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Eclipse Will Provide Training for Grid Operators

The brief reduction in PV power production will give utilities a chance to test modeling and prepare for the future

A solar eclipse will lower output from photovoltaic installations. Although brief, the outages will allow utilities to learn more about accommodating fluctuating power levels in a future that will be more dependent on solar.
Image Credit: T. Shigesa / CC BY-SA 2.0 / Flickr

Monday’s solar eclipse will be a field day for the country’s astronomy buffs as the moon completely blocks the sun in a swath stretching from Oregon to South Carolina. For utility grid operators, the event will be much more than a good show — it will be a chance to see how well they’re prepared for power fluctuations in a solar energy future.

The eclipse starts just after 9:00 a.m. in Oregon, according to a NASA website devoted to the event, with the sun completely obscured during a roughly 2-minute window beginning at about 10:20 a.m.

Columbia, South Carolina, will see the eclipse about four hours later. Those on either side of this narrow “path of totality” will see lesser amounts of the sun covered by the moon.

Grid operators, who are responsible for making sure there’s enough online capacity to meet demand, will be wondering how well they’re prepared for this brief but in some cases significant loss of electrical output from utility solar farms and residential rooftop PV.

In California, for example, more than 5,600 megawatts of solar capacity is expected to disappear, The New York Times reports. That’s nearly one-third of the state’s total solar capacity, which provides about 10% of the state’s power.

“As the eclipse carves a long shadow over California on Monday morning, it is expected to knock offline … a big chunk of the 19,000 megawatts of solar power that currently provide one-tenth of the state’s electricity,” the New York Times reports. “The California I.S.O. [Independent System Operator] plans to fill the void by ramping up natural gas and hydroelectric power plants. Then, a few minutes later, when the eclipse passes, all those solar panels will come roaring back to life, and grid operators will have to quickly make room for the sharp rise in generation by scaling back gas and hydropower.”

In North Carolina, Duke Energy’s Randy Wheeless says output will drop from 2,500 megawatts to about 200 on Monday afternoon.

Solar energy now provides less than 1% of the country’s total electricity, so Monday’s brief outages should be manageable. But as solar steadily becomes a more important part of the country’s energy mix, grid operators will have to get adept at balancing fluctuations in output, and Monday will give them a chance to see how well they are prepared.

Making up the difference with other sources

“An eclipse is obviously not something we see every day, but this is going to be a good exercise for us,” Wheeless told The Times. “There’s no doubt more solar power is going to come onto the grid in the future, and that does increase the challenge of balancing the grid even on days when there’s not an eclipse.”

As solar output declines, grid operators turn to other sources of power to meet demand. That often means turning on gas turbines, or other “peakers,” which gets expensive. California also has agreements with neighboring states to send in small amounts of energy to balance out these losses, what’s called an “energy imbalance market.” The state also uses pumped hydro storage in which excess electricity generated during times of low demand is used to pump water uphill. It can be released later in the day to generate power in a turbine when demand is higher.

These alternate source of energy, and the systems needed to distribute it, will become more important as California and other states add even more solar capacity to their grids. Monday’s eclipse will be a chance to “dip our toes in the water and see what that looks like,” says Eric Schmitt of the California Independent System Operator.

An eclipse in Europe in 2015 gave grid operators there a chance to see what happens with a loss of solar energy, and U.S. operators watched with interest. Another solar eclipse will affect the eastern U.S. in 2024.

“We would anticipate to have a lot more solar generation at that time, so our hope is to use this as a pilot for the 2024 event, which is only seven years off,” Ken Seiler of PJM Interconnection Inc. told the website

If you plan on watching Monday’s eclipse, be sure to follow NASA’s safety recommendations so you don’t damage your eyes.


  1. Expert Member
    Dana Dorsett | | #1

    Not a real problem for CA
    The predictability of the eclipse makes it pretty easy to manage the ramp rates as the PV output falls using demand response and spinning up the reserves. The more interesting challenge (and still not a real problem for CA) is what happens on the other end. Ramping down the peakers too fast will damage them, so a combination of demand response and GRID STORAGE will be a significant part of managing that ramp, to avoid having to curtail the PV output until the peakers are safely parked.

    California has about 2600 megawatts of pumped hydro storage to help smooth this out, as well as 400 megawatts of grid battery, (not counting electric vehicles on smart chargers. There is already a PV curtailment plan in place on the trailing edge to allow the peakers to ramp down at a safe rate. The EV & smart charger fleet is still too small to be very effective in load management for the CAISO grid as a whole, but by the 2024 eclipse event rolls out there will be more battery storage than pumped hydro, as well as enough EVs to make a measurable dent too.

    Some of these details were covered in today's GTM blog:

    This is pretty small in comparison to the 2015 eclipse in Germany, which has a larger fraction of PV, but also far less responsive fossil-burners for managing the ramp, almost no grid storage, and less transmission line connectivity for power import/export, but they managed just fine. The line frequency and voltage swings were pretty wild during the ramps, dropping to nearly 40Hz (from a nominal 50Hz) early in the event, but the timing of the eclipse relative to the normal AM grid load up-ramp helped them out on the PV upswing ramp on the other end. The grid didn't collapse, and no equipment was damaged/ Even two years later they are already in better shape for managing these things in Germany, but nowhere near where as good a shape as California is for this eclipse. The world learned a lot about how to manage it in 2015, which makes this an easy training exercise for the CAISO operator.

    It'll be a lot more interesting in 2024, effecting the ERCOT grid region most greatly, but also the ISO-NE, PJM, and NY-ISO areas. See:

    The anticipated PV boom in Texas will have been in full swing by then (with no pumped hydro to speak of), and the ongoing policy push for renewables in the NY/NE region will deliver an order of magnitude more PV then than it does today. The region has some pumped hydro storage, but not nearly the same magnitude as California, but Massachusetts Energy Storage Initiative guarantees at least 600 megawatts of storage in that time frame, while many of the large regional utilities have recommended targets adding up to more than 1500Mw. Given the intensity with which New York is pursuing non-wires alternatives to grid upgrades now, it's likely that NY will have both a substantial amount of storage and an even larger portfolio of demand response assets for managing the 2024 eclipse. With p lenty of time to prepare and a lot of resources to spare it should be a cake-walk in the northeast, but it's less clear how PJM is going to make out.

    It's not clear how much storage or demand response will be in Texas in 2024, (there is currently less than 50 megawatts of grid-battery, most of it in one place) but April is a lower grid-load season, and managing the ramps with wind & solar curtailment would surely work, given the magnitude of the wind resource today (which will be even larger then.) There have already been times when wind alone was delivering about half the total power going onto the ERCOT grid.

    The MISO grid region will be also affected, but they have more pumped hydro than Texas, and even more wind power that could be curtailed for managing the ramps.

    And EVERY grid region will have more demand response to work with than there is right now.

  2. Reid Baldwin | | #2

    Why is this hard?
    Grid operators have been managing a total loss of solar generation every night since solar started. Is this characteristically different because the ramp rate is so much higher? Intuitively, this seems like it would be way easier to handle than an equipment failure that requires them to unexpectedly take a base load plant offline.

  3. Expert Member
    Dana Dorsett | | #3

    It's absoloutly all about the ramp rate @ Reid
    The rate of change of an eclipse is roughly an order of magnitude faster than what happens at sunset & dawn. This is compounded by the fact that an eclipse always happens during daylight hours, when both the load and the PV output is higher than the 24 hour average load. The "backup" capacity isn't a problem, but the ramp rate is.

    The unexpected loss of a gigawatt nuke requires sufficient having spinning reserves & controls sufficient to compensate, but grid operational planners don't plan on having multiple gigawatt nukes going down at the same time faster than fast-ramping peakers can pick up the slack. As more PV goes onto the grid an eclipse has much a bigger effect on the grid operations than a gigawatt nuke failing, but unlike the nuke it's timing is predictable and plan-able.

    But the ramp rate is still the issue, particularly on the emergence from the ramp, since thermal plants can ramp up faster than they can ramp down without damage. Having controllable loads (aka "demand response") it is a key aspect of mitigating that issue, as well as some on the beginning ramp when PV output begins to fall sharply.. A million 4.5kw water heaters that can be turned on in phases can do a LOT for this providing load to manage the overage as PV output increases rapidly, as can a a million controllable electric vehicle chargers. With the resources currently available this isn't a real problem for CAISO or other grid operators, but if they don't plan for it the 8 April 2024 eclipse could be problematic for the ERCOT grid. (But they of course WILL plan for it, and data from today's grid management in other regions will provide a good planning template.)

  4. Expert Member
    Dana Dorsett | | #4

    Looks like Nest didn't miss the marketing opportunity!

    This is just one example of how distributed automated demand response can take the rough edges off the ramp rates and quell load to shave peaks or deal with diminished PV output events.

    Looks like CAISO took about a 6 gigawatt hit in PV output during the worst minutes, but there were sufficient demand response, hydro and storage resources to manage it.

    Compare the above curve to the forecast:

    There will surely be more meta-analysis on the usual utility blogs and renewables websites on Tuesday.

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