July 2013 ISO-NE Heat Wave and Historical Peak Days

For anyone who was outside during the week beginning July 15th, 2013 anywhere east of the Rocky Mountains, it was hot, like really hot. As a result, its highly likely that many commercial and industrial customers set their Installed Capacity (ICAP) or Peak Load Contribution (PLC) tags that week. We explained the ISO-NE capacity market several blog posts ago and sometime soon, we plan to cover the capacity markets for NYISO, PJM, and Ontario. In this post, we plan to focus on ISO-NE and the summer load patterns to date.

The ISO-NE grid is strongly summer peaking and the hottest days result in system loads much higher than average. The table below shows the daily peak system loads on the top 10 highest load days of the last eight years.  In this time period, all top 10 load days occurred during the summer months and the last time a top 10 load day occurred during winter was 2004. Note the separation between the peak day for each year and the 5th highest or tenth highest day. Its a significant difference in most years. Also note that the highest load day ever recorded in the ISO-NE load zone was August 2, 2006 when the system load peaked at 28,130 MW. 

ISO-NE Top 10 Load Days.png

For interval metered customers, knowing the peak load hour is significant because the customer's demand during the peak load hour will determine their ICAP Tag. For more information, check out our previous blog post on this topic here. Many large industrial customers try to predict when the ISO-NE peak load hour will occur so that they can preemptively shed load. These customers do this to reduce their ICAP tag and for each MW of ICAP tag that a customer can avoid they are able to save approximately $36,000. Predicting the peak load hour can be tricky on the hottest days because its not uncommon for the load to deviate from the forecast given by ISO-NE in the morning. Any of the following can cause significant deviations in the actual load vs. the forecast: late day thunderstorms; activation of demand response; an afternoon sea breeze at the coast; or the secondary effect of large customers shedding load when they think a peak is imminent. 

While its impossible to know if we've already set the system peak for 2013 (we won't know for sure until the summer is over), its very likely that it happened on Friday, July 19 between 4 - 5 p.m. The graph below shows ISO-NE daily peak loads through July 24th, 2013, color coded for weekdays vs. weekends. The load pattern for the summer was pretty typical for June and early July. There were four days where the load exceeded 24,000 MW prior to July 15th, 2013 and these high load days coincided with high temperatures in Southern New England ( about 90° F). In each case, the high temperatures did not last more than two or three days and the load dropped significantly when temperatures receded. The week beginning July 15th, 2013 contained a heat wave of an exceptional intensity and duration. New England experiences heat waves like this only two or three times per decade. The graph shows clearly how each day of the week experienced a higher peak load than the day before despite there being similar temperatures each day. This incrementally increasing load each day during a heat wave is a common phenomena since building envelopes absorb and retain increasing amounts of heat as the heat wave progresses. As a result, air conditioning systems have to work much harder on the third or fourth day of a heat wave than the first to overcome the heat present in the building envelope. 

ISO-NE July13Load.jpg
ISO-NE Historical Peak Days.png

The table to the right shows the ISO-NE system peak load days and hourly intervals over the last several years. Many people consider the summer of 2009 to be a total outlier given how cool and rainy that summer was. Although the week beginning July 15th, 2013 did not set an all-time peak in ISO-NE (as it did in NYISO), it did record two of the top 10 load days in ISO-NE's history and Saturday July, 20th was the highest weekend load ever recorded. Although its likely that the system peak for 2013 has been set, early August can get quite hot so we'll have to wait and see.