What? More Solar Means More NOx? No One Saw that Coming!

Duke Energy solar farm

by James A. Bacon

The surge in solar power production in North Carolina has caused an increase in nitrogen oxide (NOx), a serious air pollutant, North Carolina’s Duke Energy has concluded. Without changes to state regulatory policy, according to a report by North State Journal, carbon dioxide (CO2) emissions also could increase.

These counter-intuitive findings stem from the fact that solar power is an intermittent source of power, which must be offset by on-again, off-again generation from fossil fuel sources, primarily natural gas. The on-and-off cycling of power stations leads to inefficient combustion and higher NOx emissions. The effect on CO2 emissions is less clear, although utility officials raised the prospect of a “slight increase” in CO2 at the plant level under certain conditions.

I have no idea if Duke’s conclusions will stand up to close scrutiny. For sure, they will be attacked by those who are committed to intermittent renewable energy sources at any cost. But the debate in North Carolina is highly relevant to Virginia. North Carolina has the largest installed base of solar power of any state outside of California. But Virginia is adding solar capacity rapidly, and the Northam administration has set a goal of attaining a zero-carbon electric grid by 2050.

Let me be very clear. I am not advocating a dial-back in Virginia’s commitment to solar. But I do say, if we are going to aggressively expand our reliance upon an intermittent energy source, we need to know what we’re getting into.

Here’s how the North State Journal explains the increase in NOx:

Traditional power plants — including cleaner burning natural gas plants — must scale back electric generation to accommodate solar energy surging onto the system when the sun rises, and power back up when the sun sets and solar energy dissipates. That starting and stopping reduces efficiency and incapacitates emission control devices, increasing pollutant levels.

On other days solar energy is erratic and can result in more frequent cycling of reserve sources, further decreasing power plant efficiency. This increased cycling can result in increased emissions and undue wear and tear on the expensive equipment. …

Under its current permits in the heavily regulated market, Duke must completely shut down the backup combustion turbines when solar peaks under a full sun, then restart them when the sun recedes.

Duke wants DEQ to issue new permits allowing combustion turbines to throttle up and down from a “low load” idling operation instead of switching completely off and on as solar waxes and wanes. In its permit applications Duke said that would lower pollutant emissions and reduce stress on equipment.

In a series of e-mail exchanges for this article, [Duke spokesperson] Crawford provided information from a team of Duke subject matter experts confirming NOx emissions would be lower if there were no solar power on the electric grid.

Without any solar power in the mix, “a typical combined cycle combustion turbine emits NOx at approximately 9-11 lb/hr, assuming 24 hours of ‘normal’ operation,” Crawford said. That is equivalent to 264 pounds of NOx emissions daily. When those same plants are operated to supplement solar power facilities, daily emissions more than double to 624 pounds a day, based on a table in Duke’s application.

If DEQ agrees to Duke’s alternative operating scenario, a combustion turbine would emit 381 pounds of NOx daily — still 44% more pollution than operating without any solar power on the grid.

Duke is working under a different regulatory regime in North Carolina than Dominion Energy and Appalachian Power Co. here in Virginia, so it’s not clear if the same regulatory remedies would apply. But the phenomenon of on-again, off-again generation of fossil fuel plants is common to both states, indeed to every electric grid anywhere.

While the evidence for higher NOx levels seems solid, the conclusion that increasing solar might increase CO2 levels, which are said to be a driving force behind global warming, is more tenuous.

“We expect a slight increase in CO2 emissions at the plant level from turndown versus shutting down and restarting,” Crawford said.

In general, she said, increasing solar generation tends to decrease CO2 emissions if nuclear generation and other factors remain constant.

“As the amount of solar generation increases, however, this effect will diminish and could reverse at some point due to decreasing room for more efficient generation,” said Crawford. More lower efficiency generators, designed for short cycles, could ultimately be used to provide contingency power for intermittent solar, according to Crawford.

Clean energy is a beautiful idea. But only in the last few years has any state or country generated enough electricity from intermittent energy sources for us to learn that integrating intermittent energy sources in a grid that supplies electricity when people want it — not when the sun shines and wind blows — is more complex and requires more trade-offs than originally thought. That’s not an argument against solar and wind power. But it is an argument to proceed with caution and learn from the experience of others.

The instinct of some will be to denounce Duke and dismiss its data. A wiser approach would be to learn from Duke’s experience and design a grid for Virginia that avoids North Carolina’s flaws.

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17 responses to “What? More Solar Means More NOx? No One Saw that Coming!”

  1. Steve Haner Avatar
    Steve Haner

    It will be very hard to set up a grid to avoid the issue, as the sun goes down daily everywhere pretty much as scheduled, and then even when up gets blocked by clouds on and off. Just more proof that there are always trade offs, always downsides along with benefits. No NOx from nukes, eh? Clean energy is a slogan, cleaner energy something which can be achieved.

  2. vaconsumeradvocate Avatar

    Storage options are improving every day. It’s truly amazing what the researchers and industry have on the market and coming to the market soon. We need to think about pairing storage with solar and wind, not just assume we’ll turn to traditional resources. Folks are working hard on this. We do not need to assume so much NOx.

    1. Agreed — energy storage is the key. Economical energy storage will be a game-changer. By all means, let us try to accelerate the development of new energy-storage technologies. But we can’t build an electric grid around the hope that such technologies will come to fruition in a timely manner. When such technologies are proven to be viable, we can begin integrating them into the grid at that point.

  3. Philip Shucet Avatar
    Philip Shucet

    Interesting. Good reporting, Jim.

    1. I’d like to claim credit for reporting, but I am merely a humble scribe passing along the reporting of others (and then commenting upon it).

  4. Steve Haner Avatar
    Steve Haner

    There will remain a need for baseload generation, and a substantial amount at that. We can all remember periods of winter storms, spring rainy seasons that lasted days and days. But engineers may also be able to reduce that NOx spike that happens when a gas plant cycles, if the motivation is high enough.

  5. LarrytheG Avatar

    Re: “humble scribe” – calling all gullible minions!

    This is a valid issue and Steve is right – there are always trade offs.

    There is no silver bullet – not even for “green”

    Keep in mind – every day right now day – there are peak hours when electricity demand increases rapidly over base load demand and it’s accommodated with forecasts and the availability from PJM of additional “spinning reserve” (yes thats a phrase) power without firing up the dirtier peaker plants ) however the newer gas plants can run in both modes if needed.

    so… just like we “know” that when folks get up in the morning and come home from work in the evening that demand will increase – we also KNOW when night comes and for that matter even when the forecast is for clouds and rain.

    Another thing to keep in mind – solar is not all on and all off… it varies according to how much usable daylight there is so it does not go from 100% to 0% at night fall…it actually works a lot like people getting up in the morning and demand steadily increasing.

  6. Good report. But this is not news at all to utility folks or grid operators. Duke acts like they have discovered something surprising here, but the NOx emissions are a well known function of any steam boiler/turbine which is cycled on and off, as it does not run as efficiently or as hot when first turned on, and it’s that lower heat which causes the incomplete combustion and therefore the elevated NOx emissions.

    Mechanical engineers know there are gas-fired cycling units which minimize this by design. But you have to remember, most of these newer high-efficiency gas units were built to run 24/7, not to be cycled, because, fifteen or so years ago, many utilities did not foresee the rise of solar, or simply based their cost estimates on the past regardless of what they foresaw. There is a tradeoff here between cycling efficiency and maximum 24/7 baseload efficiency; if you designed the unit for the latter, it might not be as good at the former. So I point the finger at Duke for poor planning that caused them this problem.

    In addition, remember that Duke is not in PJM and not part of the PJM planning process. To my knowledge Dominion has anticipated this problem correctly and built the appropriate (for PJM) gas units for maximum cycling efficiency.

    That said, there is no perfect package. Anytime you cycle a fossil-fueled generating unit, there is some NOx emitted during startup. That’s simply part of the tradeoff for using the solar power when available. The solar power, of course, has zero emissions of any kind and is the cheapest (near-zero marginal cost) when it’s available, so solar+cycled gas generation is cheaper and better for the environment overall than non-cycled gas generation alone.

  7. Re: “design a grid for Virginia that avoids North Carolina’s flaws.” This problem is not solved by the design of the grid but by the design of the individual generating units built by their owners. PJM runs the grid markets, and as of now generating units are paid their startup costs and for their energy, but there is no reduction in the positive payment at present tied to NOx or CO2 emissions. There is a lot of effort at the FERC right now going into fixing new rules for the appropriate way for all the ISOs (PJM is our ISO) to price low carbon emissions and low NOx emissions in their wholesale energy markets in order to appropriately reward zero emissions plants like nuclear (which are close to uneconomical to operate these days without some sort of additional payment stream beyond what they make off capacity and energy sales). Those rules hopefully will also address NOx caused by this natural gas cycling problem.

    Let me add, there is a potential major problem at the FERC which could halt action on this rulemaking, caused by the Trump administration’s refusal to nominate two new Commissioners for Senate approval for the current vacancies on that Commission. The administration has offered to appoint one Republican; but by statute the current vacancies are one each for each major party and have been filled in pairs for years (except for one seat on the Commission which switches parties at the next vacancy if the presidency switches). If another Commissioner retires, the FERC will not have a quorum.

  8. LarrytheG Avatar

    Another thing folks may not realize – coal and nukes cannot be ramped up or down quickly. It can take hours for a coal plant to come up or go down and days/weeks for a nuke and for that reason, they are incompatible with solar/wind which can various much quicker than coal/nukes can ramp up or down.

    Coal and Nukes ran 100% 24/7 – they just took the turbines offline when demand dropped below generation output but the smokestacks still spewed out pollution.

    Gas was uber expensive and not used except when absolutely necessary usually when demand exceeded coal/nukes capacity.

    And that’s where Smart meters came from – the idea being to charge customers more when gas was used – but the meter would warn them and let them reduce use – which in turn reduced the need to burn gas.

    Nowadays, Smart meters are not really relevant. The opposite has happened – there will be times when solar/wind are abundant and cheaper and combined-cycle can pick up the slack. Seriously, does anyone know why Smart Meters would be useful these days?

    Like a lot of things now days – energy and electricity are complicated and there are no shortage of folks who just plain misrepresent basic facts… because they know some folks will believe it.

  9. Reed Fawell 3rd Avatar
    Reed Fawell 3rd

    Actually, I did see this problem coming a year or more ago and reported it here. Indeed I believe this was the experience in Germany as reported a couple years back, even without that country having to bring on line new coal plants to buttress its grid instability. Why. Because their long existing their fossil fueled units had to work much harder and more inefficiently then before by reason of solar’s introduction, and its wild and unreliable swings back and forth. Hence this problem was known by the industry, who largely refused to admit to it for the usual reasons.

  10. LarrytheG Avatar

    One of the ways to understand the issue is how you’d use solar on an island that has no gas and almost none do – it’s not native and it’s way too expensive to bring in with ships.

    The fuel of choice on islands is NOT Nukes and NOT Coal – it’s Diesel.

    Electricity from Diesel costs 20-30 cents per KWH. The advantage of diesel over the other fuels is that it IS cheaper!

    But integrating it with solar is apparently not as easy thing to do because even though there are thousands of inhabited islands only a small handful have both diesel and solar and NONE are 100% solar 24/7… they have power when solar is up and not when it is down.

  11. HB Atkinson Avatar
    HB Atkinson

    Big picture here, fellas, is our environment. And you guys are not looking at the big picture. No one has mentioned the cost of these panels; you are touching on the economic cost but the environmental cost has yet to be addressed. It is argued by some that it takes more energy to make a photovoltaic panel than it will collect in its lifetime. The info I can find is skewed and muddy so I don’t know which side of that fence to come down on. But no matter which side I come down on, I’m still mighty close to the fence. In other words, at best our photovoltaics might, just maybe, come out 20% to the good in the balance of actually “saving our planet.” (Energy used vs energy saved.) Do the research yourself (and I can only hope you can find better info that I can find) study how much heat it takes to melt down the sand, process it, then remelt it again. Study the energy used to dig out the materials, ship them, process them, and after many energy consuming steps, finally get them on the ground here in America. I’m guessing that, if you study deeply enough, you will find the coal trains rolling through our fine little city of Lynchburg are hauling coal to go on another empty ship headed back to China to be used to make solar panels to ship right back here.

    1. You raise a very good point. It is critical how one draws the bounds of the analysis. For example, environmentalists say that it is not sufficient to consider the CO2 output of natural gas generating plants (which is half that of coal) but must consider the entire supply chain that includes methane leakage from drilling and transportation of natural gas. Fair enough. What they don’t do the same thing for solar. They don’t consider the entire supply chain — as you observe — of creating solar panels, which includes not only the energy-intensive production of glass but the energy-intensive refining of rare earth metals used in solar panels.

      Sometimes I wonder if environmentalists have made up their minds that they want solar energy and are prepared to configure their arguments in any way needed to justify that conclusion. Whether solar actually reduces CO2 emissions when viewed in a systems perspective may not be something they wish to think about. … Or so it seems. But I will confess that I have not reviewed the literature on this subject, so my observation is tentative, and I am willing to stand corrected.

  12. HB Atkinson Avatar
    HB Atkinson

    Thanks Mr. Bacon. I have a recurring thought when I look at the perceived wackiness of the environmental movement and how strong it has become:

    Everybody’s got to worship something.

  13. Missed this article.
    Jim- Natural Gas elec power should be quite close to zero(0) emissions except CO2. NOx should be almost solely due to coal and diesels.

  14. Reed Fawell 3rd Avatar
    Reed Fawell 3rd

    Lord Jim says: “Sometimes I wonder if environmentalists have made up their minds that they want solar energy and are prepared to configure their arguments in any way needed to justify that conclusion.”

    Today’s Wall Street Journal, in a fine opinion piece, illustrates the how certain groups of environmentalists and their crony capitalists rent seeking allies who now fund those environmentalist groups will stop at nothing in pursuit of their sacred ideology. Here again, like with sneakers and basketball, lost sons of growing numbers of powerful politicians, and much increasingly unreliable university research, the key to understanding and revelation, is to follow the money trail. Here are the opening paragraphs of WSJ piece.

    “America has a severe plumbing problem. When it comes to crude oil and natural gas, the country can’t seem to replace its aging pipes or put in new lines to meet market demand on a timely and cost-efficient basis. For all the current administration’s vocal support of the industry, the situation has worsened under President Trump.

    Over the past decade, as the U.S. shale revolution has opened up new sources of domestic oil and gas production, midstream takeaway capacity has struggled to keep pace. Building new interstate lines has become a costly, open-ended process, as climate-change politics have overwhelmed what used to be a straightforward permitting exercise. American consumers will pay the price in fuel shortages and higher costs, and the environment won’t benefit.

    Since Mr. Trump cleared the way for the completion of the beleaguered Dakota Access Pipeline after taking office in January 2017, an opposition front of environmental lobbyists and like-minded Democrats has stymied every long-haul pipeline project across the country, regardless of sponsorship or operator track record. The list of currently stalled pipelines includes three major oil systems in the Midwest—headlined by Keystone XL, which has been stuck in regulatory purgatory since 2008—and six natural-gas transmission lines across the Eastern U.S., mainly in New York and Virginia. These nine projects total roughly $27 billion of U.S. infrastructure investment.

    Pipelines are the safest and most environmentally friendly means of transporting oil and gas around the country, especially compared with alternatives such as rail or trucking. The 2.8-million-mile system of oil, product and gas pipelines currently blanketing the lower 48 states has a safe delivery rate of greater than 99.999%, based on Transportation Department data.

    But in the running battle against fossil fuels, the environmental warriors now want to cut supply lines and thereby strand assets in the ground. Climate-change animus is overriding objective science.

    Led by environmental activists, Native American tribes and, increasingly, blue-state governments, the protest complex now decries the unique dangers of piping “fracked gas” and “oil tar sands” around the country. Twinning an existing pipe allegedly poses a new and far greater threat to the ecological order. Somehow, pipelines buried more than 100 feet below riverbeds or encased in concrete tunnels still retain the power to pollute. …” End Quote.

    For more on this fine and informative editorial opinion written in today’s Wall Street Journal by Paul H. Tice, please see:


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