March 9, 2013
Fossil Fuels Needed To Produce Grid-Scale Batteries For Clean Energy Storage Negate Benefits Of Green Power
Lee Rannals for redOrbit.com — Your Universe Online
Stanford University scientists have begun figuring out what grid-scale battery technologies are doing to the carbon footprint.
"We calculated how much energy it will cost society to build storage on future power grids that are heavily supplied by renewable resources," Charles Barnhart, a postdoctoral fellow at Stanford's Global Climate and Energy Project (GCEP) and lead author of the study, said in a statement. "It turns out that that grid storage is energetically expensive, and some technologies, like lead-acid batteries, will require more energy to build and maintain than others."
Researchers wrote in the journal Energy & Environmental Science about their findings on exactly how environmentally friendly these wind and solar farms are.
"Wind and solar power show great potential as low-carbon sources of electricity, but they depend on the weather," said co-author Sally Benson, a research professor of energy resource engineering at Stanford and the director of GCEP. "As the percentage of electricity from these sources increases, grid operators will need energy storage to help balance supply with demand. To our knowledge, this study is the first to actually quantify the energetic costs of grid-scale storage over time."
Only about three percent of electricity used in the US is generated from wind, solar, hydroelectric and other renewable sources. During the study, the authors considered a future where 80 percent of the electricity comes from these renewables.
The team used hydroelectric storage as an example in the study. When demand for electricity is low, surplus electricity is used to pump water to a reservoir behind a dam. Once demand becomes high, the water is released through turbines that generate electricity. The researchers compared the amount of energy required to build a pumped hydro facility with the energetic cost of producing five promising battery technologies, including: lead-acid, lithium-ion, sodium-sulfur, vanadium-redox and zinc-bromine.
"Our first step was to calculate the cradle-to-gate embodied energy," Barnhart said. "That's the total amount of energy required to build and deliver the technology — from the extraction of raw materials, such as lithium and lead, to the manufacture and installation of the finished device."
They used data collected by Argonne National Laboratory to help determine the amount of energy required to build each of the five battery technologies. This analysis revealed that all five batteries have high embodied-energy costs when compared with pumped hydroelectric storage technology.
"This is somewhat intuitive, because battery technologies are made out of metals, sometimes rare metals, which take a lot of energy to acquire and purify," Barnhart said. "Whereas a pumped hydro facility is made of air, water and dirt. It's basically a hole in the ground with a reinforced concrete dam."
After discovering this, the team calculated the energetic cost of maintaining the technology over a 30-year timescale.
"Ideally, an energy storage technology should last several decades," he said. "Otherwise, you'll have to acquire more materials, rebuild the technology and transport it. All of those things cost energy. So the longer it lasts, the less energy it will consume over time as a cost to society."
They came up with a new mathematical formula they called energy stored on investment, or ESOI. This formula is the amount of energy that can be stored by a technology, divided by the amount of energy required to build that technology. The higher the ESOI value, the better the storage technology is energetically, according to Barnhart.
After considering the ESOI formula, the team found the five battery technologies fared worse than the hydroelectric storage method.
"That means a conventional lead-acid battery can only store twice as much energy as was needed to build it," Barnhart said. "So using the kind of lead-acid batteries available today to provide storage for the worldwide power grid is impractical."
He suggests the best way to reduce a battery's long-term energetic cost is to improve its cycle life, which is the number of times a battery can charge and discharge.
"Pumped hydro storage can achieve more than 25,000 cycles," Barnhart said. "That means it can deliver clean energy on demand for 30 years or more. It would be fantastic if batteries could achieve the same cycle life."
He said a primary goal of the study was to encourage the development of practical technologies that lower greenhouse emissions and curb global warming.
"There are a lot of benefits of electrical energy storage on the power grid," he said. "It allows consumers to use power when they want to use it. It increases the amount of energy that we can use from wind and solar, which are good low-carbon sources."
Barnhart broke it down by saying the longer something lasts, the less energy you're going to use.
"You can buy a really well-made pair of boots that will last five years, or a shoddy pair that will last only one," he added. "I would like our study to be a call to arms for increasing the cycle life of electrical energy storage."
A study published in the journal Environmental Research Letters suggests large-scale wind farms may not be as energy efficient as previously thought. According to the study, large wind farm installations of 62 or more square miles might only be able to generate less than one kilowatt per square mile. Previous estimates showed these farms could generate as much as seven kilowatts per square mile.
“Our findings don´t mean that we shouldn´t pursue wind power–wind is much better for the environment than conventional coal–but these geophysical limits may be meaningful if we really want to scale wind power up to supply a third, let´s say, of our primary energy," said Harvard School of Engineering and Applied Sciences (SEAS) physicist David Keith.
He says he hopes the research gives a realistic expectation about wind as a source of energy.
“The real punch line,” he adds, “is that if you can´t get much more than half a watt out, and you accept that you can´t put them everywhere, then you may start to reach a limit that matters.”