This is a collector where the absorber plate is made into a corrugated shape. Its advantage lies not only in directional selectivity but also in that the absorptivity for solar radiation is greater than the emissivity. This is because direct solar radiation entering the V-shaped grooves undergoes multiple reflections before leaving the grooves, while thermal radiation is hemispherical. Reference (3) indicates that if a corrugated absorber plate with a spectrally selective absorbing surface is used, the selective radiation characteristics will be further improved. Additionally, since the air channel formed by the absorber plate and the bottom plate is inverted V-shaped, the heat transfer coefficient between the air flow and the absorber plate can also be increased.
4. Laminated Glass Type Collector
The absorber plate of this type of collector consists of a set of laminated glass placed between the transparent cover plate and the bottom plate. The tail of each layer of glass along the airflow direction is coated with black paint. When air flows through the channels between the laminated glass layers, it will be heated by the tails of each glass layer coated with black paint.
5. Porous Absorber Type Collector
Metal mesh, gauze, or loosely packed metal shavings, fiber materials, etc., are all porous absorbers. Placing a porous absorber obliquely between the transparent cover plate and the absorber bottom plate forms a porous absorber type collector. This type of collector has a very high volumetric heat transfer coefficient. If designed properly, the resistance of air flow through the porous absorber will be very small.
6. Corrugated Absorber Plate Collector with Small Holes
Unlike the corrugated absorber plate collector, in the corrugated absorber plate collector with small holes, the airflow does not flow in the inverted V-shaped grooves, but flows through the small holes on the inverted V-shaped walls.
7. Flat Absorber Plate Collector with Small Holes
The absorber plate is an ordinary flat plate, but it has many small holes. The air inlet is located above the absorber plate, and the air outlet is located below the absorber plate.
In addition to the above, there are some collectors with different absorber plate structures, which will not be described one by one. But in essence, solar air heaters are divided into two types according to whether the air passes through the absorber. The first type is that the air does not pass through the absorber, but only flows above or below the absorber, or simultaneously above and below the absorber, which can be called an impermeable absorber air heater. The second type is that the air passes through the absorber, and this type of collector is usually called a porous absorber solar air heater.
