Characterizing Curtailed and Uneconomic Renewable Power in the Mid-continent Independent System Operator

As power grids incorporate increased renewable generation such as wind and solar, their variability creates growing challenges for grid stability and efficiency. We study two facets: power the grid is unable to accept (curtailment), and power that is assigned zero economic value by the grid (negative or zero price). Collectively we term these "stranded power" or SP. We study stranded power in the Midcontinent Independent System Operator (MISO), characterizing quantity and temporal structure. First, stranded power is available in the MISO grid 99% of the time, and often in intervals >100 hours, with characteristic seasonal and time-of-day patterns. Average stranded power often exceeds 1 GW, with duty factors as high as 30%. About 30% of all wind generation in MISO is stranded. Examination of the top 10 individual sites shows stranded power can be as high as 70% duty factor and 250MW. Trends over the past 3.5 years suggest stranded power is a persistent phenomenon. The study characterizes opportunities to exploit stranded power. We consider using energy storage to increase the utility of stranded power. For a range of power levels and uniformly-distributed storage, adding 5 hours of storage doubles duty factor to 30% at 4MW, but another 95 hours is required for the next 15% increase. At 4MW with 50 hours of storage, only 3 of 200 sites reach 100% duty factor, and with 100 hours required for the next 10 sites. Higher power levels require 100's of hours. Storage at the top 10 sites is more productive, 5 hours increases duty factor to 70% at 4MW, but further storage has diminishing benefits. Studies of the amount of power served by storage show that distribution to the best sites provides 2 to 3.7-fold advantages over uniform distribution.