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![]() ![]() Volume 9 Issue 3
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July 2006
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How does the tilt angle and/or orientation of the PV panel affect system performance?Photovoltaic (PV) panels collect solar radiation directly from the sun, from the sky, and from sunlight reflected off the ground or area surrounding the PV panel. Orienting the PV panel in a direction and tilt to maximize its exposure to direct sunlight will optimize the collection efficiency. The panel will collect solar radiation most efficiently when the sun's rays are perpendicular to the panel's surface. The angle of the sun varies throughout the year, as illustrated in Figure 9. Therefore, the optimal tilt angle for a PV panel in the winter will differ from the optimal tilt angle for the summer. This angle will also vary by latitude.
In some PV lighting systems such as solar garden lights or small post-top luminaires, the PV panels are incorporated directly into the luminaire housing and cannot be moved or oriented in a particular direction. In these types of systems, the PV panels are typically oriented horizontally, facing the sky. However, many larger PV lighting systems are designed to allow a system installer to tilt the PV panel at an angle from horizontal and to orient the PV panel in a particular direction. In these types of systems, a system specifier or installer should first determine the optimal tilt angle and orientation of the PV panel for the system's location. The first step in determining optimal PV panel orientation and tilt angle is to review the site where the PV lighting system will be installed. Trees, large buildings, or other structures or obstructions surrounding the site might cast shadows onto a tilted PV panel at various times of day or during winter months when the sun is at a low angle in the sky. Therefore, it may be best to orient the PV panels horizontally to face the sky directly. This may allow the panels to collect the maximum amount of solar radiation with the least obstruction. However, a horizontal panel will get dirty faster. However, if the site surrounding the PV lighting system is relatively free of obstructions, a lighting specifier can orient the system's PV panel in a particular direction and up at a selected angle. In this case, the PV panel should always face toward the equator. In the Northern Hemisphere the panel should face south and tilt from horizontal at an angle approximately equal to the site's latitude (NREL 2005). For example, if the system were located in San Diego, California, the PV panel should face south and tilt up at an angle of approximately 33°. These recommendations for tilt angle represent an average, taking into account the angle of the sun over the entire year. However, if a PV lighting system at a northern latitude is designed to be used throughout all four seasons, it may be advantageous to tilt the PV panel at an angle that optimizes its performance in the winter, when solar radiation is likely to be at its lowest. As a general rule, to optimize the performance of PV panels in the winter, they should be tilted up from horizontal at an angle 15° greater than the latitude. For example, the calculation (in "What is the process to determine the appropriate size of PV panels for a particular application?") shows that a panel tilted to 48° in San Diego (latitude 33°) will allow a reduction of panel size by 43%, compared to a horizontal panel. Conversely, if a PV lighting system is going to be used only in summer, (e.g., at a campground or state park that is used seasonally), it may be most advantageous to optimize the performance of the PV panel for summer. For optimal summer performance, the panel should be tilted 15° less than the latitude (NREL 2005). Unfortunately, good historical data does not exist on the actual improvement in system performance that can be achieved through proper orientation and tilt of a PV panel. Most recommendations for panel orientation are made based upon computer simulations and mathematical models. Estimates of performance improvements based on optimizing PV panel orientation and tilt angle range from 10-40% (Landau 2002).
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