Renewable Energy Curtailment 101: The Problem That’s Actually Not a Problem At All

June 25, 2019
Public Domain
Mark Specht
Senior Energy Analyst

It’s that time of year again. The snow has all but melted and vivid memories of spring flowers begin to fade into the past. Once again, news stories start making the rounds proclaiming record amounts of renewable energy production in California. Renewable energy curtailment has also returned as a frequent early-summer news topic. But why?

It’s quite simple: in the spring and early summer, abundant sunshine, blustery winds, and rushing rivers all coalesce to produce ample amounts of renewable electricity. But all this happens at a time when mild temperatures mean that people aren’t using much electricity in the first place (You don’t usually need to crank the AC in the spring). Since the electric grid must always be balanced so that electricity generation exactly equals electricity usage, inevitably, there are times when there is more electricity available than we can possibly use. This excess electricity results in curtailment of renewables, which is a purposeful reduction in renewable electricity output below the levels that could otherwise have been produced.

So this is the time of year when we resume the annual discussion of all the curtailment records that have recently been broken and opine on the “problem” (or lack thereof) of all that wasted renewable electricity. While curtailment of renewable energy has been on the rise over the past few years in the California Independent System Operator (CAISO) service territory (see graph below), renewable energy curtailment still only amounts to a couple percent of all the renewable energy generated.

But let’s take a step back and ask a more basic question: why does all this curtailment happen in the first place?

Renewable energy curtailment in the CAISO has steadily increased over the past few years as California adds more wind and solar to the grid. However, only 2% of total solar energy in the CAISO was curtailed in 2018, and the CAISO expects only 3-4% of total solar energy to be curtailed in 2019. (The vast majority of curtailed energy is from solar, not wind.)

Causes of curtailment

There are two main reasons behind renewable energy curtailment: system-wide oversupply and local transmission constraints.

  1. System-wide oversupply is what most people think of when explaining renewable curtailment. This kind of curtailment occurs when, on a large scale, there is simply not enough demand for all the renewable electricity that is available. Examples of this occur frequently in California during the spring months when renewable energy production can exceed electricity demand.
  2. Local transmission constraints are an oft forgotten reason for renewable curtailment. This kind of curtailment occurs when there is so much renewable electricity in a local area that there is insufficient transmission infrastructure to deliver that electricity to a place where it could be used. A great example of this is in Texas, where wind energy curtailment fell from 17% in 2009 to 0.5% in 2014 mostly due to construction of additional transmission lines to move that wind energy out of local pockets to places where it could be used.

The CAISO, which operates the grid in most of California, keeps track of how much curtailment happens due to these two reasons. Surprisingly, in the first five months of 2019, just over half of all curtailment occurred due to local transmission constraints. And this isn’t an anomaly – roughly three-fifths of all curtailment in 2018 was due to local transmission constraints as well.

Reducing curtailment

In the first five months of 2019, over half of renewable energy curtailment in the CAISO was due to local transmission constraints.

Curtailment at low levels is more of a fact of life than a problem. In most cases, it simply does not make economic sense to build all the infrastructure (e.g. transmission lines or energy storage) that would be required to utilize every last drop of renewable electricity. But as levels of curtailment rise, instead of viewing curtailment as a problem, all that clean energy that would otherwise be wasted actually poses an incredible opportunity.

The CAISO has put forth a list of eight solutions (shown in the figure below) that could put excess renewable electricity to good use. The Union of Concerned Scientists has supported many of these solutions. For example, our 2015 study demonstrated that curtailment can be drastically reduced if renewable resources, such as solar and wind, are operated flexibly to provide the types of grid services that are currently provided by dirty natural gas power plants.

CAISO’s Eight Solutions to curtailment.

Let’s be smart about curtailment

This brings me to my main point, so listen up: to reduce curtailment, the type of solution required depends on the type of curtailment you are trying to reduce.

While any of the CAISO’s eight solutions could help address system curtailment, it’s a whole other story when it comes to local curtailment. Let me give a few examples:

  • Storage, demand response, and electric vehicles could all help alleviate system curtailment (provided they are operated to do so), but they won’t do much to alleviate local curtailment unless those technologies are deployed in the local area where curtailment is occurring.
  • Expansion of the Western Energy Imbalance Market could help alleviate system curtailment, but it won’t help very much with local curtailment. This expansion would allow excess renewable energy to be sold throughout the western United States, but local curtailment occurs because renewable electricity is trapped in a specific location and there’s not enough transmission to deliver it elsewhere. Increasing the number of entities participating in this west-wide market would not help reduce local curtailment because that electricity would still be trapped in local areas.

What this all means is that we need to be smart about how we approach the “problem” of curtailment. Every year, California sets a new record for the amount of renewable energy that’s curtailed. But that doesn’t mean you can use up all that energy by plugging in a battery at any old place on the grid. This means that, when investing in resources such as storage, we need to be smart about where we are making those investments, because putting it in the right place could increase the amount of excess renewable energy that’s available for the storage to soak up.

A word to the wise

So, in conclusion, curtailment isn’t a problem, it’s an opportunity. But for those of you who have been fantasizing about putting California’s excess renewable energy to good use by setting up a bitcoin mining operation or plugging in thousands of toasters to start an avocado toast factory, I have some bad news for you. No matter where you set up your new business, you won’t be able to take advantage of all of California’s curtailed renewable energy. However, if you put your avocado toast factory in exactly the right place, you might be able to soak up a little more renewable energy that would otherwise be curtailed.