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Guest Post: The Value of Short-term Energy Storage for Renewable Energy

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This is a guest post by Robert Fares, a graduate student at The University of Texas at Austin researching the benefits of grid energy storage as part of Pecan Street Inc.’s ongoing smart grid demonstration project. Robert is contributing a series of guest posts discussing grid storage technologies, and how storage could benefit the electric grid. You can read the first post in his series here.

A lot of the time, grid energy storage is discussed in the context of renewable energy. Storage is cited as a necessary solution to the diurnal nature of solar and wind energy. “We need storage because the sun doesn’t shine at night.” “Wind energy is greatest at night, so we need storage to shift wind energy to the daytime.” While there is some truth to these statements, they don’t reflect the real need for energy storage on our present grid.

Even intermittent sources of renewable energy like wind and solar tend to adhere to a predictable output schedule. The availability of wind, for example, naturally conforms to a probabilistic Rayleigh distribution. Using statistical analysis and other prediction tools, Texas’ grid operator has been able to integrate more wind energy with the grid than ever before, setting an instantaneous wind record of 8,521 megawatts in November. Furthermore, the temporal nature of wind energy varies based on where a wind farm is located. Experience in Texas has shown that wind farms on the West Texas plains output most at night, while coastal wind farms output most on summer afternoons. At the same time, research has shown that west-facing rooftop solar panels can produce more energy in the late afternoon than south-facing solar panels. The varying characteristics of different renewable energy sources and configurations permit grid planners to build a mix of renewables that loosely lines up with our demand for electricity.

Different intermittent renewable energy sources can be combined to loosely line up with electric load, so long-term storage is not a necessity. However, fast-ramping technologies like energy storage are required to compensate for the short-term volatility of wind and solar power production.

So why do we need storage then? As I discussed in my last post, our present electric grid operates totally on demand. The amount of electric energy generated must match the demand for electricity at every moment in time. Because of this fact, intermittent forms of renewable energy can have a destabilizing effect on the grid. Presently, renewables make up just a small part of our generation mix, so other generators can usually compensate for the intermittent nature of renewable energy to balance electric supply and demand. However, there are times when the volatility of renewable energy outpaces present electric generators. Because conventional generators rely on slow thermal and mechanical processes, they sometimes cannot match the pace of renewable energy fluctuations. A number of such destabilizing events have occurred on Texas’ wind-heavy grid. For this reason, the Federal Energy Regulatory Commission (FERC) issued order 755, which prompts organized electricity markets to enable the integration of novel, fast-acting resources capable of tightly regulating the balance between electric supply and demand.

Smaller-scale storage technologies like flywheels and batteries fill the niche opened by FERC order 755. Because a flywheel storage device operates by continuously spinning the shaft of an electric generator, it can compensate for renewable shortfalls almost instantaneously. A flywheel system manufactured by Beacon Power has been deployed in California to demonstrate the fast-ramping capabilities of flywheel storage devices.

Like a flywheel, a battery can rapidly produce electricity to compensate for shortfalls in renewable energy. A battery operates through chemical reactions, so it can adjust its power output in milliseconds to seconds, compared with minutes to hours for electric generators. Furthermore, a battery only needs a small amount of storage capacity to effectively back up renewables. For example, a purpose-built battery recently unveiled by Xtreme Power holds just enough energy to discharge for fifteen minutes. What does this mean? Smaller, less-expensive battery systems can fill a niche on our present electric grid.

By complementing renewable energy at this early stage of our transforming grid, storage establishes its role as a key grid technology going forward. For highly scalable technologies like redox flow batteries, the space opened by order 755 could pave the way to large-scale electricity storage in the future. This development could fundamentally decouple electric supply and demand in time, making the grid more robust and enabling the widespread use of renewable energy.

Photo credit: the figure in this post comes from the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy’s Solar Energy Technologies Program. It can be found in this document, which explains the ideas behind this post in more detail.

Robert Fares is Ph.D. student in the Department of Mechanical Engineering at The University of Texas at Austin. As part of Pecan Street Inc.’s ongoing smart grid demonstration project, Robert’s research looks at how energy storage models can be used with large-scale data and optimization for economic operational management of battery energy storage. Robert hopes to develop novel operational methods and business models that help to integrate distributed energy generation and energy storage technologies with restructured electricity markets and retail electric tariffs. Through his research, he hopes to demonstrate the marketability and technical compatibility of these new technologies.

The views expressed are those of the author and are not necessarily those of Scientific American.

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