BTE Solar Flat Plate Collectors: Components, Technology & Applications for Efficient Low-Temperature Solar Thermal Systems

Basic components for low-temperature solar thermal utilization

Flat plate solar collectors are essential components for low-temperature solar thermal utilization. They are non-concentrating components in solar thermal systems that receive solar radiation and transfer heat to a heat transfer medium. Flat-plate solar collectors primarily consist of a heat absorber, a transparent cover, an insulating layer, and an outer shell. The heat absorber is essentially a flat plate. When a flat-plate solar collector is in operation, solar radiation passes through the transparent cover and is projected onto the heat absorber, where it is absorbed and converted into heat energy. This heat is then transferred to the heat transfer medium within the absorber, raising its temperature and providing useful energy output from the collector.

Components and Features

Flat-plate solar collectors are essential components for low-temperature solar thermal utilization and have long been a leading product in the global solar energy market. They are widely used in a variety of applications, including domestic water heating, swimming pool heating, industrial water heating, building heating, and air conditioning. Flat-plate solar collectors primarily consist of a heat absorber, a transparent cover, an insulating layer, and an outer shell. A flat-plate solar water heater is a water heater using a flat-plate solar collector, and a flat-plate solar water heating system is a hot water system using a flat-plate solar collector.

They feature simple construction, a large heat absorption area, high pressure resistance, a long service life, resistance to damage, and ease of architectural integration.

Heat absorbing plate

This is the component within a flat-plate solar collector that absorbs solar radiation and transfers heat to the heat transfer medium. It is essentially a flat plate. 

Structure

A flat plate heat absorbing plate is typically arranged with tubes and headers. Tubes are arranged longitudinally on the heat absorbing plate and form the fluid flow path; headers are the components connecting several tubes transversely at the upper and lower ends of the heat absorbing plate, forming the fluid flow path. Heat absorbing plates can be made from a variety of materials, including copper, aluminum alloy, copper-aluminum composite, stainless steel, galvanized steel, plastic, and rubber. Structural types include:

1. Tube-sheet type: The tubes and flat plate are connected in a specific manner to form a heat absorbing strip, which is then welded to upper and lower headers to form the heat absorbing plate. This is the most commonly used type both domestically and internationally.

2. Fing-tube type: A heat absorbing strip with fins attached to each side of a metal tube is formed using a die extrusion and drawing process. This strip is then welded to upper and lower headers to form the heat absorbing plate.

3. Flat box type: The heat absorbing plate is formed from two separate metal plates by die-stamping and then welded together. Snake type: The metal tube is bent into a snake shape and then welded to a flat plate to form a heat absorbing plate. This type of structure is widely used abroad.

Solar Absorbent Coatings

To maximize the absorption of solar radiation and convert it into heat, the absorber should be covered with a dark coating, known as a solar absorbent coating.

Solar absorbent coatings can be divided into two categories: non-selective and selective. Non-selective coatings are those whose optical properties are independent of the wavelength of the radiation; selective coatings are those whose optical properties vary significantly with the wavelength of the radiation.

Selective absorbent coatings can be prepared using a variety of methods, including spraying, chemical, electrochemical, vacuum evaporation, and magnetron sputtering. Most selective absorbent coatings prepared using these methods can achieve a solar absorption ratio (SAR) above 0.90, but their achievable emissivity ranges vary significantly. From an emissivity performance perspective, the order of merit among these methods should be: magnetron sputtering, vacuum evaporation, electrochemical, chemical, and spraying. Of course, each method has a specific range of emissivity values, and the actual emissivity of a coating depends on the degree of optimization of the coating preparation process.

Materials

TP2 copper is used for headers and branch pipes. TP2 copper-phosphorus deoxidized copper is a high-purity raw material produced by melting. The oxygen generated in the molten copper is deoxidized with oxygen-affinity phosphorus (P), reducing its oxygen content to below 100 ppm. This improves its ductility, corrosion resistance, thermal conductivity, weldability, and workability, while also resisting hydrogen embrittlement at high temperatures. Characteristics and Applications: It boasts extremely low oxygen content, high purity, excellent electrical and thermal conductivity, excellent ductility, low air permeability, and minimal or no hydrogen embrittlement. It also offers excellent workability, weldability, corrosion resistance, and cold-weather resistance.

Transparent Cover Plate

The transparent cover plate is a transparent (or translucent) plate-like component that covers the absorber plate in a flat-plate collector. It has three main functions: first, it transmits solar radiation and directs it onto the heat-absorbing plate; second, it protects the heat-absorbing plate from dust, rain, and snow; and third, it creates a greenhouse effect, preventing the heat-absorbing plate from dissipating heat to the surrounding environment through convection and radiation when the temperature rises. 

Material

There are two main types: flat glass and fiberglass. Flat glass is currently more widely used both domestically and internationally.

Flat glass has low infrared transmittance, low thermal conductivity, and excellent weather resistance. However, solar transmittance and impact strength are two key considerations for flat glass. Currently, the most commonly used transparent cover material is 3-5mm thick flat glass, ultra-clear low-iron tempered glass, or ultra-clear low-iron textured tempered glass. These glass have high transmittance, are resistant to hail and impact, and are safe and reliable. Common glass thicknesses are 3.2mm and 4.0mm. Ultra-clear glass is a type of ultra-transparent, low-iron glass, also known as low-iron glass or high-transparency glass. FRP sheets (i.e., glass fiber reinforced plastic sheets) have high solar transmittance, low thermal conductivity, and high impact strength; however, for FRP sheets, infrared transmittance and weather resistance are two issues that require attention. The monochromatic transmittance versus wavelength curve of FRP sheets shows that the monochromatic transmittance not only has a high value within 2pm, but also has a high value above 2.5pm. Therefore, the solar transmittance of FRP sheets is generally above 0.88, but its infrared transmittance is also much higher than that of flat glass. FRP sheets can reduce the degree of damage caused by ultraviolet rays by using high-bond energy resins and gel coats. However, the service life of FRP sheets cannot in any way be compared with that of flat glass, which is an inorganic material. FRP sheets are rarely used as cover plates for collectors and are currently only used in some low-end products.

Insulation Layer

The insulation layer is a component in the collector that prevents the heat from the absorber from being lost to the surrounding environment through conduction. Materials used for the insulation layer include rock wool, glass wool, polyurethane, and polystyrene. Glass wool is currently the most commonly used.

High-Efficiency Insulation Materials

Phenolic Foam (PF) is a new type of insulation material that can improve the efficiency of flat-plate solar collectors. It is gradually being adopted by manufacturers. Phenolic foam is a closed-cell rigid foam plastic made by foaming and curing phenolic resin with a variety of substances, including emulsifiers, foaming agents, curing agents, and other additives, through a scientific formula.

The characteristics of phenolic foam are summarized as follows:

1. Excellent thermal insulation performance, with a thermal conductivity of <0.03 W/m·K.

2. High operating temperature. Phenolic foam can operate for long periods of time in temperatures between -200°C and 160°C (with transient temperatures of 250°C permitted) without shrinkage.

3. Excellent weather resistance. Even under long-term exposure to high temperatures, it maintains excellent thermal insulation properties and does not release any volatile substances that could block solar radiation.

4. Non-flammability. Phenolic foam (100mm thick) resists flames for over an hour without being penetrated, and emits no smoke or harmful gases. When exposed to open flames, phenolic foam forms a structural carbon layer on its surface, preventing dripping, curling, or melting. After burning, a graphite layer of structural carbon forms on the surface, effectively protecting the foam's internal structure.

5. Environmentally friendly. Using fluorine-free foaming technology and no fibers, it meets national and international environmental standards.

Thickness

The thickness of the insulation layer should be determined based on factors such as the material used, the collector's operating temperature, and the climatic conditions of the area of use. As a general rule, the greater the material's thermal conductivity, the higher the collector's operating temperature, and the lower the temperature in the area of use, the thicker the insulation layer should be. Generally, the bottom insulation layer is 30-50mm thick, with the side insulation layers roughly the same thickness.

Housing

The housing protects and secures the absorber, transparent cover, and insulation layer within the collector. Depending on its function, the housing requires a certain level of strength and rigidity, good sealing and corrosion resistance, and an aesthetically pleasing appearance. 

Materials used for the housing include aluminum alloy, stainless steel, carbon steel, plastic, and fiberglass. To improve the housing's sealing, some products utilize a single-shot compression molding process using carbon steel. Currently, the most commonly used material for the housing (frame) of a flat-plate collector is aluminum alloy and carbon steel, formed using a single-shot compression molding process. 

Aluminum Alloy: 6063T5 aluminum alloy profiles are commonly used. 6063 series aluminum alloys are widely used in the frames of aluminum doors, windows, and curtain walls in buildings. To ensure high wind resistance, assembly performance, corrosion resistance, and decorative properties, the comprehensive performance requirements for aluminum alloy profiles far exceed those of industrial profiles.