The vacuum tube is the core of the solar water heater. Its structure is like an elongated thermos flask with a vacuum between the inner and outer layers.
The surface of the inner glass tube is coated with a spectral selective absorption coating using a special process to maximize the absorption of solar radiation energy. After sunlight is irradiated, photons hit the coating, solar energy is converted into heat energy, water absorbs heat from outside the coating, the water temperature rises, the density decreases, the hot water moves upwards, while the cold water with high specific gravity drops.
