Solar Eclipse in a Solar-based System?

Solar PV generation output often changes quite fast because of fluctuations in solar irradiance due to cloud cover, weather patterns, or other environmental factors. And of course, the sun goes up and down every day. Power system operators have to deal every day with those operational challenges. A special and quite extreme effect is a solar eclipse as the amount of solar radiation that reaches the Earth's surface is reduced, which causes a decrease of solar power generation. This reduction in power generation can be significant, especially in areas where the eclipse is total or near-total. On 20 March 2015 there was a well-documented solar eclipse, find the interesting ENTSO-E report here.

The power system must respond quickly to all power generation changes to maintain a secure supply. I will compare the power ramp rate for power generation, which we successfully managed during the eclipse, with the future normal situation in 2037.

We looked at a sunny August day in 2037 in the Germany power system with a scenario of about 350 GW of solar PV installed (more details here). The generation curve could look like this:

The generation is much higher than the load and you can see a sharp increase of Solar PV generation in the morning and a similar decrease in the afternoon. At this stage I do not worry that more power is generated than load as I only want to look at the power ramp rate of the Solar PV generation:

The black lines indicate the power ramp rate. In the morning from 8:00 hours to 9:00 hours we have the fastest generation increase with a power ramp rate of about 40 GW per hour. In the afternoon we see a generation decrease with a the power ramp rate of about 30 GW per hour.

Solar Eclipse 2015

Now let’s look at the solar eclipse from 20.3.2015. During a solar eclipse, the power ramp rate of power generation in a solar-based power system will also be rapid, but in a different way. As the eclipse progresses, the amount of solar radiation that reaches the Earth's surface decreases gradually over several minutes, and then increases again gradually as the eclipse ends. This means that the power generation from solar power plants will decrease and increase at a relatively predictable rate over several minutes. In addition, in its course the solar eclipse will affect the whole power system across Europe.

In 2015 about 90 GW Solar PV were installed in the region affected by the solar eclipse. Let me quote the report from ENTSO-E about the effect: “The estimated PV injection at the start of the eclipse (approx. 09:30 CET) was approx. 22 GW. During the maximum of the solar eclipse at 10:00 CET the feed-in decreased to approx. 14 GW. The estimated PV injection at 12:00 CET was 35 GW. Hence the change in injection between 10:30 and 12:00 was approx. +21 GW.”

And here is the picture. The time day of the solar eclipse was quite critical as the load usually increases at about the same time, while solar generation was reduced through the solar eclipse:

The good news is, that the power system functioned very well. The Transmission System Operators prepared over several month well for the event. For example, they planned ahead by increasing the generation capacity from other sources during the eclipse period. They made special training of the control room operators and many more actions – as you can read in the above-mentioned ENTSO-E report.

Now we compare the power ramp rate of solar generation: During the solar eclipse the solar generation was reduced by 14 GW in 30 minutes, or 28 GW in 1 hour. That is quite similar to the power ramp rate of 30 GW of decrease per hour that we can expect on a normal sunny day in 2037. The increase of solar power generation was “only” 21 GW in 1.5 hours; in 2037 we will see something like 40 GW per hour.

Key learnings

• There was significant preparation in 2015 to ensure safe system operation.
• In 2015 the power system coped very well with the Solar Eclipse.
• The extreme event in 2015 will be the norm in 2037.

Conclusion

What was an extreme event in 2015, with long preparations, will be the norm on every normal sunny day in the next decade. Through thorough analysis we learned how the system reacted and what needs to be improved. There are big challenges ahead and they need action (see the ENTSO-E report) – many things are being implemented. To cope with the future of a weather dependent system we will also need many innovative solutions to ensure secure system operation. Great challenges for engineers.

Personal Note: I am quite relieved that the next total solar eclipse in Germany will be in 3. September 2081 – sufficient time to prepare. Partial solar eclipses in 2026 and 2027 will test the resilience of the power system on the way.