The smoke from wildfires in Quebec that caused unhealthy air quality in the region last week also posed challenges for PJM’s load forecasters.
Beginning June 6, hazy conditions throughout the RTO decreased visibility, reducing solar output, and kept temperatures several degrees lower than usual.
“Typically in the summer, the load – or customer demand – is based around the time of day, day of the week, holiday periods, and various weather impacts including humidity, cloud cover and temperature,” said Marcus Smith, Lead Engineer – Load Forecasting.
PJM does not normally see dips in load during peak sunlight hours when temperatures are very warm in summer. But if there is more cloud cover or reduced visibility, and the temperatures are not that hot, the overall load will be suppressed by fewer air-conditioning units running.
In this event, even though people may have run air conditioners to filter the unhealthy air, the load still acted more like on a normal cloudy, temperate summer day, Smith said.
Smoke Affects Solar Output
Smith explained that when there is thicker cloud cover or, in this case, a reduction in visibility from smoke during daylight hours, light from the sun is obscured or blocked. With less irradiance, solar production is reduced compared with periods of full sun and unlimited visibility.
At midday Tuesday, grid-connected solar reached about 4,800 MW, not far off its typical peak of 5,000 MW, but tapered off through the afternoon.
On Wednesday, the peak dropped about 400 MW to around 4,400 MW. There was a sudden reduction of 1,000 MW from 1–2 p.m., after which solar output continued to decline until sunset.
On Thursday, solar peaked a bit higher, around 4,500 MW.
The smoke particles lessened on Friday, and solar generation returned to near typical values around 5,000 MW.
Saturday was a particularly sunny day, and solar generation peaked above 5,000 MW in the RTO. On Sunday, there was more cloud cover in the RTO, with solar generation peaking above 4,200 MW.
PJM does not have direct measurements for behind-the-meter solar.
Wildfires can be very erratic, and there are uncertainties surrounding how much and how deep the smoke from fires translates into the atmosphere and then downstream from that fire, Smith said. This can be determined by temperatures and winds not just at the surface but vertically in the atmosphere.
It is difficult to single out the effect of smoke alone, especially when PJM has not seen an expansive plume like this, Smith said.
However, the cooler temperatures and decreased visibility last week were similar to what PJM experienced during the period of July 19–21, 2021, when the RTO was covered with smoke from wildfires in the western U.S.