Cool the house with sunlight

  At first glance, it seems to be a fantasy. Sunlight will only make the house hotter, how can it be cooled?
  Don’t worry, this requires a high-tech coating. Although the paint molecules will absorb the photons of sunlight, at the same time, they will spontaneously emit higher-energy photons; since the emitted energy is more than the absorbed energy, the temperature naturally drops.
  In fact, this technology draws on the working principle of laser cooling. The lowest temperature reached by human beings is achieved by laser cooling. The laboratory uses laser cooling, and the cooling range can reach 150℃.
  It is said that the inventor of this coating also initially wanted to use laser cooling. However, it is obviously unrealistic to install a laser on your roof, so they want to make the material work even in the sun. But there is another problem: the wavelength range of sunlight is obviously much wider than that of laser light. In other words, compared to laser, the purity of sunlight is too poor.
  In the end, they overcome this difficulty. The paint they developed has an outer layer and an inner layer. The outer layer can filter out some wavelengths of sunlight, which is equivalent to improving the purity; the inner layer can absorb sunlight photons and emit higher-energy photons; so that it can cool itself to below the ambient temperature.
  You may ask: Why can the inner material emit photons with higher energy after absorbing solar photons? Isn’t energy not conserved anymore?
  The reason is this. The molecules of the inner layer material have three energy levels from low to high energy: the lowest energy ground state energy level A, the next highest energy excited state energy level B, and the highest energy excited state energy level C. After the material molecules absorb heat from the surrounding environment, they jump from the ground state energy level A to the excited state energy level B. The excited state energy level B jumps to the excited state energy level C after absorbing sunlight photons. But the excited state energy level C is unstable. After it automatically emits a photon, it returns to the ground state A. The energy of the photon emitted is the difference between the two energy levels of C and A, so the energy is naturally higher than the absorbed photon. After the material molecules return to the ground state, they absorb the surrounding heat again… repeat the above process again. In this way, the surrounding heat is continuously absorbed and then continuously emitted in the form of photons, playing a cooling role.
  The paint has passed the test in the laboratory. As long as there is sunlight, it can continue to cool down. After using this kind of coating on the roof, the top layer can be lowered by 10°C compared with the unused coating. However, it is not cheap, and the cost of 100 square meters of paint needs 300 US dollars.
  The paint can also be used in space. You might say that space is cold enough, is it necessary? It is true that in space where there is no source of heat, it is very cold, but things that heat themselves can become extremely hot. This is because there is no air convection in space, and the heat is not easily dissipated. It is very slow to dissipate heat only by thermal radiation. For example, in order to dissipate heat, the International Space Station uses reflective cloth to resist solar radiation, while inside, it uses a heat exchanger to remove excess heat generated by electronic equipment. But if a layer of this kind of paint is applied to the outer surface of the space station, it can directly use sunlight to cool the internal equipment.