CPUC proposes rising fixed O&M for retained gas, repowering option and seeks input on geothermal derates and interday storage sharing

Staff closed the INA workshop by describing several modeling inputs still under development and seeking stakeholder data and comments.

Gas fleet retention and repowering: Sam and colleagues said Resolve will continue to economically optimize retention of baseline gas units but propose two changes: (1) baseline gas fixed O&M costs will be modeled to rise over time (staff proposed a placeholder of ~1% per year starting at an assumed age) rather than remain static; and (2) a repowering candidate resource will be created to allow Resolve to replace an existing baseline gas unit on a 1:1 basis with a repowered unit at additional capital and O&M cost. Staff reviewed historical retirement-age distributions and proposed 40 years as a representative repowering age; staff proposed brownfield repower capital costs at roughly 86–90% of greenfield costs and fixed O&M equal to new-build assumptions, and sought comment on these values.

Geothermal ambient derates: staff flagged evidence that some geothermal plants experience seasonal and diurnal output reductions at higher ambient temperatures, especially air‑cooled plants, and asked for data. Staff proposed three modeling options to capture this effect: a monthly Pmax derate, an hour-by-month Pmax profile, or a temperature-dependent derate similar to the ambient derate model used for thermal plants in ServM. Staff requested stakeholder data to parameterize any chosen approach.

Interday sharing for long-duration storage: staff described ongoing work to allow Resolve to consider storage state-of-charge continuity across representative sample days (mapping 36 sample days onto chronological days drawn from 23 weather years) so that long‑duration storage can charge and discharge across multiple days rather than being constrained to intraday cycles. Staff asked for guidance/data on minimum state-of-charge assumptions, self-discharge rates and other parameters for LDES technologies (pumped hydro assumed 0% min SOC in the draft; other LDES archetypes currently modeled with a 10% minimum SOC in staff slides).

Why it matters: Rising O&M or repowering costs change the economics of retaining gas for local capacity. Geothermal derates and interday storage sharing can materially change the seasonal adequacy valuation of geothermal and long‑duration storage resources.

Ending: Staff asked stakeholders to provide data and comment on repowering-age and cost assumptions, proposed fixed‑O&M trajectories, geothermal ambient-derate data and LDES operational parameters.