Ultracapacitors are awesome. But could they viably replace batteries in future electric vehicles?
Ultracapacitors have significant advantages over batteries, after all, they are much lighter, faster to charge, safer, and non-toxic. However, there are some areas where batteries wipe the floor with them. At least for now.
With recent acquisitions of ultracapacitor manufacturers by the likes of Tesla, ultracapacitors could be on the verge of ousting batteries as the go-to power source for electric cars.
What is an ultracapacitor?
Ultracapacitors, also called supercapacitors, double-layer capacitors, or electrochemical capacitors, are a type of energy storage system that has been gaining popularity in recent years. They can be thought of as a cross between an ordinary capacitor and a battery, yet they are different from both.
Ultracapacitors have a very high capacitance compared to their traditional alternatives -- hence the name. Just like a battery, ultracapacitor cells have a positive and negative electrode separated by an electrolyte. But unlike batteries, ultracapacitors store energy electrostatically (in the same way as a capacitor) rather than chemically like a battery.
Ultracapacitors also have a dielectric separator dividing the electrolyte - just like a capacitor. This internal cell structure allows ultracapacitors to have a very high energy storage density, especially when compared to a normal capacitor.
Ultracapacitors do store less energy than a similarly-sized battery. But they are able to release their energy much more rapidly, as the discharge is not dependent on a chemical reaction taking place.
Another great benefit of ultracapacitors is that they can be recharged a huge number of times with little or no degradation (in excess of 1 million charges/discharge cycles is not uncommon). This is because no physical or chemical changes occur when they recharge.
For this reason, supercapacitors are often used in applications requiring many rapid charge/discharge cycles rather than long term compact energy storage, such as car booster packs and power banks.
The most commonly used electrode material for ultracapacitors is carbon in various forms, such as activated carbon, carbon fiber-cloth, carbide-derived carbon, carbon aerogel, graphite (graphene), and carbon nanotubes (CNTs).
How to charge an ultracapacitor?
When a voltage differential is applied to the positive and negative plates of the capacitor, it begins to charge. According to Battery University, "This is similar to the buildup of electrical charge when walking on a carpet. Touching an object releases the energy through the finger."
Some of the very first examples of this technology were developed in the late-1950s at General Electric, but there were no viable commercial applications at the time. It would take until the 1990s for advances in material science and manufacturing to improve the performance of ultracapacitors and lower their cost enough to make them commercially viable.
How do ultracapacitors work?
As touched upon above, ultracapacitors work by delivering quick bursts of energy during peak periods of power demand, then capture and quickly store excess energy that may otherwise be lost.
For this reason, they are a great compliment for primary energy sources, as they charge and discharge very rapidly and efficiently.
While batteries can hold large amounts of power, they tend to take hours to recharge. In contrast, capacitors, and especially ultracapacitors, charge almost instantly, but they can store only small amounts of energy.
For this reason, ultracapacitors are the perfect solution when a system needs to charge rapidly and does not need to store electricity for long periods of time. They also weigh less than batteries, cost less, and generally don't contain toxic metals or harmful materials.
Can ultracapacitors replace batteries?
The answer to this question depends very much on what they will be used for. There are advantages and disadvantages to each. As previously mentioned, batteries have a much higher energy density than ultracapacitors.
This means that they are more suitable for higher energy density applications, or when a device needs to run for long periods on a single charge. Ultracapacitors have a much higher power density than batteries. This makes them ideal for high-drain applications like powering an electric vehicle.
As mentioned above, ultracapacitors also have a much longer lifespan than batteries. A regular battery can handle around 2000-3000 charge and discharge cycles, while ultracapacitors can usually sustain more than 1,000,000. This can represent huge savings in materials and costs.
Ultracapacitors are also much safer and considerably less toxic. They contain no harmful chemicals or heavy metals and are much less likely to explode than batteries.
In addition, ultracapacitors have a much greater operating range than batteries. In fact, they beat batteries hands down in this area, as they can operate within ranges of between -40 to +65 degrees Celsius.
Ultracapacitors can also be charged and discharged much more rapidly than batteries, usually within seconds, and are much more efficient at self-discharge than batteries.
Many ultracapacitors also have a much longer shelf life than batteries. Some, like SkelCap cells, can be stored for as long as 15 years at a time with little to no decline in capacity.
As with most technology, the main driver for the application of ultracapacitors is their cost to benefit ratio. Ultracapacitors tend to be the more economical choice over the long run for applications needing short bursts of energy.
Batteries, however, are a much better choice for applications that require constant, low current over time.
Could ultracapacitors replace batteries in future electric cars?
As we have seen, ultracapacitors are best suited for situations where a lot of power is needed in a short span of time. In terms of electric cars, this would mean they would have advantages over batteries when the vehicle needs bursts of energy - like during acceleration.
In fact, this is just what Toyota has done with the Yaris Hybrid-R concept car, which utilizes a supercapacitor for use during acceleration.
PSA Peugeot Citroen has also started employing ultracapacitors as part of its start-stop fuel-saving systems. This allows for much faster initial acceleration.
Mazda's i-ELOOP system also uses ultracapacitors to store energy during deceleration. The power stored is then used for the engine's stop-start systems.
Supercapacitors are also used to rapidly charge the power supplies in hybrid buses as they go from stop to stop.
When hybrid energy is used purely for performance, issues such as range and the ability to hold charge aren’t as important – and so some high-end manufacturers, such as Lamborghini are also starting to incorporate supercapacitor-powered e-motors in their hybrids.
However, ultracapacitors are not a substitute for batteries in most electric vehicles - yet. Li-ion batteries are likely going to be the go-to power supply for EVs for the near to distant future.
Many believe it is more likely that ultracapacitors will become more commonplace as power-regeneration systems during deceleration. This stored power can then be re-used during periods of acceleration rather than direct replacements for batteries.
However, according to this study, they could also have applications in hybrid vehicles in place of batteries when, "the power demand is less than the power capability of the electric motor; when the vehicle power demand exceeds that of the electric motor, the engine is operated to meet the vehicle power demand plus to provide the power to recharge the supercapacitor unit."
Recent research into graphene-based supercapacitors could also lead to advances in supercapacitor use in electric cars. One study by scientists at Rice University and the Queensland University of Technology resulted in two papers, published in the Journal of Power Sources and Nanotechnology.
They proposed a solution consisting of two layers of graphene, with an electrolyte layer between them. This resulting film is strong, thin, and able to release large amounts of energy in a short time.
These factors are a given-it is a supercapacitor after all. What makes this study different is that the researchers suggest that the new, thinner ultracapacitors could replace bulkier batteries in future electric vehicles.
This could also include integrating the ultracapacitors into body panels, roof paneling, floors, and even doors, for example. In theory, this could provide the vehicle with all the energy it needs, and make it considerably lighter than battery-powered electric vehicles.
Such an EV would also charge considerably faster than current battery-powered vehicles. But, like all ultracapacitors, this solution still can't hold as much energy as standard batteries.
"In the future, it is hoped the supercapacitor will be developed to store more energy than a Li-Ion battery while retaining the ability to release its energy up to 10 times faster - meaning the car could be entirely powered by the supercapacitors in its body panels,” said the study's co-author Jinzhang Liu.
"After one full charge, this car should be able to run up to 500km (310 miles) - similar to a petrol-powered car and more than double the current limit of an electric car.”
Interesting times ahead it seems. Watch this space.