Heat wave recap: Reliable operations through holiday heat & humidity

8/22/18 update: Peak load values updated.

Power system operates smoothly during the June 29-July 5 heat wave

Fireworks weren’t the only things ablaze during the Independence Day holiday week–temperatures surged throughout New England as well, causing high demand for air conditioning and, consequently, the electricity that powers it.

Despite the unusually hot, humid conditions during the June 29–July 5 heat wave, the timing of the holiday, coastal breezes, and abundant solar power kept electricity demand in check. With a peak of around 24,000 megawatts (MW), ISO New England’s system operators were able to rely on a high-performing fleet of resources to keep the grid running smoothly and reliably throughout the heat wave.

What Is a Heat Wave?

A heat wave in New England is defined as a period of maximum temperatures at or above 90°F for three or more consecutive days, and a typical New England heat wave generally lingers in the low 90s. Although dew point levels—a measure of humidity—are not criteria for a heat wave, dew points in the mid-60s or greater make hot temperatures feel more uncomfortable and contribute to an increased electric load as cooling demand is higher. In fact, when the dew point is above 70, every one-degree increase can cause load to rise by about 500 megawatts.

Daily peaks during heat wave

The highest hourly demand for electricity during the heat wave (and as of right now the 2018 summer peak) occurred on July 5 during the hour from 5 to 6 p.m., at 24,436 MW. This is comparable to recent summer peak loads in New England, at 23,708 MW in 2017; 25,466 MW in 2016; and 24,326 MW in 2015.

While the current peak doesn’t quite make the ISO’s Top 10 Demand Days, the peak on Sunday, July 1 ranks 8th among weekend days. The highest-ever peak in demand, at 28,130 MW, occurred during the hour from 2 to 3 p.m. on Wednesday, August 2, 2006, during a heat wave. See Table 1 for each day of the heat wave week’s hourly peak load.

The highest hourly locational marginal price (LMP) occurred Thursday, July 5, during the hour from
2 to 3 p.m., at $258.63/megawatt-hour.

Table 1. Peak load from June 29 – July 5

Peak Load
(Day & Time)
Electricity Use
Friday June 29, 2018, 5 to 6 p.m. 20,726
Saturday June 30, 2018, 5 to 6 p.m. 20,595
Sunday July 01, 2018, 5 to 6 p.m. 22,798
Monday July 02, 2018, 5 to 6 p.m. 23,454
Tuesday Jul 03, 2018, 4 to 5 p.m. 24,264
Wednesday July 04, 2018, 5 to 6 p.m. 21,880
Thursday July 5, 2018 5 to 6 p.m. 24,436

Holiday hero

A key factor in the ISO’s ability to keep the grid reliable throughout the heat wave was its arrival during a holiday week. Because July 4 fell on a Wednesday, the holiday had an impact on the previous and following peak days of July 2, 3, 5, and 6, as some people and/or businesses chose to take the first or second half of the week off. With less demand required to power office buildings and industrial facilities on these days, there was less strain on the grid’s operations.

Weather wavered

Although this past heat wave did not break records for intensity or duration, it was significant in that it lasted a week in some locations rather than the more common duration of 3–5 days. Concord, New Hampshire, saw its longest stretch of days at 90°F or hotter—7 days to be exact—since 2002, and two other cities experienced single-day record highs on July 2 and 4.

Burlington, Vermont, experienced a new record of 97°F on July 2, breaking the day’s previous record of 96°F set in 1963; Portland, Maine, achieved its Independence Day record high with a 93°F thermometer reading, breaking the previous July 4 record of 90°F set in 2010. See Table 2 for weather stats for the period of June 29–July 5 in the eight primary cities that ISO New England monitors for its regional weather forecast.

Dew point temperatures on June 29 and 30 were moderately muggy, averaging in the mid-60s throughout the region, but increased to the extremely humid low- and mid-70s during July 1–5.

Table 2. Major 8 New England city weather stats for June 29 – July 5

City Duration of 90°F
or higher (Days)
Hartford, CT 7 consecutive 98°F on July 1
Bridgeport, CT 4 (3 consecutive) 92°F on July 1
Boston, MA 5 (3 consecutive) 98°F on July 3
Burlington, VT 6 consecutive 97°F on July 2
Concord, NH 7 consecutive 96°F on July 1
Worcester, MA 2 93°F on July 1
Providence, RI 4 (2 consecutive) 95°F on July 1
Portland, ME 1 93°F on July 4

Notably, however, Worcester, Providence, and Portland did not have three consecutive days of 90°F or higher and therefore did not experience a heat wave. Sea breezes generally kept temperatures lower in coastal Maine, New Hampshire, and northern Massachusetts, mitigating these typically load-intensive areas. 

Solar surged

Solar generation peaked each day at around 1 p.m. during the heat wave, providing approximately 2,000 MW to homes and businesses with solar arrays. The vast majority of New England’s photovoltaic (PV) arrays are located at customers’ sites, “behind the meter” on the distribution system. ISO New England can’t “see” what’s happening on the distribution system—which arrays are generating, which are not, and how much—but the impact of all those behind-the-meter PV systems is seen in demand levels. Because those homes and businesses were getting their electricity demand met by their own PV systems, their demand for power from the regional power system was lower by about 2,000 MW at the solar peak. Before so many PV arrays were installed in New England, summer peak demand typically occurred around 2 or 3 p.m. As more and more PV is meeting mid-afternoon demand for power, the peak in regional demand for power is occurring later in the day, as the sun goes down.

“Solar contributes to the reduction in the peak load and also contributes to moving the peak that we see here at ISO New England to that 5 to 6 p.m. timeframe,” said John Norden, ISO New England’s Director of System Operations. “Peak load usage, or the total power used in the region, is still occurring in the afternoon, but some of it is behind-the-meter, so we don’t see it.”

Power plant fleet performed

With demand evading high levels, ISO New England had to call only minimally on older coal and oil resources.

“The performance of the fleet in general has improved over the years because we’ve got a much more modern and efficient fleet,” said Norden. “The peak that we’re capable of meeting is 28,000 MW, but the peak during the heat wave was 24,000 MW. We can meet a large proportion of that level of demand with the region’s efficient combined-cycle natural gas facilities, which are very reliable.”

While solar generation amped up in the presence of hot temperatures, that isn’t to say that the atypical conditions didn’t impact photovoltaic performance.

“Solar is affected by the weather just like any other power plant. The higher the temperature during very hot weather, the less efficient the panels become because they heat up,” said Norden.

Forecasting fervor

While they certainly welcomed the holiday’s help, the ISO operators have been gearing up for any strenuous grid conditions this summer could hold far before the season began. Part of the preparation for this particular heat wave depended on careful attention to load and weather forecasts.

Keeping tabs on weather is a vital aspect of preparing for a heat wave’s impact on the grid. Staff meteorologist Mike Fontaine studies the forecasts provided by three weather vendors, applies his own knowledge and experience, and produces an ISO New England forecast. Fontaine’s forecast correctly predicted that cool sea breezes would result in lower coastal temperatures than other forecasters expected. Before and during the heat wave, he closely monitored dew points, as high humidity causes people to blast their air conditioning, placing greater pressure on the grid. Read about how weather is the biggest factor in predicting electricity use. Also see our 2018 summer outlook.

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