PV Primer

Talkin’ shop
Learning the jargon of solar photovoltaics (PV) is essential for understanding their basic operation. Starting with the smallest unit, a PV cell is the silicon wafer of about 4 to 5 inches in diameter that you can usually see beneath the glass panel. The PV cells are wired together, fixed to a substrate, covered with glass, and contained in an aluminum frame to form the PV module.

The number of modules is normally specified when you purchase a PV system. PV modules are wired together and held in another frame to form a PV array, which can be mounted on a roof or mast. The nominal PV system voltage may be 12, 24, or 48 volts DC, depending on how the cells and modules are wired.

The real deal
Photovoltaic modules come in many different sizes and each has a nameplate DC power rating. For example, the PV module that we use in my Energy Systems course at Chico State has dimensions of approximately 4 feet by 6 feet and is rated at 315 watts. But the highest power output that we’ve ever recorded in full sun is only 240 watts. You might think that this is an example of false advertising, but it’s well known within the PV community that modules must be “derated” to determine their real-world power output.

Derating is necessary because manufacturers test their modules under ideal laboratory conditions and use the tests to establish the nameplate DC power rating. Ideal conditions usually correspond to full sun shining on a perfectly aligned module (i.e. 1000 watts of solar irradiation per square meter), a high-clarity atmosphere, a clean cover glass, and a PV cell temperature of 77 degrees Fahrenheit. These conditions rarely occur in practice, and there are other energy-robbing effects when modules are wired into an array and connected to an inverter (DC-to-AC converter).

Losing out
These other effects include wiring losses within the array, between the array and inverter, and between the inverter and the connection to the local utility service. There are also losses due to inverter inefficiency, dust on the cover glass, degradation due to age, and variation in manufacturer specifications. Not everyone computes these losses in quite the same way, but the overall effect usually produces a PV-derating factor of around 77 percent. This means that we should expect a maximum power output of about 245 watts for a PV module with a nameplate DC power rating of 315 watts that is wired into a grid-tied system.

Typically omitted from the derating factor is the effect of temperature on the PV cell. Silicon PV cells operate most efficiently when they are cold—the cooler, the better. However, the electric current flowing through the cells and modules causes heating. The resulting temperature depends on the ambient air temperature and wind speed but it is almost always higher than the standard testing condition of 77 degrees.

Class project
When my class tested our PV system last week when the afternoon air temperature was about 95 degrees, the module was 149 degrees! High cell temperatures cause a further reduction in performance—as high as 0.5 percent per degree above 77 degrees. This probably explains why our class experiments yielded less power than that calculated with the standard derating factor. Furthermore, homeowners often notice that their PV systems function more efficiently in the winter than the summer due to this effect.

More info …
The National Renewable Energy Laboratory (NREL) has a sophisticated calculator called PVWatts that estimates hour-by-hour and annual PV array power output for locations all around the world. It includes derating factors and cell temperature effects. Check it out at www.nrel.gov/rredc/pvwatts.