MISO staff last week reiterated that they can likely reliably operate the grid with a 50% renewable energy mix but warned that variables like generation retirements, fuel prices and solar siting could send energy prices soaring at certain times.
The findings were the latest in the grid operator’s ongoing, multiphase Renewable Integration Impact Assessment (RIIA). Staff’s findings focused on MISO’s ability to provide energy during all operating hours throughout the year as renewable penetration increases.
During a virtual workshop Friday, MISO Senior Policy Studies Planner Chen-Hao Tsai said that for the most part, the RTO can operate the system reliably with 50% renewable penetration, but only if it has the dramatic transmission expansion it identified earlier in the study. (See MISO Renewable Study Shows More Tx, Tech Needed.)
Tsai said escalating fuel prices, increases in solar generation and the pace of generation retirements play pivotal roles and could stand in the way of a 50% systemwide renewable portfolio. Any of those variables alone could set off a rise in LMPs, MISO said.
At a 50% penetration, MISO said prices predictably spike in the evening, when peaking units — mostly combined cycle gas units — step in to fill ramping needs. However, Tsai said that if natural gas prices more than double from about $2.50/MMBtu to about $5.60/MMBtu, more coal units will be used for ramping needs, driving up LMPs.
Multiple stakeholders observed that steady, lumbering coal units aren’t designed to provide quick on-and-off daily ramping. Some said the cycling MISO contemplated in the study would cause wear and tear on coal plants and possibly lead to more forced outages.
Other stakeholders pointed out that MISO is still leaving storage out of the study equation, ignoring the technology’s capability to dull ramping needs. Tsai said the RTO plans to reveal new study results with storage considerations in late August or early September.
A spate of thermal unit retirements might also threaten the reliability of a 50% renewable fleet, Tsai said. High retirements paired with a 50% renewable fleet also produce high prices in the evening, he said, with July evening hours yielding systemwide prices as high as $300/MWh.
“In many cases, many peaker units received over $1,000/MWh,” Tsai said. “We still make the penetration target, but we pay high system prices.”
On the other hand, the study indicates that increased solar capacity expected in MISO South would drop LMPs by midday and make the region an exporter to the rest of the RTO’s footprint during those hours. The trend also leads to significant middle-of-the-day solar curtailment during spring and fall months.
Tsai said the solar buildout “creates a new stressed operating point during the shoulder load periods, which may need further review.”
MISO’s earlier RIIA results, released in June, predicted dramatic solar generation expansion. (See Study Foresees MISO Solar Eclipsing Wind.)
The RTO is also accounting for hundreds of gigawatts of renewable energy in its 20-year transmission planning. It foresees as much as 137 GW of renewables added and up to 114.5 GW in generation retirements by 2040. (See MISO Foresees Massive Shift to Renewables by 2040.)
Again, stakeholders said MISO should investigate how storage devices could soak up excess solar capacity. Some also asked the RTO to revisit the transmission needs it identified late last year to accommodate a 50% renewable mix. Tsai said the study’s transmission expansion portion is too involved and time-consuming to redo.
MISO plans to wrap up the RIIA study by the end of the year.
