FLAT-PLATE AND VACUUM TUBE SOLAR COLLECTORS

02/03/2014 - 13:48
Renewable Energy Development Institute - REDI

Solar collectors are exceptional devices which transform solar energy into thermal energy. Unlike solar panels, solar collectors use the suns energy in the form of heat, rather than electricity, and are used to heat water and buildings. The first solar collector was built in 1767 by a Swiss botanist Horace de Saussure. Since then it has been developed and become a complete unit reducing up to 60% of the energy demand.

At present, the most common types of solar collectors are flat plate and vacuum collectors. The most important task of solar systems is the absorption of solar energy, which is then converted into thermal energy. Collectors are an essential and integral component of solar systems.

Efficiency of the system depends on; the number of collectors installed, absorption surface, installation place and hydraulic system connection. In water heating systems, solar panels are connected to the water tank battery which accumulates heat.

To ensure a proper operation of the entire system, there is a need to choose the appropriate elements such as: water tank, a group of control and pump, insulation, and proper installation.

Flat-plate collectors are characterized by the simplicity and reliability of the structure. On average, the operating time of a solar collector is about 20 years. The majority of producers of flat plate collectors use similar technology. Collectors are usually constructed of:

- light casing
- transparent insulation (made from solar glass, translucent sunlight)
- copper absorber
- fastening elements and insulation

Collector casing's are generally made out of an aluminum frame. In order to reduce the weight of the panel, the rear wall is produced from light metal or plastic, while the number of ribs placed in the frame ensure structural rigidity. The front wall is made of a solar glass low in iron content, which transmits light and solar energy. The lower the content of iron applied in the manufacture of solar glass, the more infrared transmittance obtained in the solar spectrum, and thus a higher efficiency of the collector. Some solar energy is lost as a result of light being reflected from the glass surface, and the some solar energy is absorbed by the material of the panel. The efficiency of the collector is at a level of about 50%. The installation of flat plate collectors is recommended mainly in areas characterized by a mild climate and warmer winters.

Vacuum tube collectors are made up of tubes filled in a vacuum. In the center of each tube there is a flat absorber which is responsible for the absorption of solar energy. The Vaccum protects the tube from loss of energy, and as a consequence the loss of energy is almost reduced to 0%. The Collector structure allows for high efficiency even in diffuse sunlight (during cloudy days) and during conditions of low air temperature. Research has shown that during a year, vacuum tube collectors maintain a 30-40% higher efficiency compared with other collectors.

The appropriate structure of a vacuum tube collector allows the sun falling on the absorber into acute angles. This reduces the amount of reflected light and increases the energy yield. Even on a cloudy and rainy day, the system still provides heating and hot water distribution in a building. Tubes are made of durable and hail resistant safety glass and the design allows for the fall of snow and rain between the tubular elements of the collector, therefore, these devices are successfully installed in areas with higher rainfall and stronger winds. In addition, mechanical damage to the pipes does not suspend operation of the system. A special system of „dry connection” provides safety and easy servicing. The high reliability and durability of the system provide free heat and hot water over many years.

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