The California Independent System Operator (CISO) published a chart in 2013 that has become a standard tool for discussions about the large-scale deployments of solar photovoltaic power. The duck curve, named for its resemblance with a duck, shows the difference between electricity demand and solar energy available throughout the day. Solar energy floods the market when the sun shines but falls off in the evening as electricity demand increases. The duck curve shows a snapshot from 24 hours in California during springtime. This effect is at its most extreme when it is sunny, and temperatures are cool.
The duck curve is a transitional point for solar power. This was the first time a system operator acknowledged that solar energy wasn’t a niche technology anymore and utilities needed to plan for increased amounts of solar power. This is particularly true in places where solar adoption is high, like California, where solar generated nearly 40% of the state’s electricity on a single day last March.
UTILITY CHALLENGES
California Independent System Operator
The high adoption of solar energy creates a problem for utilities balancing supply and demand. The reason is the increased demand for energy generation by electricity generators when the sun goes down, and PV’s contribution drops. High adoption of solar energy can also lead to PV producing more power than is needed at any one time. This phenomenon is called over-generation. System operators are forced to reduce PV generation to maintain economic and environmental benefits. Curtailment minimally impacts PV benefits when it occasionally happens over the year. However, higher PV penetration levels can have a significant effect.
The Solar Energy Technologies Office of the U.S. Department of Energy has been studying strategies for many years. Most projects funded by SETO’s subprogram help grid operators deal with the challenges of the duck curve.
DUCK CURVE SYSTEMS
Solar combined with storage technologies can reduce and even eliminate the risk of excess generation. When extra energy is stored and used during peak demand, curtailment doesn’t need to be done. SETO launched several projects in 2016, which paired researchers with utilities. The aim was to determine how storage might make it easier for utilities to rely on solar energy to meet customers’ needs 24/7. This research will allow utilities to integrate solar energy into the grid while addressing their challenges.
SETO launched a 2012 Research Program to help utilities, grid operators, and solar power plant owners better predict when, where, and how much solar energy will be produced. Forecasts allow electric system operators and utilities to understand solar resource patterns better and maximize their solar resources. IBM’s machine-learning technologies enabled a 30% improvement in prediction accuracy. As the amount of solar power connected to our grid continues to increase at an alarming rate, it will be necessary to improve predictive accuracy.
BRINGING IT BACK THE DUCK CURVE
The duck curve has many possible solutions. SETO’s lessons will be crucial in improving grid flexibility and addressing the over-generation risk as solar grows across the country. According to the Energy Information Administration (EIA), the amount of PV installed is expected to triple in 2030, potentially moving the duck curve out of California. New and improved technology will allow PVs to meet a more significant percentage of the total electricity demand and provide capacity on demand.