10-04-2017, 07:53 PM
literature review of microcontroller based solar dual axis tracking system
ABSTRACT
The main goal of this project is to develop and implement a prototype of two-axis solar tracking system based on a PIC microcontroller. The parabolic reflector or parabolic dish is constructed around two feed diameter to capture the sun's energy. The focus of the parabolic reflector is theoretically calculated down to an infinitesimally small point to get extremely high temperature. This two axis auto-tracking system has also been constructed using PIC 16F84A microcontroller. The assembly programming language is used to interface the PIC with two-axis solar tracking system. The temperature at the focus of the parabolic reflector is measured with temperature probes. This auto-tracking system is controlled with two 12V, 6W DC gear box motors. The five light sensors (LDR) are used to track the sun and to start the operation (Day/Night operation). Time Delays are used for stepping the motor and reaching the original position of the reflector. The two-axis solar tracking system is constructed with both hardware and software implementations. The designs of the gear and the parabolic reflector are carefully considered and precisely calculated.
INTRODUCTION
Currently, many alternative energy sources appear to be technically feasible. One of them is solar energy (Kreider and Kreith, 1981). The panels are the fundamental solar-energy conversion component. Conventional solar panels, fixed with a certain angle, limits their area of exposure from the sun during the course of the day. Therefore, the average solar energy is not always maximized. Solartracking systems are essential for many applications such as thermal energy storage systems and solar energy based power generation systems in order to improve system performance (Saxena,. and Dutta,. 1990; Koyuncu and Balasubramanian, 1991; Harakawa and Tujimoto, 2001). The change in sun s position is monitored, and the system always keeps that the plane of the panel is normal to the direction of the sun. By doing so, maximum irradiation and thermal energy would be taken from the sun. The elevation angle of the sun remains almost invariant in a month and varies little (latitude 10 ) in a year. Therefore, a single axis position control scheme may be sufficient for the collection of solar energy in some applications (Konar and Mandal, 1991. Yeong-Chau, et al., 2001. Wilamowski and Xiangli, 2002). Efficient collection of maximum solar irradiation on a flat panel requires adjustments of two parameters of the energy collecting surface namely the angle of azimuth, and the angle of tilt, , of the surface to be illuminated in Figure 1 (Davies, 1993; Macagnan, et al., 1994). This paper deals with controlling the solar panel at two axis (or two angles) by using LDRs as sensors, stepper motors as actuators (SM1, SM2) and microcontroller as a controller. In order to keep the design as simple and cheap we have chosen PIC16C71 as a micro-controller-unit (MCU). In addition to this, to observe position of the solar panel, PC based system monitoring facility is included in the design
DESIGN OF THE SOLARTRACKING SYSTEM
The tracker control system contains a control board, a control program, a power supply board, one motorinterface board and a set of sensors. The main idea of design of the solar-tracking system is to sense the sun light by using four light dependent resistors (LDRs). Each LDR is fixed inside the hallow cylindrical tubes. A pair of them, controlling the angle of azimuth, are positioned East-West direction and the two of them, controlling the angle of tilt, are positioned South-North direction. The LDR assembly is fixed onto the flat-solar panel, shown in Figure 3. The tubes are making a degree of 45 with the plane of panel; so, the angle between the tubes is 90 . The differential signals of each pair of LDRs representing the angular error of the solar panel are employed to re-position the panel in such a way that the angular errors are minimized. A micro-controller system with PIC16C71 used as the controller of the position control scheme offers up to 10MHz clock frequency, four ADC channels with 8-bit, 1Kx14 EPROM memory and thirteen I/O ports (Anon., 1998a). Since the apparent speed of the sun is very slow, the panel will also move very slowly. Therefore, a crystal with a frequency of 4MHz is used as a clock signal generator for MCU. The signals, taken from voltage divider consisting of resistors and LDRs (S1, S2, S3, S4), are applied to I/O port lines of MCU (RA0, RA1, RA2, RA3) respectively. These analog signals are converted to digital signals and compared with each others (S1-S2, S3-S4). If the difference between S1 and S2 (or S3 and S4), error signal, is bigger than a certain value (tolerance), MCU generates driving signals for stepper motors. If the error signals are smaller than or equal to the value of tolerance, MCU generates no signal; which means that the solar panel is facing the sun and the light intensities falling on the four LDRs are equal or slightly different. Schematic representation of the solar-tracking system is given in Figure 4. In order to drive two-stepper motors, an 8-bit shift register CD4094 is used (Anon., 1998b). Data is shifted serially through to shift register on positive transition of the clock signals generated by MCU. Four-output signals (Q1, Q2, Q3, Q4) drive the firststepper motor and the rest of the signal (Q5, Q6, Q7, Q8) drive the second-stepper motor. In this system, eight darlington transistors (TIP122) are used to drive the SMs. The control commands generated by MCU for driving of SMs and positioning of the solar panel are given in Table 1. As can be seen from the Table 1, when SM2 is driven, four-driving signals for SM1 is set (1111XX), and visa-versa. The software is developed in an assembly programming language. It is compatible for PIC16C71 and is a machine level language. The software consists of a main module and a few subroutines. Simplified flowchart of the assembly program is given in Figure 5, where e denotes error (tolerance). In general applications, the panel should be rotated towards East direction in order to make it ready for operation on the next day. The proposed system sends no signals during night times by sensing low or none sunshine intensities and stays as pointing at west direction after the sun set. The micro-controller unit is also skipped to sleep mode and consumes low energy. During the sun rise, the LDR senses the sunlight automatically and the panel is moved towards East direction in a short time.
EXPERIMENTAL RESULTS
The solar tracking system presented here is tested experimentally. The experimental study is realized at Gazi University in Ankara, Turkey. Two solar collector panels, one of which is stationary and the other is rotary, are employed in the test. The stationary panel is tilted at a fixed elevation angle and is oriented in some azimuth angle. The LDRs assembly is mounted on the rotary one. Temperature of the panels versus time is measured with a minuteinterval of 11.75' in the period 7:48 to 17 : 24. So, a total of measurement time is 9:36'. In this interval, a total of 50 data were captured from the stationary panel and the rotary panel with approximately an angle interval of 5.2 According to the results: while the sun rises in the morning, the temperature of each panel also increases linearly and the difference is almost constant until about 11 o clock because of being illuminated from the sun with different azimuth and tilt angle ( , ). After this point, the temperature of panels decreases until about 11:30 due to weather condition. The temperature, then, increases to the normal condition again, and then keeps decreasing slightly afternoon. The temperature of average value of the stationary panel between the hours of 7 : 48 and 17 : 24 is measured as 34.9 C and that of the rotary panel is 43.88 C. It is very obvious that there is approximately a distinction of 9 C between rotary and stationary panel. This result shows and verifies that the rotary panel containing solar tracking system takes more light density than the stationary panel. System monitoring software in PC is developed in Pascal program. Thus, LDRs information depending on light intensities, the angles of azimuth and elevation, and position of the panel can be simultaneously monitored.
CONCLUSIONS
In order to collect the greatest amount of energy from the sun, solar panels must be aligned orthogonally to the sun. For this purpose, a new solar tracking technique based on micro-controller was implemented and tested in this study. The tracking system presented has the following advantages: The tracking system is not constrained by the geographical location of installation of the solar panel since it is designed for searching the maximum solar irradiance in the whole azimuth and tilt angle (except hardware limitations ) during day times; namely, the angle of elevation does not need to be adjusted periodically. The operator interference is minimal because of not needing to be adjusted. The tracker provides also PC based system monitoring facility. Since the tracking system is controlled completely by MCU; the PC, used for monitoring the panel only, may not be employed. A drawback of the tracker is being effected by temporal variations in the atmospheric refractions caused by rain, cloud, fog, etc. Thus, the system may give an erroneous detection in the direction of the sun, and lead to wrong positioning of the solar panel.