Non-concentrated solar thermal technologies can provide heat up to 180°C. They do not need direct solar radiation to work and can thus be utilised all year round, no matter the season or weather conditions.
Within non-concentrated technologies there are different types of solar collectors.
Flat plate collectors
Flat plate collectors are the most popular solar thermal collectors, especially in the buildings sector. In 2022, 72% of all installed solar thermal collectors in Europe were flat plate collectors. They are easy to install and well suited to lower supply temperature (up to 80°C) applications. Due to the large absorber surface of the collector, they can capture diffuse radiation, allowing them to work also in cloudy, dusty or humid environments.
Glazed flat plate collectors are the most common version of flat plate collectors. They are usually constructed with a glass cover, copper tubes, absorber plates, thermal insulation and aluminium casing. Anti-reflective glass is also used to increase performance by most of the manufacturers of high-performance collectors. A distinction can be made between single and double glazing for flat plate collectors. At higher temperatures, the performance of a double glazed flat plate is better than of a singe glazed collector, as the additional glass reduces heat losses to the environment. Double glazed large-scale collectors are often used for district heating applications.
Unglazed flat plate collectors, also called Wind and Infrared Solar Collector (WISC), are not covered by glass and directly exposed to the ambient air. Therefore, their energy performance is more sensitive to the effect of the wind and radiative losses. They are suitable for applications with lower temperatures, for example pool heating and water preheating, and may also supply a heat pump.
A special variation of flat pate collectors are high-vacuum flat plate collectors, which reach the highest level of temperature for non-concentrated technologies. There is no loss of heat due to a vacuum insulation technique and they are very well-suited also for bad weather conditions. Due to their high-vacuum insulation and ultra-transparent glass, they can efficiently deliver high temperatures of up to 180°C without tracking the sunlight. Their most popular use is in district heating and industry.
Evacuated tube collectors
Evacuated tube collectors use vacuum technology, which minimises heat loss and increases efficiency. They are well suited for applications where higher temperatures (up to 150°C) are required.
There are two basic types of evacuated tube collectors:
The so-called Sydney tube collectors and the single glass tube collectors. The Sydney tube collectors consist of double glass tubes where the inner absorber tube is coated with a selective material. The space between the two tubes is evacuated. The single glass tube collectors are made of one evacuated tube with a conventional metallic absorber inside the tube.
Evacuated tubes with CPC (Compound Parabolic Concentrator) collectors are a special design of an evacuated tube collector. They use concentrating mirrors to direct the solar radiation onto the absorber, resulting in higher energy yields. Due to the geometry of the parabolic mirror, it reflects the direct radiation onto the absorber tube at any angle of incidence. Therefore, no tracking is necessary and a small row spacing can be used, as the shading without tracking and the flat geometry is comparatively low. Evacuated tubes with CPC are very efficient and well suited to medium-high supply temperature applications.
