that appears, and the salt difference energy that occurs with chemical energy, are extremely large, and any one of them is larger than the total power consumption or even the total energy consumption in the world today. Unfortunately, "there are great difficulties", and there is not much ocean energy available today. But don't sigh and do nothing. Technological invention is a skill that human beings are good at, and perhaps in the future, human beings' ability will make people astonished today.