Solar Tracking Controls

Solar tracking control means that by controlling the rotation direction of the solar panel, the solar panel is always facing the sun, thus absorbing more solar energy to improve the power generation efficiency of the solar PV module.

Due to the rotation of the earth, the angle of the sun's light changes from time to time during the spring, summer, autumn and winter seasons of the year, as well as at sunrise and sunset every day, so the efficiency of the photovoltaic power generation system will be optimal if the solar panels are effectively oriented to the sun at all times relative to a fixed location.

Solar tracking control methods

Currently, various types of solar tracking control systems can be divided into two categories: mechanical tracking systems and electronically controlled tracking systems. Mechanical tracking systems are generally pressure differential, while electronically controlled tracking systems can be divided into photoelectric sensing tracking control systems and solar motion track tracking systems. The following is a brief introduction to these systems respectively:

(1)Pressure difference tracking

Pressure differential tracking refers to the fact that when the human-radiated sunlight is deflected, the two sides of the closed container will produce a certain pressure difference due to the different light receiving area, and under the action of the pressure difference, the tracking device will be re-aligned with the sun. According to the different media contained in the closed container, the pressure differential tracking can be divided into gravity differential, air differential and hydraulic type.

The basic working principle of the system is: when the tracking system is not aligned with the sun, i.e., the solar light does not shine vertically into the system, the two sides of the closed container inside the system are subject to different light areas, and the media will undergo corresponding physical changes due to the different light, resulting in different pressure, thus forming a pressure difference on both sides. Under the action of this pressure difference, the tracking control system does the corresponding direction of movement and readjustment until the pressure on both sides is the same. At this point, both sides of the container are subject to the same light and the system is aligned to the sun. According to the medium stored in the closed container, the pressure differential solar tracking system can be divided into hydraulic differential, air differential, gravity differential, etc. This type of tracking control system is simple in structure, low in cost, without electronic control parts and external power supply, and is a pure mechanical control system. However, this system has limitations, generally can only be used for single-axis tracking system, tracking accuracy is very low. Therefore, this system is only used in the low demand of general users.

(2) Photoelectric sensing solar tracking control

Photoelectric sensing solar tracking control system uses photo-sensitive silicon phototubes, silicon photocells and other components, common photoelectric devices are photoelectric cells, photo diodes and photo triodes. At present, the more commonly used domestic photoelectric tracking systems are electric, gravity, electromagnetic type. These photoelectric tracking control systems are using photosensitive components as sensors. In this kind of tracking control system, the sensor is generally installed in the light board or fixed position, through the rotation of the motor to adjust the position of the light board so that the light board is facing the sun. When the sun moves to the west, the light board also follows the offset, the photoelectric sensor will output a certain value of voltage or current because of sunlight, as the input signal, amplified by the amplifier circuit, the motor rotation to adjust the angle of the solar light board so that the tracking system aligned with the sun. The photoelectric sensor type tracking has the advantages of high sensitivity and fast response, and the mechanical structure design is relatively simple, but it is easy to be affected by the weather. If there is a cloudy day or clouds cover the sun, the sun rays will be scattered, which will lead to the tracking control system can not be aligned with the actual position of the sun, and even cause the wrong action of the actuator, so that the tracking fails.

(3) Sight-day motion trajectory tracking control

According to the number of axes of the tracking system, there are two kinds of single-axis and dual-axis tracking. Single-axis tracking: ① tilt arrangement east-west tracking; ② focal line north-south horizontal arrangement, east-west tracking; ③ focal line east-west horizontal arrangement north-south tracking. These three ways are single-axis rotation of north-south or east-west tracking, the working principle is basically similar. The axis (or focal line) of the tracking system is arranged north-south, and according to the change of the solar declination angle calculated in advance, the parabolic mirror of the column is turned around the axis for pitching and tilting to track the sun. With single-axis tracking, only the midday sunlight is perpendicular to the bus of the column paraboloid, when the heat flow is maximum; and the sunlight is oblique in the morning or afternoon. The advantage of single-axis tracking is the simplicity of the structure, but the effect of collecting solar energy is not very satisfactory because the human-emitted rays cannot always be parallel to the main optical axis.

Dual-axis tracking is the ability to track the sun on both changes in solar altitude and declination angle. Dual-axis tracking can be divided into two ways: polar-axis full tracking and altitude angle-azimuth angle full tracking.

Polar axis full tracking means that one axis of the mirror points to the north pole of the earth, which is parallel to the earth's rotation axis, so it is called polar axis; the other axis is perpendicular to the polar axis, which is called declination axis. When working, the reflecting mirror orbits the polar axis, and its rotational speed is set at the same size and opposite direction as the Earth's rotation, which is used to track the sun's apparent solar motion; the reflecting mirror rotates around the declination axis for pitching and tilting in order to adapt to the change of declination angle, which is usually adjusted periodically according to the change of seasons. The polar axis full tracking method is not complicated, but the weight of the reflector does not pass through the polar axis in the structure, and the design of the polar axis support device is difficult.

Height angle-azimuth solar tracking is also known as two-axis tracking in the ground plane coordinate system. When the azimuth axis of the collector is perpendicular to the ground plane, the other axis is perpendicular to the azimuth axis, called the pitch axis. When the PV system works, the collector rotates around the azimuth axis to change the azimuth angle according to the sun's apparent solar motion, and changes the tilt angle of the collector by pitching around the pitch axis, so that the main optical axis of the reflecting mirror surface is always parallel to the sun's rays. This tracking system is characterized by high tracking accuracy, and the weight of the collector device is kept in the plane where the vertical axis is located, and the design of the support structure is relatively easy.

Currently, most existing solar collectors have a fixed angle toward the sky, and PV system designers need to calculate an optimal local angle in order to achieve the maximum possible solar energy collection. Since the sun's altitude angle varies over time in practice, the use of solar tracking devices in photovoltaic systems can greatly improve the utilization of solar energy.

Solar tracking control types

Solar tracking control can be divided into several categories based on the type of control, one for open-loop control, another for closed-loop control, in addition to a hybrid control that combines the two.

Open-loop control in control theory refers to a . A class of control without feedback. This type of control is a time-based control method that does not require the use of sensors to sample the light intensity, but is based on the information of local longitude and latitude, and adjusts the solar panel according to different times of the day in order to track the sun. The open-loop tracking control is based on the fact that the position of the sun at any time of the day is determined. This type of timing assumes that the Earth's trajectory around the sun is a standard circle to facilitate time calculations. However, since the Earth's trajectory around the Sun is elliptical and the Earth-sun distance is constantly changing throughout the year, the calculation assuming the Earth's trajectory around the Sun is a standard circle is subject to certain errors.

Closed-loop control is a control method with feedback compared to open-loop control, which is based on photosensitive sensor control. The control system detects the intensity of the sunlight at a certain moment through the sensor, and then adjusts the direction of the solar panel according to different weather conditions in order to achieve the desired target alignment. A simple closed-loop control method for solar tracking is to test the light intensity on the front side with a photosensitive element, and constantly adjust the angle of the panel in one direction (east-west or north-south), rotating it by a small angle in the same direction each time, and rotating it in the correct direction when the measured light intensity keeps strengthening. When the measured light intensity suddenly changed from increasing to decreasing, the opposite direction back to rotate the same angle, when the panel in the direction of the sun. A major drawback of this tracking control is that if there is an occluding object during the tracking cycle, it may lead to a positioning error, which affects the accuracy of subsequent tracking and positioning. To overcome this problem, bi-directional tracking is required, i.e., not only searching in one direction but also in the opposite direction, which increases the computational complexity and requires additional energy consumption.