Heat and time take toll on solar panel performance

Solar arrays at NREL.

The widely held belief of solar systems is that once the initial cost is paid off, the rest is gravy.

Or more specifically, that the power harvested from the sun is free for those who own their systems. And that's true.

However, there's an important detail to consider, especially if the cost of materials and labor are financed over a long period.

"Numerous studies have shown that degradation rates for silicon modules are typically less than 1 percent per year," says analysis from the National Renewable Energy Laboratory in Golden, Colo.

That means power output diminishes minutely each year and could be significantly less for a system once the up-front costs are paid off after financing. That could mean a photovoltaic solar array produces 40 percent less power than the day it was installed once a 40-year loan is paid. A highly technical NREL study says declines could actually be less, ranging from .5 percent to .7 percent.

Of course degradation and performance rates depends on multiple factors, with some being solar intensity and temperature. Series resistance poses another interesting factor as does the system performing only as well as its worst cell. But that can be dealt with by connecting the panels in parallel or using other technical means.

On an NREL map of the United States outlining solar intensity, California's San Joaquin Valley sits in one of the optimal places. The region is sunny.

But with sun comes high temperatures, which also decrease electrical output somewhat. And that's something to consider in the Valley where many communities have more than 40 days of the year hotter than 100 degrees.

The reason for the heat-related performance decline has to do with conductivity, reducing the magnitude of the electric field and lowering voltage, according to solarpower2day.net. "It should be noted that a higher temperature increases the mobility of electrons, which causes the flow of current to increase slightly," the site says. "This increase is however minor and insignificant compared to the decrease in voltage."

Simply put, "the solar radiation which produces solar electricity carries heat with it that will cause the components of your photovoltaic solar panel to become altered and less able to capture sunlight effectively," says solarpanel-direct.com.

NREL has an outdoor test facility that's been operating about a decade testing all sorts of solar panels. As time marches on, more will be written about performance, maintenance and optimizing crystalline photovoltaic systems to extend their lifetimes.

I started looking into this concept after a colleague of mine who works with one of the counties I contract with mentioned solar performance issues. He said the county is likely to continue to pursue development of a significant solar project, but that it is also looking into other methods of boosting its clean energy footprint.