Array tilt and orientation affect PV system output, so they need to be considered when choosing a mounting method. Two terms—azimuth and altitude—relate to array orientation and tilt and are used to describe the sun’s position in the sky.
Azimuth is the horizontal angle from a reference direction that the sun makes throughout the day. This angle is commonly referenced from north (0°)—east is 90°, south is 180°, west is 270°.
Altitude is the vertical angle the sun makes relative to the horizon. At dawn, when the sun is on the horizon, the altitude is 0°. As the sun makes its arc across the sky, its altitude progresses to be the highest at solar noon. The altitude changes throughout the day and also varies throughout the year, reaching its highest point on the summer solstice at noon.
The sun’s azimuth and altitude depend not only on time of day and season, but also on location (i.e., latitude). The sun charts illustrate the difference in altitude angles for San Diego, California (latitude = 32.6°) and Bismarck, North Dakota (latitude = 46.8°). Notice that the sun’s altitude is lower for locations with higher latitudes. For example, at 9 a.m. in Bismarck on December 21, the altitude is less than 10°. In San Diego, at the same time on the same date, it is almost 20°. At noon on the same date, the sun in Bismarck will only get 20° off the horizon; in San Diego, it will reach about 35°. (Note: A sun-path chart for any location can be made with the this online program from the University of Oregon Solar Radiation Laboratory: http://solardat.uoregon.edu/SunChartProgram.html.)
So how do solar azimuth and altitude angles relate to a PV array’s placement? A PV array’s tilt is the vertical angle between the back of the modules and level. The array’s orientation is its horizontal angle relative to north (i.e., an array with a “south-facing” orientation is 180° from north). Ideally, we want our array’s tilt and orientation to take advantage of the sun’s altitude and azimuth angles as much as possible. While dual-axis tracking arrays can follow both of these sun angles, fixed arrays cannot. System designers rely on solar radiation data provided for a location and examine the predicted system output for various array tilts and orientations to find the combination that yields the highest production (see “Methods” in this issue).