# Measuring the costs of electrical storage

April 25, 2013

Several alternative-energy sources will only be practical if power companies have an economical way to store harvested energy. And U. S. grid operators will need methods of storing electricity to balance and maintain the national grid if alternative sources are to provide a majority of the nation’s power. With that in mind, researchers at Stanford University developed a way to calculate the costs of building and deploying a variety of energy-storage technologies.

The team evaluated pumped hydroelectric storage as well as lead-acid, lithium-ion, sodium-sulfur, vanadium-redox, and zinc-bromide batteries. (Pumped hydro involves pumping water up into a reservoir using surplus electricity during low-demand times, then running water down through generators during times of high demand.) The team determined the amount of energy necessary to build each of the five storage devices and found that batteries have significantly higher energy costs to be practical compared to pumped hydro.

“This is somewhat intuitive because batteries are made of metal, and sometimes rare metals, which take a lot of energy to acquire and purify,” says researcher Charles Barnhart. “Whereas a pumped hydro facility is made of air, water, and dirt. It’s basically a hole in the ground with reinforced concrete.”

The team next determined how much energy would be needed to maintain the devices over 30 years. They devised a mathematical term, energy stored on investment (ESOI), calculated by dividing the amount of energy a device can store by the energy needed to build it. So the higher the ESOI, the better.

Pumped hydro ends up with an ESOI of 210, meaning it can store 210 times more energy than it needs to build it. Batteries all had lower ESOIs with lithium-ion batteries checking in with an ESOI of 10 and lead-acid versions bringing up the rear with an ESOI of 2.

The team concluded that the best way to lengthen batteries’ ESOI was to lengthen their cycle life or the number of times they can be charged/discharged. Pumped hydro, for example, can endure 25,000 cycles, equivalent to 30 years or more. Lithium-ion batteries survive 6,000 cycles, and lead-acid batteries give up the ghost after only 700 cycles.

The Stanford scientists then examined the materials needed to build these storage devices. They found that materials were not as expensive as the energy requirements, with a few exceptions. For instance, cobalt for lithium-ion batteries and the vanadium needed in vanadium-redox batteries are rare and getting expensive.

The team also considered compressed-air energy storage. It entails using surplus electricity to compress and pump air into an underground cavern or aquifer, then releasing it through a turbine to generate electricity when needed. This method has the highest ESOI, 240, and the lowest material costs. But similar to pumped hydro, researchers found there are a dwindling number of geologically well-suited locations.

Resources:Stanford University,www.stanford.edu

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