CAISO on Dec. 11 kicked off a new Storage Design and Modeling Initiative intended to tackle an array of challenges related to the market participation of storage resources, including further addressing bid cost recovery (BCR) issues and developing a default energy bid (DEB) formula specifically for batteries.
The initiative piggybacks off the ISO’s prior storage BCR working group, which identified that BCR provisions don’t align with storage resources and led to passage of a proposal to modify the calculation used to determine BCR payments. (See Proposal to Refine Bid Cost Recovery for Storage Passes Unanimously.)
But several stakeholders, along with CAISO’s Market Surveillance Committee and Department of Market Monitoring, emphasized that the proposal was only a first step in addressing the number of problems identified with storage BCR and their default energy bids.
The new initiative will delve into previously identified problems, including the need for a holistic redesign of the uplift mechanism for storage and changes to the DEB that reflect the specific characteristics of the resources. It will also introduce new ideas designed to further integrate batteries efficiently into the ISO market, including a proposal to develop a state-of-charge (SOC) mechanism and a way for storage batteries to bid into the market based on their SOC.
The working group’s effort will be separated into three topic groups: The first deals with BCR, the DEB and outage management systems (OMS); the second covers all topics related to state-of-charge management; and the third deals with distribution-level and paired resource topics.
Bid Cost Recovery, Default Energy Bid Modification
While CAISO’s completed storage BCR and DEB initiative closed a major market design gap related to existing BCR for storage resources, the ISO identified a further need to address storage assets’ lack of exposure to real-time prices if they deviate from their day-ahead schedules. As a result, the new initiative will seek to redesign the storage uplift mechanism, Sergio Dueñas Melendez, storage sector manager at CAISO, said during the meeting to launch the effort.
In prior working groups, stakeholders also recommended modifications to the storage DEB and the desire to consider standard approval for storage reference level change requests, which are currently manually processed by ISO staff. Automation and standard approval would provide clarity for market participants, Dueñas Melendez said.
Lastly, the ISO is seeking to enhance the outage management system (OMS) to align with storage resources, which includes reviewing lower and upper SOC real-time biddable parameter use, clarification of how SOC physical outages impact Pmax and Pmin outages, and improvement of OMS functionality to better support outage submissions from storage assets.
State-of-charge Management
The ISO is considering developing a “system SOC” mechanism that would track total energy available across the entire storage fleet.
“Thinking about the fleet holistically may allow better optimization of that storage fleet in critical conditions,” said Dinesh Das Gupta, policy developer at CAISO.
The system SOC mechanism will work in tandem with how the market operates, so it would be an addition to the system, not a replacement.
CAISO is also considering developing a “biddable SOC market participation pathway” that would allow energy storage resources to offer charge and discharge bids in relation to their SOC.
“The vast majority of storage resources participate in the market through the non-generator resource model, which approximates values through megawatt price bid pairings,” Das Gupta said. “A new pathway option centered on bidding at a given SOC may address multiple needs that are currently not found with the non-generator model.”
Developing this new pathway would take additional time from a policy and technical perspective, Das Gupta highlighted.
The ISO also highlighted the need to modify the SOC definition and calculation, after determining that resources may face physical constraints not reported to the market that prevent dispatch. Refining how SOC is defined and calculated would improve the ISO’s confidence in a storage resource’s ability to follow dispatch signals during tight system conditions, Das Gupta said.
The working group will also consider the non-linearity of a storage resource’s SOC. Non-generator resources are modeled linearly, but energy storage resources have non-linear maximum charging and discharging abilities. Better accounting for this non-linearity, especially under extreme conditions, may improve storage resource performance, Das Gupta said.
Finally, the ISO highlighted the need to explore SOC management for capacity awards. The current SOC calculation doesn’t fully model the impacts of capacity awards, particularly for the ISO’s flexible ramping product, which could result in storage resources being unavailable for other commitments, potentially jeopardizing reliability.
“With the flexible ramping product, we’re seeing potentially serious implications given the price of the product and the high percentage of the product being provided by storage resources,” Das Gupta said.
Distribution-level Resources
Distribution-level storage assets provide wholesale energy storage to the system via the distribution network rather than through a direct interconnection at the bulk transmission level. These assets fall under both the ISO tariff and the distribution level tariff, and aligning the two would “enhance operational confidence for both resources,” Das Gupta said.
Additionally, due to significant growth in co-located resources, each with unique parameters and challenges, the ISO is also seeking to explore settlement provisions, including BCR, following increased operational experience with co-located resources.
The last effort in this topic group seeks to address the lack of a DEB for hybrid resources. Developing a bid for such resources would allow bidding up to the soft-offer cap.
Next Steps
CAISO expects to release a straw proposal for the initiative in March 2025, with a final proposal slated for July.