Super capacitors are a horrible choice for solar. Change my mind

[Will Prowse]

A "super capacitor" is a horrible choice for solar energy storage because:

- Horrible energy and volumetric density.

- The price per kwh is outrageous. Super capacitors make lithium batteries look cheap.

- Cycle life are great, but just like LTO, you need to factor in calendar aging. And solar batteries only need to be cycled once a day, so it is pretty pointless to have 1 million plus charge cycles when calendar aging will kill your pack long before cycle life is reached.

- Discharge and charge rate are amazing, but guess what?! Any normal, off the shelf lead acid or lithium battery charge and discharge rate will support loads and required charge rate in a properly designed system. No reason to get excited unless you have a tiny battery system.

Capacitors are great as a buffer for reducing noise in a circuit or pushing surges with a small battery. That's about it.

What I want to know is why so many people online are obsessed with them. They are great for specific applications, but nothing to get excited about. Can someone change my mind? Am I missing something?

 

[Rider]

The only place I would have a use for them is in conjunction with your batteries, to help lessen any surge/dip when a load is applied. Can't see using them as a battery replacement.

 

[electric]

Yes Will, you are missing something. The fact that 99% of people online are not electrical engineers and have no idea what technologies are best for specific applications. If words “super” or “nano” are used, then people will believe anything, just like snake oil of 19th century.

Also, caps have linear voltage profile, and very high leakage losses, horrible for storage applications.
You must keep boosting voltage to get most energy out of the cap.

 

[eddie1261]

If words “super” or “nano” are used, then people will believe anything, just like snake oil of 19th century.

Kale.
Salmon.
Quinoa.
Spinach.
Eggs.
Turmeric.
Lentils.
Almonds.

All this time I had no idea that I have been eating "superfoods". But now that I know they are "super", suddenly I feel better about myself!!! LOL!!!

Give it a cool name. Increase sales. Make more money.

 

[svetz]

Super-capacitors are superior to Lithium in most ways:

• lower round-trip energy losses (98 to 99% efficient)
• vastly higher C-Rates (charge discharge kWhs in seconds, not all that useful for solar storage)
• far greater cycle life (millions compared to thousands)
• longer life
• higher DoD
• Lower energy losses at high current discharges
• lower internal resistance (making it easier to add batteries in the future without degrading the new ones)
• greater operational temperature ranges (-40F to 160F)
• easier to recycle
• won't catch fire
• won't explode

There are only a few ways they are inferior, but they're pretty big ones:

• Cost, currently crazy high...but will probably come down over time and newer exotic material structures should improve characteristics.
• Volume, they're physically big for the number of Wh; an equivalent of an 18650 is about the size of a can of soda (might be an old wive's tale).
• non-constant voltage
• Self discharge is about 40% per month (compared to Lithium at 2.5-3.5% per month; although for nightly cycling of batteries the discharge is ~1.3% which is more than made up by the higher round trip efficiency).

If they were the same price and I had the physical space, I could see going caps over Lithium.

 

[electric]

• won't catch fire or explode??? my friend, you have not worked enough with capacitors to make this statement. They explode spectacularly, you will never forget it.
• almost all the benefits you listed are marginal compared to modern Li cells and Li cells are also improving constantly, so the catch up game is not a good argument.
• poor volumetric energy density and leakage are the 2 main killer reasons why no one in their right mind would use caps for storage, it's just not the right tool for the job
• also, linear voltage profile means that 50% charged cap has 1/2 the voltage, which means your inverter must be much more complex to continuously boost voltage in the widest possible range.

 

[strat-o]

Not now but soon!
From a theoretical standpoint, batteries will always be limited because you must have one atom/molecule paired with each electron you use: If you use 100 electrons you have to move 100 atoms/molecules.

Capacitors don't have this limitation. The limitation of capacitors is only limited by man's imagination and ingenuity of coming up with ways or schemes for storing electrons. Capacitors store electrons not by pairing them with molecules but by providing surfaces for the electrons to cling to. The more surface area, the more electrons can be stored. As we begin to produce capacitors that are more efficient at electron storage we should see the power to weight ratio go up as well as the power to cost ratio,

Capacitors typically do not rely on exotic nor expensive materials. Currently the best capacitors are made of carbon. Although we may see expensive (at least initially) forms of carbon being used like graphene and nanotubes. Not having to rely on rare chemicals should keep the costs down.

Also as was previously noted, capacitors have a different charge/discharge profile than batteries. I personally think that capacitors can be made to behave more like a chemical battery by using multiple capacitors arranged in a battery and controlled by a computerized capacitor management system. The capacitor manager would ensure that you have a constant voltage being supplied at any given time during the discharge cycle.

 

[Newenough]

Kale.
Salmon.
Quinoa.
Spinach.
Eggs.
Turmeric.
Lentils.
Almonds.

All this time I had no idea that I have been eating "superfoods". But now that I know they are "super", suddenly I feel better about myself!!! LOL!!!

Give it a cool name. Increase sales. Make more money.

I'll eat nano-peas....but not "regular peas". ?

 

[gnubie]

Would you eat a nano-pi though?

*oh dear I've gone off topic, sorry!

 

[gnubie]

poor volumetric energy density and leakage are the 2 main killer reasons why no one in their right mind would use caps for storage, it's just not the right tool for the job

I have read papers, most of the content passing 2km over my head, in the past discussing supercapacitors and applications of materials such as graphene and the indication at the time, 2017 (?), was that the most optimistic projection was 100 Wh / kg. Lithium is miles ahead right now, and large reputable companies such as Bosch have announced lithium storage at 400 Wh / kg. Lithium isn't standing still either so it's hard to say if capacitor storage will even catch up.

 

[electric]

Capacitors don't have this limitation. The limitation of capacitors is only limited by man's imagination and ingenuity of coming up with ways or schemes for storing electrons. Capacitors store electrons not by pairing them with molecules but by providing surfaces for the electrons to cling to. The more surface area, the more electrons can be stored. As we begin to produce capacitors that are more efficient at electron storage we should see the power to weight ratio go up as well as the power to cost ratio,

Imagination and ingenuity applies to all technologies alike. Electrons are much harder to trap due to their small size and mobility, while ions are easier to trap and release using carefully formulated materials. Electrons love to jump thru dielectric barriers, so thickness of the barrier and dielectric strength are limiting factors in increasing volumetric density.
The point is that all technologies have many pros and cons, because if they didn't, then one would have immediately emerged as a clear winner.
Just pick the right tool for the job. Caps for short hi-power needs, batts for long hi-energy needs. In a very few cases there is some overlap when both can be merged, but it's rare and must be highly optimized for cost and efficiency.

 

[Supervstech]

A small bank of them can power large demand devices in short ish bursts with rapid recharge.
So, they are ideal for job site solar use. They are totally useless for home off grid etc.
I like the kit built by the guy on YouTube... but yeah. Way too expensive.

 

[svetz]

So, they are ideal for job site solar use. They are totally useless for home off grid etc.

They'd be idea for hand tools if they had a better power density. Can you image portable tools that recharge in seconds and never need battery replacements?

 

[WoodsieLord]

[not being serious]
Suppose your only source of power must be solar.... and you're located in a deep and narrow valley , so narrow that sun only hits your roofs for 45 minutes per day! (I know, I'm almost talking about Mordor but... bear with me!).
You can add 10 times more panels to compensate for this, but you would be oversizing your charge controller so that it can manage and supply that great amount of Amps! also, your batteries should have a great C rating to be able to receive that supply.
Instead, you could add a capacitor bank that would be able to handle a great inrush current so that in the next hours, a smaller charge controller with cheaper batteries (I mean, less strict C ratings) would be sipping power from that capacitor bank without stressing the system.
Basically, I suggest using a capacitor bank as a buffer.

[being slightly more serious]
The longevity is very attractive to people like me (that know very little about solar and different battery chemistries). This is not a very good explanation at all! But I was considering them until I saw a bunch of tests on youtube. I also think they can be very dangerous, in fact, to be able to do what I suggested in the [not being serious] section, I'm figuring that the system would require some kind of resistors to limit the charge controller input current. Resistors also mean losses, and they must be calculated correctly and such.

In other words, unless you have a family member that works at very good position in a super capacitor factory... they're not very feasible as energy storage, I suppose

Regards

 

[SolarRat]

I use them to power my intergalactic proton phaser.

 

[electric]

I use them to power my intergalactic proton phaser.

That’s a very appropriate application, right tool for the job.

 

[svetz]

[not being serious]
Suppose your only source of power must be solar.... and you're located in a deep and narrow valley , so narrow that sun only hits your roofs for 45 minutes per day! (I know, I'm almost talking about Mordor but... bear with me!).

There is such a place, except they never get sunlight: https://www.theguardian.com/world/2013/nov/06/rjukan-sun-norway-town-mirrors

 

[svetz]

..almost all the benefits you listed are marginal compared to modern Li cells ...

We'll have to agree to disagree. Lower round trip losses, higher C rates, longer life, far longer number of cycles, greater thermal range, easier to recycle don't seem all that marginal. Although for solar I'll give you super high C-Rates aren't all that important, and 80 vs 100% DOD isn't that big.

...linear voltage profile means that 50% charged cap has 1/2 the voltage...

Watts is watts; put a voltage regulator on it and call it done.

• won't catch fire or explode??? ...They explode spectacularly, you will never forget it.

Most small value capacitors are simple sandwiches of conductor and insulator and when the voltage exceeds the dielectric strength of the insulation, they short out and burn, crack, pop, open, or smoke. Explosions are rare for these. Popping open is more likely.

Video of exploding capacitors:

So, design plays a big role in the risk and if they get a UL listing I'd feel confident they were at least as safe as the other junk in my house.
See also:

1. How to Explode a Capacitor, note the design to prevent unforgettable explosions.
2. Supercapacitor batteries are safer than ordinary batteries when mistreated. While batteries are known to explode due to excessive heating when ... ref
3. FastCAP designed its first-generation product, for the oil and gas market: a high-temperature ultracapacitor that could withstand temperatures of 150 C and posed no risk of explosion when crushed or damage ref

Not that someone won't find a way to explode a supercapictor someday; there are very innovative and determined people out there.

 

[grizzzman]

Well, they would make a great jump starter! 2.7 volt 500 f supercapacitor bank cost can be 35.00 with a balance board or DIY for 18.00 ()If you don't mind waiting for parts) And 27.00 with a balance board. Charge for a minute( or less ) And your ready to go!

 

[gnubie]

There are actually commercial jump starters based on capacitors already. There's one handy device that you keep in your boot (trunk). If you find your battery is low enough that it won't start the car but not completely dead, there is still a lot of energy stored there. The battery just isn't capable of delivering the big whack to crank the motor over. The starter pack slowly draws power out of the battery to get the best out of it, and can then dump it all back out in short order to allow you to start the car and be on your way.