The objective of the smart DAST system is to obtain the maximum perpendicularity between the incident rays of the sun and the PV panel surface, so that to obtain maximum power from the PV panel. The descriptive diagram of blocks of smart DAST is shown in Figure 3. The function of the tracking system is based on the program embedded in the ATMega328 microcontroller of the Arduino UNO board. The ATMega328 microcontroller, the brain of the entire system, converts the analog values from LDR sensors into digitals and provides two output channels to control the rotation of PV panel through two servo motors. Then, the rotation movements occur in two axes: vertical and horizontal, from east to west (azimuth tracking) during the day and from south to north (elevation tracking) during the seasons. Figure 4 shows the circuit diagram of the proposed smart DAST system in ISIS Proteus. This Figure illustrates the connections between the Arduino UNO board and sensors/actuators of the system. As an embedded system, the controller of the smart DAST is divided into two parts: hardware and software.
DC motors, stepper motors or servo motors are highly used in the solar tracking systems to motorize the PV panel. In this work, two 180° servo motors are used and Table 1 presents their characteristics. A servo motor (SG90) for the solar tracker's vertical movement and a micro servo motor (MG996R) for the horizontal movement. A servo motor is able to wait for predetermined positions in the instructions given to it and then to maintain them, so it works in a closed loop. It consumes power when it turns to the desired position, otherwise, no energy is consumed. Whereas, stepper motors continue to consume energy to maintain the commanded position.52 The advantage of the servo motor is that we can control its stop, run, the direction of rotation, and speed using a single low current wire connected directly to a PWM output of the microcontroller, there is no need of interface circuit. The used servo motors are controlled by the ATMega328 microcontroller via three-wire electrical cable as shown in Figure 4, two wires for supply, and a PWM input for transmitting position commands.
As the specialty of the proposed smart DAST is active, that is, the movement of the solar tracker is determined instantaneously according to the position of the sun; four photo resistors LDRs (Cds GL5528) are used to sense the position of the sun, which are very low-cost. LDR or photoresistor is a resistor whose resistance decreases with increasing light intensity incident onto its surface. The LDR sensor circuitry is designed as a voltage divider circuit as shown in Figure 6 in order to provide an output voltage. So, as the LDR resistor varies with light, the analog voltage at series resistance (Vout) also varies. Then the ADC of the microcontroller converts the analog to digital value between 0 and 1023, because it is coded in 10 bits, and according to this value, it is possible to know the level of light.
LDR sensors position in DAST system
Different positions of LDRs have been employed to track the sun's movement in two directions. Figure 7 presents the most used configurations. For instance, as shown in Figure 7A, a solar sensing device, which comprises a four-quadrant LDR sensor and a cylindrical shade, has been used in Ref.46 This device is designed based on the use of the shadow. If the PV panel is not perpendicular to the sunlight, the shadow of the cylinder will cover one or two LDRs resulting in a differential of light intensity. The best orientation of the PV panel is achieved when intensity on the east LDR is equal to that on the west LDR, and the intensity on the north LDR is equal to that on the south LDR. In Ref 53 a solar sensing device, which includes a four-quadrant LDR sensor separated by two barriers as shown in Figure 7B, has been used. Here, the best orientation of the PV panel is achieved when intensity on LDR1 and LDR2 is equal to that on LDR3 and LDR4, and the intensity on LDR1 and LDR3 is equal to that on LDR2 and LDR4.
Both of sensing devices presented in Figure 7A,B can be placed at the top, left, or right of the PV panel. When using either of these devices, the length of the cylinder (or barrier) and the distance between LDR sensors and the cylinder (or barrier) should be well-dimensioned. In Figure 7C, the LDRs are positioned on the center of each sides. The top and bottom LDR are used to track the sun's movement south/north direction and the right and left LDRs are used to track the sun's movement in east/west direction.44 Figure 8 shows the configuration used for LDRs in this work. This configuration is similar to that presented in Figure 7B but without the use of barriers so that LDRs are placed in the four corners of the PV panel. LDRs are positioned as follow: one in the top, one at the bottom, one in the left and one in the right. The advantage of this configuration compared to others is in the four positions of the sensors, as they cover the entire area of the PV panel. Hence, we will ensure that the PV panel is perpendicular to the sunbeam when the sunlight intensity on the top LDRs is equal to that on the bottom LDRs and the intensity on the right LDRs is equal to that on the left LDRs.
The software implementation consists of coding the algorithm of tracking system in the Arduino IDE environment and uploaded it in the microcontroller. The flowchart describing the microcontroller operation is presented in Figure 10.
The algorithm is based on the analog values returned by the left LDR and the right LDR, as well as the top LDR and bottom LDR. For azimuth tracking, the average values from two right LDRs and two left LDRs are compared and if the left set of LDRs receive more light, the horizontal servo motor will move in that direction (Rotates Clockwise (CW)). The servo motor will continue to rotate until the difference result is between a positive threshold value (10) and a negative threshold value (-10), which means that the solar tracker is approximately perpendicular to the light source. If the right set of LDRs receive more light, the horizontal servo motor moves in that direction (Rotates Counterclockwise (CCW)) and will continue to rotate until the difference result is between 10 and -10. The same way is used for elevation tracking. We also determined the average radiation between the four LDRs, the idea being that at the end of the day, when the solar projection is null, the solar tracker returns to its initial position, waiting for a new day. At noon, when the sunlight is at maximum, the servo motors must be stopped. However, according to Table 2, we found that the resistance values of the LDRs are not the same, even if they have the same reference (Cds GL5528) and have been placed at the same right at noon in front of the sun. This means that the readings of the LDR voltages are not equal. Therefore, the difference result between the average value of the left set of LDRs and the average value of the right set of LDRs will be unstable around zero, in which case the servo motor will constantly turn. This explains the use of the threshold value as a hysteresis band in the algorithm, which aims to reduce the power consummation and assuring smoothly moves of the servo motor. That is mean if the difference result is in the hysteresis band, the horizontal servo motor always stops. And if the difference result is outside the hysteresis band ([-10, 10]), the servo motor will start to rotate CCW or CW. In Figure 11, a timing diagram describing the principle of the horizontal servo motor operation is shown. The same principle is used for vertical servo motor operation.