Green Illusions – Why Wind and Solar Intermittency Matter
For the first article in my Green Illusions series, “Why Wind and Solar Power Cannot Displace Coal” I focused on the normal swings in electric power demand over the course of a typical day and explained the differences between base-load power from coal and nuclear, peaking power from natural gas, and intermittent power from wind and solar. Today I’m going to drill a little deeper into the detail and try to show readers how intermittent power from wind and solar is integrated with base-load and peaking power assets.
The following graph uses data from July 15, 2015 to show power supply and demand on a random mid-summer day in the ERCOT grid which services most of my home State of Texas, gets almost 12% of its power from intermittent wind resources and has one of the highest concentrations of wind power in the country.
The purple band at the top represents intermittent wind power, which started at 5,900 MW during the morning rush hour, declined to 3,100 MW at 1 p.m. and then climbed back up to 8,800 MW by 6 a.m. on the following day. The blue and red bands at the bottom represent base-load nuclear and coal power which remained stable at 4,500 MW and 11,150 MW, respectively, during the entire period. The big green band in the middle represents base-load and peaking power from natural gas, which started at 17,800 MW during the morning rush hour, climbed to a peak of 44,400 MW by 5 p.m., and declined to a base of 14,000 MW by 6 a.m. on the following day. While solar accounts for less than 1% of ERCOT’s electricity mix, the dashed vertical lines bracket the period when solar power can be relevant if the weather cooperates.
The next graph comes from ERCOT’s daily Wind Integration Report: July 16, 2015. It compares ERCOT’s total “Integrated Load” with its “Actual Integrated Wind Output” for the preceding week. The graph is a little complex because the green load line is read in relation to the left hand vertical axis, which scales from 20,000 to 70,000 MW, while the blue wind power line is read in relation to the right hand vertical axis, which scales from 0 to 10,000 MW.
While the scale differences visually distort the importance of wind’s contribution to the Texas power mix, they highlight the biggest shortcoming of wind power. In most cases, intermittent wind power is most plentiful at night when electricity demand is low and least plentiful during the day when demand is high. For the week of July 9th through July 16th, wind supply and system demand were almost perfectly out of synch. ERCOT’s daily wind integration reports since August 2010 are all available on-line. If you can find a week when wind supply and system load weren’t wildly out of synch, I’d appreciate a link in the comment section.
The bottom line is that all power generation technologies are not created equal – they can’t all perform the same functions. In a maintenance-free world, 1 MW of generating capacity would produce 8.76 GWh (8,760 MWh of electricity in a 365-day year (24*365=8,760). While perfection can’t happen in the real world, the ratio between capacity and potential annual power output allows us to use a factor of 8.76 as a Generation Productivity Index, or “GPI,” that facilitates head-to-head comparisons power generation assets. The following table is based on data from the U.S. Energy Information Administration. It categorizes each class of generating assets as base-load, peaking, or intermittent; summarizes total capacity in Gigawatts and total electricity production in Terawatt-hours for each class in 2010 and 2015; and calculates an annual GPI factor for each class.
