It is well known that in theory, 41% more sunlight is available by tracking the PV module to follow the daily course of the sun, relative to fixed installations. The overall objective of this study is to develop a control algorithm that improved performance and reliability the two-axis solar tracker. To achieve this goal, this study concentrate on optimizing the LM3S811 based controller board, drive hardware and software.
Keywords: embedded system design, two-axis sun tracking, control algorithm.
ıntroductıon
Solar energy systems and equipment such as PV and day lighting systems, solar collectors, and solar-powered heat engines work best when their collectors aim directly at the sun. Adding a solar tracker to these systems increases their efficiencies at the expense of initial and operational costs and system complexity. It has been estimated that the use of a tracking system, over a fixed system, can increase the power output by 20% - 40% with cost increase 10%-30% [1-3].
Since the sun's position in the sky changes with the seasons and the time of day, tracker is used to align the collection system to maximize energy production. Several factors must be considered when determining the use of trackers. Some of these include: the solar technology being used, the amount of direct solar irradiation, feed-in tariffs in the region where the system is deployed, and the cost to install and maintain the trackers. Concentrated applications like concentrated photovoltaic panels (CPV) or concentrated solar power (CSP) require a high degree of accuracy to ensure the sunlight is directed precisely at the focal point of the reflector or lens.
The two basic categories of trackers are single axis and dual axis. Single axis solar trackers can either have a horizontal or a vertical axis. In concentrated solar power applications, single axis tracker is used with parabolic and linear Fresnel mirror designs. Dual axis solar trackers have both a horizontal and a vertical axis allowing them to track the sun's apparent motion virtually anywhere in the world
This paper presents a control strategy for a two axes solar tracker that is executed in an ARM based Stellaris L3S811 microcontroller. Correct sun position is inferred from the GPS. The proposed control strategy consists of a combination between; an open loop tracking strategy, and a closed loop strategy. The overall objective of this study is to develop a control algorithm that improves performance and reliability the two-axis sun tracker. To achieve this goal, this study concentrates on optimizing the controller board, drive hardware and software.
Two-axıs sun tracker
The sun's rays can be decomposed into two components, one perpendicular to the panel surface, and the other parallel to the surface, where only the former radiation can be received by the panel. Thus, the angle between the sun's rays and the normal of the panel which is called the incident angle should be as small as possible. Incidence angle changes with the diurnal and seasonal variations. Therefore, the fixed-installed solar collectors cannot fully absorb the solar radiation energy. If at any time by automatically tracking solar collectors, panel position can be adjusted according to the sun's trajectory to reduce the incidence angle; it will be able to absorb more solar radiation energy than the fixed panels in the same irradiation conditions. The panel of dual-axis sun tracking system rotates around the two mutually perpendicular shafts, azimuth shaft and elevation shaft, shown in Fig 1. It will track the sun's azimuth angle and elevation angle, so that the panel can achieve incident angle of 0.
Two methods are commonly used in solar tracking to identify and follow the position of the sun at any instant of time between sunrise and sunset: the closed loop control method and the open loop control method. The closed loop control method uses several feedback sensors such as LDR, photodiode, light-intensity sensors, reference cells and a signal processing circuit [4-6]. The signal processing circuit compares the output signals of the sensors and operates on a feedback loop with the desired signal condition. The goal of the loop is to produce maximum total error signal from sensors by continuously adjusting the tracker direction until the shadow on the sensors is the minimum. A drawback of the closed loop control method is that it cannot effectively track the sun on a cloudy day without a robust algorithm.
Fig. -Structure of the two-axis sun tracker.
The open loop control method uses the longitude and latitude data of the solar tracker location to determine and track the position of the sun [7, 8]. It has the advantages of easy programming and high accuracy. The system is simpler and cheaper than the closed-loop type of sun tracking systems [9]. It does not observe the output of the processes that it is controlling. However, a fixed starting direction of the tracker at sunrise every day is required in this method. Thus, the starting direction of the tracker must be corrected from time to time. Consequently, an open-loop system cannot correct any errors so that it could make and may not compensate for disturbances in the system.
Defining Elevation and Azimuth Angles
The algorithm for sun tracking uses the solar elevation, ï±e and azimuth, ï±A angles computed at the solar tracker location. The tracker must be aligned horizontally to determine the elevation and azimuth angles accurately along with the hour and declination angles with respect to the celestial equator or plane as depicted in Fig.1. Solar elevation, ï±e is the angle between the horizon and the line connecting the origin and the sun that is, the complement of the zenith angle. Solar azimuth, ï±A is the angular displacement of the projection of the line to the sun onto the horizontal plane from the south axis.
The solar elevation angle, ï±e, of the orientation system in the vertical plane, θe, can be calculated as follows [9]
sinï±e=sinï¤ sinï¦ +cosï¤ cosï¦ cosï· (1)
Where
θe is the elevation angle of the system
ï¦ is the latitude.
ω is the hour angle (15° / hour), where ω = 0 at local noon.
δ is the solar declination, where δ is calculated from
Cooper's equation,
(2)
N is the day of the year (1 - 365) with N = 1 representing the 1st of January.
Fig. - The relation between elevation and zenith angles.
The azimuth angle of the system in the horizontal plane, θA, is calculated as [9].
(3)
The solar tracking system usually returns to its initial rest position after sunset, and starts to track the sun after the sun rises above the horizon. The sunrise and sunset times can be calculated using [13] for system location.
Desıgn and application
3.1. Mechanical Design
The panels' support structure was designed with two degrees of freedom in order to vary the inclination and orientation. Also, the conception of the panel's support as well as the system that allows this support to revolve around the two axes was developed. The tracker is composed of a fixed base which is directly on the ground, having a mechanism that connects the base to the supporting structure of the panels. This mechanism consists of two parts, which have a degree of freedom (from each other) in two axes. For changing the inclination of the structure, additional linear actuator is mounted to the solar tracker control system.
Linear actuators are extremely precise by design, especially when compared to pneumatic and hydraulic solutions. Screw based mechanical linear actuators allow to advance or retreat the motive rod by extremely small increments, which is required for the exact positioning of solar tracker. Electric linear actuator consumes extremely low electricity and are available in 12 Volts d.c. it can be powered by the solar panel itself supported by a battery. Linear actuators can be unusually small, especially when considering the range of motion that is required for moving the sun tracker. Photograph of the mechanical structure is shown in Fig. 3.
Fig. - Mechanical structure of the sun tracker.
3.2. Hardware Design
The hardware design combines the embedded microcontroller with two DC motor drivers, rotational DC motor, DC motor controlled linear actuator, solar rotation mechanism, GPS, pyranometer, anemometer, tilt switches and MEMBS based inclinometer. A general block diagram of the control system is shown in Fig. 4.
Global positioning system (GPS) is connected to the microcontroller via a standard serial RS-232 port. GPS sends to the microcontroller sentences, that contains a string of characters, continuously. These sentences mainly include longitude, latitude, altitude, date and time for location where GPS is placed. Since microcontroller has the real time clock circuitry, it is reasonably accurate over short periods, but it needs calibration periodically. As a result, the GPS clock signal is used to update the microcontroller's internal time periodically and thus effects of the long term errors are eliminated.
As part of the effort to improve solar tracker reliability and better understanding performance, a pyranometer is being added to solar tracker. This pyranometer allows the data acquisition system to measure precisely the irradiance witnessed by the PV modules on that tracker, and thus better monitor the impact of the tracking algorithm on the energy output of the system.
Solar tracker measures tilt angle with potentiometer that has long-term reliability problem. A higher reliability alternative is a solid-state inclinometer. It has three main advantages; inherently higher reliability, higher resolution less than 0.1°, direct measurement of angle. In this project, micro electromechanical systems based on electronic inclinometer ADXL345 is used [11]. Digital output data is formatted as 16-bit two's complement and is accessible through either a SPI (3- or 4-wire) or I2C digital interface The inclinometer would typically be mounted directly underneath a tracker's plane, from where the inclination can be measured.
Fig. - Sun Tracker control system block diagram.
The solar tracker is fitted with limit switches to ensure robust operation. A micro roller switch mounted on the base of the solar tracker prevents multiple revolution windup of the azimuth tracking stage. The solar collector also includes two more limit switches on the zenith tracking stage to prevent over travel damage to the linear actuator mechanism. The initial reset balance use tilt switches. The mechanism include four tilt switches (east, west, south and north) To protect tracker components from over wind speed, system also requires an anemometer to measure wind speed.
Consequently, we need powerful and cost-effective microcontroller to connect all these parts and manage to track the sun. It must have two serial port, (UART) one for communicate computer the other one for GPS, two PWM signals for motor A and motor B, one I2C port for solid-state inclinometer, hardware counter input for anemometer, analog input for pyranometer, at least four digital inputs for tilt switches. In addition, these features we need software development tools for microcontroller. Regarding the calculation of the mentioned before, 32-Bit Stellaris microprocessor LM3S811 which is optimized for small-footprint embedded applications, fits best to the sun tracker system.
TI Stellaris LM3S811 microcontroller has a Reduced Instruction Set Coding (RISC) ARM core, internal oscillators, timers, UART, USB, SPI, pull-up resistors, PWM, ADC, analog comparator and watch-dog timers are some of the features [10].
Software Design
The developed sun tracking algorithm enables high-precision determination of sun angles and times for sunrise, solar noon and sunset year-round. The flowchart of the algorithm is drawn in Fig. 5. The calculation of the sun angles with the sun tracking algorithm software simply requires the specification of the date, time and exact longitude, latitude and elevation of the location through a GPS system.
Fig. - Flowchart of sun tracking algorithm.
The algorithm we developed for the control of the sun tracker is composed of two main sections. In the input section, the solar elevation and azimuth angles as well as sunrise, sunset, solar noon and present solar times are calculated according to section 2 and used as shared variables in other parts of the software. When the system starts, Stellaris first sets tracker to the home position and then takes GPS information to calculate the sun set and rise times. The present solar time is compared with the sunrise and sunset times to determine whether tracking should start or stop. At night time, it waits next sample time. Sample time period may be defined according to technical constraints. First constraint is GPS hot start time that is 1 second for GPS. It cannot be shorter than this value. The other constraint is the energy consumed by motor A and B during one tracking step. We set sample time to 2 minute during experimental work. The present solar time between sunrise and sunset time's Stellaris reads pyronometer value to check if there is enough solar radiation to generate power. Otherwise, sun tracker stays at home position until solar radiation rise to lower limit of solar radiation. After solar radiation reaches the desired value, then algorithm reads anemometer value to define whether the sun tracker can move safely. If not, sun tracker stays at home position at least during one sample time. Otherwise, it starts tracking the sun.
In the output section of the algorithm, the software takes azimuth, θA, and elevation, θe, angles from shared variables and converts them to motor motion. The calculated angles θe, θA, are then subtracted from the previous position values. According to the obtained angle difference and their signs, microcontroller sends PWM and direction signals to the motor controllers. Motor A and motor B takes solar panel to the new position.
Motor A can drive solar panels to turn in the horizontal plane in order to track the changes in azimuth angle; its positive position is westward. Similarly, motor B can drive solar panels to turn in the style of pitching so as to track the changes of solar elevation angle; its positive direction is downward. At the end of tracking, the position of mechanisms needs to be defined. The electronic inclinometer ADXL345 sends x, y, z axis values to the Stellaris. This digital axis values are converted to angles by the microcontroller. By comparing the measured angles with calculated angles, the two motors take different movements to finish come up to the desired position of the solar panel. Finally, when this day is over, the system backs to the home position to wait for the next day.
ResultS and dıscussıon
Sun tracker was tested both in the laboratory and outdoors using SM-55 solar panel [12]. During the outdoor test, the sun tracking system was moved to outside of a building so that we could compare the results between fixed position and two-axis sun tracking systems. During a 24-hour trial period, the two-axis solar tracker was required to operate for approximately 2 minutes every hour to maintain proper alignment with the sun.
Fig. - Outdoor test results of sun tracker.
During the test process, solar panel charged to the battery, and solar panel current and voltage values were measured and stored every minute using a data logger. End of the day, the solar panel, charged the battery up to 408.2 Watt-hour energy for about eleven hours. The recorded data on the day 5.5.2012 proved that the two-axis solar tracking PV panel produced more energy than the fixed one with about 40.7%.
Conclusıon
In this study, a cost effective two-axis sun tracker has been developed. The ARM Cortex-M3 core microprocessor successfully calculated the tilt angle of the solar panel in order to investigate the accurate sun elevation angle. The positioning technique, which has been investigated by the DC motor and linear actuator, reduced the error in locating the elevation azimuth angles to 0.1°. The proposed tracker has increased the energy collected by 40.7%.
