Coal is suffering death by a thousand cuts, what with cheap natural gas and plummeting costs for wind and solar power, and now here come our friends from the Energy Department’s Pacific Northwest National Laboratory to deal the knockout punch. They’ve come up with a clean, safe energy storage solution that also doubles as a much-needed way for cities to shore up their power resiliency in case of grid outages.
Since you already know this special something is a flow battery (check that headline again if you missed it), you might also know that along with our sister site Gas2.org, we’ve been fangirling over this new flow battery or that one for some time now. This latest effort from Pacific Northwest National Laboratory (PNNL) offers yet another new angle.
Energy Storage For 21st Century Cities
Now that we’re all thisclose to climate change and the damage from extreme weather events is piling up, US communities are experiencing the drawbacks of a centralized, regionally connected, and aging grid system that can cut off your juice right when you need it the most.
As part of the solution, the Energy Department has been gung-ho on getting communities to adopt more locally generated renewable energy, the latest example being the new SPARC initiative.
The missing piece of the puzzle is onsite energy storage, and that’s where things get sticky. In tight urban spaces where land is at a premium, the opportunities for large-scale local energy storage are rare.
Lithium-ion batteries could provide a solution, but although the technology has a good safety record in autos and portable devices, scaling them up for community use in densely populated areas typically presents a hazard best avoided if at all possible.
That’s where flow batteries come in. Like the name says, flow batteries literally work from movement. When certain liquids flow next to each other, separated by a membrane (or sometimes without, as the case may be), they can generate an electrical current.
For those of you keeping score at home, the liquids are called electrolytes (in a conventional battery, the electrolyte is the part that holds the charge).
When not in motion, the two electrolytes are stored in separate tanks, and that setup offers some huge advantages over conventional batteries for large-scale energy storage. The liquids don’t interact with each other, reducing the chance of unintended or accidental reactions that could lead to fire or other hazards. Hazards are further reduced if non-toxic liquids are at play, and flow batteries can sit idle for a long time without losing their charge, making them ideal for emergency backup.
Until recently, flow batteries were huge, bulky affairs, but next-generation flow batteries are much more compact and versatile.
A Countertop Flow Battery With A Powerful Punch
To be clear, the new PNNL flow battery is still in the lab — the working model was literally built on a countertop, but the folks at the lab are optimistic about the potential for scaling up.
Here’s how it works. Before the PNNL battery receives a charge, both of its storage tanks hold the same liquid electrolyte. It consists of positively charged zinc ions and negatively charged iodide (iodide is a form of iodine, but with a different charge).
When charged, one of the tanks gets an additional negative ion. That tank is the negative side, but let’s have some fun and call it the dark side.
Once you put the two liquids together (separated by that membrane, of course), the zinc ions in the other tank come over to the dark side and transform into metallic zinc. That’s the meat of the process that generates electricity.
The countertop model has a 12-watt-hour capacity, which PNNL compares to about two iPhone batteries, so we’re a long ways away from powering entire cities.
However, according to PNNL, those 12-watt-hours stack up against the competition:
The demonstration battery put out far more energy for its size than today’s most commonly used flow batteries: the zinc-bromide battery and the vanadium battery. PNNL’s zinc-polyiodide battery also had an energy output that was about 70 percent that of a common lithium-ion battery called a lithium iron phosphate battery…
Lab tests revealed the demonstration battery discharged 167 watt-hours per liter of electrolyte. In comparison, zinc-bromide flow batteries generate about 70 watt-hours per liter, vanadium flow batteries can create between 15 and 25 watt-hours per liter, and standard lithium iron phosphate batteries could put out about 233 watt-hours per liter.
As for comparisons with other flow batteries, PNNL is touting the model’s lack of acidity, relatively low cost, and ability to operate efficiently in climate extremes ranging from -4 to +122 degrees Fahrenheit.
Dendrite: The Dark Side Of The Dark Side
We weren’t actually kidding when we said the dark side. A key issue that PNNL is going to have to resolve is a problem common to this sort of battery — the formation of dendrite, which is a hairy, fernlike buildup of metallic zinc on the membrane.
So far, the solution has been to douse the dendrite with alcohol, so the next steps — in addition to building a 100-watt-hour model — include fine tuning that approach.
Meanwhile, last year, we were sniffing around the potential for using flow batteries for electric vehicles, and it looks like PNNL has that covered as well.
According to PNNL, it takes about 350 watt-hours for a typical electric vehicle to go one mile in urban driving. Based on the results of the lab tests, PNNL estimates that the new battery could crank up to 322 watt-hours per liter of electrolyte. File that under “n” for “not there yet but maybe some day.”
Image Credit: PNNL flow battery courtesy of Sandia National Laboratory.
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