A heat collector is a device used to collect the radiant energy of the sun and convert it into heat energy, also known as a heat collector.
In theory, there are several classification methods for collectors.
(1) According to the type of heat transfer working medium of the collector, it is divided into liquid collector and air collector.
(2) According to whether the solar radiation entering the daylighting port changes its direction, it is divided into concentrating collectors and non-concentrating collectors.
(3) According to whether the collector tracks the sun, it is divided into tracking collectors and non-tracking collectors.
(4) According to whether there is vacuum space in the collector, it is divided into flat plate collector and vacuum tube collector.
(5) According to the working temperature range of the collector, it is divided into low temperature collector, medium temperature collector and high temperature collector.
In fact, the various solar collectors classified above are intersecting each other. The following mainly introduces three kinds of solar collectors that are currently widely used.
①. Flat plate collector
The main body of the heat absorbing part of the flat plate collector is a flat plate coated with a black absorbing coating. According to the difference in structure, it can be divided into direct sunlight flat plate collector and transparent cover plate collector, as shown in Figure 1 (a) and Figure 1 (b) respectively.
Direct sunlight flat plate collector, the heating surface is one or more flat plates, coated with high-absorption, low-emission selective coating, direct sunlight to the flat plate coated with absorption coating, water pipes and other heat transfer structures are placed On the backlit side of the heat collecting plate, heat is transferred to the water tank through water circulation.
The transparent cover plate collector is designed according to the “hot box principle”. The side of the “hot box” facing the sun is a transparent cover plate, which can be made of glass, fiberglass or plastic film; the other sides are airtight insulation layers, and the inner wall is painted black. Sunlight enters the box through the transparent cover plate, is absorbed by the inner wall coating, and is converted into heat energy. The heat collection medium in the hot box can be air or water.
The appearance of the flat plate collector is shown in Figure 2. The area of this type of collector receiving solar radiation is equal to the area of the heat sink itself. Due to the low energy flow density of solar energy, the working temperature of the heat collection medium is generally relatively low, and in order to receive enough solar energy, a large heat collection area is often required.
②. Vacuum tube collector
If a vacuum is formed between the heat collecting plate of the transparent cover plate collector, the transparent cover plate and the side wall, and the structure is made into a circular tube shape, it becomes a vacuum tube heat collector, as shown in Figure 3 and 4 shown. The core component is the vacuum tube. According to the material, there are two types: all-glass vacuum tube and metal vacuum tube. The more common one is the vacuum glass tube with black coating.
The evacuated tube heat collector is a relatively new type of solar heat collection device, which uses vacuum heat insulation and adopts a selective absorption coating. It has high heat collection efficiency and low heat loss. The temperature of the heat collector is also high, and it can generally be used all year round. At present, vacuum tube collectors have obtained large-scale commercial applications.
③. Focusing collector
The focusing type heat collector adopts a specific focusing structure to concentrate solar radiation on a small heat collecting surface, so as to obtain a high energy flow density and heat collection temperature. This type of collector has a more complicated structure and a higher cost.
Common focus structures include the following (Figure 5).
(1) Point focusing structure, such as compound parabolic mirror (condensing magnification 1.5~10), Fresnel lens (100~1000) and heliostat type condenser (1000~3000), etc.
(2) Line focusing structure, such as trough parabolic reflector and cylindrical parabolic reflector (15~50).