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Substantial variability in US solar performance

An anomaly study from 3TIER shows how solar irradiance in the USA varied from its long-term norm this summer.

By Kari Williamson

3TIER's solar performance maps of the US for June, July, and August 2011 shows strong correlation with a series of weather events that impacted the US this past summer.

Solar energy is not immune to climatic variability and these risks needsto be factored into the financial structure of projects and regularly monitored, 3TIER says.

“The 2011 study shows that some areas experienced above or below normal irradiance levels at extremes of ±30-40%,” says Dr Mark Stoelinga, Senior Scientist at 3TIER.

“In comparing the results of 3TIER’s 2010 solar performance study, this summer’s irradiance anomalies, both positive and negative, were much stronger than last summer. This shows us that high amounts of variance, even in peak production months, are not unusual.”

Findings from three-month solar irradiance study:

  • June: This month saw the most significant, widespread solar irradiance anomalies of the summer, highlighted by a broad area extending from Texas and Oklahoma to the Carolinas experiencing direct normal irradiance (DNI) values ranging from 15-40% above normal. This was caused by a high-pressure pattern that also produced hot, dry, drought conditions and wildfires in Texas. Meanwhile, DNI values ranging from 15-30% below normal were seen in the West Coast, interior Pacific Northwest, and Upper Midwest. In the West this was due to an upper-level trough that has been in place since January causing cool, cloudy weather;
  • July: Positive anomalies over Texas and Oklahoma persisted in July, but were less noted than in June, with DNI values ranging from 10-20% above normal. Farther east, the Deep South transitioned to a below average solar irradiance regime connected to wet weather along the Gulf Coast. Along the East Coast, positive anomaly shifted north to the mid-Atlantic and New England. The lingering trough in the Northwest continued along with below average DNI values, although parts of California and the Northern Rockies transitioned to normal or above normal solar irradiance; and
  • August: This month the breakdown of the persistent upper-level trough over the Northwest finally occurred, yielding above normal DNI values, particularly in the interior Pacific Northwest where DNI ranged from 10-20% above normal. However, the ridge over the eastern part of the country persisted, maintaining hot, dry weather and DNI levels 20-30% above normal over Texas and the Ohio Valley. The Gulf Coast transitioned to dryer weather, yielding well above normal DNI. Meanwhile in Florida and the Northeast, Tropical Storm Emily and Hurricane Irene contributed to DNI values 10-20% below normal.

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