Looking ahead to a really severe duck curve, what are the most effective solutions? Learning to make “baseload” power plants more flexible

Excerpt, Learning to make “baseload” power plants more flexible, Vox.com Aug 2018

David Roberts

When you look ahead 10 years to a really severe duck curve, what do you see as the most effective solutions?

Paul Denholm:

A lot of people in the industry use this concept of a “flexibility supply curve,” which is you deploy the most cost-effective flexibility option first.

Thinking back to the early part of the 2000s, when wind people started worrying about this, storage was nowhere near cost-effective. There was a lot of strong disagreement between the storage people and the wind people, with wind really resisting storage, I think legitimately. They said, wait a second, there are way more cost-effective measures to deal with wind curtailment and integration challenges.

So for the first eight years of my time here, I was focused on more cost-effective measures. A lot of them are pretty boring and don’t get a lot of attention. “Learn how to ramp your coal and gas plants better” — that’s a real thing, it’s really important, but it’s not sexy.

David Roberts

You mean ramp them better technically or economically speaking?

Paul Denholm

Both. You’ve got coal or gas plants that were designed as baseload, or intermediate load with a little bit of load-following capability, and the operators just weren’t used to moving these plants over a large range.

They technically could. You look at the spec sheets, what the manufacturers say they can do, and they say the plants can comfortably ramp over 50 percent of their range. But the operators were never forced to do that, so there were a lot of institutional issues, quite frankly.

Work by NREL and others has promoted the economics, saying hey, if you ramp your plant an extra 10 points per day, yeah, it’s going to impose a little bit of extra cycling cost, but the benefit of avoided curtailment vastly outweighs those costs.

If we’d had this conversation five years ago, I would have said the institutional culture needs to start changing. But that’s underway. There’s a lot less resistance now. These utilities talk to each other now, learning what they can do and what the challenges are.

coal plant

David Roberts

You’re probably aware of the heated debate about just how flexible nuclear power plants can be, whether they can be “load following.” What’s your take on that?

Paul Denholm

Look, the French have been ramping their nukes up and down for years. They do it every day. I’ve talked to people from Électricité de France multiple times and they say it’s no big deal. It’s part of the institutional culture.   But I’ve talked to some nuclear folks here in the US and they have legitimate concerns. We haven’t done [load following] here. There are safety issues, thermal stresses on the plant, and some of these plants are reaching the end of their life.

So in general, I’m gonna give nuclear plants a pass. There aren’t so many nuclear plants in this country that they’re the highest priority. The higher priority is maximizing the inherent flexibility of the fossil thermal fleet.  Covering a wider geographic area with renewables and energy markets

David Roberts

Faster ramping can’t keep up forever, though, right?

Paul Denholm

That’s right. It’s not any single solution; it really is a suite of options.  One thing we talk about is spatial diversity [i.e., spreading renewables out over a larger geographical territory] and the role of larger footprints [i.e., larger connected energy markets] — things like the energy imbalance market (EIM), where California can sell surplus energy to its neighbors.

If there’s a way you can share the sun with places that don’t have as much sun, that helps everybody out. The wind folks have been talking about that for more than a decade.

There are limits to spatial diversity. We’ve only got three or four hours of solar spatial diversity across the entire country, east to west — when it’s 4 am in Maine, it’s midnight in California — so you’re only going to get so far.  But let’s do all these things that don’t cost very much. Implementing markets is a pretty darn cheap solution.

David Roberts

To what extent are utility regulatory models a barrier to solving this problem?

Paul Denholm

[laughs ruefully] The spread of [deregulated] ISO/RTO markets has provided a lot of benefits to system operation. We’ve got more efficient economic dispatch of the system, much greater transparency.

But this is one case where vertically integrated utilities do have a slight advantage. They can look at the system as a whole — not just this individual power plant, but 12 power plants, plus their fleet of wind and solar. They can look at the total cost of all of this. There are challenges when you have a single-plant owner. 

More ways to fatten or flatten the duck: flexibility, EVs, and demand shifting

David Roberts

What about reducing inflexible baseload capacity [coal and nuclear plants] to make more room for renewables?

Paul Denholm

We talk about “fattening the duck” or “flattening the duck.” Fattening is doing all the things that let the belly of the duck grow, and reducing inflexibility is one of those things. If you’re talking about existing nuclear plants, it’s a little weird to back them off to increase solar — you’re trading one carbon-free resource for another.

David Roberts

How big of a role will demand shifting play in flattening the duck? Can we move a lot of demand over underneath the belly, to soak up some of that power?

Paul Denholm

Obviously I would love to have super-flexible demand. There needs to be planning around that, as well as socially acceptable methods to empower people to use electricity in the most economically efficient way.

I want to know, when we start exposing customers to some kind of varying price, what are we going to get? The preliminary numbers I’ve seen are disappointingly low. It will be really interesting to see how much demand we can shift.

David Roberts

What about electric vehicles (EVs)? Intuitively, it seems like having an enormous, dispersed fleet of batteries could help soak up renewable energy during times of excess.

Paul Denholm

[NREL has] done quite a bit of work on that, looking at how you send the right price signals, so people are incentivized to charge the right way.

vehicle to grid
Doing it right? Shutterstock

The general idea is, you get home, you plug in your car, and your intelligent car is sending signals. It’s saying, don’t charge right now. It’s 5, 6 pm, now is the worst time to charge. The car is going to know that most of the time, its person doesn’t need the car until 6 in the morning, so it will hold off [charging] until midnight or 1 in the morning, hopefully mostly or completely from off-peak wind.

You drive to work and plug in, it’s 8, 9 am, there isn’t quite enough solar yet, so [the car] is going to hold off again until 11 am. It’s going to know prices are low and charge until 2, 3 pm.  If you do that math, EVs help a lot. Fortunately, the way that people use their cars fits. We’re just going to need the right intelligence in the system, sending the right economic signals [i.e., charging varying rates throughout the day, more when power is expensive, less when it is cheaper].

Wind is complementary to solar, but not quite enough

David Roberts

Is there any kind of analogous animal curve caused by wind power?

Paul Denholm

Wind is just different. The thing about solar, which makes it harder and easier at the same time, is that it just has a big blast in the middle of the day. Wind is a lot more spread out, and you don’t see a characteristic pattern from one day to another.

The biggest challenge with wind is it tends to blow more at night, off peak, which does lead to this synergy between wind and solar — wind covering more of your nighttime load, PV covering more of your daytime load.

That is true to a certain extent, but I don’t want to oversell that phenomenon. We have so much wind in the spring, and so much solar in the spring, when demand is low [because of milder temperatures], so yes, they are complementary in some sense, but the seasonal nature of the wind and solar means it isn’t as awesome as a lot of people would like it to be. But I’ll take it!

David Roberts

So wind and solar are complementary, but not enough so to solve the problem or cover the gaps.

Paul Denholm

We’re not going to get to 100 percent [renewable energy] with just wind and solar, without doing something else.

Why we may have to throw away some solar power

David Roberts

The duck curve is only going to get worse, and it doesn’t seem like solutions are coming online nearly as fast as solar, which is continuously falling in price. Is something going to break?

Paul Denholm

We have to be really careful when we talk about something breaking. The only thing that’s going to break is the economics of solar. The lights are gonna stay on. All it comes down to is more curtailment.

If it was all rooftop solar, it would be a huge problem, because the ISO can’t control that. Fortunately, a large enough fraction of the solar is utility scale that the ISO ultimately has the ability to turn off. And that’s just what’s going to happen, more and more curtailment until people say enough is enough [and stop building solar] or economic solutions are deployed.

solar curtailmentKQED

That’s where I’m glad to be an optimist about the price of energy storage. I want to see all these things happen, improvement on the flexibility side, growth of the energy imbalance market, all of the semi-boring things I was talking about. But ultimately, the biggest hammer in the toolbox is energy storage.

All the projections I see say that by 2020, four-hour batteries should be competitive with peaking resources in much of California. Then you potentially have a multi-megawatt or even multi-gigawatt sink for at least some of this curtailed solar energy. That’s — fingers crossed — the solution that looks like it might come into play.

But we are going to have to get used to the world of curtailment. As we move to a renewables-rich world, large amounts of curtailment in the spring is going to become the normal operating mode.

David Roberts

Curtailment seems crazy to me. Surely any use of solar power is more economical than throwing it away. Do you see curtailment as a sign of failure?

Paul Denholm

I have a sign over my desk that says, “Don’t be afraid of curtailment.” [laughs] I’ve been dealing with this for so long.

Is it an indication that something’s not quite right? Yeah. If you expose the average consumer to the real prices of electricity — say that it’s going to be dirt-cheap at noon and $0.50/kWh at 6 pm — then yeah, you’re going to see a shift in consumer behavior.

But what fraction of the nation’s consumers of electricity are exposed to actual, wholesale, real-time prices? If you’re not a large industrial consumer, it doesn’t matter.

I’ve got a button on my dishwasher that lets me delay the start by two or four or six hours, and I have no incentive to push that button [because rates are “fixed,” unchanging throughout the day]. Here in Colorado, I know we’re curtailing wind, but I have no way to know when. One night [the utility] could be throwing away wind; another night it could be coal on the margin. I would rather do something with that free wind, but I have no idea when my local utility is throwing it away. Let me push that button!

wind power
Throwing it away.

Still, you have to think about how much curtailment there is and what it means. If we’re curtailing 2 percent of the wind or solar and 98 percent of the time we’re not, the world is fine.

We never talk about “curtailing” coal or gas plants. A mine-mouth coal plant, some of those plants used to be able to generate for $5/MWh and they’d still have to ramp because there wasn’t enough demand. Instead of having a 95 percent capacity factor, maybe they’d run with 80 or 85. Nobody was screaming about that, even though we were “curtailing” the capacity of a coal plant.

David Roberts

When you curtail the coal plant, though, you still have the fuel. You’re not wasting it.

Paul Denholm

But what fraction of the LCOE [levelized cost of energy] of a mine-mouth coal plant is the fuel? It’s a small fraction. The cost of the plant is the major component of the LCOE, so when I curtail generation from that coal plant, I’m losing the opportunity to recover the cost of investing in that really expensive asset. That’s one of the reasons I say don’t be afraid of curtailment. It can be part of the economics of the power system. It’s when you start curtailing 30 or 40 or 50 percent, that’s a problem. But these small amounts of curtailment here and there, I’m not too worried.

They are indicators that we need to do something about it, it’s gonna get worse, so let’s be intelligent, let’s get the economists to design appropriate markets and rate structures, let’s get the engineers to come up with solutions, let’s let industry come up with interesting and new ways to use electricity.

The holy grail: affordable energy storage

David Roberts

It is frequently argued that a system based on wind and solar will need an enormous amount of storage — not just hourly, but daily or even seasonal storage — and that batteries aren’t up to the task. So we’ll either have to limit the scale of renewables or find some other cheap, large-scale, long-term storage. What’s your take?

Paul Denholm

We spend a huge amount of time talking about this topic here around the lunch table — a lot of calories are spent on it. So I’ll tell you what I’d say is the informal general consensus about ultra-high-penetration renewables scenarios.

The consensus is emerging that we can probably do 80 percent [renewables] with some combination of spatial diversity and short-duration storage.

We can deal with diurnal shifts with short-duration storage, and not too much of it. When we did our Renewable Electricity Future study back in 2012, we got up to 80 percent renewables with only about 100 GW of additional storage. It’s not that much.

NREL 80% renewables scenario
The load curve in an NREL 80 percent renewables scenario.

So what’s the last 20 percent?

Some people might say, well, why isn’t 80 percent good enough? Eighty percent renewables and 20 percent gas, you’ve largely decarbonized the electricity sector, you’ve electrified the transportation fleet, everybody’s happy. But what if that’s not good enough and we need to go even further?

For 100 percent, I don’t think we actually know what the right cost-optimal solution is. The seasonal nature of wind and solar is a problem.

David Roberts

But even getting to 80 percent requires a nationwide transmission grid, right?

Paul Denholm

We did build a lot of transmission. We did do some cases where we didn’t allow as much transmission, and we just built more PV and more storage. Obviously, the social concerns about new transmission really worry me. I’d like to see more transmission.

But yeah, the seasonal nature of the problem will require something. Maybe it is [synthetic] fuels production, maybe it’s seasonal storage, I don’t know. If I had the answer worked out, I probably wouldn’t have a job anymore.

We’re not at 100 percent yet. We’re not even at 80 or 50. Let’s keep going with what we know, continue development of solar and wind and batteries, other no-regrets strategies. I think we can continue on this pathway, and I’d like to think we’ll get there.

Flattening the “duck curve” to get more renewable energy on the grid

“Hm? You want to do what to me?”

Wind and solar power are pretty great, especially the part about how they don’t create pollution that kills people and threatens the stability of advanced civilization.

But they do have their challenges.

In my previous post, I discussed one of those challenges: the “duck curve,” i.e., the effect wind and solar energy have on daily demand for utility electricity.

Here’s the key graph:

california's duck curve.CAISO

Those lines show “net load” — which is total demand for electricity minus whatever renewable energy is on the grid — over a typical spring day in California.

As you can see, as more and more solar energy comes online during the sunny daytime hours, demand for utility electricity is pushed down further and further. That’s the duck’s belly.

But just as the sun is setting and solar energy is declining, people are getting home from work and turning on all their appliances. So net load rises very rapidly (the duck’s neck) to an early-evening peak (the duck’s head).

Grid operators do not like these big valleys and peaks, and they really don’t like the steep, rapid ramping in between them. Keeping a power grid stable is hard enough already.

So the duck curve is a problem. The question is how can the duck be flattened? How can those peaks and valleys be smoothed out?

duck in flight
Flatten out, duck. Flatten out.

It’s a fun puzzle, to be honest. You have these two variables, supply and demand, that have to be balanced at every moment. To some extent both can be controlled, moved around, mixed and matched. It’s like a real-time Rubik’s cube, only the solution keeps changing every hour.

Vis–à–vis duck flattening, I’ll touch on two big, visionary ideas and then 10 smaller, more immediate, more practical measures that use existing technologies.

The main point to make is that we have a decent (if somewhat hazy) understanding of the long-term solutions to the duck curve, the kind of stuff we’ll be dealing with in 2050 when wind and solar are getting toward 60, 70, 80 percent of grid energy.

But we have a very clear understanding of the sort of immediate steps we could take to accommodate more wind and solar than we have today. There’s no reason the duck curve should delay the growth of renewables.

The two big, long-term solutions to duck curvature

The first big strategy to flatten the duck is interconnection. The more grids can be connected with one another to form larger grids over larger areas, the more spread out the potential for wind and solar will be and the more spread out load will be, both of which will serve to smooth out the peaks and valleys in the curve.

Theoretically, you can push the problem back for a long time with this strategy, hooking up more and more grids. Maybe someday we’ll get to the much-discussed global grid.

Transmission today, transmission tomorrow, transmission forever.

But that’s a long way off, and in practice, though the technology of high-voltage transmission is well-understood, the politics of it — how to get funding and rights of way, how to deal with NIMBYs — is not.

The second big strategy is energy storage. If you can store some of that wind and solar energy rather than automatically sending it to the grid, you make it “dispatchable,” meaning you can time it. It becomes a movable piece of the puzzle.

A sufficient amount of energy storage would, almost by definition, flatten the duck and remove any limits on the integration of wind and solar. But at least at current prices, that would be prohibitively expensive.

Advocates of these two strategies are always arguing with one another about which is the ultimate long-term solution.

Researchers from NOAA recently published a studyin Nature Climate Change showing that by a) siting renewables where they have the highest potential, and b) building high-voltage transmission lines to those areas, the US could cut greenhouse gas emissions 78 percent from 1990 levels in 15 years — with no new energy storage.

Wind power potential across the US.
Wind power potential across the US in 2012.

By linking the entire US into a single high-voltage grid, you can keep the duck flat enough to accommodate, to be technical about it, a metric shit-ton of new renewables. (As a bonus: According to NOAA, it can be done at comparable costs to a fossil-fueled grid.) Thus, NOAA researchers say, there’s no need to wait for storage to scale up and get cheaper.

And that’s true. But, as storage fans respond, getting high-voltage transmission lines built is extremely difficult and time-consuming. Meanwhile, storage is rapidly getting cheaper and seeping into the grid to fill the cracks.

It’s kind of a pointless argument: They’re both right. Both new transmission and lots of storage will eventually be necessary to get the grid to zero carbon. (Transmission will help wind more; storage will help solar PV more; see this study.)

But it’s important to remember that we don’t have to wait on new transmission lines or a scale-up of energy storage. There’s a great deal that can be done immediately, with existing tools and techniques.

10 steps to flatten the duck quickly

flying duck(RAP)

To guide us through this somewhat nerdy territory, we have “Teaching the ‘Duck’ to Fly,” by utility guru Jim Lazar of the always excellent Regulatory Assistance Project.

(The paper was originally released in 2014; since then, utilities have tried out many of the ideas, experts have offered feedback, and the new and improved second edition was just released.)

Lazar starts with a hypothetical California-like grid with a duck problem:

duck curve(RAP)

The blue line is total electricity demand. The green line shows how wind and solar duck it up.

Lazar then offers 10 practical ideas to start flattening the duck. I’ll walk through them quickly. (Their relative contributions, and much supporting detail, can be found in the report.)

1) Target energy efficiency to the hours when load ramps up sharply 

If load ramps up quickly during a certain time of day (mainly between 4 and 7 pm), then target the energy uses that cluster in those hours.

Lazar focuses on “residential lighting, air conditioning, and office building lighting controls.” Technologies to substantially cut energy use in these applications is widely available and affordable, and would cut demand when it’s most needed.

2) Acquire and deploy peak-oriented renewable resources

Policies that require utilities to purchase renewable energy generally do not say anything about the timing of when that energy is produced. But timing turns out to be important (a regular theme in the duck-flattening literature).

If renewable energy production needs to be pushed back a few hours later in the day, there are particular resources that can help. Most big hydropower facilities have some storage (“pondage”) that allows them to alter the timing of their power production; they could be induced to push it later.

hoover dam
Saving some power for later.

Some wind sites produce energy more regularly in the evening hours; they could be favored in procurement. Solar panels could be turned to face west rather than south, which somewhat decreases their total production but, importantly, keeps them producing for up to two hours after south-facing panels stop.

And concentrated solar plants (CSP), which use the sun to heat fluid that turns a turbine, can store some of that heated fluid and delay some of their production.

In short, renewable energy can be pushed back into evening, when it will be more help with the peak (er, duck’s head).

3) Manage water and wastewater pumping loads

This one sounds boring, but it turns out pumping water consumes 7 percent of total electricity in the US (and much more in California, where lots of water gets moved over long distances).

Current electricity rate structures encourage water and wastewater utilities to use small pumps that operate continuously, at a low level, throughout the day. Tweaks in the rate structure (I’ll spare you the details) could instead encourage them to use large pumps that operate only in off-peak hours, shutting off during high ramp or peak times.

4) Control electric water heaters to reduce peak demand and increase load at strategic hours

Hot water is most intensively used in the morning and evening. But there’s no reason water heaters should heat the water during those hours. Heated water can be stored a long time.

hot water heater
The hot water heater: one of clean energy’s more humble champions.

Theoretically, the 45 million electric water heaters in the US could be connected to the grid and controlled; water heating could be shifted to those times when excess wind or solar energy needs to be absorbed.

Not only could load be shifted, but those same water heaters could effectively be treated as batteries that provide grid frequency and voltage control.

This would require simple load-control technology on the heaters and some new institutional arrangements with customers, but it’s entirely doable. Lots of utilities are already doing it.

5) Convert commercial air conditioning to ice storage or chilled-water storage

Air conditioners are a huge part of peak load for most utilities. And a big part of that is commercial air conditioners, those giant units you see on malls and university buildings.

But just as water heaters don’t need to heat water during the hours it’s being used, commercial air conditioners don’t need to make cold during the hours people use it. They can make it beforehand, when power is cheap, in the form of ice (or chilled water). Then when demand is peaking and power is expensive, they can use the stored ice to cool buildings (as a bonus: much more quietly).

In this case, ice is the battery that allows an air conditioner to shift its time of peak power consumption.

Ice-storage air conditioners are a widely available and affordable technology.

6) Rate design: Focus utility prices on the “ramping hours” to enable price-induced changes in load

This one’s pretty straightforward. If you want people to shift their energy use from certain hours to certain other hours, reflect that in the price of energy. This is known as “time of use” pricing, whereby the price of electricity varies throughout the day.

Studies have shown that customers voluntarily shift their load in response to price signals. And eventually, much of that process can be automated by “smart” appliances and home energy control software.

7) Deploy electrical energy storage in targeted locations

Unlike thermal storage (hot water, ice) or mechanical storage (pumped water), electrical storage is still pretty expensive, though the cost is falling rapidly.

However, there are certain targeted places on the grid where compressed airpumped hydro, or battery storage make sense. These need to be places where storage can produce multiple value streams:

  • Defer investments in new grid infrastructure
  • Time-shift power use, storing power when it’s cheap and releasing it when it’s expensive
  • Provide spinning reserves and frequency or voltage control to the grid
A pumped-hydro energy facility just outside Los Angeles.
A pumped-hydro energy facility just outside Los Angeles.

8) Implement aggressive demand response programs

I explained demand response in this post. It basically amounts to finding lots of little ways that customers can shift their demand, aggregating all those customers together, and treating the sum of their demand-shifting capacity as dispatchable power — “negawatts” instead of megawatts.

9) Use inter-regional power exchanges to take advantage of diversity in loads and resources

This is Lazar’s nod to interconnection, only it’s not about building new power lines; it’s about using existing interconnections more strategically.

There are places in the US where separately managed grids already have links with one another. Those interconnections can be better used to trade power. Different regions have different times of production and different peak loads; they can help smooth out each other’s peaks and valleys.

10) Retire inflexible generating plants with high off-peak must-run requirements

There are some big, older power plants (coal, nuclear, and natural gas) that are extremely inflexible. They cannot ramp up and down quickly. And it’s very expensive to shut them completely off and restart them, so they must always be kept running at a minimum level. When renewable energy starts expanding too much, it bumps into these limits and is curtailed.

coal plant
A dinosaur in twilight.

What a modern grid needs above all is flexibility. Over time, these inflexible plants should be retired and replaced with flexible resources — newer natural gas, storage, and (some) renewables. The more flexible the overall grid is, the more wind and solar can be integrated.

So that’s the 10 steps!

(It’s worth pausing to acknowledge, as Lazar does, that every region and utility is different and each will need its own customized set of solutions.)

Supply and demand curves are not fated — they can be reshaped

Here’s what all these measures put together do for Lazar’s hypothetic grid:

duck curve, flattened
A flat duck.

That dotted line is the old net load curve — the duck. The thick green line is the new and improved net load curve, after all Lazar’s strategies have been applied.

As you can see, the belly no longer hangs so low and the head no longer pokes up nearly so high. There are no more steep ramps. The duck has been flattened. It’s flying!

And it’s been accomplished using existing technologies and demonstrated policies. No breakthroughs or grand schemes required.

It’s just a matter of breaking supply and demand down into their constituent parts, assessing which parts can be controlled or shifted (which is more and more of them), and rearranging them so that they balance throughout the day.

It’s not rocket science. It’s, uh, duck science. And it can enable any utility that’s ready and willing to accommodate a lot more clean, renewable energy.

Further reading:

If you can’t get enough of this ducking stuff, here are some ways to dive deeper:

These are all about the duck curve specifically. On the somewhat broader topic of integrating more wind and solar into the grid, a reading list would run several (kabillion) pages. Just start with this post and this post, which contain links to lots of other resources.