Can the combination of "salt" and "mirror" generate electricity? So easy! 24 hours a day!

Review expert: Zhu Guangsi, member of Beijing Science Writers Association

Have you ever heard of using "salt" and mirrors to generate electricity? In Dunhuang City, Gansu Province, there is such a molten salt tower power station: it was independently designed and built by a Chinese enterprise, and was officially put into use at the end of 2018 to achieve grid-connected power generation. It is also the world's tallest molten salt tower power station with the largest concentrating area. Now, follow the editor to see how it works!

Source: Xinhuanet

Can the combination of "salt" and mirrors generate electricity?

The "salt" mentioned here is not the edible salt used for cooking in our daily life, but "molten salt". Molten salt is a molten body formed by the melting of salts. It is composed of metal cations and non-metal anions , such as alkali metal, alkaline earth metal halides, nitrates, and sulfates. There are more than 80 kinds of cations and more than 30 kinds of anions that can constitute molten salts, and the combined molten salts can reach more than 2,400 kinds. Molten salt has good heat transfer and energy storage characteristics. It can absorb the heat energy contained in the sunlight reflected by the mirror, and complete the energy transfer in the process of heating and cooling. Engineers used 12,000 mirrors to form many concentric circles of different radii, and built a 260-meter-high molten salt heat absorption tower at the center of the circle. The light reflection area of ​​a single mirror is 115 square meters, and the total reflection area of ​​the mirror field can reach more than 1.4 million square meters. Even when there is no sunlight at night, molten salt can release the energy absorbed during the day to achieve 24-hour uninterrupted power generation.

Working principle of molten salt heat absorption tower

Source: See watermark

The molten salt in the heat absorption tower will circulate in the cold molten salt tank and the hot molten salt tank.

The molten salt continuously absorbs the heat energy from the sunlight reflected by the concentric mirrors and continues to heat up. It releases heat in the hot salt tank and is used to heat water, vaporizing it into steam, which drives the generator to rotate and generate electricity. The molten salt that has released its heat energy enters the cold salt tank and waits for the next cycle.

Figuratively speaking, it is to use the heat absorbed by molten salt to "boil water", which is the same as the thermal power plant using the heat released by burning coal to heat water to turn it into steam. However, using molten salt to generate electricity is cleaner and will cause less damage to the environment.

After the water is heated and vaporized into steam, the atmospheric pressure in the container increases rapidly, driving the steam turbine to rotate; then the steam turbine drives the generator to rotate, converting mechanical energy into electrical energy; the electrical energy is transmitted to all parts of the country through the power transmission grid. This is a complete power generation process.

What is the power of a molten salt tower power station?

It is not an easy task to build such a molten salt tower power station. As the pitch angle of sunlight changes with the rotation of the earth, each mirror has an angle control device. As time goes by, the orientation angle of the reflector changes with the angle of the sun, always maintaining the optimal reflection angle and focusing as much sunlight as possible. The concentric circle design can make the sunlight reflected by all mirrors shoot to the central heat absorption tower, so that the molten salt in the heat absorption tower can store the heat energy in the sunlight with maximum efficiency.

Source: Xinhuanet

In Dunhuang, the annual sunshine duration can reach more than 3,000 hours . The long-term light-to-heat conversion plus the huge array of reflective mirrors have greatly increased the power generation of this power station, with an annual power generation of up to 390 million kilowatts .

At the same time, since solar thermal energy is a clean energy, such a 100-megawatt molten salt tower power station can reduce carbon dioxide emissions by 350,000 tons per year, actively responding to the country's "low-carbon environmental protection, energy conservation and emission reduction" strategy.

How to build a molten salt tower power station?

Of course, there are many difficulties in maintaining such a large-scale molten salt tower power station. The harsh climate environment such as sandstorms, lightning, strong winds and high altitudes in the northwest region can easily cause mirror dust, damage, control device failure and other problems, resulting in reduced power generation efficiency. Therefore, the reflectors used to build the concentric mirror array are specially customized at a huge cost, and their dust and corrosion resistance are greatly improved compared to ordinary mirrors. In addition, fluctuations in daily light duration and light intensity can also cause the mirror to reflect too much light energy, causing overheating and damage to the device. The technicians control the cold molten salt pump and the temperature of the molten salt at the outlet of the heat absorber in a linked manner. When the molten salt pump reaches the maximum flow rate and the outlet molten salt still overheats, the mirror field is required to remove the corresponding mirror. In order to control the temperature of the heat absorber pipe wall, the technicians must also adjust the flow rate of the molten salt in real time to ensure that the molten salt heat absorber pipeline is fully cooled to avoid overheating and causing the heat absorber pipe to burst. In June 2019, this molten salt tower power station in Dunhuang achieved full-load power generation. Through the west-to-east power transmission, electricity generated by a renewable energy source such as solar thermal energy was delivered to thousands of households across the country.

Power transmission tower

Source: pixabay

Clean energy helps “dual carbon”

In order to achieve the "dual carbon" goal as soon as possible, clean energy plays a mainstay role. Traditional thermal power plants burn a lot of coal, but in this process, a large amount of harmful gases and greenhouse gases are released, which aggravates the greenhouse effect and is extremely harmful to the environment. Clean energy is often renewable, including light energy, tidal energy, wind energy, etc. Due to its renewable characteristics, these forms of energy do not require a long time span to be produced again like traditional fossil energy such as coal and oil, so they are not easy to be exhausted. Unlike solar and wind power generation, molten salt can store thermal energy and can continue to generate electricity even in rainy weather and at night when sunlight is scarce or disappears . Solar panels can only generate electricity when there is sunlight, and wind power generation also requires a windy environment. According to national planning, China's molten salt power generation will achieve a container capacity of 5 million kilowatts during the 13th Five-Year Plan period.

5MW offshore wind turbine.

Source: Wikipedia

In the process of coordinating renewable energy and traditional energy and realizing energy interconnection, we can use multi-energy complementarity to give full play to the advantages of multiple energy sources, increase the consumption of renewable energy, and thus accelerate the process of replacing fossil energy with clean energy. In the multi-energy complementary system, since the tower molten salt solar thermal power station is equipped with molten salt that can store energy, it can be used as a peak-shaving power source to smooth load fluctuations and increase the grid-connected capacity of wind power and photovoltaic power generation. In the future, we will definitely use clean energy more frequently. Let us look forward to the new world of clean energy that is coming.