The duck curve is named for its resemblance to a duck, with its peaks and valleys highlighting the effect solar production has on the power demanded from thermal generators and hydropower resources throughout the day. Advancements in energy storage technology are providing a new method for narrowing the timing imbalance.
Since 2006, solar energy production has grown at an annual rate of 59%, according to the Solar Energy Industry Association. Solar energy’s share of total U.S. energy generation skyrocketed from just 0.1% in 2010 to almost 2% in 2017. In the past three years, it has comprised an average of about one-third of all new energy capacity additions.
This rapid growth has resulted in a fundamental challenge for system operators by creating an imbalance in the supply and demand of energy on the grid known as the “duck curve.” The duck curve (Figure 1), which derives its name based on a chart published by the California Independent System Operator (CAISO) in 2013 resembling a duck, highlights the sharp midday drop in energy demand resulting from peak solar energy production followed by a short, steep ramp-up in the early evening hours as demand for energy increases and solar energy production falls.
In its report, CAISO noted three major problematic conditions affecting grid management that are exemplified by the duck curve. They are:
- ■ The creation of short, steep ramps caused by system operators being forced to quickly bring on or shut down energy generation to meet demand over a short period of time. To address this condition, CAISO stated that a flexible energy resource is necessary to quickly react to adjust energy production to meet sharp changes in demand.
- ■ The fact that peak midday solar production exceeding demand results in the risk of oversupply, which must be managed to avoid the costly consequences associated with overgeneration, including increased costs from curtailing energy production and reduced environmental benefits as a result of such curtailment.
- ■ The management of such oversupply during the midday hours results in decreased frequency response capabilities, which are caused by fewer energy resources being available to automatically adjust energy generation to maintain grid reliability. That is, as renewable generation resources (which typically do not have automated frequency response capability) deliver energy, conventional generation resources (which can typically provide automated frequency responses) are displaced. As a result, the grid becomes less reliable and is increasingly subject to disruptions. To avoid this risk, the grid needs access to automated frequency response systems that can quickly and automatically ramp up or down in the event of sudden interruptions.
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