Solar sensors

To work properly the concentration systems cannot operate without following the sun.
There are various techniques to do this: it is possible to use passive systems, based on a knowledge of the position of the sun in relation to the geographic location and to the time, or it is possible to use active systems, which indicate the position of the sun.
The figure below shows an example of the calculation of the position of the sun at different times of the day for the latitude of Florence in the months of March, June, August and December.

Passive systems are not suitable for systems that require high accuracy, such as high concentration photovoltaic cell systems or fiber-optic coupled systems.
However, they are useful as an auxiliary system, in fact the combination of the passive system with the active one gives flexibility to the system and allows to overcome the following problems:

• In the case of a passive system only, a great precision is required in the orientation of the device time axis which must be in the North-South direction (parallel to the earth's axis). If the device must be able to work even with an approximate placement it is useful to have the active system to correct the wrong positioning.
• The system must orient itself in the direction of the sun even in cloudy conditions, when the active system is not functioning properly or at all.
• With only the passive system the speed of the motors must be synchronized with that of the sun. The speed value must take into account the geographical position, the inclination of the collector and the day of the year.
• When there is a change from cloudy sky to clear sky, the image of the sun will be aligned with the target only if the speed of the motors is as expected and the initial placement operation has been performed correctly. Otherwise it is useful to have an active pointing system that will realign it.

During our activities, the Laboratory designed and tested different optical sensors for sun tracking still based on the same principle.

Considered a cylinder with a hole f, if the source is not aligned with the side of the cylinder the bright spot on the other basis will be decentralized. The relationship between the coordinate of the image a,the height L of the cylinder and the angle of the source θ is:

The appropriate choice of the size of the cylinder determines the field of view of the pointer (FOV). To choose a greater length L corresponds to a sensor at a tighter angle of view, while choosing a smaller L value means having a sensor with a wide angle of view.
The size of the hole f instead determines size and intensity of the sun image.

A 4-quadrant sensor is placed at the bottom of the cylinder. The processing of the 4 output voltages from the sensor allows to find the coordinates of the center of the image a.

Our laboratory has developed "custom" sensors based on this principle, adapted to the needs of particular concentration systems, whether these systems with uniaxial or biaxial tracking.