A company in Austin (Texas), named EEStor, will soon have developed a super capacitor that is close to production. This super capacitor with a weight of 180 kg (396 lbs) and would be able to store 52 kWh of energy. This energy is usable for many purposes: for example, electric automobiles, but also to be used to store and release on command the fluctuating energy coming from wind turbines or PV solar cells. This latter would improve the availability of solar-energy, wind-energy and other intermittent sources of energy on demand, which will increase the usefulness of these sources enormously. It could then lead to a situation where we do not need fossil fuels anymore for our energy production.

EEStor’s claims

The company, which does not want publicity, has filed a patent, and collaborates with other companies to start in 2008 with production. What is known about the super capacitor is the following:

• It is a parallel plate capacitor with Barium Titan for dielectric
• The selling price for the first year would start at $3,200 and lowered to $2,100 in high-volume production
• The product weighs 180 kg (396 lbs) and stores/delivers 52 kWh
• The capacitor charges in minutes instead of hours
• The EEStor technology is tested up to 1,000,000 cycles with no material degradation; compare this with lead-batteries that last 500 – 700 charge cycles
• It is no chemical battery, which means no overheating risk
• EEStor plans its own production line (Cedar Park Texas) to prove the battery works and then license the technology to other production companies for mass production
• EEStor’s technology can be used for more application areas: electrical-only vehicles, backup power, big scale energy storage for intermittent energy sources as wind and sun.
• They have an exclusive agreement with Feel Good Cars, the Canadian producer of the ZENN, a low speed electric car, to sell to them the high-power-density ceramic super capacitors. FGC then has the right to use these for all transport applications below 15 kW traction-power and for vehicles with a weight below 1200 kg (2640 lbs) without batteries.

A closer look at the claims

It is probably best to recall two formulas for the explanation below:

1. The parallel plate formula for a capacitor:
   With C = capacity in F (Farad), ε the dielectric constant in F/m, A the surface of the flat plates in m² and d the distance between the plates in m. ε also also written as ε_rε₀ and also as kε₀ (the k-value).
2. the amount of energy in a capacitor:
   With E the energy in J (Joule) and V the voltage over the capacitor in V (Volt).

Barium Titan ls dielectric

The k-value of BaTiO₃ and of BaSrTiO³ is very high; up to 6,000 (Brandmayr et al.). This is very high compared with other materials that are below 100. The problem has been to make this ceramic material reliably. Perhaps EEStor has now put this under control. You can see from the formulas that a high k-value will lead to a high capacity, and a high capacity will lead to a high energy content.

One million cycles

If this would be tested, and a cycle would take 15 minutes to charge and discharge 52 kWh, 1,000,000 cycles would still take 28 years to test, which I’m sure they have not yet done. In the patent, they talk about connecting 2,000 identical basic modules into one 52 kWh super-capacitor. This would be chargeable in 5 minutes. This means that 1 or 2 modules would only need 1 second, and 1 million seconds is 11 days of testing.

The product’s weight is 180 kg (396 lbs) and stores 52 kWh

In the patent it becomes clear that we talk about one capacitor with a capacity of 31 F used by a voltage of 3,500 V. The voltage has to be that high to reach the mentioned 52 kWh: (formula 2) E = 0.5*31*3500²= 189.875.000 J = 52.7 kWh.
This is a lot compared to other super- or ultra-capacitors. Look at the site of Maxwell, who just make high capacity capactors as their boostcap ultra-capacitors. De BoostCap 2600 uses 2.7 V at a capacity of 2600 F, resulting in 9.5 kJ or 2.6 Wh. The EEStor stores 20,000 times more.

How can we compare? Take a BoostCap 2600 (which is for sale). This weighs 480 grams (16 oz), so when the EEStor weighs 180 kg (396 lbs), that would be equivalent to 363 BoostCap 2600s. So one EEStor has 20,000/363 = 55 times more storage than 363 BoostCap 2600s. This difference then has to come from the k-value or in the voltage. To have a factor of 55 times more storage, then the voltage needs to be the square root of 55 times bigger (formula 2), meaning 7.4 times higher. This means that a EEStor has a voltage of 7.4 * 2.7 = 20 V. Maybe this is possible.

No safety risk

In the patent you can see that the voltage is very important to come to an energy content of 52 kWh; we need a 3,500 V. This value is high and therefore also dangerous. Also, when a breakthrough effect occurs in one of the small modules, this shall not lead to any dangerous destruction of the complete capacitor, where high currents can lead to high internal losses and hence temperatures.

No overheating possible

See the previous explanation and there might still be a risk. Compared to chemical batteries, these Eestors surely will have a lower ESR (equivalent series resistance). For those interested, I have here an article that explains how to select your charge current to lose less energy in the ESR.

The super capacitor charges in minutes

This would require very high currents- To charge 52 kWh in 5 minutes requires a charging current of 600,000 A. These require very thick cables and switches.

A $3000 price and Usable for more application areas

At this price, the usage in a car is very possible since the costs will be low compared to gas. Also one can try to use it at home, to use night tariff (if applicable, as in Europe) for his electricity and charge the capacitor during the evening at a lower tariff and use this power during the day.
But why are we only looking at costs? There is a huge advantage when using electric energy for transportation, which is that we will not have pollution directly around the cars. So imagine much cleaner air in the streets, on highways etc. etc. This is an improvement in well being which is indispensable! Bikers will not need the caps anymore to protect themselves from the bad exhaust gases. For all those living in cities, they also will have not to clean their windows to remove the fine black dust on it, which also accumulates in our lungs!!! We should use the marketing effort to focus on these very important effects and not only the direct accountable costs!

Their own production line

I have understood they have their own website, since July 2006 under contruction. This doesn’t look like a good sign. Also in a specific blog I read a message from September that introduction will be delayed for another year.

Conclusion

I do not consider it impossible, but maybe 2008 is too optimistic. I still hope the super capacitor will arrive soon with the above mentioned cost and capacity and weight. This would mean a real boost for a more reliable and available use of intermittent durable energy sources..