For countries to switch their electricity generation from fossil fuel to renewable sources, they'll also have to dramatically reconfigure their electrical grids to be able to store electricity when there is oversupply. However, a study looking into the effectiveness of different batteries has found that the environmental savings from switching over may be negligible until better storage technology is developed.

When a wind turbine or solar panel generates power, it's not necessarily when that electricity is needed -- it could be the middle of the night, or during a holiday when lots of people are outside. That power needs to be stored somewhere so that it can be used, otherwise renewable energy can't ever replace coal, oil, nuclear or similar plants that can output a reliable level of power whenever needed.

For that reliability, there are three main options: pumped hydroelectric storage (PHS), where water is pumped upwards into a reservoir where it can be released later; compressed air energy storage (CAES) where the air can be expanded again through turbines when needed; and batteries, of which there are many different types, each with their own maximum number of effective charge cycles.


Currently, roughly 12.1 percent of the US's energy comes from wind, solar and other renewable sources, while the national grid has a storage capacity of only one percent. Climate and energy researchers Charles Barnhart and Sally Benson from Stanford University were curious as to which of these technologies would be best for a national grid if 80 percent of energy comes from renewables sources, taking into account the energy it would take to actually build each kind of storage. They compared PHS, CAES and five types of batter: lead-acid, lithium-ion, sodium-sulphur, vanadium-redox and zinc-bromine.

The researchers then worked out the cost of both building the technology and maintaining it over a 30-year timescale, chosen as the kind of length of time any realistic technology should be looking to deliver over to minimise replacement costs. They derived the "energy stored on investment" for each of the seven technologies to work out how much more energy it could store over its lifetime than it took to build and maintain.

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By far the best technology was PHS, which could store 210 times as much energy as is took to construct. The best lithium-ion batteries, by comparison, only managed a score of ten, while the worst batteries, lead-acid, had a measly value of two. Effectively, switching to these batteries is almost self-defeating.

This is in large part because battery technology currently can't handle enough charge cycles. Lithium-ion batteries can handle at most around 6,000 cycle, lead-acid batteries only 700, compared to more than 25,000 cycles for a PHS facility. Even though the material costs for large-scale batteries are more prohibitive than for PHS (rare-earth minerals versus what is often no more than concrete and steel), it's the lifecycle of batteries that we'll need to work on if we want to be able to rely on them as affordable parts of the grid.


Benson said: "The most effective way a storage technology can become less energy-intensive over time is to increase its cycle life. Most battery research today focuses on improving the storage or power capacity. These qualities are very important for electric vehicles and portable electronics, but not for storing energy on the grid. Based on our ESOI calculations, grid-scale battery research should focus on extending cycle life by a factor of 3 to 10."

As much as PHS might sound like a great idea, the reality is that you need a relatively hilly or, even better, mountainous landscape in which to build it. Those locations tend to be limited -- often they are within national parks or have other environmental qualifications, and those that aren't take up a huge amount of room. The largest such facility in the UK, Dinorwig in Wales, consists of 16km of tunnels but only generates 1.7GW of electricity at peak capacity -- that's only roughly two percent of the UK's total energy demand. Plus, people tend not to live in the mountains, so there are transmission issues in getting that power to far-away towns and cities.

There's better news for CAES, which had the highest ESOI value of 240. However, it will require huge tanks or caverns to fill with air, which again limits its viability to a limited number of areas.

That leaves us with the necessary task of improving battery life.

Barnhart said: "I would like our study to be a call to arms for increasing the cycle life of electrical energy storage.


It's really a basic conservative principal: 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."

The study has been published in Energy & Environmental Science.

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