An electromechanical device whose work platform can be aligned with the sun in real time is called a sun tracking device. The sun tracking device is composed of two parts: mechanism and control system. According to the number of degrees of freedom of the mechanism, the sun tracking device is divided into two types: single axis and dual axis; according to the control mode of the control system, the sun tracking device is divided into two types: photoelectric sensor tracking and sun trajectory tracking; according to the information feedback mode of the control system, it is divided into three types: closed loop, open loop, and mixed control.
Single-axis and dual-axis sun tracking device
According to the number of degrees of freedom of the mechanism, the sun tracking device can be divided into two types: single axis and dual axis.
1. Single axis tracking device
The single-axis tracking device has 3 layout modes: ① Tilt layout, east-west tracking; ② Focal line horizontally arranged north-south, east-west tracking; ③ Focal line horizontally arranged east-west, north-south tracking. These three methods are single-axis rotating north-south or east-west tracking, and their working principles are basically similar. The principle of the third tracking method is: the axis of rotation (or focal line) of the tracking system is arranged in the east-west direction, and the cylindrical parabolic mirror rotates pitching around the axis of rotation to track the sun according to the change of the sun’s declination angle calculated in advance. With this tracking method, only the sun’s rays are perpendicular to the generatrix of the cylindrical paraboloid at noon in a day, when the heat flow is the largest; and the sun’s rays are oblique in the morning or afternoon. The advantage of single-axis tracking is that the structure is simple, but because the incident light cannot always be parallel to the main optical axis, the effect of collecting solar energy is not good.
2. Two-axis tracking device
When the sun’s altitude and declination angle change, if the sun tracking device can track the sun in real time, more solar energy can be obtained. The dual-axis tracking device is designed to meet such requirements. According to the different types of coordinate axes, the dual-axis tracking device can be divided into two methods: polar axis type and height angle azimuth angle.
(1) Polar axis tracking device. The principle of the polar-axis tracking device is: one axis of the condenser points to the north pole of the celestial sphere, that is, it is parallel to the rotation axis of the earth, so it is called the polar axis; the other axis is perpendicular to the polar axis and is called the declination axis. When working, the reflecting mirror rotates around the polar axis, and its rotation speed is set to be the same as the angular velocity of the earth’s rotation, but in the opposite direction, so as to track the sun’s daily movement; the reflector is pitched around the axis of declination to adapt to changes in the angle of declination, and is usually adjusted regularly according to seasonal changes. This tracking method is not complicated, but the mass center of gravity of the reflector does not pass through the axis of the polar axis on the structure, and the design of the polar axis support device is more difficult.
(2) Elevation angle and azimuth angle tracking device. The altitude angle azimuth angle sun tracking method is also called the horizontal coordinate system two-axis tracking, and its principle is similar to other tracking devices. The azimuth axis of the worktable is perpendicular to the ground plane, and the other axis is perpendicular to the azimuth axis, which is called the pitch axis. During operation, the worktable rotates around the azimuth axis to change the azimuth angle according to the sun’s daily movement, and pitches around the pitch axis to change the tilt angle of the worktable, so that the main optical axis of the reflector is always parallel to the sun’s rays. This kind of tracking device is characterized by high tracking accuracy, and the weight of the workbench device is kept in the plane where the vertical axis is located, and the design of the supporting structure is relatively easy. The sunlight incident angle θ on the lighting surface of the workbench is calculated from the solar declination angle δ, the solar hour angle ω, the workbench inclination angle β, the workbench azimuth angle γ, and the latitude Φ of the test site. The calculation formula is as follows:
The solar declination δ on the nth day in 1a is calculated as:
It can be seen from equations (1) and (2) that when δ, ω, and Φ are determined, the values of the inclination angle β and the azimuth angle γ of the worktable determine the incident angle θ of the sunlight. Therefore, as long as the angle of the workbench is controlled to have a proper inclination and azimuth angle, it is possible to ensure that the sunlight incident angle θ is 0, so as to maximize the collection of solar energy.