Obviously, 2017's total solar eclipse will affect photovoltaic energy production - reducing solar electricity generation by several gigawatt-hours - but did you know it could also affect wind power? Here's why...

Wind Power Is a Form of Solar Power

The wind is caused by uneven heating of the Earth's surface due to sunlight hitting parts of the planet with different albedo (reflectivity) and thermal mass, as well as random clouds blocking sunlight. Warm air rises because it's less dense; the warm air rising and cool air falling - that's wind. (Okay, that's a simplified explanation. You can read a more detailed one here.)

In effect, wind power IS solar power.

I've experienced three partial solar eclipses, and in every case, the wind seemed to increase as the eclipse progressed. I figured it was due to the change in temperature caused by the partial blocking of the Sun. So when I started seeing articles about how much solar production would decrease during the eclipse, my gut feeling was that wind power would actually INCREASE, and I wondered if it would be enough to compensate for the loss of solar. As I researched wind levels during solar eclipses, I was reminded of the difference between gut instinct based on anecdotal evidence and reality based on empirical data.

You see, it turns out that the wind decreases during an eclipse. What I was experiencing was an increase in ground level, turbulent wind. But the decrease in sunlight has an overall cooling effect. With less warm air rising, there's less wind, especially higher up, where utility-scale wind turbines have access to sustained, high-speed winds.

How much less wind? That's difficult to estimate, but during the eclipse of 2015, which affected the UK and parts of mainland Europe, UK wind speeds dropped by about 0.8 m/s (1.8 mph). The average wind velocity in the US (at turbine-level heights) is around 8 m/s; a 10% decrease in wind speed could lead to a 25% loss in wind power generated by an individual turbine. That's because wind power varies exponentially with wind velocity.

The images below show the eclipse path and a map of wind farms in the US. Many wind farms will be in the path of totality and many more will experience a partial eclipse.

The good news is that the 10% decrease in wind speed isn't nearly as dramatic as the difference in sunlight. The entire continental US will experience at least a 50% blockage of sunlight - more as one gets closer to the path of totality. Also, wind turbines don't usually operate at their full capacity. In order to maintain compatibility with the grid frequency and to protect the turbines from spinning too rapidly in very high winds, turbines have electronic control systems that vary the blade pitch and rotor torque, allowing the turbine to spin faster or slower as required by external conditions, including what's happening on the grid at any given instant. So there's some compensation built right into the turbine itself.

How Will The Grid Compensate for the Decrease in Solar?

Looking at the map of US solar farms, we see that California and North Carolina have the highest PV density in the affected regions. Grid operators plan to increase hydroelectric power and use natural gas-burning peaker plants to make up for the loss in solar power. Both methods are already used to supply extra power during peak demand times and to compensate for baseline power plants that unexpectedly go offline. During the European 2015 solar eclipse, hydroelectric and natural gas plants made up for the loss in solar power with no overall effect on customers.

Since the eclipse is happening in the middle of the afternoon, when electricity consumption is already near its peak, utilities are also asking customers to voluntarily cut back on energy use that afternoon. Utilities that have adopted time-of-use pricing may "encourage" customers to conserve by raising electric rates during the eclipse.

Storage?

Energy storage systems, like the massive battery banks sprouting up in California, are gradually replacing gas-fired peaker plants. They'll also be called on to supplement grid power during the eclipse, but on the grand scale, the grid-level battery systems are too small to make a significant impact on their own. But as solar grows, so will energy storage systems.

The next solar eclipses to cross a major section of the US will occur in 2023 (southwest) and 2024 (south-central to northeast), and it's safe to say that by then, a more significant chunk of our electricity will come from solar. Utility operators will use the data from the 2017 eclipse to help prepare for the impending decrease in solar-generated electricity during those events. I'll let the scientists collect data during the eclipse; I plan to just enjoy the show!

_______________________________________________________________