"Even though the wind does not blow nor the sun shine all the time, careful management, readily available storage and other renewable sources, can produce nearly all the electricity North Carolinians consume," says John Blackburn, former Chancellor of Duke University.

His report, Matching Utility Loads with Solar & Wind Power in North Carolina: Dealing with Intermittent Electricity Sources, says only 6% of electricity would have to be purchased from outside the grid or produced at conventional plants.

“The truth is that solar and wind are complementary in North Carolina,” he explains. “Wind speeds are usually higher at night than in the daytime; they also blow faster in winter than summer.”

“Solar generation, on the other hand, takes place in the daytime; sunlight is only half as strong in winter as in summertime,” he adds. “Using wind and solar in tandem is even more reliable; together, they can generate three-fourths of the state's electricity.”

When hydroelectric and other sources of renewable energy are added, the gap that must be filled is “a surprisingly small” 6%, he notes.

Solar and wind helped by hydro, co-gen and efficiency

Blackburn used hourly wind and solar data for a total of 123 days in the months of January, April, July and October, with samples taken from three wind and three solar sites across North Carolina.

Solar and wind were scaled up to represent 80% (40% each) of average utility loads for the sample months, with the rest coming from existing hydroelectric capacity (8%) and 12% from assumed biomass co-generation.

The study calculated energy efficiency by assuming an annual utility load of 90 TWh, lower than North Carolina’s current load of 125 TWh, and by calculating average hourly loads from the 2006 load profile of Duke Energy with modifications to show reductions in summer and winter peaks due to greater efficiency in buildings.

It also assumed increased storage capacity from a smarter electrical grid.

“Those reluctant to endorse a widespread conversion to renewable energy sources in the US frequently argue that the undeniably intermittent nature of solar and wind power make it difficult, if not impossible, to provide reliable power to meet variations in demand without substantial backup generation,” he says.

Dispatchability is not a constraint

Several studies, concentrating on regions with ample wind and solar resources, have suggested that a combination of the two, when spread over a sufficiently wide geographic area, could be used to overcome the inherent intermittency of each separately, reducing the need for backup generation.

“Moreover, since the backup power is required at more or less randomly distributed times, the availability of baseload power, so strongly entrenched in utility circles, becomes more or less irrelevant,” he concludes.

“North Carolina has several means of evening out differences between variable generation and load from hour to hour within days, but very limited ability to carry stored energy forward from day to day,” the report notes.

“The hydroelectric system is already used as a means to meet peak demands with a generation system heavily oriented toward baseload generation.

“As smart grids are developed, some customers will be able to respond to real-time pricing, offering still more opportunities to shift loads during the day,” it adds. “Still other storage opportunities may arise when plug-in hybrid vehicles are in use and have two-way communications with grid operators.”

“The important conclusion from all of the calculations is that a system with annual sales of 91 TWh can be run with 76% of total generation coming from intermittent solar and wind sources,” the report explains. The intermittent sources would be assisted by 2 GW of biomass generation or cogeneration, 2.5 GW of hydroelectric capacity, and 1.5 GW of pumped storage.

Aggregate benefits reduce constraints

If the system has ice storage for summer, load control and access to vehicle batteries, “it can be run with some modest outside-of-system purchases and 2700 MW of auxiliary gas-fired capacity,” it adds. “Purchases and auxiliary generation are needed for 6% of electricity loads.”

“A corollary observation is that the concept of baseload generation is more or less irrelevant to its successful operation of such a system,” it states.