Basic Structure
Heat Absorber
The heat absorber is a component in a flat-plate solar collector that absorbs solar radiation and transfers heat to the heat transfer medium. It is basically in the shape of a flat plate
On the flat-plate-shaped heat absorber, there are usually row pipes and headers. The row pipe refers to the component that is arranged longitudinally on the heat absorber and forms a fluid channel; the header refers to the component that connects several row pipes horizontally at the upper and lower ends of the heat absorber and forms a fluid channel. There are many types of materials for heat absorbers, including copper, aluminum alloy, copper-aluminum composite, stainless steel, galvanized steel, plastic, rubber, etc.
Materials Used For Flat-Plate Solar Collector Absorber
⑴ Flat-plate solar collectors and branches use TP2 copper. TP2 copper-phosphorus deoxidized copper is a high-purity raw material for melting. The oxygen produced in the molten copper is deoxidized by oxygen-philic phosphorus (P) to reduce its oxygen content to below 100PPm, thereby improving its ductility, corrosion resistance, thermal conductivity, weldability, and drawing processability. Hydrogen embrittlement does not occur at high temperatures.
⑵ Flat-plate solar collector strips (whole plates) use copper or aluminum TU1 oxygen-free copper standard (GB/T5231-2001)
Material characteristics and application range of flat-plate solar collector absorber: extremely low oxygen content, high purity, excellent electrical and thermal conductivity, excellent ductility, low air permeability, no “hydrogen disease” or very little “hydrogen disease”; good processing performance, welding, corrosion resistance and cold resistance.
According to the different structures of the heat absorber, it can be classified into: tube-sheet type, wing-tube type, flat box type, round tube type and heat pipe type.
⒈Tube-sheet heat absorber
The tube-sheet heat absorber is a heat absorber strip formed by connecting the row tubes and the flat plate in a certain combination, and then welded with the upper and lower headers to form a flat plate solar collector heat absorber. This is the most commonly used heat absorber structure type at home and abroad.
⒉Wing-tube heat absorber
The wing-tube heat absorber is a heat absorber strip with wings on both sides of the metal tube made by die extrusion and stretching process, and then welded with the upper and lower headers to form a heat absorber. The material of the flat plate solar collector heat absorber is generally made of aluminum alloy.
3.Flat box heat absorber
The flat box heat absorber is a heat absorber made by molding two metal plates separately and then welding them into one. The heat absorber material can be stainless steel, aluminum alloy, galvanized steel, etc.
Usually, spot welding is used between fluid channels, and rolling welding is used around the flat box heat absorber of the flat plate solar collector.
Coating: In order to maximize the absorption of solar radiation energy by the flat solar collector and convert it into heat energy, the absorber should be covered with a dark coating, which is called a solar absorption coating.
Absorption Coatings Can Be Divided Into Two Categories:
⑴ Non-selective absorption coatings and selective absorption coatings.
⑵ Non-selective absorption coatings refer to absorption coatings whose optical properties are independent of the radiation wavelength; selective absorption coatings refer to absorption coatings whose optical properties vary significantly with the radiation wavelength.
Generally speaking, it is not very difficult to simply achieve a high solar absorption ratio. The difficulty lies in maintaining a high solar absorption ratio while achieving a low emissivity. For selective absorption coatings, as the solar absorption ratio increases, the emissivity often increases; for the commonly used blackboard paint, its solar absorption ratio can be as high as 0.95, but the emissivity is also around 0.90, so it belongs to a non-selective absorption coating.
Selective absorption coatings can be prepared by a variety of methods, such as spraying, chemical methods, electrochemical methods, vacuum evaporation methods, magnetron sputtering methods, etc. Most of the selective absorption coatings prepared by these methods can achieve a solar absorption ratio of more than 0.90, but there are obvious differences in the range of emissivity they can achieve. From the perspective of emissivity performance, the order of advantages and disadvantages of the above methods should be: magnetron sputtering method, vacuum evaporation method, electrochemical method, chemical method, and spraying method. Of course, the emissivity value of each method has a certain range, and the actual emissivity value of a certain coating depends on the degree of optimization of the coating preparation process.
Transparent Cover Plate
The transparent cover plate is a plate-shaped component that covers the heat absorbing plate in the flat plate collector and is composed of transparent (or translucent) materials. It has three main functions: one is to transmit solar radiation and project it onto the heat absorbing plate; the second is to protect the heat absorbing plate from dust and rain and snow; the third is to form a greenhouse effect to prevent the heat absorbing plate from dissipating heat to the surrounding environment through convection and radiation after the temperature rises.
When the operating temperature of the flat plate solar collector is high or it is used in areas with low temperatures, such as solar air conditioning in southern China or solar heating in northern China, the flat plate solar collector should use a double-layer transparent cover plate.
In general, 3 or more transparent cover plates are rarely used, because as the number of layers increases, although the convection and radiation heat loss of the collector can be further reduced, the actual effective solar transmittance will be greatly reduced at the same time.
Insulation Layer
- The insulation layer is a component in the collector that inhibits the heat absorption plate from dissipating heat to the surrounding environment through conduction
- The thickness of the insulation layer
The thickness of the insulation layer should be determined according to the type of material selected, the working temperature of the collector, the climatic conditions of the use area and other factors.
The thickness of the insulation layer should follow this principle:
The greater the thermal conductivity of the material, the higher the working temperature of the flat-plate solar collector, and the lower the temperature in the use area, the greater the thickness of the insulation layer is required. Generally speaking, the thickness of the bottom insulation layer is selected to be 30~50mm, and the thickness of the side insulation layer is roughly the same.
- Insulation layer materials for flat-plate solar collectors
The materials used for the insulation layer are: rock wool, glass wool, polyurethane, polystyrene, etc. At present, glass wool is more commonly used.
Glass wool is a material that is formed by fiberizing molten glass to form a cotton-like material. Its chemical composition belongs to the glass category. It is an inorganic fiber with good molding, low volume density, low thermal conductivity, good thermal insulation, sound absorption, corrosion resistance, and stable chemical properties.
Phenolic foam is a new type of high-efficiency thermal insulation material that can improve the flat-plate solar collector. It is a closed-cell rigid foam plastic made of phenolic resin and emulsifiers, foaming agents, curing agents and other additives, which are foamed and cured by scientific formula.
Shell
The shell is a component in the collector that protects and fixes the heat absorption plate, transparent cover and insulation layer.
Technical requirements of the shell: According to the function of the flat-plate solar collector shell, the flat-plate solar collector shell is required to have a certain strength and rigidity, good sealing and corrosion resistance, and beautiful appearance.
Shell materials: The materials used for the shell include aluminum alloy, stainless steel plate, carbon steel plate, plastic, fiberglass, etc. In order to improve the sealing of the flat-plate solar collector shell, some products have adopted the one-time compression molding process of carbon steel plate.
At present, the most commonly used materials for the flat plate collector shell (frame) are aluminum alloy and carbon steel plate molded in one step. Aluminum alloy: 6063T5 aluminum alloy profiles are generally used. 6063 series aluminum alloys are widely used in the frames of building aluminum doors, windows, and curtain walls. In order to ensure that doors, windows, and curtain walls have high wind pressure resistance, assembly performance, corrosion resistance, and decorative performance, the requirements for the comprehensive performance of aluminum alloy profiles for flat plate solar collectors are much higher than the industrial profile standards.
