Is it true that the mini refrigerator is a distant relative of the nuclear power plant?

Today's story starts with a small refrigerator.

In the hot summer, you can sweat profusely in just five minutes. In order to drink a can of iced cola in the hot weather, I spent 68 yuan to buy a mini refrigerator on Taobao. It can not only cool but also become a heating box in winter.

Are today's refrigerators so versatile? Something must be wrong. A refrigerator that can heat or cool and is cheap is definitely not a real refrigerator. We must investigate it carefully!

01

Are mini fridges “proper” fridges?

Ahem, let me announce the results of my investigation: this refrigerator and the refrigerator at home, except for the same name, their refrigeration principles are not significantly different, but completely irrelevant. There is no need to explain the refrigeration principle of ordinary refrigerators here. The core components of those colorful small refrigerators on the Internet are semiconductor refrigeration chips .

The semiconductor refrigeration chip looks like this:

(Image source: Wikipedia)

Its structure is very simple, a small square piece and two wires. When the cooling piece is powered, one side of the small square piece will heat up and the other side will become cold. After changing the power supply direction, the heating side and the cooling side will also be reversed. The small refrigerator using the semiconductor cooling piece can cool and heat.

(Image source: self-made by the author)

In addition, the voltage required by the semiconductor refrigerator is also very low, and it can be powered by a power bank. Most car refrigerators are also semiconductor refrigerators.

The semiconductor cooling sheet is made by bonding two different semiconductor materials together, and then a wire is led out from each of the two materials. When the wire is connected to a DC power supply during operation, heat will be transferred from one material to the other, which is manifested as cooling on one side and heating on the other.

This phenomenon is called the Peltier effect . It was discovered by French physicist JCA Peltier in 1834. Peltier was originally a watchmaker, but at the age of 30, he began to experiment and observe physics. His daily life was either boiling water in the Alps and measuring the temperature of boiling water, or observing waterspouts at the seaside and thinking about how tornadoes are formed.

However, it is precisely because of these seemingly "useless" measures that many "useful" scientific principles have emerged, so let us say together: Thank you Peltier!

02

Thermoelectric Phenomenon Big Three

As physics has become more and more sophisticated, scientists have grouped the Peltier effect and the effects discovered by two other scientists together, collectively known as the thermoelectric phenomenon. Now machines made using these three phenomena are all over the world, and some have even gone beyond the earth (don't worry, you'll know what they are when you see the rest).

03

Who are the three giants of thermoelectric phenomena?

The Peltier effect has been mentioned before. Now let me introduce the Seebeck effect : when two different conductors are connected into a loop and the materials on both sides have different temperatures, a current will be generated in the circuit. This effect is the reverse effect of the Peltier effect .

That is to say, take a semiconductor refrigerator and create a temperature difference on both sides (for example, stick ice cubes on one side and put an insulation bag on the other side). This semiconductor refrigerator becomes a generator that uses the temperature difference to generate electricity.

The last phenomenon is the Thomson effect : when an electric current flows through a uniform conductor with a temperature gradient (a piece of iron wire with one end hot and the other end cold), the conductor will absorb or release a certain amount of heat.

Semiconductor refrigerators made using the Peltier effect allow us to enjoy ice cola and hot milk tea, but power generation depends on the Seebeck effect: temperature difference + thermoelectric converter = generator.

During World War II, Soviet guerrillas had no place to charge their radios, and ordinary generators were bulky and noisy. So they commissioned scientists to develop a kettle thermoelectric generator based on the Seebeck effect. It looks no different from an ordinary kettle, but it uses the temperature difference of about 300 degrees between campfire and water to generate electricity, and the charging power of this generator can reach 6W.

(Photo source: pikabu.ru)

Later, the TKG-3 generator that used the heat from kerosene lamps to generate electricity appeared. People could charge their radios by lighting kerosene lamps at night.

(Photo source: pikabu.ru)

04

Small "nuclear power plant" - RTG

Because the thermoelectric generator itself has no moving parts and is durable, it has become a favorite in the aerospace industry.

When conducting deep space exploration missions, the spacecraft may be far away from the sun or in a shadow area, with severe lack of light and facing a low temperature environment. In this environment, the power supply of the spacecraft is a big problem.

The solution to this problem is the radioisotope thermoelectric generator , or RTG (Radioisotope Thermoelectric Generator). RTG uses the Seebeck effect to convert the heat energy generated by the decay of radioisotopes into electrical energy.

The power generation process of RTG is divided into the decay heat collection stage and the temperature difference power generation stage. Taking the typical RHU/RTG as an example: radioactive elements (plutonium-238) release radiation when they decay, and the high-energy particles that make up the radiation are blocked and absorbed when they interact with the surrounding matter, and this process generates heat.

The device is equivalent to a very long-lasting heat source (can continue to generate heat for decades). With a long-lasting heat source, the power generation capacity of the RHU/RTG is also very long-lasting and stable, and the waste heat generated during power generation can be used to keep the spacecraft warm, killing two birds with one stone.

RHU: radioisotope heating unit

RTG: Radioisotope Thermoelectric Generator

(Source: Reference 1)

The RTG installed on the Voyager 2 probe (launched in 1977, flew out of the heliosphere in 2018, and became the second spacecraft to enter interstellar space) has been working stably for 41 years. The Curiosity Mars rover and the Chang'e 4 lander are also equipped with RTG.

The RTG at the rear of the Curiosity rover

(Source: Wikipedia)

Of course, RTG can even overcome the extreme environment in the universe, so the extreme environment on Earth is a piece of cake for it.

During the long polar nights of the Arctic winter, the lighthouses rely on RTGs for energy supply. As of 1992, the US military also used RTGs to power its military facilities in the Arctic. Miniaturized plutonium-238 batteries were once used as pacemaker batteries.

With the emergence of flexible circuits and new materials, the performance of devices made using thermoelectric phenomena will be greatly improved. Perhaps future smart watches will be able to obtain sufficient electricity using the temperature difference between human skin and air, or flexible semiconductor cooling devices can be implanted in clothes to create a cooling suit that is not afraid of summer.

References:

[1] Niu Changlei, Tang Xian, Li Xin, et al. Development and prospects of ~(238)Pu isotope heat/power source[J]. Atomic Energy Science and Technology, 2020, 54(S01):9.

[2] Cai Shanyu, He Shunyao. Review of the development of space radioisotope batteries and their application prospects in the new century[J]. Nuclear Science and Engineering, 2004, 24(2):8.

[3] Ritz F, Peterson CE. Multi-mission radioisotope thermoelectric generator (MMRTG) program overview[C]// Aerospace Conference. IEEE, 2004.

[4] Hammel T, Bennett R, Sievers B. Evolutionary upgrade for the multi-mission radioisotope thermoelectric generator (MMRTG)[C]// 2016 IEEE Aerospace Conference. IEEE, 2016.

[5]https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

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