Less solar, more coal: How Duke kept power on in a heat wave

In mid-June, much of the United States was baking in a heat wave. At 3:50 a.m. on the morning of June 24, in response to an emergency request from Duke Energy Carolinas, the US Department of Energy (DOE) issued an emergency order authorizing Duke to exceed environmental emissions levels if needed to dispatch enough electricity generation for maintaining power provision during the extreme temperatures.

The order by Secretary of Energy Chris Wright acknowledged the unusual combination of heat and humidity in Duke’s service area from June 23 to 25, “with heat indices in the range of 100˚F to 110˚F,” that would lead to “unusually high” electricity loads. The order helped Duke maintain “maximum reliability on its system” and “identify and dispatch generation necessary to meet load requirements” to avoid “curtailments or outages, presenting a risk to public health and safety.” The order authorized Duke to obtain power from additional generation units “regardless of emissions or other permit limitations” through 10 p.m. on June 25. Duke’s request had anticipated possible “exceedance of emissions of nitrogen oxide and particulate matter.”

The need for such an order reiterated a lesson learned the hard way recently by Spain. Grid reliability requires power plants that can answer the call of mankind, not fall victim to the whims of nature.

Nothing tests the reliability of the grid more than extreme weather, which is also when people need access to electricity the most: heating during bitter cold and cooling during heat waves. An unreliable grid is not only inconvenient — it’s deadly. And an arbitrary emissions number is less important than people’s needs, as the DOE recognized.

The following table and graph show how Duke’s power generation from different sources changed from the day before the emergency order (June 23) to the last day of the order.

DUKE ENERGY CAROLINAS DAILY ELECTRICITY GENERATION BEFORE AND DURING THE JUNE 24–25 HEAT WAVE, BY SOURCE (MWH)

The unwavering performance of nuclear is the most striking takeaway from the chart. Nuclear provided 174,898 megawatt-hours (MWh) on June 23 and 174,581 MWh on June 25, almost identical amounts each day and by far the most of any generating source. Under the emergency order, Duke required nearly 20% less solar and about 12% less hydropower than when the utility had to be careful to stay below emissions limitations. At the same time, Duke was able to utilize over 6% more natural gas and almost 30% more coal power.

Crucially, no outages or blackouts were reported.

Duke was able to keep the power on by shifting away from solar generation (20% less) and relying more heavily on natural gas (6% more) and especially coal (30% more). Duke required about 27,000 more MWh of electricity on June 25 than on June 23, and most of that was provided by additional coal generation. Solar provided only 7,192 MWh (just 1.8% of total generation needed) on June 25. Helped by the emergency order, coal provided 101,117 MWh on June 25 (25.2% of total generation). Nuclear — a zero-emissions resource with an approximately 100% productivity factor in North Carolina — was providing peak electricity generation throughout (43.6% of total generation on June 25).

The worry here is that under the state’s Carbon Plan, all of North Carolina’s coal plants will be closing. Policymakers deliberately chose expensive new facilities (especially unreliable solar and wind resources) over inexpensive, paid-for facilities (coal plants) because they prioritized reducing carbon dioxide (CO₂) emissions over continuing to provide reliable, least-cost electricity. They also decided that, as it pertains to coal power, technological improvement in the coming decades is impossible.

The mandate also forecloses the possibility of a breakthrough in carbon-capture technology. As a fuel source, coal is cheap, abundant, and reliable. With respect to the CO₂ emissions reduction goals, the only problem with coal is its emissions. Meanwhile, companies in Caryand Charlotte have been awarded grants by the Biden administration’s Department of Energy to test carbon-capture technologies.

We cannot know for sure if that or some other disruptive innovation might make plentiful and inexpensive coal a low-emissions resource.

But if our coal plants are all closed, how are they going to be replaced? Will they be replaced with expensive solar and wind that leave North Carolinians at risk for sudden outages even without extreme weather — as happened in Spain days after the grid operator announced that the Spanish grid had achieved 100% renewable energy provision? Or will they be replaced with baseload natural gas and nuclear?

As Mitch Rolling of Always On Energy Research observed, forcing the retirement of reliable power plants is a recipe for blackouts:

The lesson here is that North Carolina still needs its coal power plants. Faced with the choice between loss of their power and higher emissions, people would choose the latter, especially during extreme weather. Duke and the DOE wisely made the right choice to keep the power on.