A quantum leap in electrical energy storage

By on Jan 7, 2009 in Physics |

Something I heard about on Security Now (Ep.117) has got me rather excited and I wanted to report about it on here.

Batteries vs Capacitors

Standard mobile phone Li Ion battery

Standard mobile phone Li Ion battery

Currently, the state of the art for energy storage in contemporary electronic devices is Lithium Ion batteries. Despite the material used (the electrolyte), batteries are all pretty much the same, in as much as a chemical reaction occurs inside the battery which drives electrons that our electronic devices can use to do work. Rechargeable batteries use a reversible reaction, so that once you do electrical work on the battery (i.e. charge it), the reaction is reversed and can begin again to power your devices.

There are a number of disadvantages to this. Firstly, because we are reversing the chemical reaction of the battery, it takes quite a while to charge them. Also, again because of relying on a chemical reaction, disposing is difficult because the contents are toxic. Also, rechargeable batteries degrade over time, so there are only so many times that you can discharge and recharge them.

Various capacitors

Various capacitors

In electrical circuits energy can stored in components called capacitors. Capacitors aren’t used in place of batteries because the amount of energy you can store for a given size (i.e. their energy density) has been tiny. A little bit of physics for you here, which you need to know to understand the significance of what I’m leading up to telling you! The standard unit of capacitance is the Farad. Most capacitors you come across are rated at the micro Farad level (i.e. a millionth of a Farad), ranging up to a few hundred micro Farads. In my personal experience I have only seen 1 Farad capacitors for car stereo systems, although the things are massive and expensive.

Diagram of a capacitor

A capacitor is two parallel metal plates (connected to a circuit) sandwiching a non-conducting material called a "dielectric".

Capacitors work differently to batteries. The general model of a capacitor is two parallel metal plates separated by a non-conducting material called a “dielectric“. The idea is that when you apply a voltage to a capacitor no current can flow through, but instead the electrons pile up at the cathode and create an electric field within the dielectric. Therefore, energy is stored in a capacitor as an electric field. Hence, a desirable dielectric material is one in which electric fields can easily form (which is a physical property known as permittivity). There are some limits in which you can rely on capacitors, first all there is something called the “breakdown voltage“, which is the limit of voltage you can apply to the capacitor before the current starts forcing its way through (and thus damaging) the dielectric. The amount of voltage you can safely apply to a capacitor is the limiting factor in how quickly you can charge it to its full capacitance. There is also a “leakage current“, which is the rate at which the capacitor looses energy, due to the dielectric having a very small amount of electrical conductivity.

The advantages of all this over batteries is that you can charge up a capacitor much more quickly than an electrochemical battery due to there being no chemical reaction to reverse, and there is no limit to the number of times you can charge and discharge a capacitor. The latter point is great because you have a much more durable storage system and you don’t have to worry about disposing of toxic chemical batteries.

The next generation

Now, I can hear you saying wouldn’t it great if we could have a capacitor that could store as much energy as a Lithium Ion battery, in the same size package? Well, funny you should say that, because a patent as been filed by a company called EEStor for something called a Super Capacitor that has a huge capacity, huge breakdown voltage, and twice the energy density of Lithium Ion technology.

An EeStor Cell

An EeStor Cell

The patent that Eestor has filed details a device with a capacity of 31 Farads (remember before we were talking in millionths of a Farad?), with a breakdown voltage of 3,500 Volts (and it could even tolerate up to 5,000 volts). The amount energy this device can store is around the 52KWh (187 Mega Joules), with a volume of around 33 Litres, this has an energy density of around 1 Mega Joule per cubic metre, which is about double that of Lithium Ion. You could charge this thing at 3.5MV 3.5KV in around 5 minutes! While Lithium Ion batteries slowly discharge themselves at around 5% a month, according to EEStor, the super capacitors will only discharge at 0.1% per month.

The Tesla Roadster

The Tesla Roadster

Just to give you some sense of scale of this, consider the first commercial electric car that is currently being made, the Tesla Roadster. Currently, its Li Ion battery can be charged with 53KWh of energy in 3.5 hours at 375 volts, and has a range of 250 miles. With a super capacitor, it could be charged in 5 minutes and go twice as far.

What’s more, EEStor claim to have taken one of the super capacitor units (a small subsection of the 52KWh model), and performed one million charge/discharge cycles, then tested the unit’s performance – No degradation!

Potential uses

Of course, when we are all charging our electric cars at home in the future, we are not going to have megavolt power supplies. Although the idea is that these super capacitors could be used as energy buffers. You slowly charge up a super capacitor in your home, over night while the electricity is cheap. Then in the morning, you can zap all the power from your home capacitor into your car’s capacitor. This will really help power companies smooth out the power demands.

Energy buffers won’t just be useful for homes, think about renewable energy sources like wind power or solar. This could be an efficient way to store energy which is coming in at unpredictable times and amounts too. Come to think of it, attach solar panels to your house and they could top up your home capacitor during the day!

As I mentioned above homes won’t be equipped to charge these large capacitors in super fast time, and will instead have to slowly trickle charge on their domestic supplies. Although I imagine we’ll have the electrical equivalent of petrol stations, equipped with high voltage supplies to zap power into our car capacitors as quickly as possible.

Other interesting features of this technology is that it has been designed to be mass producible and scalable, based on integrated circuit technology. This means that the same technology can make up much smaller energy storage devices to power our mobile phones and laptops, etc. . This is where things get fun because these sorts of devices do operate on domestic voltages. Think about it we could get two to three times as much life span out of a charge on our mobile phone or laptop, but have a full recharge in 5 seconds!

Plausibility

From what I have read, there are some good reasons to think that this is not some sort of grandiose wishful thinking. Firstly, Kleiner Perkins Caufield & Byers have put venture capitol into this. This is the same company who first funded Amazon and Google. EEStor has signed a contract with Lo√°ckheed-Martin for military and homeland security uses. Also, the material they are using as their dielectric (based on Barium Titanate) is in the same group of materials that other researchers are researching into for the same goal.

  • This blog post has been based on information I have gathered from episode 117 of Security Now and the Wikipedia article for EEStor.

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