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As solar capacity grows, duck curves are getting deeper in California

As solar capacity grows, duck curves are getting deeper in California
July 10, 2020 - The Flatirons Campus substation is part of the Power Generation Upgrade Project changing the source of utility power at the Flatirons Campus from the distribution network that feeds businesses and houses to the transmission network that ties all the power plants and substations together. In that process, the campus’ transmission limitation increased from 10 megawatts (MW) to 19.9 MW, enabling twice as much power transmission as before, allowing researchers to test multiple types of technologies on the same electrical grid with enough power to run them effectively and add more testing scenarios with greater electrical stability. (Photo by Dennis Schroeder / NREL)

As solar capacity in California continues to grow, the midday dip in net load (or duck curve) is getting lower, presenting both economic and grid stress challenges for grid operators, according to the California Independent System Operator (CAISO).

The duck curve presents two challenges related to increasing solar energy adoption. The first challenge is grid stress. The extreme swing in demand for electricity from conventional power plants from midday to late evenings, when energy demand is still high but solar generation has dropped off, means that conventional power must quickly ramp up electricity production to meet consumer demand.

That rapid ramp up makes it more difficult for grid operators to match grid supply with grid demand in real time. In addition, if more solar power is produced than the grid can use, operators might have to curtail solar power to prevent overgeneration.

The other challenge is economic. The dynamics of the duck curve can challenge the traditional economics of dispatchable power plants because the factors contributing to the curve reduce the amount of time a conventional power plant operates, which results in reduced energy revenues.

If the reduced revenues make the plants uneconomical to maintain, the plants may retire without a dispatchable replacement. Less dispatchable electricity makes it harder for grid managers to balance electricity supply and demand in a system with wide swings in net demand.

The duck curve, however, has created opportunities for energy storage, according to analysis from the Energy Information Administration (EIA). The large-scale deployment of energy storage systems, such as batteries, allow some solar energy generated during the day to be stored and saved for later, after the sun sets. Storing some midday solar generation flattens the duck’s curve, and dispatching the stored solar generation in the evening shortens the duck’s neck.

Battery storage is being constructed in California; it’s grown from 0.2 GW in 2018 to 4.9 GW as of April. Operators plan to build another 4.5 GW of battery storage capacity in the state by the end of the year, according to EIA’s Preliminary Monthly Electric Generator Inventory.

EIA said that grid operators at CAISO have observed a drop in net load in the middle of the day when solar generation tends to be highest. When graphed for a typical day, the pattern created by the midday dip in the net load curve, followed by a steep rise in the evenings when solar generation drops off, looks like the outline of a duck, so this pattern is often called a duck curve.

Unlike conventional power plants, solar and wind resources can’t be fully dispatched at will to help meet demand, and utilities may have to curtail them to protect grid operations. Solar power is only generated during daylight hours, peaking at midday when the sun is strongest and dropping off at sunset.

As more solar capacity comes online, conventional power plants are used less often during the middle of the day, and the duck curve deepens.

The duck curve is not unique to California, EIA said. It’s increasingly occurring in other parts of the country and around the world in places where the share of solar generation is increasing compared with generation from conventional sources. In addition, a duck curve is becoming visible at the national level in the United States.

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