Overgeneration in California emphasizes the need for more energy storage

March 25, 2015

A recent announcement by Governor Jerry Brown ramped up the state's goals for renewable energy adoption, setting California's sights for 50 percent renewable capacity by 2030. As the nation's leader in renewables, it only seems logical that California would aim for such a high bar. However, the reality of the state's journey toward 50 percent capacity is rife with complications.

The pressing issue of overgeneration, for example, is being caused by an overabundance of renewable capacity. As recently as April 2014, operators reported instances of overgeneration in the state's grid, according to Breaking Energy. Thankfully, energy storage solutions already in use in the state can also be used to help resolve California's load issues.

All according to the duck curve
Greentech Media noted that California's rate of renewable overgeneration is well ahead of schedule, reaching predicted capacities from five years in the future along a graph of projected overgeneration levels referred to as the "duck curve."

The California ISO created future scenarios of net load curves to illustrate changing grid conditions. According to the CAL ISO, net load is the difference between forecasted load and expected electricity production from variable generation resources. During certain times of the year, as renewable energy ramps up, these curves produce a "belly" appearance in the mid-afternoon. Then when renewable energy drops off, traditional long start generational resources must quickly ramp and "arch" up similar to the neck of a duck - hence the name duck curve.

Essentially, the duck curve shows that overgeneration happens when renewable energy, in particularly solar, ramps up mid-day. More electricity is supplied than is needed to satisfy real-time electricity requirements. Therefore, the "belly" gets bigger and takes a bigger portion of the load. California's grid is currently flexible enough to withstand the extra power running through its systems, but this trend could spell trouble in the future for grid operators and renewable generators, especially if the state is gunning to accelerate its adoption of renewable energy.Eventually California will need to curtail renewable energy generation during certain periods to stop transmission lines from being overloaded.

In the past, overgeneration normally occurred in the morning when demand for energy was low  and power plants were coming online. With the advent of renewables however, overgeneration is expected in the middle of the day too.  Later during the day, when the renewable energy drops off, there is another problem--a sharp incline up to the neck of the duck.  This ramp up is the problem as slow starting traditional generation must act quickly to take over the load that renewables cannot meet. Energy storage is dispatchable and could help by replacing the lost renewables resources thereby reducing the slope of the duck's neck.

Thermal energy storage offers a potential solution
Another resource from Greentech Media emphasized that thermal energy storage could provide a suitable solution to the overgeneration problem. The proven solution of thermal energy storage offered in California and throughout the world could hedge peak air-conditioning use on hot summer days. Just a few hours of thermal storage operation could flatten the duck curve. With proper planning or demand response, buildings could  match AC cycling with solar load while reducing peak demand. A similar large scale deployment of thermal energy storage could serve as a means of storing renewable energy that exceeds the daily load and redistributing that power somewhere else.

Executives and end-users alike turning to storage
The thermal energy storage industry is coming into 2015 with plenty of momentum, and there are several signs that the technology will come into play to help solve the power grid's biggest problems.  A report by Transparency Market Research predicted that investments in thermal energy storage will continue to rise, enjoying a compound annual growth rate (CAGR) of 17.6 percent between 2014 and 2020, said Press Rocket Release. The research also noted that commercial and industrial applications lead thermal energy storage growth in the past, while rising demand for backup power along with rising energy prices will continue to drive growth.

A survey performed by Utility Dive showed that this energy storage optimism is shared by a large number of utility executives, according to Grist. Over 50 percent of respondent ranked energy storage as the top emerging technology that their company should be investing in going forward. The same executives also noted that aging infrastructure was their top problem, another indication that more energy storage investments are around the corner.