With the growth in renewable resources, short-term supply variability has increased. To address this issue some ISOs have introduced a new ancillary service that pays supply sources for the capability to ramp up or down between load-following intervals without using up AGC resources. These products are sometimes designed to “look forward” across five-minute intervals to ensure sufficient ramp capability in future intervals within an operating hour (as opposed to AGC and reserves requirements, which are set at the start of a given hour). Typical units providing flexible ramp capability are hydropower and natural gas combustion or combined-cycle turbines. In some cases, it may also be provided by battery storage and flexible loads. Other terms for flexible ramp capability include flexible ramping product and ramp capability.
The need for flexible ramp is demonstrated by the “duck curve,” which shows the change in net load (total load less renewable reserves) across one day in California. Flexible ramp is needed to meet late afternoon/early evening loads as solar energy declines as the sun goes down.