An analysis shows that PV output peaks hours before the grid really needs it
Most rooftop photovoltaic (PV) panels face south because the owners of the panels want to generate the most electricity possible. But a recent report says that shifting more PV panels to the west would produce electricity at a time when the electricity is much more useful to utilities, reducing the need for utilities to buy costly power to meet peak loads.
Writing in an Outlier blog for Opower, a company that analyzes energy data for utilities, Barry Fischer and Ben Harack note that most PV-equipped homes are producing much more power than they consume during the middle part of the day. The grid, however, really needs the power boost in the late afternoon.
Fischer and Harack said they looked at Opower data from 25,000 solar homes in the western U.S. along with public data about 110,000 residential projects installed in California since 2007.
“Our statistical results reveal a key disconnect between today’s solar panel landscape and the broader power system,” they write.
More west-facing panels would generate more power in the late afternoon and give utilities a “compelling alternative” to bringing additional power plants online.
That’s the same conclusion reached in a report from the Pecan Street Research Institute at the University of Texas at Austin last December. That report, however, was based on a much smaller sample — just 52 homes.
Lots of midday output
During the sunniest parts of the day, an average of 93 percent of the solar-equipped homes export electricity to the grid because the panels generate more electricity than the homes use. On one day in particular, a hot day in May of this year, the authors charted power exports in 25,000 western homes from about 8:00 in the morning until just before 4:00 in the afternoon.
After that, the solar homes actually used more than non-solar homes. The authors explain the situation this way: “Wondering why solar homes’ use of grid electricity shoots way above average after the sun goes down? It’s likely related to the elevated energy needs of their owners’ lifestyles: the typical solar home in our dataset is 34% larger than the typical non-solar home, 2.6 times as likely to own a pool, and 2.7 times as likely to enroll in an electric vehicle rate plan.”
At midday, between 11:00 a.m. and 1:00 p.m., the average solar home produces enough power to run itself and two non-solar homes at the same time.
But peak demand on the grid occurs about 5:00 p.m., when the output of south-facing panels has fallen sharply. By 4:00 p.m., only 27 percent of solar homes are exporting power to the grid; by 5:00 p.m., that’s fallen to 6 percent.
“Could solar homes be more helpful in satisfying peak electric demand on the grid?” the authors write. “The answer is certainly yes. And a small subset of homes appears to be leading the way.”
Panels face south for a reason
In California, only 9 percent of solar panels face within 10 degrees of due west, the blog says. A western orientation reduces their total output by between 10 percent and 20 percent when compared with south-facing panels, and that means less electricity for homeowners and lower earnings from net-metering.
Peak output for west-facing arrays is 2:00 p.m., the blog notes, two hours later than for south-facing panels.
And that helps explain why some utilities are offering financial incentives to homeowners to make the switch. The study says some plans pay customers as much as 35 cents per kWh for power produced late in the afternoon vs. 12 cents per kWh for power exported to the grid at noon.
“Offering a handsome incentive for well-timed solar power (or well-timed reductions in usage) can be a smart play for any utility seeking to avoid a painful alternative: paying notoriously high marginal costs to source electricity from ‘peaker’ power plants (e.g. 3 to 5x the normal price level),” the report says.
“An appropriate time-of-use rate framework could enable west-facing systems to achieve compelling monetary returns despite their reduced annual energy output,” it continues. “Our recent analysis of time-varying rates — specifically those that encourage nighttime electric car charging — suggests they can have a strong effect in shaping consumer behavior.”
The California Energy Commission recently decided to award subsidies of up to $500 for the installation of west-facing panels.
Source: Green Building Advisor