Flexibility Inventory for Western Resource Planners

For utility planners, one criterion for choosing a portfolio of resources to meet future needs is that a portfolio has sufficient flexibility. Flexibility indicates the capability of the system to accommodate variability and uncertainty in demand, production from variable renewable resources like wind and solar, and other unforeseen events. Historically, flexibility has not been a primary concern and has not been systematically evaluated in utility planning studies. Growth in the share of energy produced by variable renewables will increase variability and uncertainty, potentially making flexibility more important in the future.

In order to better gauge the flexibility of planned resource portfolios, we developed a way to measure, at a screening-level, the overall flexibility of a portfolio. Our flexibility inventory is based on a methodology developed by the International Energy Agency (IEA) as part of a cross-country comparison of the potential to accommodate growing shares of variable renewables.The key inputs to the flexibility inventory are the capacity of existing and planned resources, forecasts of peak demand, and several key parameters that are discussed in the full report.

The primary use of the flexibility inventory is to show trends in the balance between flexibility supply and flexibility demand over the planning horizon. Flexibility supply measures the capability of generation or demand to change in response to system conditions over various time scales relevant to power system operations (specifically we consider four time intervals of 15 min, 1 h, 6 h, and 36 h both in the up and down direction). Contributors to flexibility supply include conventional generation, demand response, bulk energy storage, and transmission interconnections. Flexibility demand is the amount that the net demand is expected to change over those different time scales, the degree to which those changes can be predicted ahead of time, and the contingency reserves.

The flexibility inventory can act as an “early warning” system. If planned resources lead to large changes in the balance of flexibility supply and flexibility demand, then additional detailed studies may be warranted to ensure the system will be sufficiently flexible in the future. Because it is a high-level analysis, and not a detailed study, it is just as important to understand what the flexibility inventory developed in this report does not do, as summarized in Table ES1.