PV system owners get very excited about the number of kilowatt-hours produced by their systems. And for good reason—this electricity is helping to offset their utility bills or, in off-grid scenarios, providing most or all of their electricity. Anything that can help or hinder production should be investigated, and racking methods are no exception.
Frank Vignola and his team at the University of Oregon’s Solar Radiation Monitoring Laboratory have been collecting environmental and PV system performance data across the Pacific Northwest (see http://solardat.uoregon.edu). This data has enabled PV designers to accurately predict PV array output for a variety of installation conditions—including mounting methods.
Because high temperatures adversely affect a PV system’s performance, it’s particularly important to try to implement best practices when feasible. In general, arrays that are mounted parallel to the roof surface with less than 6 inches of space between the array and roof will experience cell temperatures of about 35°C (63°F) above ambient temperatures. For rack-mounted arrays, where the back of the array is tilted off the roof surface greater than 6 inches, cell temperatures are estimated to be about 30°C (54°F) above ambient. Top-of-pole mounted arrays will operate at approximately 25°C (45°F) above ambient temperatures. In general, PV array output takes a 0.5% hit for every 1°C rise in temperature.
Two other critical array mounting concerns for optimizing system production are the array’s orientation (relationship to true south) and the tilt (the array’s angle off of horizontal). As a general rule, for a fixed array to produce the maximum amount of energy annually, it should be oriented toward true south (after correcting for magnetic declination), with its tilt angle fixed to correspond to the site’s latitude.
However, this general approach can be fine-tuned based on site specifics such as what time of year the most potential solar energy is available, or if the interconnection agreement with the utility is based on annual, rather than monthly, system output. For locations in western Oregon, for example, an array will produce the most energy when oriented slightly west of south at a tilt angle of approximately 30 degrees (approximate latitude minus 15 degrees). In the eastern half of the state, where there is a greater solar resource, the ideal array position is approximately a 35-degree tilt (approximate latitude minus 10 degrees) and either a true south or slightly east-of-south orientation.