The device that absorbs solar radiation and transfers the generated heat energy to the heat transfer medium is called a solar collector, which is a key component of various solar water heating systems. The solar collectors currently used in China can be roughly divided into two categories: flat-plate solar collectors and evacuated tube solar collectors.

Flat-plate solar collector

Flat-plate solar collectors are the most commonly used type of collectors in Europe. It is composed of four parts: heat absorption plate, cover plate, insulation layer and shell. Sunlight shines through the cover plate (glass) on the heat-absorbing plate coated with a high solar absorption rate coating; the heat-absorbing plate heats up and transfers the heat to the heat-transfer working medium in the heat-absorbing plate tube, and insulation materials play a role in reducing heat dissipation. At present, copper has been widely used as the heat-absorbing plate material in China, and aluminum alloy, steel, galvanized sheet, etc. are also used. The structure diagram of the collector is shown in Figure 1.

The working principle of the flat-plate solar collector is to let sunlight shine through the light-transmitting cover plate on the heat-absorbing plate coated with a high solar absorptivity coating on the surface, and the temperature of the heat-absorbing plate increases after absorbing the solar radiation energy. On the one hand, it transfers heat to the working fluid in the heat collector to increase the temperature of the working fluid, which can output useful energy as a heat carrier, and its annual solar energy utilization rate is up to 50%; on the other hand, it also dissipates heat to the surroundings. The cover plate plays the role of allowing visible light to pass through, while infrared rays cannot pass through, forming a greenhouse effect, allowing the working fluid to take away more heat and improving the thermal efficiency of the solar collector.

The temperature of the hot water produced by the flat-plate collector is 30~70℃. It has a simple structure, reliable operation, low heat flow density, and can operate under pressure. It is mainly used for domestic hot water preparation and district heating, and can be combined with buildings in many ways. However, due to the air interlayer between the heat absorbing plate and the cover plate, convection heat dissipation will occur, and the metal heat absorbing plate and the metal frame will conduct heat outward, so the flat-plate collector has the problem of large heat loss and low heat collection efficiency in a low temperature environment.

Vacuum tube solar collector

1. All-glass vacuum solar collector tube

The structure of the all-glass vacuum solar collector tube is shown in Figure 2. It is composed of two coaxial glass tubes, and a vacuum is drawn between the inner glass tube and the cover glass tube. The solar selective absorption coating is deposited on the outer surface of the inner glass tube to form a heat sink, which converts sunlight energy into heat energy and heats the heat transfer fluid in the inner glass tube. The all-glass vacuum solar collector tube adopts a single-ended open design, and the open end is connected by the inner glass tube and the cover glass tube by fusion sealing. The other end of the inner glass tube is a closed hemispherical shape, and a spring clip with a getter supports the round end of the inner glass tube with a heat sink on the inner surface of the exhaust end of the cover glass tube. When the heat-absorbing coating of the inner glass tube absorbs solar radiation and causes the temperature of the inner glass tube to rise, the round head end of the inner glass tube can form a free end of thermal expansion, which solves the stress problem at the ring seal of the glass tube and the cover glass tube at the open end of the all-glass vacuum solar collector tube caused by working. The getter spot welded on the spring clip is evaporated by high-frequency induction heating, and a getter film is formed on the outer surface of the inner glass tube at the round head end of the heat collection tube and the inner surface of the cover glass tube, which is used to absorb the gas released by the glass, spring clips and heat absorption coating of the all-glass evacuated heat collection tube during operation, and the gas that penetrates into the vacuum interlayer of the heat collection tube through the cover glass tube and the inner glass tube, to maintain the vacuum degree of the vacuum interlayer between the inner glass tube and the cover glass tube.

Due to the large heat loss coefficient of the flat plate collector, this limits its effective heat gain at higher operating temperatures. The all-glass vacuum tube is composed of two layers of glass tubes inside and outside, and a selective absorption film with high absorptivity and low emissivity is attached to the surface of the inner tube. Sunlight passes through the outer glass tube and irradiates the inner tube surface coated with a selective absorption film, and is converted into heat energy to heat the heat transfer medium flowing directly in the glass tube. This kind of heat collector has high operating efficiency, but it cannot be operated under pressure, is easy to burst, easy to scale, and is not easy to integrate with buildings for integrated design.

2. Heat pipe vacuum tube collector

The heat pipe type vacuum heat collecting tube is a kind of metal heat sink vacuum heat collecting tube. It is composed of heat pipe, metal heat-absorbing plate, glass tube, metal cover, spring bracket, evaporative getter and non-evaporable getter, among which the heat pipe includes two parts of evaporation section and condensation section, as shown in Figure 3.

When the heat pipe type evacuated collector tube is working, the solar radiation passes through the glass tube and is projected on the metal heat absorbing plate. The heat absorption plate absorbs solar radiation energy and converts it into heat energy, and then conducts it to the heat pipe tightly integrated in the middle of the heat absorption plate, so that the working fluid in the evaporation section of the heat pipe is quickly vaporized. After the working fluid vapor rises to the condensation section of the heat pipe, it condenses on the cooler inner surface, releasing latent heat of evaporation, and transferring the heat to the heat transfer medium of the solar collector. The condensed liquid working medium flows back to the evaporation section of the heat pipe by its own gravity, and then repeats the above process.

The heat pipe is an enhanced heat transfer element that uses the latent heat of vaporization to efficiently transfer heat energy. Its heat transfer coefficient is several orders of magnitude larger than the thermal conductivity of a metal rod of the same geometric size. The heat pipes used in the heat pipe vacuum heat collection tube are generally gravity heat pipes, also known as thermosyphons. At present, most of China uses copper-water heat pipes, and other countries also use organic materials as the heat pipe working fluid, but the compatibility of the working fluid and the heat pipe shell material must be met.