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Wind and solar will slash MISO emissions but increase grid complexity, report says

MISO’s system could approach 30% of annual energy from wind and solar generation within five years, and renewable penetration levels may increase on the order of 10% every five years after that.

Wind and solar generation in the Midcontinent Independent System Operator (MISO) region are projected to serve 60% of annual load by 2041, reducing emissions by nearly 80% compared to 2005 levels but also increasing “sharply” the complexity of reliably operating and planning the grid.

MISO’s system could approach 30% of annual energy from wind and solar generation within five years, and renewable penetration levels may increase on the order of 10% every five years after that. Such a level of renewable penetration could be significant in terms of planning and operating the grid. 

Those were among the key findings of the 2022 Regional Resource Assessment (RRA) study, published in late November by the regional grid operator. MISO manages the generation and transmission of high-voltage electricity across 15 U.S. states and the Canadian province of Manitoba, an area that includes around 42 million people.

The 2022 RRA provides a view of how members’ resource plans are evolving, and is intended to help stakeholders address reliability, affordability, and sustainability goals.

The report said that as the MISO solar generation fleet grows, the system will have a greater need for controllable ramp-up capability. It said that maximum short-duration up-ramps increase by three times by 2031 and four times by 2041, compared to current levels.

Quack like a duck curve

As solar generation capacity grows, so does the challenge of steeper ramping needs, the report said. The challenge is expected to vary by season but be most prominent in the winter months when hours and intensity of sunlight are limited. 

MISO said that to complement solar’s growth, the region will increasingly need “controllable resources” that can rapidly turn on and ramp-up or -down quickly, and perhaps cycle multiple times during a day. 

The report also said that solar generation’s capacity contribution is forecast to decline rapidly as more solar capacity is added to the system. Wind generation’s contribution, by contrast, is forecast to remain relatively stable as more wind capacity is added.

The MISO footprint includes 15 states and Manitoba in Canada.

In winter months, MISO has seen two peaks in the diurnal net load. The first occurs in the morning as people wake up, followed by a mid-day dip in demand, and then the load increases again in the evening.

With the fleet changes estimated by the RRA, the “two peak” pattern of today is expected to morph into a “duck curve” shape. The term “duck curve” has become well known in the industry after California system operators studied the potential impacts of increasing penetration of renewables, predominantly solar, on the net load. One feature of this shape is the drop in net load around mid-day due to the impact of solar production. Then, in the evening as solar production falls and electricity consumption rises, a greater need for a rapid ramp-up of production from dispatchable generation resources occurs.

The report said that, generally, wind production in MISO is better than average during the evening peak load hours, while solar production is less aligned with the evening peak load (particularly during winter months). As a result, further increases in wind penetration “minimally impacts the capacity contribution.”

The report also looked at both solar-plus-storage and stand-along energy storage options through 2041, modeling so-called hybrid units at 14.8 GW of capacity and battery units at 11 GW. It found that monthly patterns showed that the combined impact of solar-plus-storage and battery units resulted in reducing the evening peak net load.

It also found that hybrid and battery storage resources contributed to slightly steeper morning down-ramps, depending on prices. Down-ramps are anticipated to be simpler to manage relative to up-ramps. Increasing the size of the battery resource fleet would likely further increase system benefits, the report said. By contrast, additional batteries were not chosen by the model due to higher relative cost.

Accredited capacity

At a macro level, MISO members are planning what the report called “significant investments” in new resources, resulting in an overall increase in installed capacity compared to 2021 levels.  It found a planning shortfall in so-called “accredited capacity,” which it said reflects how much energy a resource is expected to produce to meet tight conditions after accounting for historic performance. 

For example, MISO members are planning to replace retiring coal capacity primarily with investments in solar and wind capacity. On a system level, natural gas units remain more or less neutral in terms of capacity expected to retire versus planned additions. But, the report noted that 1 GW of coal has a much higher capacity contribution than 1 GW of solar. Therefore, even though planned additions appear to outnumber the planned retirements, additional investment beyond what is currently being planned by MISO members will be required to “reliably achieve decarbonization targets and meet planning reserve margins,” the report said.

MISO control room. Credit:

Because the new resources that MISO members plan to build — wind and solar — have lower accredited values than the thermal resources being retired, the estimated accredited capacity is expected to fall below current levels through 2041.

The region’s combined levels of existing resources and planned resources are seen as meeting the anticipated load-plus-reserve level on a region-wide basis for the next four years, albeit with a small margin of error. It cautioned that the risks and timing of potential capacity shortfalls differ across MISO’s 10 Local Resource Zones (LRZs). 

The report cautioned that the risk of capacity shortfalls will increase if load growth exceeds assumptions or if retirement schedules are accelerated without sufficient replacement. Risks will be further heightened, it said, if any planned resources are delayed beyond their currently scheduled in-service dates and other solutions are not promptly implemented.

Read the full report here.

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