Don’t count on batteries to be available when they are most needed!
By Ziad Alaywan, P.E.
Hybrid and Co-Located Resources
The CAISO has two options for representing resources that combine variable energy resources (VERs) with battery energy storage systems (BESS), which are designed to shift energy from the middle of the day during when solar energy is abundant to evening to assist with mitigating the Duck Curve.
The first option is a hybrid resource which combines VER and BESS resources into a single resource. This allows the Scheduling Coordinator to determine how best to optimize use of the VER and BESS by scheduling or bidding an integrated resource into the CAISO markets. The CAISO dispatch says, “Give me 50 MW” and the Scheduling Coordinator decides how much of the 50 MW comes from the VER and how much comes from the BESS. It sounds straightforward, but there is a catch. The CAISO views the hybrid resource as a single dispatchable resource that is expected to hold its dispatch operating target (DOT) regardless of VER output. VER output is variable, so when the DOT is 50 MW, the CAISO expects the BESS to make up the difference between the 50 MW DOT and the VER generation. The CAISO receives a firm 50 MW of generation from the hybrid resource in response to its 50 MW DOT, but the consequence is that the BESS is forced to use energy stored in its reservoir to stabilize VER production rather than using the energy for the primary purpose of injecting energy onto the grid during the evening hours.
The second option is a co-located resource whereby the CAISO sees two separate resources, one VER and one BESS. The VER and the BESS operate independently. The VER generates unfettered unless curtailed by the CAISO and the BESS is dispatched as scheduled or bid into the CAISO markets. This allows the BESS to respond to DOTs based solely on CAISO market signals reflected in the DOT as opposed to reserving energy and state-of-charge capacity to firm up VER generation. It is not perfect because the VER introduces additional variability that the CAISO has to address. However, the primary reason these types of resources are being deployed is to mitigate the Duck Curve. Modeling these types of resources as co-located is aligned with that objective while modeling as a hybrid reduces the ability to fully satisfy it.
Most of the energy storage that will be deployed is hybrid and may not help to deal with the Duck Curve if it is chasing intermittency all day long and unavailable for the evening ramp. Caution when estimating the benefits of energy storage in reducing the “shape of the Duck”!
Published June 30, 2022, to the Western Power Trading Forum's "The Friday Burrito"