Supercapacitors / Ultracapacitors

Discussion in 'General' started by Domenick, May 10, 2018.

  1. Domenick

    Domenick Administrator Staff Member

    Supercapacitors to store energy for electric vehicles have long been a dream.

    This thread is meant to track various efforts at developing this peculiar (in the best sense of the word) technology.
  2. Domenick

    Domenick Administrator Staff Member

  3. Martin Williams

    Martin Williams Active Member

    You might like this:

    They are about 100 times better than conventional supercaps and the developers seem to have demonstrated energy densities greater than Lithium ion batteries. Whether they are commercially viable is another matter. I worry, too, about large amounts of electrical energy stored as charge in a small volume but capable of discharging in microseconds. It could well turn out to be similar to a bomb! We shall see. Certainly an improvement on electrochemical batteries, however.

    They are not prone to degredation of capacity, and will happily work over a wide temperature range. They don't need cooling and can accept and supply huge currents. I have one on my desk that is about the size of a D cell which will supply 1,200 Amps. Not for long, of course, but there is enough energy to resistance weld 1/16" aluminium plate! I use six of them crank the diesel on my boat!

    I imagine these things can be charged as fast as the charger can supply the current, but practical considerations will probably limit it.

    One small considerations with all supercapacitors is that they are low voltage devices - typically 2.7v although these hydrophilic ones will go up to 4 volts I think. Certainly, conventional supercaps are destroyed if you allow the voltage across it to exceed this. In a series string of them, this can easily happen so each cell has to have a shunt circuit which clamps the voltage across it to below the maximum voltage. I imagine the same will apply to these new ones.

    See also: This company was set up by the University of Bristol to exploit this technology.
    Domenick likes this.
  4. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    There certainly is a great potential there if supercaps can be made with an energy density approaching or exceeding that of li-ion batteries currently in use in EVs. As Martin said, there some great advantages to supercaps (supercapacitors, aka ultracapacitors) over chemical battery cells. Advantages such as being able to cycle them tens of thousands of times with very little if any degradation, the ability to charge and discharge them almost instantly, and -- at least for traditional supercap tech -- a wide range of operating temperatures far beyond that of battery cells, which are dependent on chemical reactions to store or discharge electricity. Specifically, traditional supercaps work just as well in bitterly cold temperatures as they do at room temperature.

    But the "if" in "if only supercaps had much better energy density" has been a very big if, and anyone familiar with EEStor's completely disproven claims along that line should be highly skeptical of similar claims. I don't know why it is that the energy storage field seems to attract so many wildly exaggerated claims and even outright fraud and scams, but unfortunately that's the way it is. :(

    Therefore, while I remain interested in all claims for radically improved supercaps, just like I remain interested in all claims for radically improved battery cells, I won't actually believe any such claims unless and until they are verified in independent third-party testing by a properly qualified laboratory, or until such devices appear in an actual product you or I can buy on the market.

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  5. Martin Williams

    Martin Williams Active Member

    It's straightforward to work out some of the possible features of a supercapacitor powered car, making a few sensible design guesstimates

    Assume we want to store 200 kWh, which I reckon is the minimum needed for a car that will have good range without having to worry about whether the heater can be used etc. 200kWh is 720 million Joules. Supercapacitors are NOT constant voltage devices and will exhibit zero volts when discharged and full voltage when fully charged. The energy they contain in Joules is proportional to the voltage squared, so one could - for instance - construct a capacitor pack which will be fully charged at 200 volts, and design the power conversion electronics to run between 50 and 200 volts. This means the capacitor would contain only one sixteenth of a full charge when the car would no longer be able to run. So let's assume we design the capacitor to be fully charged at 200v and to contain 800 million Joules.

    To store 800 million Joules at 200 V implies 40,000 Farads

    The 'D' cell I have in front of me is 350F, at 2.7 volts (max) It is 33mm in diameter and 70mm long. It will, therefore, hold about 1275 Joules of energy when fully charged.

    If the claims about the new hydrophilic dielectric ones being a factor of 100 better than this, we can expect a 'D' cell of these dimensions, therefore, to be capable of storing 127,500 Joules, and we can divide this into the 8 million needed for the car to find that we will need 5650 of these D cells. I understand the new supercaps to work to 4V so to contain the same energy, 127,500 Joules, they work out at about 16,000Farads. One imagines the battery pack to be organised as a number of serial strings of fifty cells in parallel. You'd need 125 such strings making a total of 6250 of the new cells.

    Of course one can make the cells bigger and you would probably need less and reduce the space by doing so, but this gives the rough order of the problem.

    Charging a capacitor like this is best done at constant current. The time taken then is linearly related to the current provided. Double the current, cut the time in two and so on. I calculate that capacitors of this size can be fully charged at 1500 Amps in an hour, the voltage rising linearly from 50 to 200 volts. To charge it in 30 minutes would take 3,000 Amps, Fifteen minutes would need 6,000 amps, 7.5 minutes would need 12,000 Amps and so on. I have neglected resistive heating losses, but it is worth remembering that resistive heating losses are proportional to the SQUARE of the current. Double the current and the losses go up by a factor of four. double it again and the losses are sixteen times and so on.

    However miraculous you make the 'battery', I feel that the physics of getting this sort of energy into a car in electrical form is going to make charging a much longer business than filling a tank. Whether the public will accept this is doubtful. My feeling is that they will if there is no alternative, but at the moment there IS the alternative of petrol, and people are taking it.
  6. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Well, perhaps you chose premises which are unnecessarily disadvantageous. Why limit the supercap pack to 200 volts maximum? For some of the newer "supercar" EVs, my understanding is they've already moved to 800 volts.

    So instead of 4v x 50 = 200 volts, we'd need 4v x 200 = 800 volts, allowing the pack to be charged at (a maximum of) 800v, thus charging faster and allowing us to use smaller diameter, less expensive copper (or aluminum) wires and cables.

    Last edited: May 12, 2018
  7. Martin Williams

    Martin Williams Active Member

    I am afraid you understand neither the physics nor the implications of it, Pushmi.

    Yes, you can push up the voltage, but this increases the danger associated with it. The point is that the old lady filling her tank with 100 litres of diesel or petrol in a couple of minutes is operating at a power level of about 30 MegaWatts in perfect safety. To do that in electrical form in the same time involves 30,000 volts at 1,000 Amps or some similar combination of current and voltage. Not only is this too dangerous to let people get near, it requires massive (and massively expensive) electrical infrastructure to do it.

    The disposition of capacitors in the car is arbitrary, but however they are arranged you still have 800 million joules to get into it.

    Operating at 800 Volts means a current of 250 Amps for an hour. A demand of 200kW
    Or 500 Amps for 30 minutes. A demand of 400kW
    Or 1000 Amps for 15 minutes. A demand of 800kW
    Or 2000 Amps for 7.5 minutes. A demand of 1,600 kW
    Or 4000 Amps for 3.25 minutes. A demand of 3.2MW

    Now imagine a filling(charging) station with ten of these stations and go and research the size of the substation needed to supply it!

    The diameter of copper needed to carry these current is a further problem. To carry 250 Amps requires #0 AWG which means - in solid copper - a rod 0.325" in diameter. This, even in stranded form will not be very flexible, to put it mildly! Operating at 800V you need fairly hefty electrical insulation and this will also reduce the rate at which resistance heating escapes, so it will have to be even thicker. I will leave you to work out what sort of thickness you need if you want to reduce the time from an hour.

    I'm not spreading FUD here, but rather pointing out that the universe inflicts restrictions on what can be safely done. The poor charging rates of batteries has disguised this problem, but if you use capacitors you can charge at much higher rates and these problems become real ones which have to be addressed rather than glossed over.
  8. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    For once, I'm not going to be drawn into refuting your same ol' tired FUD arguments yet again. The horse is dead, and no matter how long you keep beating it, it will remain dead.

    You're suggesting that a 1.6 MW charger wouldn't be practical for BEVs, yet you claim that hydrogen-powered fuel cell cars will replace gasmobiles... despite the fact that renewable hydrogen fueling stations would need to pull about 3.5 times as much energy for every car filled as BEV fast chargers! In fact, even more in your scenario, in which you claim that fool cell cars can be filled as fast as gasmobiles -- in 2 minutes!

    Contrariwise, I think a 6-9 minute recharge time for BEVs would be perfectly fine, since most charging will be slow charging done at home or at work, where a mere 220 volt Level 2 charger will be perfectly adequate. Since the majority of ultrafast charging stops for BEVs would be needed only for long trips, a few extra minutes for a "fill-up" shouldn't be a significant concern for most drivers. Chances are they will need a rest room break anyway, so a 2-minute charge wouldn't do them any good.

    So, Mr. Fool Cell fanboy, it's you who is weaving some fantasy of impossibly high power provided to future fueling stations -- not me!

    ProTerra BEV bus chargers already charge at up to 500 kW. They don't use flexible cables, they use sliding solid rods or bars to carry the power.

    Sorry you're so locked into "Thinking inside the box". Fortunately, electrical engineers are more flexible in their thinking.

    And the Earth is flat, and fool cell cars will make electric cars obsolete, and -- what was that astoundingly ridiculous thing you said just a few days ago? "On the basis of Tesla accidents so far, 75% of them have burned."

    :p :p :p
    Last edited: May 12, 2018
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  9. Martin Williams

    Martin Williams Active Member

    Well, actually a hydrogen station would get its hydrogen by road tanker. They're a lot cheaper and more efficient than a substation and transmission lines.

    If you want to mandate a standard charging system using busbars for cars, go ahead and try it! They don't exist at the moment and I suspect you'll have major fun and games trying to get manufacturers to agree a standard! Technically feasible of course, but politically and economically rather less easy.


    The following quote:
    Should, of course, read
  10. Domenick

    Domenick Administrator Staff Member

    So, if I have this right, one only needs 5,000 of the D capacitors for a 200 kWh pack?
  11. Martin Williams

    Martin Williams Active Member

    Well, the figure was 5,600. But otherwise you are correct, provided the claims of the developers can be carried through to production and a D cell with 100 times the capacitance of the one on my desk can be produced. Its a big 'if' of course.

    But you can get away with only 50 cells, albeit 100mm in diameter rather than 33mm as I've argued above.

    But I still have reservations about the safety of these things. There is a lot of energy there, and capacitors can discharge it VERY quickly into a dead short. With 50 cells, each one holds 4kWh of energy. If it develops an internal short circuit and discharges it in a second, that is an average power level of 14.4 MegaWatts. Bit like a bomb going off!

    But these are early days. We'll just have to wait and see how they work out in practice.
  12. Martin Williams

    Martin Williams Active Member

    A further advantage of using supercapacitors is that you can get a super accurate measure of the amount of energy left, as the energy is directly proportional to the square of the voltage. You can measure the voltage and square the result with a microprocessor, and get the result in Joules, kWh, or even an estimate of how many miles you have left at your current rate of consumption.

    It's worth noting, too, that the capacitor will not be damaged by running it down to zero, so if the electronics can handle lower voltages than the minimum of 50v, then you might be able to have an emergency 'crawl' mode capable of taking you that last mile to the charging point. Or, alternatively if you have really miscalculated and are waiting for a tow truck (or a charging truck) you will at least still be able to listen to the radio!

    Let's all keep our fingers crossed that it proves possible to get these things to achieve their promise. It'll be bad news for the battery industry, but great news for us!
  13. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    That just moves the problem back a step.

    The problem is that generating enough renewable H2 to be the first link in the chain of fueling hydrogen fuel cell cars would take about 3.5x as much energy as needed to charge the same number of BEVs with electricity from the wall.

    You keep harping on how much more power and energy it would take to charge BEVs than the grid is currently set up to handle, while ignoring the rather obvious problem with your preferred solution, which would make that situation far, far worse -- not better!

    Last edited: May 13, 2018
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  14. Martin Williams

    Martin Williams Active Member

    This thread is supposed to be about supercapacitors. Move your post to a more relevant thread and I will be happy to reply there.
  15. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Oops, that was sloppy of me. Merely generating the H2 would not take 3.5x as much energy; it's the entire well-to-wheel supply chain for H2 that takes about 3.5x as much energy as charging a comparable BEV for driving the same distance.

  16. Martin Williams

    Martin Williams Active Member

    Choose an appropriate thread.
  17. Gary Lea

    Gary Lea New Member

    Sigh...first things first, okay? Adding super/ultra capacitors to any plug-in or hybrid powertrain will enable more efficient braking regeneration storage and acceleration, reducing battery cell abuse. Reducing thermal loading upon batteries reduces cooling requirements, therefore additional weight as well as extending service life.
    Domenick likes this.
  18. Domenick

    Domenick Administrator Staff Member

    This would be especially cool if that power could be fed straight back to the motors via inverter, instead of being put back into the battery, so it could maintain a higher efficiency.
    Gary Lea likes this.
  19. Gary Lea

    Gary Lea New Member

    Well, certainly! Depending on leakage characteristics of such capacitors and vehicle usage, the vehicle can trickle charge the battery pack (long-term steady-state driving; park & shut down) rather than allowing charge to dissipate. As cost decreases and capacity/density increases, such devices shall segue from reducing battery demand to replacing batteries.
  20. Loaddown

    Loaddown New Member

    Is this what you were looking for:

    I believe it takes time to develop and commercialize a 'disruptive' technology and many are even destroyed by the selfish, mendacious, devolution syndrome of the current commercial/political leadership. For example, the Wright brothers could not impress the US, but finally got manufacturers in France to recognize the value of their accomplishment.

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