[Reading Science] Heated Clothing, Directly Coating Solar Cells onto Fabric?

Thermal underwear is more effective when you raise your body temperature through exercise.
[Photo by YouTube screen capture]

[Asia Economy Reporter Kim Jong-hwa] Heated clothing that keeps you warm even when going out in the cold winter is the trend. When a person exposes their skin in a cold place, the air around the skin is warmed by the heat emitted from the skin, causing the body’s heat to be taken away, which makes one feel cold. Until now, winter clothing has developed in a way to reduce heat loss.

Padded clothes filled with cotton, duck down, or goose down maximize the heat retention effect by applying the principle of insulation, where the filling traps air to prevent body heat from escaping outside. Nowadays, heat-generating materials that keep you warm even when thin have become mainstream. Winter clothing has evolved from reducing heat loss and maintaining body temperature to generating heat by itself.

The material of this heated clothing is metal. Although metal is used as a fiber material, it is not as hot as the popular hot packs used in winter. Hot packs usually maintain 50~60℃, but heated clothing is lower than that.

In the case of photo-thermal fibers that absorb solar heat and convert it into thermal energy, the temperature rises by more than 10℃ within 1 to 2 seconds after exposure to sunlight, and after about 10 to 20 minutes, the body temperature feels like it has increased by about 1 to 2℃. This is because the self-heating technology has not yet reached a complete stage.

Of course, there are clothes that raise the temperature using external batteries, but self-heating clothing is currently limited to that extent. On very cold days, even wearing heated underwear may not feel very warm because you are not moving to produce sweat or because the temperature does not rise significantly enough to feel the difference.

A recently spotlighted technology is 'light-absorbing heat storage fiber technology.' Light-absorbing heat storage fibers accumulate solar thermal energy and then send it back to the human body to raise the temperature. This technology is possible because of the metal 'Zirconium (ZrC),' which absorbs sunlight.

Zirconium is a material with a high absorption rate in the short wavelength range above infrared and visible light, which accounts for 95% of sunlight. It uses the principle of converting sunlight into thermal energy and storing heat. When clothing is made using zirconium, it absorbs 95% of sunlight within the clothing, converts it into energy, and radiates it as far-infrared rays.

In the far-infrared range, the reflectance is high, so energy is accumulated inside the clothing. Light-absorbing heat storage fibers are made using the sunlight-absorbing properties of 'zirconium carbide' and 'zirconium oxide.' The principle is simple. Nylon or polyester yarns are mixed with zirconium carbide at the core to make fibers, and clothing is produced from those fibers.

A representative fiber made with light-absorbing heat storage fiber technology is 'Solarina.' Solarina is the first heat storage fiber developed in Korea, applying technology that absorbs and accumulates sunlight in the fiber and then converts it into thermal energy to radiate to the human body. It is said to have a heat retention effect about 4~5℃ higher than general fabrics due to solar heat absorption and the infrared insulation effect of body temperature.

'Constant heating' is also standard these days. Constant heating refers to technology that simultaneously has solar heat generation function and self-heating function that radiates heat by reflecting the human body’s temperature. It was developed to enable heating even at night without sunlight.

It amplifies solar heat outdoors to provide warmth more than 5℃ higher than duck down and has a self-heating function that radiates heat by reflecting body temperature at night without sunlight. While previous heated clothing could only generate heat when exposed to sunlight, with the development of constant heating technology, heating is possible indoors or at night without sunlight.

There is also technology that uses indoor lighting as an energy source. Aeroheat technology uses far-infrared rays emitted from sunlight or indoor lighting to generate heat within the fiber itself. Mineral-containing yarn absorbs external light and radiates heat energy and far-infrared rays, making it a semi-permanent photo-thermal material.

'Solarball' is also one of the notable heated fibers. Solarball is a next-generation filling material where specially developed nano-chemical particles collide with each other when exposed to sunlight, generating heat by themselves. It is inserted as filling between fibers. Indoors and at night without light, it amplifies the body’s far-infrared rays to maintain warmth.

Electrodes are made using a loom, and heating clothing coated with nano solar cells on these electrodes has also been developed, with attempts at commercialization underway.
[Photo by Korea Electrotechnology Research Institute]

Solarball is up to three times cheaper than duck down but has a higher air content than general filling materials, resulting in greater heat retention. It is still attracting attention as an 'ethical product' that can replace duck or goose down.

Recently, the Nano Convergence Technology Research Center at the Korea Electrotechnology Research Institute developed the world’s first woven-structure fiber-type flexible solar cell, which has become a hot topic. Although called a solar cell, it has flexibility, mechanical durability, light weight, and impact resistance like fabric. In a weaving machine, metal fibers, glass fibers, natural fibers, and other fabrics are woven into a three-dimensional textile form to directly create electrodes, and nano materials are coated on these electrodes to manufacture the solar cells.

The fiber, that is, the fabric itself becomes a solar cell, creating wearable heated clothing. If this woven solar cell technology is commercialized, it is expected to once again change the landscape of the heated clothing market.