Types of Solar Air Heaters
According to different absorber plate structures, solar air heaters can be divided into various types.1. Flat-plate absorber collector
This is the simplest and most widely used collector. It consists of one or two transparent covers on top, insulation material at the bottom, and a flat-plate absorber between them. Depending on design requirements, air can flow above or below the absorber plate, or both above and below.
2. Flat-plate absorber collector with ribs
To enhance heat transfer between air and the flat-plate absorber, ribbed absorber plates can be used. The ribs not only increase the contact area between the air flow and the absorber plate but also enhance turbulence, thereby improving heat transfer.
3. Corrugated absorber plate collector
This is a collector with a corrugated absorber plate. Its advantages include directional selectivity and a higher absorptance than emittance for solar radiation. This is because direct solar radiation entering the V-shaped grooves undergoes multiple reflections before leaving, while thermal radiation is hemispherical. Reference (3) indicates that using a corrugated absorber plate with a spectrally selective absorbing surface further improves selective radiation characteristics. Additionally, the air channel formed by the absorber plate and bottom plate is inverted V-shaped, which increases the heat transfer coefficient between the air flow and the absorber plate.
4. Laminated glass collector
The absorber plate of this collector consists of a set of laminated glass layers between the transparent cover and the bottom plate. The trailing edge of each glass layer along the airflow direction is coated with black paint. As air flows through the channels between the glass layers, it is heated by the painted trailing edges.
5. Porous absorber collector
Metal mesh, screens, or loose metal shavings and fibrous materials are porous absorbers. Placing a porous absorber obliquely between the transparent cover and the absorber bottom plate forms a porous absorber collector. This type has a high volumetric heat transfer coefficient, and if properly designed, the airflow resistance through the porous absorber is very low.
6. Corrugated absorber plate collector with holes
Unlike the corrugated absorber plate collector, the airflow in this collector does not flow in the inverted V-shaped channels but through holes on the inverted V-shaped walls.
7. Flat absorber plate collector with holes
The absorber plate is a flat plate with many holes. The air inlet is above the absorber plate, and the air outlet is below the absorber plate.
In addition to the above, there are other collectors with different absorber plate structures, which are not described here. In essence, solar air heaters are divided into two types based on whether air passes through the absorber. The first type is where air does not pass through the absorber but flows above, below, or both above and below the absorber, known as non-porous absorber air heaters. The second type is where air passes through the absorber, commonly called porous absorber solar air heaters.
