A prime parameter in the specs for photovoltaic system installation in the Connecticut Clean Energy guidelines refers to shading of the panels: to wit, no shading allowed during the normal "solar day," reckoned to be between 9 Am and 3 Pm. It's a pretty stiff requirement here in tree-covered New England, and it may seem unfair to disqualify a potential roof site because a tree shades it for part of the day. But here, in brief, is the danger of shading and the logic behind zero tolerance for it.
A solar PV array is configured in "strings," or source circuits, of two to 12 panels, according to system voltage. The string of panels is connected from one to the other via the connected module leads so that the current through the string is constant, and the voltage of each module adds up to the nominal system voltage, anywhere from 24 volts for small battery-connected systems to nearly 5oo volts for high-output grid-tie systems. And in that string, or series circuit, a little patch of shade can limit the current of the entire string to a small fraction of capacity. Diodes are installed to permit current to bypass shaded or malfunctioning modules or cells, but the effect is still significant on performance.
Shading analysis in the planning stages is critical to predictable and maximum performance. If an area selected for panel installation is shaded, the time and extent of the shading must be calculated and deducted from the expected output of the system. Sometimes module choices are affected by shading analysis; "amorphous" crystalline cells are slightly more shade tolerant than other module types.
Non-grid tie systems suffer at least as much, if not more than grid-tied arrays. If batteries are matched to the output of the array, a small shaded area alerts the Maximum Power Point Tracking device in the inverter, which senses the efficiency and total output of the system, simply shuts down and waits for the shading to pass. For the duration of the shading, the system sits idle.
Shade analysis, then, is a vital part of planning when photovoltaic arrays are being sited on rooftops or on the ground. The panel manufacturers and government agencies aren't kidding when they say that zero shading is the proper amount. And we, installing professionals, may be advising you to trim or remove trees, or purchase costly racking systems to relieve shading conflicts; we're not just upselling the job. Shade is your enemy in the solar game, whether it's for hot water or photovoltaics. And for photovoltaics, a little shade can be deadly.
A solar PV array is configured in "strings," or source circuits, of two to 12 panels, according to system voltage. The string of panels is connected from one to the other via the connected module leads so that the current through the string is constant, and the voltage of each module adds up to the nominal system voltage, anywhere from 24 volts for small battery-connected systems to nearly 5oo volts for high-output grid-tie systems. And in that string, or series circuit, a little patch of shade can limit the current of the entire string to a small fraction of capacity. Diodes are installed to permit current to bypass shaded or malfunctioning modules or cells, but the effect is still significant on performance.
Shading analysis in the planning stages is critical to predictable and maximum performance. If an area selected for panel installation is shaded, the time and extent of the shading must be calculated and deducted from the expected output of the system. Sometimes module choices are affected by shading analysis; "amorphous" crystalline cells are slightly more shade tolerant than other module types.
Non-grid tie systems suffer at least as much, if not more than grid-tied arrays. If batteries are matched to the output of the array, a small shaded area alerts the Maximum Power Point Tracking device in the inverter, which senses the efficiency and total output of the system, simply shuts down and waits for the shading to pass. For the duration of the shading, the system sits idle.
Shade analysis, then, is a vital part of planning when photovoltaic arrays are being sited on rooftops or on the ground. The panel manufacturers and government agencies aren't kidding when they say that zero shading is the proper amount. And we, installing professionals, may be advising you to trim or remove trees, or purchase costly racking systems to relieve shading conflicts; we're not just upselling the job. Shade is your enemy in the solar game, whether it's for hot water or photovoltaics. And for photovoltaics, a little shade can be deadly.
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