As the total U.S. solar eclipse approaches and the amount of renewable energy in the country continues to rise, many people have concerns about grid reliability.
We reached out to Troy Miller, S&C Electric’s director of grid solutions, to find out more about how battery storage can solve intermittency issues, and how contractors can prepare for the solar eclipse in August.
SPW: Can you define grid intermittency?
Miller: Traditional generation is either coal-fired or natural gas-fired. It essentially is a big, spinning generator. Once it gets going, it’s hard to slow down, so it provides a very level output until it’s turned off.
When you’re talking about renewable resources, if you’ve got a bright, sunny day with direct sun overhead and a 100-MW PV plant, most likely you’re going to get close to the full output of that PV plant, which could be 85 or 90 MW. However, if clouds come over at any point, either large or small, you’re going to have darkness shade portions of your PV farm. You will have a very rapid drop-off of the production of solar from previously 85 or 90 MW down to 40 or 30 or 20 MW, and then it’ll bump back up to 80 very quickly as the cloud cover goes away.
That can wreak havoc on the voltage output of the overall system as those large clouds go overhead and you have multiple tens of megawatts that drop offline. You can have problems with voltage variations and in some cases, frequency variations.
SPW: How could a microgrid be helpful for intermittency?
Miller: The way S&C defines a microgrid is alternate sources of generation, typically renewables, and then switching and protection mechanisms to allow for islanding so you can actually disconnect from the grid. It includes a microgrid controller to control the assets and then bring you back on to the grid when it comes back. Then typically you need energy storage for a microgrid to provide a voltage source for the overall system. It becomes the grid for the system.
When you’re talking about how you can solve these intermittency issues, typically they’re not solved by a microgrid, they’re solved by putting in energy storage along with your solar installation. Energy storage is nothing more than large batteries connected at the same point of interconnect as the solar plant.
The batteries essentially can discharge during those periods of intermittency. When the generation goes from a high level to a low level, the battery can fill in those gaps. You charge the battery when it’s really high, when you’ve got a lot of sunshine.
SPW: What’s going to happen to PV during the August 21, 2017, eclipse?
Miller: It’ll have the same effect as clouds, meaning solar production is going to drop off. So there are a number of things that you can do to take care of that.
One thing that’s different about an eclipse compared to clouds is it’s predictable. You can go on a website right now and see exactly where the solar eclipse is going to happen and where it’s going to cause total darkness.
Certain things you can do to help with the intermittency is, if the plant has energy storage, you can provide a more predictable output, similar to a traditional form of generation. So rather than being 80 MW one minute and 42 MW the next, if you’ve got an energy storage system there, you can actually level out at a predictable generation output, which is much better for the voltage control of your grid.
You may be able to plan for this break in generation by ramping up alternate forms of generation to fill in. So you could theoretically curtail your PV and fill it in with alternate forms of generation including traditional peaking power plants or storage.
SPW: If our country were to rely more heavily on solar, would we have to add more battery storage?
Miller: The one thing about our grid is that it’s kind of the most complex machine in the world. At any time, you have to match the available generation with the load out there. If you have 3 GW of load, you need to immediately match that with 3 GW of generation—the way that it’s currently structured. And if you don’t, you rely on something that’s called inertia in the system to slow those ramp rates. The inertia can be provided by big spinning generation machines.
As you slowly retire those traditional assets and replace them with more renewable types of generation, you will need energy storage to act as a buffer and an equalizer, a levelizer for that generation.
And so, yes, as we rely more and more on intermittent resources, we will need to put in the subsequent amounts of energy storage. It’s not a 1:1 ratio, it’s more like 20 to 30% storage to solar to be able to handle that intermittency.
SPW: What should contractors keep in mind regarding intermittency when they’re selling and installing systems?
Miller: You can really increase the overall value of your system if you’re adding energy storage, depending where you are in the country.
It allows you to be able to island, and it also allows you to take advantage of more of the production of the day, as well as do alternate things like bill reduction and participation in things that are called ancillary services markets. Certain parts of the country pay for you to provide correction to frequency deviations—this is called frequency response.
Adding storage to a solar plant allows you to get a lot of different cost avoidances along with potential new revenue streams.