A 2 MW battery the AES Huntington Beach power plant.

Energy storage (through batteries) is something we use everyday in our cell phones and computers. So it may be a little surprising that when it comes to the electric grid, storing energy is something that’s rarely done.

California’s grid is designed to deliver electricity on a real-time basis. Every four seconds, the grid operators at the California Independent System Operator have to ensure that the energy supply meets the demand in the state – something that’s known as “balancing the grid.” (You can check out today’s electricity forecast on their site). As a result, they coordinate the one piece of the system that they have control over: the generators, like natural gas plants.

Luckily, most generators produce a steady power supply. But California is adding increasing amounts of solar and wind power to the grid each year.  Since the output of a solar or wind farm depends on the sun or wind, the power they produce is variable (here’s a time-of-day profile of renewable energy on the grid today).  That causes problems for the grid operators on a number of levels.  Wind farms produce most of their power at night, but that’s when demand for power is lowest. Solar farms using photovoltaics can drop off substantially when the sun disappears behind clouds. And large solar thermal farms ramp up extremely fast when they are first hit by the sun in the morning.

Energy storage is one of the ways that utilities and grid operators can address this intermittency.  By having some extra electricity on hand, they can smooth out the bumps caused by these renewables.  Just how to store energy is another issue.  Here are some of the ways it can be done.

Pumped Hydro

In the energy storage world, this is as old school as it gets.  Hydro power uses water and gravity to generate electricity.  Storage is added by pumping that water back uphill to the reservoir, so it can generate power again.  Of course, it takes electricity to run the pumps, but usually this is done a night when there is cheaper or excess power on the grid. California’s largest pumped hydro facility is PG&E’s Helms Pumped Storage Project outside of Fresno, which has a 1.2 gigawatt capacity (for more on how it works, check out this powerpoint). PG&E is reportedly looking at 2 gigawatts of new pumped storage at two other sites in California.


There are a number of different kinds of batteries that can be used in grid-scale installations. I visited a 2 megawatt battery in Southern California that uses lithium-ion cells, much like a hybrid car uses. Southern California Edison is working on an 8 MW battery project near the Tehachapi wind farms.  But lithium-ion technology has plenty of competitors, many of which have been awarded federal stimulus funding.  The primary barrier for batteries is the cost. A Beacon Power flywheel.


This technology uses rotational energy to store power. Flywheels have an internal rotor that uses electricity to spin at high speeds.  When energy is needed, the rotor slows down and generates electricity through a motor.  This is used for what’s known as “frequency regulation” on the grid.  Since they can charge and discharge power on a second-to-second basis, flywheels can smooth out the short-term fluctuations on the grid. Beacon Power has installed flywheels in Tehachapi, California as part of a demonstration project there.

Compressed Air

Using energy produced at non-peak times (at night), compressed air energy storage projects pump air into large underground caverns. When demand for energy is high, it’s released to run power turbines. PG&E is now planning a 300 MW compressed air facility in Kern County.

Of course, for all these technologies, cost is major issue, not mention the siting and planning considerations. For a good comparison, check out thesetechnology comparison charts from the Energy Storage Association.


Listen to Energy Storage: The Holy Grail radio story online and check out the rest of our stories in the 33×20 renewable energy series.

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Reporter’s Notes for Energy Storage: The Holy Grail 2 October,2015Lauren Sommer


Lauren Sommer

Lauren is a radio reporter covering environment, water, and energy for KQED Science. As part of her day job, she has scaled Sierra Nevada peaks, run from charging elephant seals, and desperately tried to get her sea legs - all in pursuit of good radio. Her work has appeared on Marketplace, Living on Earth, Science Friday and NPR's Morning Edition and All Things Considered. You can find her on Twitter at @lesommer.

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