Energy Storage Systems Episode 5 : Supercapacitors, better than batteries?
Hey guys, I hope you're all doing well in this very fine day! I am back with another article in the Energy Storage System series, I hope you'll enjoy it!
Remember when you first had sex? The dream of all teenagers and a moment you had been waiting for your entire life, your heart was pounding, it was YOUR time to shine! But, one thing led to another and before you knew it, it was already over. Wait, was that just me?
Aaaanyways, that’s pretty much how a supercapacitor works.
In 1957, H. Becker of General Electrics patented what is known today as a supercapacitor, it stored much more energy than a regular capacitor but there was a slight problem: he didn’t quite understand the reason behind it.
It is not known exactly what is taking place in the component if it is used for energy storage, but it leads to an extremely high capacity. - H. Becker
However, the product wasn’t marketed by GE. Standard Oil of Ohio came up with another version of the invention while working on an experimental fuel cell design and once again the technology was not commercialised. Instead, SOHIO licensed it to NEC who decided to bring it to the market as a backup power for computer memory.
Long story short it was not very popular and sales only took off in 1978 when Panasonic started producing its famous Goldcaps - also used for memory backup.
But let’s not get ahead of ourselves, we have other things to cover!
If you haven’t already noticed, supercapacitor is composed of 2 words: super and capacitor. And while we all know what super means, capacitor is a little more complicated.
What is a capacitor?
To put it simply, a capacitor, just like batteries, can charge and discharge. The major difference is the absence of chemicals used in the storage process. Instead, the energy is stored as static electricity; think of the times when you rubbed your feet on the carpet to zap your sibling.
To make this possible, two electrical conductors – such as metal plates – are put extremely close to one another and are separated by a dielectric insulator.– ceramic, mica, dry air or even a vacuum are possible insulators. This configuration allows a capacitor to store energy as static electricity.
What I said at the very beginning of this article for supercapacitors also works for capacitors. Meaning that they release an important amount of energy in a reduced lapse of time. Thus, they are mainly used in devices which need a sudden an important amount of current, a good example would be the flash light in a digital camera.
How does a Capacitor work?
Ok, so we already know that capacitors store energy as static electricity and that it is composed of 3 main elements: 2 plates and a dielectric insulator. But how does everything come together?
First, you need to understand that both conductors have a neutral charge as there are as many positive charges as negative ones. This all changes during the charging process.
Indeed, positive charges accumulate on one of the plates (p1), thus positively charging it. What about the other one?
As you might already know, 2 identical electric charges tend to repel each other. Therefore, both plates being extremely close to one another, the positive charges of the second plate (p2) are repelled and negative charges are attracted, thus negatively charging p2.
Now we have an electric field between both plates! But that’s not all!
The dielectric insulator is polarized by the field and so, its negative charges line up next to p1 and its positive charges line up next to p2. The effect is an overall increase in the amount of energy stored by the capacitor.
When discharging, the whole system is reversed, and the plates are once again neutrally charged.
Easy Peasy!
Why go Super?
Capacitors have shitty capacitance.
Capacitance, property of an electric conductor, or set of conductors, that is measured by the amount of separated electric charge that can be stored on it per unit change in electrical potential. Definition by Encyclopaedia Britannica
In other words, capacitance tells us if a capacitor – or supercapacitor – can store a lot of energy. And the answer often is hell no! Just to give you an idea, the farad is the unit used to measure capacitance and usually, capacitors have a capacitance in the micro, nano or even picofarads.
This order of magnitude pales in comparison to everyday AA batteries which can store around 9000 Coulombs of charge and release it at a steady 1.5V, which equates to a capacitor of 6000 farads charged to 1.5V.
Although at full charge a capacitor is closer to 5V, 6000 farads still equates to 6 million microfarads. So yeah, there was a certain need to go super.
But how can we improve capacitance? The latter is directly related to the distance between the plates, the size of the plates and the relative permittivity of the dielectric insulator. So by acting on these 3 factors, we've got a good shot at improving the amount of energy a capacitor can hold.
What is a supercapacitor and how does it work?
Let’s start with the plates:
They are bigger, meaning that more charges can be stored on each plate.
They are coated with a porous substance such as powdery activated charcoal or carbon nanotubes which has the same effect as increasing the size of the plates.
They are closer together, which increases the strength of the electric field and once again increases the number of charges of the same kind on each plate.
All these seem steps seem quite easy, so why didn’t we think of it earlier?
In order to get these plates closer together, you need to dip both plates in an electrolyte. The electrolyte just like the dielectric insulator in a capacitor is non-conductive but contains a bunch of ions and acts as a super-thin dielectric - as small as one molecule thick.
When charging the supercapacitor, the anions go to the electrode of positive charge and the cations to the electrode of negative charge. This creates a double layer of charges, giving supercapacitors the name of Electrochemical Double Layer Capacitor – AKA EDLC.
Most of the overall characteristics of supercapacitors are similar to that of regular capacitors:
Voltage drops linearly, meaning that at full charge the volts written on the supercapacitor is the volts you’ll get when discharging it but as time passes, the voltage will reduce.
Supercapacitors are limited by the breakdown voltage of their electrolyte which is usually around 3V, whereas capacitors can go up to 30kV.
There is no need for full charge detection in both capacitors and EDLCs.
How do supercapacitors hold their own
Advantages
Supercapacitors can charge extremely fast, in China, some buses are powered by supercapacitors which can recharge in just 10 seconds which is miles ahead of any type of battery. Moreover, while they charge, temperature doesn't matter as much as for Li-ion batteries.
The cycle life is also much greater for a supercapacitor, in some cases it can reach a million charges, far more than any batteries out there. Mainly because of the lack of chemicals involved, which also allows outstanding power density - measured in W/kg – meaning that it outputs great amounts of energy for a reduced weight.
But as you have guessed, in life, there are always trade-offs. So for one great quality, most of the times there is an equally great flaw.
Disadvantages
The biggest of all the flaws a supercapacitor has is its incredibly low energy intensity – measured in Wh/kg – meaning that it cannot store as much energy as batteries. Usually around 10% of the capacity of a Li-ion battery. The cost per Watt is also something rather problematic as it is 10 times more expensive than that of its chemical foe.
While the cycle life of a supercapacitor is outstanding, its self discharge is equally depressing losing over 50% of its charge in a month.
And finally, the voltage is low compared to Li-ion batteries and extremely low compared to its non-super friend. Moreover, as we've seen before, supercapacitors suffer from linear voltage discharge meaning that as it sends out electricity, the voltage decreases.
Use cases and future
Setting the record straight
A common misconception is to think that supercapacitors and batteries are used for the same tasks. Indeed, their characteristics are very different, for instance, you will use supercapacitors when you need to store or release energy instantly and batteries when speed is not a priority.
A perfect example of this is regenerative braking, where vehicles store the energy used when braking and release it when accelerating. Another is that of backup power for data centers which need to be supplied instantly when a power outage occurs, therefore, making it possible for backup generators to get started and take over.
Unlike batteries, most of the use cases of supercapacitors do not directly affect us in our daily lives, however this might change soon.
CSR Zhuzhou Electric Locomotive and China's electric buses
In 2015 a Chinese train manufacturer, CSR Zhuzhou Electric Locomotive, made its electric bus public in Ningbo. The bus is equipped with supercapacitors which allows it to travel 5km on a single charge. Not interesting you might say! But on the contrary, the bus line is only 11km long and has over 24 stops.
Still not interesting? What if I tell you that the bus can full charge at each stop in only 10 seconds.
Although this is far from being compatible with personal EVs, it is a completely viable solution for public transport! Not only that, but the supercapacitors used in the buses can last over 12 years and consume 30 to 50% less than the batteries used in any other electric vehicle.
We are far from completely replacing chemical batteries but it is an interesting step forward for supercapacitors which might greatly affect vehicles in the future.
Closing Words
It is important to understand that, mainly because of capacitance and linear voltage discharge, capacitors can absolutely not replace batteries. However, this does not mean that they are useless! They are completely different and unique and have probably saved databases from completely crashing ore than once.
Nonetheless, who says we won't see supercapacitors powering our cars and electronic devices in the future. It is something worth investing in, and many big companies and start ups are betting big on this technology, making bold claims regarding the near future.
A future in which graphene might have a huge impact - but more on that in another article.
So, there you go guys!
I Hope you've all enjoyed this Article, please leave a comment if you feel like it and an upvote if you like what you've felt! Resteems and new followers are also greatly appreciated!
Also, I would like to know what YOU think about this energy storage system? Is it worth investing in? Or do you believe it doesn't stand a chance against the competition?
Sources
| N° | Type | Source |
|---|---|---|
| 1 | Article | ExplainThatStuff - Supercaps |
| 2 | Article | ExplainThatStuff - capacitors |
| 3 | Article | BatteryUniversity - Supercaps |
| 4 | Article | Murata - capacitors |
| 5 | Article | Supercapacitor buses |
| 6 | Article | Wikipedia - capacitors |
| 7 | Article | Wikipedia - supecaps |
| 8 | Article | Wikipedia - capacitance |
| 9 | Article | Wikipedia - CSR Zhuzhou |
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