Radical New Flow Battery: Look Ma, No Membrane!
A team of researchers at MIT has come up with a membrane-free flow battery, and that could mean that the solar powered future we’ve been looking forward to is even closer than we thought. Flow batteries have a huge advantage when it comes to utility-scale energy storage, especially for intermittent sources like solar power and wind, too, for that matter.
The problem has been that conventional flow batteries work by separating two streams of liquid (hence, “flow”) with a membrane, and coming up with an efficient but cost-effective membrane has been one of the obstacles to engineering a commercially viable system.
A Membrane Free Flow Battery
The MIT flow battery solution is elegantly simple: get rid of the membrane! Now, that’s been tried before with mixed results, but the MIT team seems to be on to something. So far, their palm-sized prototypes have achieved a power density significantly higher than other membrane-less designs, and “an order of magnitude” beyond lithium-ion batteries.
To understand how it works, let’s have a brief recap of basic flow battery technology. The idea is to pump a solution of metal ions dissolved in an electrolyte through an electrochemical cell, in which another liquid awaits. Separated by a membrane, the two liquids exchange ions to create an electric current.
The advantages almost hit you over the head. Aside from the membrane, almost the entire system consists of tanks and pumps, which means that you have the potential for an extremely low maintenance, long-lifespan piece of equipment that can be scaled up with a few relatively simple engineering tweaks. In addition, flow batteries can sit idle indefinitely and be called into action quickly when needed.
As described by MIT writer Jennifer Chu, a hydrogen-bromine combination is potentially ideal for flow batteries, since both are relatively cheap and abundant. However, hydrobomic acid likes to munch on membranes, which severely curtails the battery’s lifespan.
To get rid of the membrane, the MIT team relied on a form of parallel flow called laminar flow, in which two liquids stay on their respective courses with little mixing, even though no separating membrane is present.
Basically, the new battery was engineered with a slim channel between two electrodes. As Chu explains:
“Through the channel, the group pumped liquid bromine over a graphite cathode and hydrobromic acid under a porous anode. At the same time, the researchers flowed hydrogen gas across the anode. The resulting reactions between hydrogen and bromine produced energy in the form of free electrons that can be discharged or released.”
At room temperature, the battery achieved a maximum power density of 0.795 watts of stored energy per square centimeter. That puts the device on track for the golden $100 per kilowatt-hour mark, which is the goal set by the U.S. Department of Energy for commercially viable, utility-scale advanced battery storage.
MIT Is Not The Only Member Of The Membrane Free Club
The membrane free flow battery thing is becoming a bit of a horse race, since a team at Stanford University has come up with an alternate approach, using a single stream of liquid (a lithium polysulfide solution) making contact with a piece of lithium metal.
The Stanford team has been working with the Department of Energy’s SLAC National Acelerator Laboratory on the project, which coincidentally has also resulted in a palm-sized prototype.
Meanwhile, over at Pacific Northwest National Laboratory, a company called UniEnergy Technologies has been working with federal researchers to develop a flow battery that packs more energy into a smaller system, based on vanadium ions and hydrochloric acid.
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Palm-sized prototype?? Very, very preliminary. Let us know when there’s a unit that can power a house reliably for a year.
Doesn’t sound like it was in a closed cycle.
Where does the hydrogen gas come from? Is it created when charged and consumed when discharged?
Good question – my understanding is that H2 must be consumed during discharge (H2 + Br2 => 2HBr + energy). Therefore this system will struggle to achieve a competitive cost per kWh for long-term energy storage due to the current high cost of H2 storage tanks/methods (which also limits the competitiveness of traditional H2 fuel cells in the long-term application). However for short-term usage (e.g. a few hours) this is less of an issue (smaller tanks) and it appears that this technology could deliver a very competitive cost per kW (power) for grid balancing. There appear to be some advantages over H2-O2 fuel cells (such as better round-trip efficiency, higher current density and cell potential) which additionally help to deliver a more cost-effective system. However for off-grid applications a system which utilises only liquid reactants would be preferable as it would provide a cost-effective capability to store much larger amounts of energy on weekly or even seasonal timescales.
“0.795 watts of stored energy per square centimeter. That puts the device on track for the golden $100 per kilowatt-hour mark”
Comparing apples and meat are we? 0.795 watts/ cm2 is the power density of the device. It has nothing to do with stored energy. Which you can’t measure in cm2 since the electrodes are fluids with volume.
While it’s not a simple unit conversion to go from watts/cm2 to $/kWh, the power density is a significant driver of cost. If you double the power density, the stack size is cut in half, so you need half of the electrode material, half of the gaskets, half of the current collectors — basically everything except the tanks and electrolyte. 0.8 W/cm2 is a very good power density. Now I am still a little bit skeptical of membraneless designs because I don’t see how to achieve very high Coulombic efficiencies, but hopefully I’ll be proven wrong. Also, I don’t like that their round trip voltage efficiency was calculated using two different electrolytes. I’d probably subtract a few percentage points from their number.
And the end of the MIT laminar flow battery:
1) WIPO rejected every claim in their patent application
https://patentscope.wipo.int/search/en/detail.jsf?docId=WO2012039977&redirectedID=true
2) MIT is still trying to license a patent with no claims:
http://technology.mit.edu/technologies/14232_laminar-flow-fuel-cell-incorporating-concentrated-liquid-oxidant