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.
