The Duck Curve
The what? Let us explain. The Duck Curve is a graph that shows the difference in electricity demand on the grid and the amount of available solar energy throughout the day. It was created by the California Independent System Operator (ISO)1 to demonstrate the electric load on the ISO grid on an average spring day when it’s sunny but temperatures are cool, meaning that demand is not as high as in height of summer when people are using AC or in the winter when heating homes is needed.
Though adapting to changing consumer demand has been an issue that utilities have dealt with for over a century, seasonal usage patterns have changed, and the actual resources used for generating electricity have evolved2. The Duck Curve illustrates an important aspect of the challenges that renewable energy such as solar poses to utility managers, and how these have evolved over a much shorter time period.
The Duck Curve has become a familiar graphic in the energy sector. The different lines you see are showing the net load—the demand for electricity minus the supply of solar energy—over a 24-hour period in California, with each line representing a different year, from 2012 to 2020.
The Duck Curve shows two high points of demand: the duck’s tail and the duck’s head. Energy demand reaches peak in the morning (6 A.M. and 9 A.M., the duck’s tail) and evening (6 P.M. and 9 P.M, the duck’s head). In other words, people wake up to get ready for their day and use a lot of energy, and then again when they come home from their workday and need to turn on lights, TVs, and other appliances. Solar energy, however, is at its best during the bright hours of the day after most people leave for work/school and before people return home in the evening.
Referring to our diurnal cycle graphic from the previous section, we recall that solar energy peaks in midday, and that even on a cloudy day its input is distributed across the late morning through mid-afternoon hours. The greater input of solar energy during these bright hours significantly lowers demand on the electrical grid.
This “lowering” is the duck’s belly on the Duck Curve, which represents the lowest point of net load. Notice that as the evening hours approach, the Duck Curve starts to climb quite a bit from that lowest point. This climbing begins to form the duck’s neck, and it illustrates the sun setting, people coming home, and energy use increasing. The grid, in turn, is now left to answer to that high demand. The steepness of the line is an indicator of how rapidly new sources of electrical power need to come on-line to meet the increased power demand and the corresponding loss of solar power in the evening. By 6 pm, the contribution of solar power is essentially zero, and we see the duck’s head as demand peaks in the evening hours from 6-9pm.
In 2012 and 2013, solar energy input only slightly levels out the demand curve and the duck’s belly has not fully developed yet. But with each consecutive year from 2012 to 2020, we can see that the duck’s belly develops more clearly. This means that with each passing year, solar energy input into the grid has become more common, utilities must cope with higher ramp needs, and the duck curve is appearing more often and getting worse3.
In addition to the increased need for utilities to quickly ramp up energy production when the sun sets and the contribution from solar falls, increased solar adoption also means that solar often produces more energy than can be used at one time, called over-generation. In this case, system operators like the ISO in California must curtail solar generation to avoid overloading and/or permanent damage to the grid, reducing both economic and environmental benefits of renewable generation.
California (CA) and Hawaii have integrated a lot of solar photovoltaic (PV) generating capacity into their grids, so at present they are the main areas where the Duck Curve is currently a problem4. According to the U.S. Office of Energy Efficiency & Renewable Energy, there were recorded days in March 2017 where solar contributed nearly 40% of electricity generation in CA for the first time ever. The Duck Curve will continue to appear as more U.S. states and countries incorporate increasing amounts of solar energy into the grid and reflect similar or greater solar generation percentages like California’s, and the effects of curtailment could have a potentially significant economic or environmental impact at greater solar penetration levels4.