Thousands of megawatt-hours of electrical energy squirreled away into utility-scale energy-storage plants to cover the spread between generated renewable energy and the lowest reliable operating limit (LROL) during the 10 a.m. to 3 p.m. “duck curve” period of the day — does this sound like a glimpse into the far future of major utilities across the United States and the world? But the future has already arrived at the Carolina operating companies of Duke Energy.
For example, the Duke Energy Progress (DEP) Company, serving customers primarily in the eastern half of North and South Carolina, managed a whopping 106 GWh of excess energy in 2017, largely generated from the vast fleet of mostly utility-scale solar-energy resources in North Carolina. “Excess energy” is generated whenever the net demand load falls below the LROL. This tends to happen during the period of the day of greatest solar energy generation, between 10 a.m. and 3 p.m., as shown in Figure 1.
A Balancing Authority (BA) is responsible for achieving continuous balance between available energy and demand for energy in real time and therefore, needs to commit generation units that can reliably meet peak demands, expected net-demand ramp rates and reserve requirements to regulate frequency and support contingency conditions.
The LROL is the minimum MW level needed to keep the required generating units online. In addition, the utility is bound by private power agreements with guaranteed “take” provisions. Between the “take” provisions and the reliability requirements enforced by NERC, the problem is two-fold: (1) maintaining sufficient local regulating reserves to manage intermittency and net-demand ramping, and (2) managing excess energy, which is not a simple matter of switching open a recloser at a large solar-generating facility or shutting down a gas-fired turbine.
All of that renewable energy has got to go somewhere. According to Kat Sico, transmission operations engineer for DEP, “When resources are available, excess energy can be managed by reducing internal generators or exporting the excess energy via the joint dispatch dynamic schedule. This energy either serves Duke Energy Carolina’s (DEC’s) retail load or utilizes DEC’s 2100 MW of pumped storage load capabilities.”
This dynamic schedule has been implemented under the Joint Dispatch Agreement (JDA), in which DEP and DEC are permitted to share “non-firm” energy in order to meet their customers’ energy demand in an economic manner. Since solar generation has guaranteed “take” provisions for DEP, the excess energy that is transferred from DEP to DEC comes from other generation resources. DEC primarily serves the western parts of North and South Carolina, a region encompassing the Appalachian mountain range, where DEC’s pumped-hydro operations are in motion.
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