Solar Tracking System

Introduction

Solar tracker, a system that positions an object at an angle relative to the Sun. The most common applications for solar trackers are positioning photovoltaic (PV) panels (solar panels) so that they remain perpendicular to the Sun’s rays and positioning space telescopes so that they can determine the Sun’s direction. PV solar trackers adjust the direction that a solar panel is facing according to the position of the Sun in the sky. By keeping the panel perpendicular to the Sun, more sunlight strikes the solar panel, less light is reflected, and more energy is absorbed. That energy can be converted into power. solar tracker, a system that positions an object at an angle relative to the Sun. The most common applications for solar trackers are positioning photovoltaic (PV) panels (solar panels) so that they remain perpendicular to the Sun’s rays and positioning space telescopes so that they can determine the Sun’s direction.PV solar trackers adjust the direction that a solar panel is facing according to the position of the Sun in the sky. By keeping the panel perpendicular to the Sun, more sunlight strikes the solar panel, less light is reflected, and more energy is absorbed. That energy can be converted into power.

Common types of trackers:

1) Single-axis trackers:

Fig: Single-axis Trackers

Single-axis trackers have one degree of freedom that acts as an axis of rotation. The axis of rotation of single-axis trackers is typically aligned along a true North meridian. It is possible to align them in any cardinal direction with advanced tracking algorithms. There are several common implementations of single-axis trackers. These include horizontal single-axis trackers (HSAT), horizontal single-axis trackers with tilted modules (HTSAT), vertical single-axis trackers (VSAT), and tilted single-axis trackers (TSAT)polar-aligned single-axis trackers (PSAT). The orientation of the module with respect to the tracker axis is important when modeling performance.

 2) Double axis trackers:

Fig: Double axis Tracker

Dual-axis trackers have two degrees of freedom that act as axes of rotation. These axes are typically normal to one another. The axis that is fixed with respect to the ground can be considered a primary axis. The axis that is referenced to the primary axis can be considered a secondary axis. There are several common implementations of dual-axis trackers. They are classified by the orientation of their primary axes with respect to the ground. Two common implementations are tip-tilt dual-axis trackers (TTDAT) and azimuth-altitude dual-axis trackers (AADAT). The orientation of the module with respect to the tracker axis is important when modeling performance. Dual-axis trackers typically have modules oriented parallel to the secondary axis of rotation. Dual-axis trackers allow for optimum solar energy levels due to their ability to follow the Sun vertically and horizontally. No matter where the Sun is in the sky, dual-axis trackers are able to angle themselves to be in direct contact with the Sun.

 Block diagram of solar tracker:

Block Diagram

Tracker type selection:

The selection of tracker type is dependent on many factors including installation size, electric rates, government incentives, land constraints, latitude, and local weather.

Horizontal single-axis trackers are typically used for large distributed generation projects and utility-scale projects. The combination of energy improvement and lower product cost and lower installation complexity results in compelling economics in large deployments. In addition, the strong afternoon performance is particularly desirable for large grid-tied photovoltaic systems so that production will match the peak demand time. Horizontal single-axis trackers also add a substantial amount of productivity during the spring and summer seasons when the Sun is high in the sky. The inherent robustness of their supporting structure and the simplicity of the mechanism also result in high reliability which keeps maintenance costs low. Since the panels are horizontal, they can be compactly placed on the axle tube without the danger of self-shading and are also readily accessible for cleaning.

A vertical axis tracker pivots only about a vertical axle, with the panels either vertical, at a fixed, adjustable, or tracked elevation angle. Such trackers with fixed or (seasonally) adjustable angles are suitable for high latitudes, where the apparent solar path is not especially high, but which leads to long days in summer, with the Sun traveling through a long arc.

Dual-axis trackers are typically used in smaller residential installations and locations with very high government feed-in tariffs.

Disadvantages:

• Trackers add cost and maintenance to the system - if they add 25% to the cost, and improve the output by 25%, the same performance can be obtained by making the system 25% larger, eliminating the additional maintenance. Tracking was very cost-effective in the past when photovoltaic modules were expensive compared to today. Because they were expensive, it was essential to use monitoring to minimize the number of panels used in a system with given power output. But as panels get cheaper, the cost-effectiveness of tracking vs using a greater number of panels decreases. However, in off-grid installations where batteries store power for overnight use, a tracking system reduces the hours that stored energy is used thus requiring less battery capacity. As the batteries themselves are expensive (either traditional lead-acid stationary cells or newer lithium-ion batteries), their cost needs to be included in the cost analysis.
• Tracking is also not suitable for typical residential rooftop photovoltaic installations. Since tracking requires that panels tilt or otherwise move, provisions must be made to allow this. This requires that panels be offset a significant distance from the roof, which requires expensive racking and increases wind load. Also, such a setup would not make for a very aesthetically pleasing install on residential rooftops. Because of this (and the high cost of such a system), tracking is not used on residential rooftop installations and is unlikely to ever be used in such installations. This is especially true as the cost of photovoltaic modules continues to decrease, which makes increasing the number of modules for more power the more cost-effective option. Tracking can (and sometimes is) be used for residential ground mount installations, where greater freedom of movement is possible.
• Tracking can also cause shading problems. As the panels move during the course of the day, it is possible that, if the panels are located too close to one another, they may shade one another due to profile angle effects. As an example, if you have several panels in a row from east to west, there will be no shading during solar noon. But in the afternoon, panels could be shaded by their neighboring west panel if they are sufficiently close. This means that panels must be spaced sufficiently far to prevent shading in systems with tracking, which can reduce the available power from a given area during the peak Sun hours. This is not a big problem if there is sufficient land area to widely space the panels. But it will reduce output during certain hours of the day (i.e. around solar noon) compared to a fixed array. Optimizing this problem with math is called backtracking.
• Further, single-axis tracking systems are prone to become unstable at relatively modest wind speeds (galloping). This is due to the torsional instability of single-axis solar tracking systems. Anti-galloping measures such as automatic stowing and external dampers must be implemented.