If you’re talking about renewable energy long enough, someone will inevitably bring up the “duck chart”—prepared by the California Independent System Operator (CAISO) to illustrate the challenge of integrating increasing amounts of variable renewable resources into the power grid. Our ability to meet this challenge will affect grid reliability and future adoption rates of renewable energy. But the implications of the duck chart take a bit of context to understand.
Let’s start with the basics. The North American power grid has been called the largest machine on earth because in many ways it functions as one integrated system. Also, electric power is not easily stored so demand must equal supply at all times. For example, when you turn on your air conditioner on a hot day, its electricity needs must be generated in real time somewhere on the power grid. During the day, electricity demand fluctuates as we get up, leave for work, get home, go to bed. Likewise, demand varies throughout the year, with largest loads typically occurring during the summer cooling months and winter heating months. To accommodate variations in demand, system operators dispatch power from a variety of controllable generating sources. This means that power plants are literally ramped up or down to match demand fluctuations. This is typically done in layers; operators start with base load power plants, such as coal and nuclear that run 24/7, and then dispatch various types of intermediate and peak load power as demand requires. So far so good.
Recent technology advances have enabled clean renewable resources such as solar and wind to become an increasing part of our power generation mix. In California, renewable portfolio standards have a goal of 33% of retail electricity from renewable power by 2020. CAISO has projected daily net load curves with variable, uncontrollable renewable resources being added to the generation mix over the years. The resulting “duck chart” shows the load on a typical March day. The X-axis shows time throughout the day and the Y-axis shows the net load in megawatts. This is the load that CAISO must provide from its controllable resources to meet net demand (total customer load minus renewable generation).
During the day, electricity demand increases in the morning, dips a bit during midday, increases in the evening, and then falls at night. Renewable resources typically become available once the sun comes up and are unavailable when the sun sets. As the amount of renewable power generation increases over the years, CAISO has ample power during the day but must quickly supply large amounts of power during the evening. The “duck curve” is created by the low early afternoon net load (the belly), the steep early evening increase (the neck), and nighttime fall off (the head and beak).
So what’s the problem? By 2020, CAISO shows a risk of overgeneration. This occurs when too much renewable power is produced in the early afternoon, implying the need to curtail baseload power. But these baseload resources are not designed for rapid ramp up and ramp down. In addition, CAISO must ramp up about 13,000 megawatts in three hours during the early evening by dispatching large amounts of controllable power from central stations. This has significant implications on costs, operations, and system reliability. (A March day was selected by CAISO to represent a worst case situation because in late winter customer electricity demand is low and solar and wind resources are high.)
CAISO must “maintain real-time system balance and reliability while also supporting the increasing levels of renewable energy resources.” Is this duck soup? Not really. Yes, there are several options that can “flatten the duck.” Power can be imported and exported to surrounding regions, electricity storage technologies could provide relief, and demand response and energy efficiency can dampen customer loads. Natural gas power plants can also help because of their quick ramp up ability. But none of the solutions are fool proof, and many require better technologies and techniques for managing distributed resources.
Simply put, grid integration—the ability to easily accommodate all grid resources and enable greater customer engagement—is likely to be the most exciting energy challenge in the next two decades.