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Solar Energy System Output
Introduction
Solar Energy System Output
Orientation
Siting
How much energy a photovoltaic (PV) or solar hot water (SHW) system produces in Wisconsin depends on the:
- Size of the system (i.e., area of the collecting surface)
- Orientation of the collecting surface
- Site characteristics (e.g., obstructions to sunlight).
- Conversion efficiency of the solar energy system
As the homeowner or installer, you can determine the size of the system, its orientation, and, to a limited degree, the site characteristics and the system’s efficiency. We’ll discuss the system’s orientation and site characteristics here.
Orientation
There are four options for orienting solar
energy panels:
- A fixed orientation
- Adjust the orientation of the panels on a seasonally, typically three times a year
- Tracking the sun each day along a single axis
- Tracking the sun each day along two axes.
Photograph 1. Two-axis tracking PV system (located near Amherst, Wisconsin).
It is rare for a solar thermal system to use any tracking system, while it is common for a photovoltaic system to use an adjustable rack, single-axis or dual-axis tracking.
Photograph 2. single-axis tracking PV system
Annual solar system efficiencies vary depending on the specifics of the system and, for solar thermal systems, how it is used. The annual solar system efficiencies stated in Table 1 are representative of high quality installations.
The first entry on Table 1 shows that a total of 128 kWh of solar radiation lands on one square foot horizontal surface over one year. A horizontal, one square foot crystalline PV module would generate 15.4 kWh over that year. This is enough power to light a 13-watt compact fluorescent light bulb for 1,200 hours each year.
Table 1. Annual total solar radiation for PV and solar hot water systems in Wisconsin at various surface Orientations 1| Annual Insolation (KWh/ft2 | Thin Film PV System 8% effic.(KWh/ft2) | Crystalline PV System, 12% effic.(KWh/ft2) | SHW System,40% effic.(ft3 natural gas/ft2) | |
|---|---|---|---|---|
| Fixed, horizontal | 128 | 10.2 | 15.4 | 171 |
| Fixed 25o tilt from due south | 142 | 11.4 | 17.0 | 190 |
| Fixed 50o tilt from due south | 138 | 11.0 | 16.6 | 184 |
| Tracking, single axis | 176 | 14.1 | 21.1 | --- |
| Tracking, double axis | 186 | 14.9 | 22.3 | --- |
Fixed solar panels are typically oriented facing due south with a slope of between 25oto 50o from the horizontal. With lower slopes, the system’s summer output is maximized. With higher slopes, the system’s winter output is increased, and the system’s ability to shed snow is improved. Seasonally adjustable racks, allow the system owner to change the tilt of the panels; these systems result in a 10% to 15% output increase when compared to a fix system.
Some fixed PV systems are installed 10o to 20o west of south to increase energy production during the late afternoon – exactly when the electric utility’s summer demand for electricity is typically greatest. Meanwhile a SHW may be oriented 10o to 20o east of south to maximize morning solar heating, after the cooling of the night.
For retrofitted roof-mounted systems, the building roof orientation dictates the orientation of the solar panels. A building with the long side facing within 30 degrees of south can take advantage of sun power. If the short end (the gable end) faces south solar options are limited.
Photograph 3. A Fixed roof-mounted PV system, facing due south inclined at 45o (located in Madison, Wisconsin).
Photo credits
Photo 1 Niels Wolter, WisconSUN
Photo 2 Michigan State Energy Office
Photo 3 Niels Wolter, WisconSUN
copyright © 2000 WisconSUN