How does the tilt angle and/or orientation of the PV panel affect system performance?

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Volume 9 Issue 3	July 2006

application - The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications. ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. compact fluorescent lamp (CFL) - A family of single-ended fluorescent-discharge light sources with small-diameter [16-millimeter (5/8-inch) or less] tubes. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. grid - The combination of electric power plants and transmission lines operated by an electric utility. lamp - A radiant light source. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. halogen lamp - An incandescent lamp that uses a halogen fill gas. Halogen lamps have higher rated efficacies and longer lives than standard incandescent A-lamps. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. luminance - The photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (square feet or square meter). glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. illumination - The process of using light to see objects at a particular location. PN junction - For light emitting diodes, the portion of the device where positive and negative charges combine to produce light. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. inverter - Also known as “power inverter.” A device used to convert direct current (dc) electricity into alternating (ac) current. irradiance - The density of radiant flux incident on a surface. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. photon - A small bundle or quantum of electromagnetic energy, including light. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels.

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.

Figure 9. Range of midday sun angles at 33° north 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).