California’s Duck: More Quack than Bite

Don’t Duck Renewables Due to Reliability Fears

If you’ve followed debates about California’s electricity system over the last several years, you’ve no doubt seen our friend the duck made out to be an evil creature ready to destroy California’s ambitious energy plans. The “duck curve,” as it is known, represents a plot of net demand (electric demand minus renewable generation) on certain spring days. The low “belly” of the duck is caused by increasing solar generation and the “neck” results from reduced solar output as the sun goes down and demand for electricity increases into the late afternoon and evening.

Source: CAISO (2013), What the duck curve tells us about managing a green grid,

The duck poses two interrelated issues that present challenges for grid operation. The first is caused by the duck’s “belly” – as (primarily) solar generation increases in the afternoon, the supply of electricity may exceed demand. Since demand and supply of electricity must always be in nearly exact balance to maintain system reliability, this means excess power needs to be stored, exported, or curtailed, a term meaning a source of energy is shut off. In the case of renewable curtailment, this sounds terrible because we’re wasting an essentially free (at the margin) renewable resource, likely in favor of a polluting plant. The second issue is the “ramp,” or the steep rise of the “neck” of the duck between 4:00 and 8:00p.m., which requires the rapid addition of other generation to quickly meet peak demand.

Though often presented as a systemic issue to be dealt with at all times, “duck days” primarily occur on sunny days in spring when the glorious California sun beats down amid relatively cool temperatures (and thus low electric demand). This rather odd net load shape is not so extreme on most days of the year to cause major issues. Further, given California’s relative abundance of quick-ramping natural gas plants combined with (current) low curtailment levels, doom and gloom should not be our first reaction when we talk about California’s ambitious renewable electricity goals.

That doesn’t mean we should underestimate the challenges, either. The numerous mitigation measures to curtailment and ramping issues should be compared against one another to find the most cost-effective and environmentally sustainable solutions; as in most areas of energy policy there are no silver “bullets” but plenty of ways we can address these issues. One that I think deserves more attention is encouraging, or even at some point requiring, renewable generators to take over the system services now provided by traditional generation, much of which is fossil-fueled. For instance, frequency response and most ancillary services, which maintain system reliability during unexpected conditions, remain pretty much the exclusive domain of traditional fossil generators like natural gas plants (and in other states, coal). We must consider how to transfer these responsibilities to renewables – solar and wind generation in particular.

Specifically, it will be important (and is technically possible, though seldom if ever practiced) for some portion of “downward reserves,” a system reserve to quickly reduce supply of electricity, to be taken over by renewable generators. This would allow fossil generators to turn down output to lower minimum levels (and/or potentially shut down), which would result in substantially decreased curtailments. This highlights the fact that while renewables are often exclusively blamed for integration issues because of their dependence on weather, it is also inflexible traditional generation that leads to renewable curtailment. These plants cannot be turned down to very low levels or off due to physical operating constraints, so rather than shutting fossil plants down system operators choose to curtail renewables instead.

Ironically, provision of downward reserves by renewables means curtailment occurs more often but at finer levels of control – this results in substantially less renewable curtailment overall. If just a portion of downward reserves can be provided by renewables, the Union of Concerned Scientists (UCS) finds that curtailment can be reduced by 44% to around 2.7% of total renewable output at 50% renewable penetration in California. It is remarkable how impactful this single measure could be.

Let’s keep in mind that the duck and increasing renewable generation does not necessitate doomsday scenarios for California’s electric grid. Sensible measures to shift some responsibility for grid reliability to renewables can make a big impact, along with other cost-effective solutions to increase net load and decrease the steep ramp on some spring days. Some amount of curtailment may even be necessary as part of a cost-effective portfolio to address renewable integration concerns. By the time California hits our goal of 50% renewable energy, we will watch the duck fly right by-through much cleaner air.

Eric Borden originally wrote this column for California Energy Markets Sept. 23 Issue.