Concentrated solar thermal technologies work with mirrors or specialised lenses that focus the direct radiation of the sun on a secondary absorber element, resulting in higher supply temperatures between 150°C and 400°C. They are therefore more suitable for large-scale applications, for example district heating networks and industrial processes.
There are different types of solar collectors making use of concentrated technologies.
Parabolic trough collectors
Parabolic trough collectors use mirrors to focus direct radiation onto an absorber along the focal line. Due to the relatively large construction, they track the sun with a single axis to maintain the right angle for high solar irradiation. The tracking also allows to regulate the solar heat supply. On sunny days, the solar yield is also distributed relatively consistently throughout the day, which makes system control easier.
A major advantage of this technology is the low heat loss due to the small surface area of the absorber tube. For this reason, they can reach very high supply temperatures (up to 400°C) and are therefore especially suitable for large-scale applications.
Parabolic trough collectors are usually not stagnation safe, meaning that overheating protection is mandatory. In general stagnation is avoided by turning the collector out of the sun. Most of the absorber tubes are protected by a surrounding glass tube and of the receiver tubes have a vacuum.
Small parabolic trough collectors can be installed on roof areas or on the ground, whereas large parabolic troughs need a suitable land area for the installation.
Linear Fresnel collectors
Like parabolic trough collectors, linear Fresnel collectors use mirrors to focus direct radiation onto an absorber element. They consist of many mirror surfaces that are tracked separately in a single axis and reflect the direct radiation onto the absorber tube. To reduce optical losses, a secondary reflector is mounted on top of the absorber tube. Due to the simple geometry of the mirrors, they are cleaned by robots at night. This ensures radiation conditions on the mirrors, while the upper side of the receiver can be difficult to clean.
The advantages of linear Fresnels are almost identical to parabolic troughs due to their similar functionality. These collectors too can deliver comparatively high temperatures efficiently. Due to tracking, the solar yield is constant throughout the day and the mirrors can be tracked away if there is a risk of stagnation. The use of linear Fresnel collectors also makes sense in sunny locations due to the high influence of direct radiation on the solar yield.
A technological variation of linear Fresnel collectors are Fresnel lenses. They use special types of lenses instead of mirrors to concentrate the sunlight. They can reach high supply temperatures and are well suited for specialised applications. Due to their compact design, they use a 2-axis tracking, which tracks the sun to keep the angle of incidence as low as possible and thus maintain a high amount of irradiation at the collector plane. This allows high temperatures to be delivered efficiently and reduces the risk of glare, as the collector always reflects towards the sun due to the 2-axis tracking.