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Super Capacitors

What are Super Capacitors ?

Supercap's (SC or CAPS) have very high power delivery, But unless they are configured into a large bank, they individually provide very small energy storage for Micro Grids.

1. Small SC units: These small SC banks come in 6~48v, units can provide 52.8Wh (0.05kW) power, this capacity SC are becoming popular in Micro Grids for smoothing of discharge/charge cycles, and are used in conjunction with other forms of energy/battery storage (lead, nickel, lithium, flow batteries etc) and they will become more important for this duty in the future!

2. Residential Standalone SC units: Currently ranging from 1.00kWh (12v), Australia only 3.55kWh (48v), 6.70kWh (48v) & 7.10kWh (48v), these  can be configured in single, two or three phase have successfully developed as the primary storage mechanism, and simply Plug & Play into any Hybrid Inverter, currently these units comes at a premium price.

3. Commercial 3 Phase Standalone SC units: Supply Continues Power from 9kW~1MW (380v±20%), these  can be configured successfully as the primary storage mechanism, also simply Plug & Play into any Hybrid Inverter, also at a premium cost.

Click Here to Calculate the SC bank you require.

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Super-Caps can be charged/discharged very fast and as often as you like, Unlike batteries, they degrade very little with use. Their life is in millions or billions of charge cycles, not thousands. This does mean that they have some very useful applications. The CSIRO/GMH E-Commodore hybrid car used supercapacitors to ease the charge cycle on the batteries, I'm a little surprised this isn't more common. In a Hybrid vehicle you can stand on the brakes, the capacitors can soak up the re-generative charge as fast as you like, hold it for a moment until you step on the throttle, all without a high current load on the battery's.

Supercapacitors are rated in farads, which is thousands of times higher than the electrolytic capacitor, and suited for energy storage undergoing frequent, rapid charge & discharge cycles at high current (Amps) and short durations, But more suited to short-term energy storage &/or burst-mode power delivery, and when used in conjunction with batteries, a supercapacitor bank will instantly supply the initial Hi-Amp demand when appliance like Water heaters, Ovens or Air-conditioners, Cloths Dryers, Welders etc. initial start, as supercapacitor reduce the direct load on batteries, they in turn reduces the depth of discharge (DOD) to improve the batteries cycle life!

Capacitor banks are available and becoming more common in energy storage, the main issue is that all CAPS discharge reasonably quickly, then they are done, But they also re-charge almost as quick, and ready for the next high Amp demand.

Capacitors are designed for a different purpose than a battery and thus are ill suited to deliver Slow discharge over long periods like we need a battery to do, But Ultra Caps have almost infinite cycle life.  In modern electronic vehicles (EV's) they are used for regenerative braking and act as a buffer between battery and more to capture the huge energy surge from braking. Then released back into either the batteries or drive train, this is why they are becoming popular in home & business Micro grids, and are not limited to just one Capacitor bank, just like batteries that can also be wired in series or parallel, depending on the voltage required.!

SC life is predominantly affected by a combination of operating voltage and operating temperature. The ultracapacitor has an unlimited shelf life when stored in a discharged state. When referring to ultracapacitor life the data sheets reflect the change in performance, typically decrease in capacitance and increase in resistance. The life specified by industry standards is a 20% decrease in capacitance &/or 200% increase in resistance. The ultracapacitor does not experience a true end of life rather the performance continually degrades over the life of the use of the product.  End of life will be when the ultracapacitor performance no longer maintains the application requirements. This may be different from that specified on the data sheets.

The typical degradation behaviour of the ultracapacitor resembles that of an exponential decay. The majority of the performance change occurs during the initial use of the ultracapacitor and this performance change then levels off over time. The most dramatic effect of the life degradation is on the internal resistance of the device.

A supercapacitor (SC) and electric double-layer capacitor (EDLC) (also called supercap, ultracapacitor or Goldcap), are a high-capacity capacitor with capacitance (measure of the ability to store electric charge) values much higher than other capacitors, But with lower voltage limits, and bridge the gap between electrolytic capacitors and rechargeable batteries.

Supercapacitor typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, and can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.

Advances made in supercapacitor capacity and energy density will ultimately lead to greater functionality and more overall presence of supercapacitor throughout the energy industry. Based on all of their inherent advantages, supercapacitor banks help reduce the costs to the customer by minimising the amount of batteries needed, as well as the frequency of battery replacement.

How do I convert from Farads to Ampere hours ?

I have a 2.7 volt, 3000 farad Supercapacitor, 1 farad = 1 Ampere second/Voltage.

So, I calculated 2.7 x 3000 to get 8100 Ampere seconds, then I divided this by 3600 (3600 seconds in 1 hour) and get 2.25 Ampere hours.

Are my calculations correct ?

Yes, if the capacitor is indeed 3000 farads, But we are correct and we are not correct !

Amp Hours in a battery are measured from fully charge to fully discharged. (see Super-Capacitor Storage Calculator)

In a chemical cell, those are carefully specified voltages. But in our example with a Capacitor, we are using the entire range from 2.7V fully charged to 0V fully discharged.  There is a problem with this - the lower the voltage, the less power available, and the lower resistance it takes to continue to draw current. So, when the voltage is getting really low, it will be very difficult to get any useful power out of the capacitor. You need to choose a terminating voltage. Perhaps 1V.

In that case, your calculations simply substitute 1.7V in place of 2.7V in your first equation. C = As/V, As = CV = 3000 x 1.7 = 5100, As Divide by 3600s/h to get 1.417Ah, But that doesn't tell the whole story. 10A drawn at the start, 2.7V, is 27W.

But by the time it is down to 1V and assuming a constant current of 10A, that is only 10W. Energy E in joules = (CV^2)/2.

But you cannot just substitute 1.7V in there or you'll get the wrong answer!

So we'll calculate the entire power from 2.7 to 0, then subtract the energy from 1V to 0V. Total E = (3000 x 2.7^2)/2 - (3000 x 1^2)/2 = 10935J - 1500J = 9435J.

If you want that in watt hours (Wh), one watt is one joule per second, so divide by 3600 to get Wh. 9435J/3600s = 2.62Wh.

If you connect this to a switching regulator to get a constant voltage, then it will draw a constant power from the capacitor, rather than constant current.  So instead of a linear drop in voltage, it will accelerate as the voltage on the capacitor drops as it draws more current to make up the same power. There are a number of switch mode boost regulators out there. You need to specify what is your maximum current.

Supercapacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible, but at a reduce service life. To get higher voltages, several supercapacitors are connected in series. Serial connection reduces the total capacitance and increases the internal resistance. Strings of more than three capacitors require voltage balancing to prevent any cell from going into over-voltage. Lithium-ion batteries share a similar protection circuit.

The specific energy of the supercapacitor ranges from 1Wh/kg to 30Wh/kg, 10–50 times less than Li-ion. The discharge curve is another disadvantage. Whereas the electrochemical battery delivers a steady voltage in the usable power band, the voltage of the supercapacitor decreases on a linear scale, reducing the usable power spectrum. (See BU-501: Basics About Discharging.)

Manufacturing Super Capacitors: At $500 a ton, Hemp fibre worldwide have been used for Clothing, Rope and even American Currency, and now the fibres are being used to manufacture Super Capacitors, see the informative video below!

Finally, Natural Hemp Fibre getting recognition for it's Great Properties !

 

Other general information:

Farad is the SI unit for capacitance. One farad is defined as the capacitance of a capacitor across which, when charged with one coulomb of electricity, there is a potential difference of one volt. In other words: C=Q/V Where "C" is capacitance in Farads. "Q" is electrical charge in Coulombs. "V" is electrical potential difference in Volts. You can rearrange the mentioned equation as follows: Q=C.V

Now, we can calculate the maximum charge your capacitor can hold: Q=3000 * 10^(-6) * 2.7=8.1 * 10^(-3)=8.1 mC (mili Coulombs).

Now let's have a look on the definition of the electrical current. Electrical current is the amount of electrical charge that goes through your circuit in a time unit. Ampere is the SI unit for electrical current. It's one of the seven SI base units and is defined as: the amount of electric charge passing a point in an electric circuit per unit time, with an equivalent charge to 6.241×10^18 charge carriers (or one coulomb). So you can write: I=Q/t Where I is electrical current in Amperes Q is electrical charge in Coulombs t is time in Seconds. You can rearrange this equation as follows: Q=I.t

Now it's clear that 1 Coulomb is equal to 1 Ampere*Second and: 1 Ampere * 1 Second = 1 Coulomb of electrical charge, so 1 mili-Ampere * 1 hour = 0.001 * 3600 = 3.6 Coulombs of electrical charge.

We can answer your question now. Your capacitor can store 8.1 mC of electrical charge at max, which is equal to 0.00225 mA.h (0.0081/3.6) And as i said before, your answer is correct and with a 3000 farads capacitor you'll get 2.25 A.h, but I doubt that such a high capacity capacitor even exists; As you can see this is a very small amount of storing capacity and you can't use a capacitor instead of a battery in practical applications. The maximum current that you can drain from the capacitor is depending on the circuit design and most importantly the frequency in which you're running the capacitor.

NB: "SI" stands for "System International" and is the set of physical units agreed upon by international convention.

 
 
 
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