Electrochemical flow capacitor: Hybrid battery-supercapacitor design targets grid storage


July 17, 2012

Drexel University research combining the best features of batteries and supercapacitors could lead to a more stable, greener energy grid (Photo: Shutterstock)

Drexel University research combining the best features of batteries and supercapacitors could lead to a more stable, greener energy grid (Photo: Shutterstock)

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Researchers at Drexel University are developing an electrochemical flow capacitor (EFC) that combines the storage capabilities of batteries with the much longer cycle life and power output of supercapacitors. The team's goal is to improve the stability of the energy grid and ease the integration of renewable energy sources.

Renewables such as wind and solar power are experiencing an exponential year-by-year growth, but integrating such an intermittent and unpredictable power source into the grid can be problematic as it calls for a highly flexible, cost-effective solution that can store vast amounts of energy and release it quickly whenever needed.

While batteries can store large amounts of energy, they cannot dispense it quickly and they can only survive about one thousand charge-discharge cycles. Conversely, supercapacitors can release energy in quick bursts and last for hundreds of thousands of cycles, but they can't store quite as much energy. Recent advances in both battery and capacitor technology have been working toward a perfect marriage of the two.

The EFCs developed at Drexel University are another step in this direction and are aimed at large scale usage. The device consists of a cell connected to two external reservoirs, each containing a mixture of electrolyte liquid and charge-carrying carbon particles. The uncharged slurry is pumped from the reservoir tanks into the flow cell, where the stored energy is transferred to the carbon particles. Once charged, the slurry can be stored in large tanks until the energy is needed, at which time the entire process is reversed.

This conceptually simple approach promises to be both scalable and cost-effective: EFCs can survive hundreds of thousands of charge-discharge cycles, their power output is comparatively large - controlled by the size of the electrochemical cell - and their capacity is proportionate to the size of the tanks that contain the carbon-electrolyte slur.

The researchers still need to overcome some challenges, chief among which is the low energy density of the slurry, which currently requires very large tanks for storage. They are now developing new slurry compositions to increase its energy density tenfold which, they believe, would be enough to make this technology practical.

A paper describing this energy storage concept was recently published in the journal Advanced Energy Materials.

Source: Drexel University

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Dario Borghino Dario studied software engineering at the Polytechnic University of Turin. When he isn't writing for Gizmag he is usually traveling the world on a whim, working on an AI-guided automated trading system, or chasing his dream to become the next European thumbwrestling champion. All articles by Dario Borghino

Maybe this is the technology that is desperately needed by solar power farms in order to release electricity overnight, and for wind power in quiet periods. What makes this an excellent solution is that electricity is not stored inside the battery, but in large tanks.


I always find myself worrying, when there is talk about pumps, that the pumping will make the whole process a pointless exercise.


digi_owl, youre comment could be amusing if you were talking about something much bigger, like life itself. almost every mammal has a pulse, created by a pump called a heart.

life can be a pointless exercise yea. but pumps do seem to be pretty efficient. they help move and mix disparate levels of things. heat pumps, chemical pumps, salt pumps. its all about motion, and if you think about a generator as creating a voltage, youre talking about pumping elecriicty.


Why low-energy density, high viscosity slurry? How about using compressed lithium-oxygen gas = high energy density. The gas mixture could be charged and compressed hydraulically as in an accumulator, storing both thermodynamic energy and electrical energy.

The system could be arranged such that a lithium ion-gas accumulator is simultaneously electrically charged and hydraulically pressurized, the compressed charged gas mixture can then be stored in a pressure vessel until needed, at which point a valve can open, releasing the compressed gas accross a flow capacitor, possibly also driving an-air motor generator at the same time = instant electricity!


Hmmmm. Automobiles have pumps and tanks. I wonder if this could be scaled down to provide an alternative to battery packs, assuming you could get the kind of quick response that a vehicle would require for acceleration, etc. The slurry might be regenerated via either a quick plug-in charge, or a trickle charge from an IC-powered generator. But even with a tenfold improvement in slurry charge density it may be too large a system.

Bruce H. Anderson

This isn't an answer just yet. It has no great energy storage capacity. Why do the creators think the electrolyte slurry have a long life? I couldn't see the answer in the article.

Guy Macher

to PeetEngineer:

Wouldn't the fact that the lithium-oxygen gas, being highly compressed result in a lower energy efficiency, as energy must be diverted into running compressors (PV=nRT --> p*(dV)=nr(dT))


lithium-oxygen gas ? sounds absolutely frightening? there was a big fire in Vancouver B.C. rather dangerous big fire. It was a Li battery recycle plant. Tell me why Li and O in gaseous form is not a disaster begging to happen.


What is lithium-oxygen gas? Does it exist?


walt ` what about the explosive properties of gasoline?

Michael Kamrath

lol, plz do your homework before post a hilarious comment. lithium-air battery has nothing to do with Li-O gas (not sure if it does exist).

Min Hyun

PeetEngineer, Lithium Oxide is a ceramic at STP. not a gas. Melting point is about 1800 degrees C. It also reacts with water, like Lithium metal does, so probably not a good choice of material. Also, a condenser is just a type of capacitor.

Questions about this process are;

cycle efficiency. How much of the stored energy is recoverable.

discharge time. How long does it take to get the energy back out.

conductivity conditions. They are basically making a large Leiden jar. If the charge of the capacitor system is held in two tanks, the charge will average out. What controls the whole charge conditions. Why bother with the pumps, just run wires into the two tanks like Ben Franklin did.

what effects will settling have on the suspension.

In free systems, the charge travels to the outside of the container/wire. What keeps the charge on the carbon particles. Water with ANYTHING in suspension/solution is a conductor. Does this system have a way of keeping the water pure, or does it really use an oil suspension?

Min Hyun, Aluminum Air batteries have the highest energy density of any battery system, but they are not rechargeable. This system is claimed to be rechargeable, but there is no information given about things like energy density, or discharge rates/times.

Speaking as a Professional Engineer, these are things that I would want to know about before recommending such a system for commercial use. Right now, battery systems are the best, specifically molten Lithium cells. But they see limited use, as the overall system efficiency is less than 50% energy in to energy back out.

Given that, you can see why in real utility use, coal, nuclear or hydro is used for base load (minimum demand conditions) and things like gas turbines or 'renewable energy' are used for peaking. Energy storage is just not efficient enough. You have to put in two megawatt-hours or more for every megawatt-hour you get back.

Pumped Hydro gives a little better than this, but it isn't feasible everywhere. Still no good solutions to make Wind/Solar actually doable. I'm eagerly awaiting the figures on actual use of pumped air storage.

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