The Magic Math of Solar Plus Storage

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By Joseph Daniel, Senior Energy Analyst

I’ve always been a math person. I find solace and comfort in its consistency. Not like grammar with all its exceptions to the rules. (What do you mean “y” is SOMETIMES a vowel?!)

So, imagine how surprised I was one day to find out that 1 plus 1 doesn’t always equal 2.

Well, that’s the magic math of solar plus storage.

A bit of background

Before we get to the magic math, there is a very important concept that you have to understand first: Effective load-carrying capability, or ELCC. My colleague Mark Specht wrote a fantastic blog on the subject that you can read here.

ELCC is a metric used by grid planners to evaluate a resource’s ability to meet demand when outages are most likely to occur. Most frequently these moments are at the time of net peak demand. Meeting net peak demand is an important part of keeping the lights on so when grid planners do long-term resource planning, they use metrics such as ELCC to make sure they will have enough resources to meet net peak demand five, 10, or even 20 years in the future.

For solar, the ELCC might start around 50 percent — that is, 1 megawatt (MW) of installed solar will contribute a half MW of power during net peak demand.

The thing is, the more solar you add to the grid, the less the next MW of solar contributes to net peak demand. This declining marginal value effect is well-known and well-understood by grid planners. So, when they do their planning, they take this into account.

Wind, solar, storage — each resource is given an ELCC value that the computer models use to make sure the grid can meet future demand.

The whole is greater than the sum of its parts

The old phrase “the whole is greater than the sum of its parts” is the perfect description for the magic math for storage plus solar.

The effective load-carrying capability of solar with storage is actually higher than the ELCC of solar plus the ELCC of storage.

The example below comes from a New Mexico proceeding where the local monopoly utility, Public Service Company of New Mexico (PNM), is already working to replace a coal plant with solar and battery storage. Now the company is asking for approval to do the same for its share of a nuclear plant.

During the PNM proceeding, an expert witness testified that if a service territory with approximately 8 gigawatts (GW) in peak load added 5 GW of solar, its net peak load would only see a 0.6 GW reduction.

Meanwhile, 2.5 GW of storage, on its own, only contributes 1.6 GW of peak load reduction.

But when you pair solar and storage, the combined effectiveness in load reduction is 2.8 GW.

So, 0.6 GW plus 1.6 GW equals 2.8 GW! That’s an extra 0.6 GW!

Figure NS-3 from Nicolai Schlag’s testimony before the New Mexico Regulation Commission, Case No. 21-04-02-UT, p. 11

Not so magic after all

It turns out that the magic math of solar plus storage is a bit like the magic of pulling a rabbit out of a hat. There’s no magic. It’s just, and … spoiler alert …

The rabbit is in the hat the whole time.

It’s not really magic.

And the additive benefits of solar plus storage are also not magic, it’s the value of resource diversity.

For resources such as solar and wind, the ELCC is dependent on other variables, like the hour of the day when net peak demand occurs. But it also depends on the mix of other resources on the grid. As you add more and more solar on the grid, the net peak demand gets shifted over but it also gets shorter, which means that it is easier for storage to do its thing and help meet net peak demand.

Solar plus storage are complementary resources.

Solar plus wind are also complementary resources. So is wind plus storage. And it doesn’t end with pairs. Portland General, for example, is working on a wind plus solar plus storage project.

Ignoring those benefits makes a portfolio of renewables appear to have less value than they really do, and utilities might choose to build more renewables if they accounted for diversity benefits.

Need for better modeling

The ability for utility resource models to account for these dynamics in the system is not the limiting factor.

What is the limiting factor?

A model operator’s willingness to make the changes. Utility companies are the ones running these models in integrated resource plans across the country and sometimes they make bold claims, such as “the model can’t do that.”

First off, if your model can’t account for the variables that are essential to planning for the gird of the future, then get a new model.

And second, and more often the case, the model can account for these changes if it has the right inputs. Sometimes that requires extra work that utilities aren’t willing to do.

Although, some are. Take Xcel Energy. Xcel hired a consultant, E3, to do a separate analysis to account for the dynamics of the ELCC of wind, solar, and storage on a grid with increasing levels of wind, solar, and storage. And while that “exogenous” analysis wasn’t perfect, it did allow the utility company to provide inputs into the model so that its modeling could account for these grid dynamics.

Another option is to model solar plus storage as a single “hybrid resource.” That’s what UCS, and many other groups, have recommended in a number of proceedings. There are pros and cons to this approach, but certainly, one pro is that it is a simple and straightforward option.

And to any utility planners reading my blog, I’d be open to other ideas. You got other ideas, I’m open to alternatives. Just don’t tell me that your model can’t do it because you are too lazy to do it yourself.

Editor’s note: An earlier version of this post incorrectly suggested that the PNM peak demand is 8GW.

Originally published by Union of Concerned Scientists, The Equation.

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