As utility-scale solar and wind have grown exponentially in recent years, most of the attention has been focused on just the growth of these technologies themselves, not necessarily on the consequences of this growth on the grid, and the grid’s capabilities to provide dependable and reliable service at the right levels, in the right locations, at the right times.

In other words, for solar and wind generation to become even more significant and useful components of the grid, thought must be given to how they will be “balanced” with other generation sources.

According to a new report from the U.S. Energy Information Administration (EIA), one ISO is already wrestling with this issue and trying to create some viable plans.

According to the report, the California Independent System Operator (CAISO), which is the grid operator for most of the state, is increasingly curtailing solar- and wind-powered electricity generation, in an effort to balance supply and demand during the rapid growth of wind and solar power in California.

“Grid operators must balance supply and demand to maintain a stable electric system,” said the EIA. The output of wind and solar generators are reduced either through price signals or rarely, through an order to reduce output under one of two circumstances: during periods of congestion (when power lines don’t have enough capacity to deliver available energy), or as a result of oversupply (when generation exceeds customer electricity demand).

In CAISO, according to the EIA, curtailment is largely a result of congestion. “Congestion-related curtailments have increased significantly since 2019. because solar generation has been outpacing upgrades in transmission capacity,” said the EIA.

In 2022, CAISO curtailed 2.4 million megawatthours (MWh) of utility-scale wind and solar output, a 63 percent increase from the amount of electricity curtailed in 2021. As of September 2023, CAISO has curtailed more than 2.3 million MWh of wind and solar output so far this year.

The EIA noted that solar accounts for almost all of the energy curtailed in CAISO – 95 percent in 2022, and 94 percent in the first seven months of 2023. “CAISO tends to curtail the most solar in the spring when electricity demand is relatively low (because moderate spring temperatures mean less demand for space heating or air conditioning) and solar output is relatively high,” said the EIA.

The report went on to note that CAISO has increasingly been curtailing renewable generation as renewable capacity has grown in California. In 2014, a combined 9.0 gigawatts (GW) of wind and solar capacity had been built in California. As of July 2023, that number had grown to 17.6 GW. Developers plan to add another 3.0 GW by the end of 2024.

In specific, CAISO is exploring and implementing various solutions to its increasing curtailment of renewables, including:

1 – Using the Western Energy Imbalance Market (WEIM), which is a real-time market that allows participants outside of CAISO to buy and sell energy to balance demand and supply. In 2022, more than ten percent of total possible curtailments were avoided by trading within the WEIM. A “day ahead” market is expected to be operational in Spring 2025.

2 – Expanding transmission capacity to reduce congestion. CAISO’s “2022-23 Transmission Planning Process” includes 45 transmission projects to accommodate load growth and a larger share of generation from renewable energy sources.

3 – Promoting the development of flexible resources that can quickly respond to sudden increases and decreases in demand, such as battery storage technologies. According to the EIA, California has 4.9 GW of battery storage, and developers plan to add another 7.6 GW by the end of 2024. Renewable generators can charge these batteries with electricity that would otherwise have been curtailed.