Solar Thermal Collector Components and Design

The Solar Absorber

The absorber is the component where the sun's energy is converted into heat. This heat is transferred by conduction to the ducts and conduits where the working fluid passes by convection. The absorber consists of thin sheets or plates (0.3 - 0.5 mm), usually manufactured from aluminum or copper, which absorb radiation and transmit it to the tubes.

Tube Distribution and Material

These tubes typically have diameters of less than 1 cm, although depending on the distribution, they can join others approximately 2 cm in diameter. The manufacturing material is usually stainless steel or copper. The tubes must be distributed evenly across the surface with a distance between them that is not too large (80 - 120 mm). They must be designed for low pressure loss to avoid increased costs in the pumping system.

Surface Coatings and Absorption

The face exposed to the sun is given a coating to improve energy absorption conditions. The most common treatments include:

• Black paint: Using specialized solar paints.
• Selective surface treatment: Modern treatments where absorption is slightly lower than with black paint, but the emission rate is significantly reduced.

Currently, many systems utilize both treatments to optimize performance.

Transparent Cover and the Greenhouse Effect

The transparent cover functions to protect the absorber and the insulation from external conditions. It assists with insulation and allows incident radiation (short wavelength) to enter while preventing the exit of radiation emitted by the absorber (long wavelength). The cover should be transparent to short wavelengths and opaque to long wavelengths so that the radiation emitted by the absorber reflects inside the collector, producing a greenhouse effect.

Thermal Insulation and Seals

The primary aim of thermal insulation is to reduce conduction losses. This insulation is placed on the back and sides of the unit. The material must possess high resistance to elevated temperatures and low thermal conductivity. Commonly used materials include:

• Fiberglass
• Glass wool
• Rock wool

The seals are located in the joints of the frame, the transparent cover, and the pipes. These components are subjected to high temperatures (up to 180 °C) and significant mechanical loads. The material typically used for these seals is EPDM.

Protective Frame and Housing

The frame serves as the sensor housing, protecting the system from mechanical damage and moisture. It is usually manufactured from aluminum, aluminum alloys, or fiberglass combined with stainless steel sheets.