There are numerous reasons why our energy future will continue evolving towards an increasingly electrified system, but perhaps the biggest is the way we have become so adept at manipulating electrons. If you need any proof of that you need simply look at your smartphone. That is largely due to advances in the micro world which have provided a myriad of capabilities that designers can now draw upon.
But there have also been some stunning advances in the macro world as well, some of which are certain to have profound implications in energy. One of those is the development of super-capacitors.
Super-capacitors — sometimes called ultra-capacitors or ultracaps — are devices that can store relatively large amounts of electric charge for short periods of time. They are constructed like batteries with an anode and a cathode and a carbon nano-structure electrolyte. Unlike batteries, however, which depend on chemical activity that ultimately depletes their constituents, ultracaps, which store energy electrostatically, do not degrade this way and thus can be used for literally millions of cycles. They are also capable of delivering very large bursts of power in a highly efficient manner. These characteristics make them complementary to batteries, particularly in many of the emerging applications that utilize electricity as a replacement for fossil fuels.
I spoke with Chad Hall, the founder and VP of marketing and product management at Ioxus, about some of the ways that ultra-capacitors can make a difference in tomorrow’s energy picture.
RP: How do you see ultra-capacitors being used in the future?
CH: Ultra-capacitors can only store something like 5 percent of the energy of a lithium-ion battery, but they have 100 times more power. We can use that to do things like start motors, or provide very fast response for peak demands. We have systems that can provide up to 4 MW for 2 seconds and designs that can go up to 30 seconds in the MW range.
RP: Aren’t some of those applications already being served with batteries?
CH: Batteries are good at storing energy. If you need both energy and power, then you need to hybridize the energy storage system using both ultra-capacitors and batteries.
RP: Because of transients?
CH: Exactly. About 90 percent of all voltage dips and sags last less then 2 seconds. Ultracaps can handle these. The last 10 percent can be handled by batteries.
RP: So it’s really like the marathon runner versus the sprinter, the tortoise versus the hare.
CH: You could say that. The ultra-cap would be the sprinter.
RP: So what are some applications?
CH: Take a look at forklifts. They can use ultra-capacitors for the initial surge of power required for lift which both drains and heats up the battery, reducing its efficiency. We are using a 20 percent smaller battery, filling that space up with capacitor bank. Now the battery never sees that peak, which avoids that detrimental heating effect. This not only improves battery life four to eight times, but also increases run time per charge by 33 percent which improves productivity. In some industrial applications vehicles can go for a full year non-stop if there is a track that provides inductive charging. You can get rid of the battery room, charging room, and storage room saving floor space.
RP: That’s great. What about other transportation applications?
CH: Micro-hybrids or start-stop vehicles use ultra-capacitors to re-crank the engines because the AGM batteries were not designed to handle that many cycles. So the ultra-capacitor starts the car and the battery then slowly recharges the capacitor. When you get up to hybrids and EVs, right now people are using large banks of Li-ion batteries. Most of that battery capacity is used for acceleration power. They could considerably reduce the number of batteries if they added ultra-capacitors for those bursts of speed. So you could change your chemistry to use a higher energy Li-ion battery, which is both safer and provides more range.
RP: Is this being done?
CH: It is in China and Europe for hybrid buses. They use ultra-capacitors instead of batteries because they have such high power density and they can recharge so quickly. These ultra-capacitor modules improve fuel efficiency by 50 percent and emissions by 75 percent and they can bring a full 62 passenger bus from 0-30 mph in about 2.5 sec.
RP: How do they improve efficiency by that much?
CH: Round trip efficiency for ultra-capacitors is in the 90 to 95 percent range whereas a battery is in the 70 percent range by itself, minus the electronics. No other form of energy storage is this efficient.
RP: I didn’t realize that.
CH: Another thing people don’t realize is that the temperature range is -40 to +65 C. That’s why they are being used in wind turbines.
RP: You mean to smooth out the generator output?
CH: Actually, they use them to power the blade pitch adjusting motors. They have gone to ultra-capacitors over lead-acid batteries because of cycle life and cold temperature problems. They are used primarily to “feather” the blades, when the wind speed gets too high. The batteries needed replacement every 18-24 months. With ultra-capacitors, it’s a 15-20-year application.
RP: Impressive. What are some other utility applications?
CH: Large scale energy storage gets very expensive and very difficult. But if you put ultra-capacitor banks on every pole, you’d not only get distributed energy storage, but you get frequency control, voltage balancing, and peak shaving all in one. V2G (vehicle to grid) is another area they can be used, both in the charging station and in the car. Ultracaps are also used in smart meters. They hold onto a small amount of power so that if the grid goes down, they can transmit that information. We’re also finding applications in solar homes. People want to be able to run motors on a lower voltage system. Even the low voltage LED lights have pretty high peaks that can throw the charging system off. So some suppliers are looking at ways to incorporate ultra-capacitors here, too.
Another thing that many people do not know is that in a solar PV system, you can store more energy if you charge an ultracap first and then use that to charge the battery. That’s because the battery needs the solar panel to reach a certain voltage before it can accept any energy. Ultracaps don’t have that restriction. You can get up to 20 percent more energy storage doing it that way, like on cloudy days or early in the morning.
RP: Lots of possibilities. What are the barriers?
CH: A lot of engineers still don’t know about it. So there are a lot of educational efforts going on.
RP: Where are you headed with this technology?
CH: We already have plenty of power, but it would be helpful if we could store more energy. Power capacity is a competitive advantage we enjoy. Ideally we’d like to get to the point where we can store 20 to 30 percent of the energy that a battery can store. Right now we can store about 5 percent of a Li-ion battery. If we can increase our energy capacity by a factor of 3 or 4, we are going to be much closer to what a battery can do, which will broaden our range of applications even further.