A power station hidden in a "mirror"?! Explore the science behind the "secret mirror"

Plane mirrors can help us adjust our attire and appearance, convex mirrors make objects appear smaller, and concave mirrors make objects appear larger. So, if I say that mirrors have an unusual function - generating electricity, can you believe it?

The 100MW molten salt tower solar thermal power station in Dunhuang is composed of 35 concave mirrors and more than 12,000 heliostats, which can gather and accurately reflect sunlight. This is the "secret mirror base" we are looking for.

So, how do these mirrors generate electricity? Does it remind people of the time when we were children, when we used mirrors to reflect sunlight? Yes, you have already guessed the beginning. In fact, this process requires four steps: light tracking, heat absorption, power generation, and circulation.

Exploring the science behind the "secret mirror"

To use sunlight, you must first choose a place with good sunshine conditions. Dunhuang, Gansu is one of the areas with the highest solar radiation in China, and its annual solar radiation is 1.28 times the national average.

Now that the location is chosen, how do we turn sunlight into electricity? We need to build a tall heat-absorbing tower with a heat-absorbing device installed on the top of the tower. A large amount of molten salt is stored in the heat absorber. This salt is not the salt we usually eat, but an industrial salt. It has the characteristics of low melting point, strong heat storage, and low price. Next, more than 12,000 heliostats surround the heat-absorbing tower in concentric circles, and together they focus the sunlight on the receiver on the top of the tower. Each heliostat is like a giant sunflower, which can automatically adjust the mirror angle to follow the sun so that the reflected light is accurately directed to the heat absorber.

When light is focused on the absorber, the molten salt will be heated to more than 600 degrees Celsius, and will melt into liquid and flow into the hot salt tank below. When electricity is needed, we pump the molten salt in the hot salt tank and the water in the water tank into the steam generator. After absorbing the heat energy of the molten salt, the water turns into high-temperature and high-pressure steam, which drives the steam turbine to generate electricity, which is the well-known "boiling water" power generation.

Finally, the molten salt that releases heat returns to the cold salt tank and then returns to the absorber at the top to wait for the next round of power generation. This is the whole process of "mirror" power generation. In fact, it uses mirrors to achieve solar thermal power generation.

Solar thermal power generation has unique advantages

Solar thermal power generation has the advantages of being clean, stable and efficient. It fully utilizes solar energy and does not consume fossil energy such as coal and oil. More importantly, molten salt and water resources can be reused. For the same kilowatt-hour of electricity generated, the carbon dioxide produced by solar thermal power generation is almost the lowest among all the current mainstream power generation methods, only 1/50 of thermal power generation and 1/6 of photovoltaic power generation.

Like wind power generation, the amount of electricity generated will also fluctuate with the wind force, while photovoltaic power generation depends on the weather and cannot generate electricity at night. So, how can solar thermal power generation break through the randomness and volatility of new energy power generation? The answer lies in the molten salt storage tank that can store heat. It is the "battery" and "power bank" of solar thermal power generation. When there is no solar heat, hot molten salt can be extracted from the molten salt tank to "boil water" to achieve 24-hour sustainable power generation. Even at night, the "Secret Mirror Base" can still generate electricity at full load for 11 hours.

In June 2019, the "Secret Mirror Base" achieved full-load power generation, with an annual designed power generation of 390 million kWh, which is equivalent to guaranteeing the electricity consumption of all households in Haining City where we are now for half a year.

This is the "Secret Mirror Base" we are exploring today. It is also the world's largest solar thermal power station with the tallest heat absorption tower, the shortest construction period, and the ability to generate electricity continuously for 24 hours - the Dunhuang 100MW molten salt tower solar thermal power station.

(The speaker is the deputy team leader of the inspection and control team of the customer service center of State Grid Wenzhou Power Supply Company)