Solar Eclipse 2017: Slow Ramping Reduces Impact to Solar Farms

Solar Eclipse 2017: Slow Ramping Reduces Impact to Solar Farms

By: Adam Baker

Why solar eclipse 2017 isn’t the end of the world for utility-scale owners and operators.

The upcoming August 21, 2017 solar eclipse reminds me of bit of Y2K. Too much over exaggeration and emphasis on the negative effects…to solar power plants.

Let’s assume the worst-case scenario for a second. Worst case: your entire site in the maximum window of generation and directly in the path of totality, which means a loss of two hours (ramp down, a few minutes of total eclipse, and ramp back up) worth of energy. When you put it like that, two hours doesn’t sound like the end of the world, does it?

According to the U.S. Energy Information Administration, “relatively little solar PV capacity lies in the path of totality…and the North American Electric Reliability Corporation does not anticipate the eclipse will create reliability issues for the bulk power system.”

Source: US Energy Information Administration

Areas with high solar penetration (such as California), will be extremely impacted. But luckily, the rate of change is slow. Because the eclipse is taking place over the course of an hour and a half, it will be an extraordinarily slow ramp up and down. And slow ramping is what the grid was designed for. Solar ramp rate will be even slower than a solar control system would ramp a site off or back on. Thermal generation, whether peaking plants or combined cycle, should be able to compensate for the solar loss.

It’s the balancing authority’s responsibility to make sure the grid is not interrupted with a large loss of generation…which means on August 21, utility-scale solar farms are just along for the ride.

In preparation for this shift, utilities may call and curtail sites proactively before the event begins. In fact, we have customers who have already been notified they’ll be getting a call on August 21 to change their output.


Examples of solar farms in the solar eclipse 2017 path

Let’s take a look at some concrete examples in areas with high solar penetration.


In the entire U.S., North Carolina has the greatest amount of installed solar farms within the 90+% obscurity band. Luckily, solar power makes up a much smaller generation total in North Carolina when compared to areas like California.

This solar farm in Conetoe, North Carolina is outside the path of totality, but still within the 90% obscurity band. If August the 21st is a clear sky day, a normal 1,000W of irradiance will be reduced to 100W. The DC from the modules will get low enough that inverters will probably shut off. The sun will begin to be obscured starting at 1:19 pm, and will be back to normal at 3:07pm. Overall, the site will undergenerate for 2 hours.

Let’s travel to the west coast. Unlike North Carolina, solar power (solar thermal and PV combined) makes up 40% of California’s total generating capacity. Though solar thermal facilities will experience less drop in output, the PV contribution is directly correlated with irradiance, which means electricity must be replaced with ramping up of other thermal generation like hydropower plants and natural gas.

This is the 550mW Topaz Solar site in Santa Margarita. Here, the eclipse starts at 9:03 am, which means the site isn’t yet at full output, and will remain under full output until the eclipse ends at 11:41 am local time. These modules will see 66% obscurity, which means 1,000W of sunlight will reduce to 330W…probably not enough to set the inverters to night mode, but certainly enough to drive the output of the site down for a while.


What can you do about solar eclipse 2017?

The utility will likely call to ask you to change site conditions, reduce output, and curtail capacity. Does your site have a control system? Owners with even the most basic controls system should be able to ramp the plant output or perform an orderly shutdown of the plant with one click. Owners without a control system will probably be interacting with the inverters individually and shutting them down manually through a mechanism like Port Forwarding (an extremely tedious process with string inverters).

Obviously, it’s too late to install solar controls system in anticipation of the solar eclipse 2017, but controls are useful for plenty of other situations. For example, using inverter VARs to correct grid voltage conditions and avoiding curtailment through analog curtailment.

Adam Baker - Affinity EnergyAdam Baker is Senior Sales Executive at Affinity Energy with responsibility for providing subject matter expertise in utility-scale solar plant controls, instrumentation, and data acquisition. With 23 years of experience in automation and control, Adam’s previous companies include Rockwell Automation (Allen-Bradley), First Solar, DEPCOM Power, and GE Fanuc Automation.

Adam was instrumental in the development and deployment of three of the largest PV solar power plants in the United States, including 550 MW Topaz Solar in California, 290 MW Agua Caliente Solar in Arizona, and 550 MW Desert Sunlight in the Mojave Desert.

After a 6-year stint in controls design and architecture for the PV solar market, Adam joined Affinity Energy in 2016 and returned to sales leadership, where he has spent most of his career. Adam has a B.S. in Electrical Engineering from the University of Massachusetts, and has been active in environmental and good food movements for several years.