Energy storage: it’s about time

2014-04-08 - Power and energy. Most of us would probably consider these terms to be synonymous most of the time, and usually that’s fine - they do mean the same thing a lot in conversation. In engineering circles, however, they mean two distinctly different things.

The difference becomes especially important if you’re talking about energy storage. “Power” refers to an instantaneous measurement, expressed in watts, of the rate of flow of electricity. “Energy” takes that rate of flow and adds a length of time so that the unit of measure is the watt-hour (or kilowatt-hour, megawatt-hour, etc.)

We might say that a battery is rated at 16 kilowatts, but that only tells us that it can deliver that amount of power at a given moment in time. In energy storage applications, that’s important, but what’s often just as important is how long the battery can keep it up, and that’s energy.

This critical distinction is what makes one type of energy storage device better than another for any given application, and is what ABB’s Stephen Clifford focused on during his presentation on battery energy storage systems (BESS) today at Hannover Messe.:

“In a nutshell, timescale drives technology,” he says. Some storage technologies lend themselves to short bursts of high power while others are better at steady supply over a long period of time.

Lithium-ion batteries have become well known for their use in personal electronics, and it might be tempting to equate six hours of battery life in a laptop computer with long life in larger applications. However, at grid scale, Li-ion batteries are best used in short-term emergency backup situations requiring discharge of an hour or less. It’s not that a BESS running on Li-ion batteries can’t run longer, only that it would take more batteries and thus incur more cost.

At the other end of the spectrum is sodium sulfur (NaS). Storage systems using these batteries can deliver power for several hours at a time, but they lack the punch required for short-term applications. To use a metaphor from the sports world, the NaS battery would be the marathon runner and the Li-ion the sprinter.

Energy storage has received increasing attention in the last few years, partly because of regulatory changes such as California’s initiative to install more than 1,000 MW (that’s power, remember?) of storage by 2020. But storage technologies are also advancing rapidly.

Going forward, it’s important to keep in mind that batteries all have particular characteristics that make them suited to various tasks. Understanding the difference between power and energy is a good place to start.

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