It has been well established that renewable penetration in market leads to changes in thermal dispatch.  Increased solar capacity in particular leads to a glut of supply in the middle of the day, drives down midday net load and stretches the net load/thermal demand evening ramp as solar falls off a cliff in the late afternoon.  We have observed the pattern clearly in ERCOT so far this year.  Most of the attention for changes to thermal dispatch behavior usually falls on an ISO’s natural gas fleet.  But in ERCOT, starting back in the final quarter of last year, we saw significant changes to how the states coal units were utilized.  This was the topic of a Special Report back in January, “ERCOT’s Hazy Horizon”.  The figure below plots the average hourly generation profiles for load, along with key thermal and renewable resources with ERCOT on a monthly basis over the past 3 years.  Starting back in October of 2023 the coal profile exhibits a change, with generation during the middle of the day dipping sharply with a steep ramp down from the morning peak and an even steeper ramp up into the evening peak.  It was remarkable that despite the relative lack of agility for coal plants, we saw ERCOT coal being operated as something more akin to a peaker plant to compensate for the growing renewable supply in the midday thanks to solar growth.

Figure 1 | Average Hourly ERCOT Load and Generation by Fuel Type