Can a Solar Eclipse cause a black out?
Author: Christiaan Klein Lebbink
Last update: Saturday, 18-Apr-2015 12:00:05 EDT

Can a Solar Eclipse cause a black out?

by Christiaan Klein Lebbink

Ths question has come up in regards to solar power plants and the March 2015 solar eclipse that falls across parts of Europe. That is, electricy power plants that make energy from the light of the Sun. Christiaan wrote the following in response to the question posted on the Solar Eclipse Mailing List.

Good question. The answer is; yes and no. Even though even when the sun is over 90% obscured, we still perceive it as 'bright' light. However, the lunar shadow does have a significant dimming effect and does affect the output of solar panels. As you mentioned; you noticed your own PV-installation generating less on cloudy days, and even a thin haze in upper atmospheric layers or other weather related issues have an effect on solar production.

In the Netherlands, there's not a lot of solar energy being produced, but in Europe as a whole, on a bright sunny day, all installed solar panels can produce some 90 GW. Comparable to 150 coal fired plants. 34 GW of this installed capacity is installed in Germany, and approximately 1 GW of it is installed in The Netherlands. On a sunny day, over 20% of all electricity produced in Germany is produced by solar panels.

The effect of the lunar shadow can, to some extent, be compared to the effect clouds have on such an installation; it still produces, but less. This effect is barely noticable by the naked eye, but very well measurable. The big difference when compared to weather patterns is the fact it only lasts a couple of hours, and happened in all of Europe (especially; all of Germany) at roughly the same time. This makes it harder to cope with than changable weather. Conventional power plants provide primary control reserve that can easily cancel out the effects of hazes of clouds, rainy days, or clear days. There have been some close calls due to changable weather over the past years, but it fortunately didn't lead to any blackouts or media exposure yet.

Though the generation of electricity is balanced locally, to keep the imports and exports to a minimum, all national power grids in continental Europe are connected via interconnectors into one synchronous network, maintinging a perfect 50 Hz frequency. The grid's heartbeat. All conventional production plants, be it coal, lignite, gas, petroleum or nuclear fueled plants, will work together to keep this pace and can nudge the grid frequency in the right direction by short responses. Same can be said for a nuclear plant; it can provide a short step response without the need to steer its control rods. In both cases, you simply exhaust the excess energy stored in the boiler by feeding more steam to the turbine - or chocke it, by feeding less steam to the turbine - a coal fired plant running baseload can deliver some extra megawatts for a short period of time, and a plant running part load (which is less efficient, thus causes more CO2 and NOx emissions per megawatt) can provide control reserve upward and downward for a longer period of time, by adding more or less fuel to it.Yet, it follows a gradient. It can take a long while to go from minimum load to part load to baseload. On average, a power plant can accelerate or decellerate by 5 MW to 20 MW per minute. Yet, because there are many plants hooked to the grid, and since all grids on mainland Europe and Northern Africa all cooperate to maintain this frequency, all responses of each plant combined make it well able to adapt to rapid changes in supply and demand. For every megawatt of energy produced by renewable sources, there has to be a megawatt available in running reserve in a conventional plant - this ensures that power is always available. Even at night, on rainy days or when there is no wind.

Per panel, the effect of the lunar shadow is not all that big, yet with all panels combined, the effect as a whole is significant. Though, of course, the ENTSO-E-report pictures a worst-case scenario (perfectly clear skies in all of Europe) - based on that, the infeed of solar generated electricity will decrease at a rate of 400 MW per minute for over half an hour. You will need more than just a handful of mid-sized coal fired powerplants ready at part load to be able to keep up with that pace, for so long. Yet the real challenge comes immediately after that. When Europe moves out of the penumbra, the installed solar panels will push more and more power into the grid. Solar panels don't care about demand; they just supply. Based on previous measurements, and experiences from the solar eclipse of 1999, the effect of all solar panels combined may, at some point, amount to an increase in output of 700 megawatts per minute - for several minutes! Main reason for this, is the fact it will be close to mid-day when the penumbra has fully passed - the resurfacing sunlight is much brighter that it was at 9 AM.

Even in a stong grid like the European synchronous grid, in which (as you may have noticed) people take constant supply of power for granted, it will be quite challenging to cope with that prolonged imbalance. All conventional power plants connected to the grid will have to lower their production. If you want to do that in a controlled fashion, you will, again, have to follow forementioned gradient. In power plants, protective circuits are in place that cause load rejections if there's a too large wedge between the desired grid frequency and the actual grid frequency. In such an event, they will simply open the circuit breaker connecting the generator to the grid. This may sounds as a quick fix to get rid of some of the excess power fed in by solar panels, for obvious reasons it is not what you want to happen.

Long story short - the excessive news coverage, indeed, is a hype. Yes, there will be a dip in the output of solar panels, and it _could_ cause blackouts, and it is even likely that it will cause some power plants to trip or interconnectors between countries to open (protecting national grids from disturbances in the greater European grid) it is highly unlikely this will cascade into a great pan-European blackout. It is, however, something that requires preparation. Which is exactly what's being done. Professionals prepare well for extraordinary events, and the media turns it into a huge bloated story.

Please do read the information provided by ENTSO-E


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