As the use of hybrid systems containing batteries and ultracapacitors has increased over the last several years, many manufacturers have found that using these systems in conjunction with an engine can help improve the overall energy efficiency of a vehicle.
Michael Liedtke, in charge of business development for Maxwell Technologies, San Diego, CA, says there are now thousands of hybrid buses worldwide using the company’s ultracapacitors and energy storage products. “We calculated that by next year, one billion people per year will be using our products because they are on so many buses.”
Since many applications don’t typically require 100% power draw 100% of the time, using energy storage systems can help supplement power demand by providing a boost of energy when necessary, says Alan Nelson, North American R&D Director at Dow Energy Materials, Midland, MI. This eliminates the need to throttle the engine all the way up, helping to reduce fuel consumption, as well as emissions.
Vehicles with a regenerative braking system, for example, can use a device such as an ultracapacitor to quickly capture the energy created during a braking event and reuse it to accelerate away from a traffic sign. Assisting in peak power demands like this not only takes some of the workload off of the engine, but also enables manufacturers to use a smaller one, which can help reduce overall vehicle weight and add further efficiency benefits.
Which device should I choose?
Knowing the application requirements is the first step to choosing whether to use a battery or an ultracapacitor. To determine which device is appropriate for a specific task, engineers must first define the power profile for the application. They must specify how much voltage is needed, how much current is needed for that voltage, and the duration of time necessary to complete the task at that specified voltage.
For applications requiring consistent power over an extended period of time, batteries are the better option. Liedtke says the electrochemical reaction which occurs inside these devices enables them to store large amounts of energy that can be accessed over several hours if necessary. Most electric vehicles use batteries for this very reason. With these vehicles, there needs to be a consistent source of energy.
Ultracapacitors, also known as supercapacitors, don’t have the energy storage capacity of a battery and are better suited for applications requiring a lot of short bursts of energy. “[An ultracapacitor] can only store about 1/10th of the energy of a battery, but has 10 times more power,” says Liedtke. “And power is just how fast you can release the energy that you store in the device.”
An electrostatic reaction within capacitors—consisting of electrons moving back and forth between defined surfaces—provides the high power output. This enables the capacitors to rapidly discharge energy, then just as rapidly recharge. If necessary, the capacitors can be continuously discharged and recharged up to one million times, which is why they lend themselves well to being used in regenerative braking systems. “You can imagine a bus constantly stopping at a bus stop, accelerating again, regeneratively braking for the next stop, and so on seven days a week over the course of a whole year,” says Liedtke. “You would kill a battery if you tried to put this kind of demand on it.”
Typically a lot of heat is generated within batteries, whereas capacitors’ low internal resistance keeps their heat generation to a minimum. To keep their batteries from overheating, many manufacturers use cooling lines. However, if the cooling causes a short, problems could occur, such as the battery catching fire.
Battery manufacturer Dow Kokam, Midland, MI, (a separate company from the aforementioned Dow Energy Materials) says it lowers internal battery heat by using a prismatic form for the cells and low Equivalent Series Resistance (ESR). When combined with a proper pack design and thermal management systems, heat buildup can be minimized even further.
Take a look inside
Much of the storage and power capabilities of batteries and capacitors are dependent on the materials and chemistries used inside the devices. According to James Burke, Vice President of Kold-Ban International (KBi), Lake in the Hills, IL, capacitors can have either a symmetric or an asymmetric design. In a symmetric design, both the positive and negative electrodes inside the capacitor are made of the same material, such as carbon, whereas asymmetric designs use two different materials for the electrodes.
KBi’s KAPower Supercapacitor employs an asymmetric design consisting of a nickel electrode and a carbon electrode. “We learned quickly there was a significant difference in the way the devices performed during engine starting, storage and temperature exposures,” says Burke. “Overall the asymmetric device outperformed the symmetric devices.” KBi also found that some symmetrically designed capacitors were using hazardous electrolytes. This required special handling during manufacturing and transportation of the capacitors because the electrolytes were flammable as well as highly toxic. The KBi capacitor, on the other hand, is more akin to consumer batteries because it uses potassium hydroxide electrolytes, also known as alkalines, which are not flammable or very toxic.
Batteries typically consist of cathodes, made of oxides or phosphates, and graphite- or carbon-based anodes. While batteries containing oxide cathodes are most commonly used in electric vehicles, Nelson of Dow Energy Materials says if safety is a key priority then phosphates are used because they have a different chemical structure and will be less likely to cause harm if damaged.
For its batteries, Dow Kokam uses nickel manganese cobalt (NMC) chemistry. Mira Inbar, senior marketing manager for commercial business at Dow Kokam, says this is because it provides the right balance of energy and density, a long lifetime as well as safety advantages. She also notes that the stacking techniques the company uses of these materials within the battery enables users to access more energy with minimal thermal build up.
Nelson says there are two key parameters for choosing materials; the capacity, or how much lithium a material can hold per volume, and the voltage at which the material operates. To further enhance these performance characteristics, the materials can be manipulated at the cell level, such as adjusting the thickness of the anodes and cathodes.
The biggest challenge, however, in optimizing these materials is the tradeoffs. “When you want to have high energy density, typically you’re sacrificing power density,” says Nelson. “The challenge for us is to try keeping all of these key parameters moving in the same direction at the same time.” Along with continuing to research new materials and chemistries, Nelson notes the importance of optimizing the battery components together, instead of individually. “Optimizing them together as a system is really what you need to do in order to meet some of the performance criteria for lithium-ion batteries and the energy storage market in general today.”
Working in tandem
Many manufacturers have begun to employ split systems which utilize both batteries and capacitors. “Whenever there’s a quick power demand, you satisfy that out of the ultracapacitor,” says Maxwell Technologies’ Liedtke, “and if there is a long-term energy requirement, you satisfy that out of the battery pack.”
With the split system, batteries can be used to power “creature comforts” such as air conditioning, while capacitors are reserved for high power needs such as engine starting. “What the industry has accepted, and known for years, is that there is no single device that does all of that very well,” says Burke. “We’ve always had to select and compromise what type of battery or energy storage device can do ‘a little bit of everything’ but ‘a lot of nothing’ efficiently.” By employing a split system, OEMs no longer have to sacrifice power for storage capacity or storage for power density. They can use batteries and capacitors together in one system to meet both requirements.
Split systems also offer the opportunity to reduce vehicle weight. Often times, an OEM will use an oversized battery, or as many as four batteries, to achieve the power density that one capacitor can provide. By combining the two devices into one system, however, the capacitor can provide the necessary power demands, enabling the battery to be downsized while still offering the benefit of high storage capacity.
Burke sees the use of different technologies within a vehicle, like the split system, as the future for designing cost effective and efficient vehicles. “A vehicle that uses the least amount of energy possible without wasting it is going to [accomplish that] by splitting up the different energy storage technologies for the application.”
As energy storage devices continue to become more mainstream, battery and capacitor manufacturers will continue working on ways to improve upon their technologies. Liedtke says Maxwell Technologies is currently looking at ways in which it can add more energy into its ultracapacitors without sacrificing their high cycling, wide temperature range and power performance characteristics. Dow Energy Materials is also investigating ways to improve energy and storage properties, as well as cost, through the use of different chemistries and compositions of materials. This includes moving beyond lithium-ion technology for batteries.
Within the next 10 to 20 years, lithium-oxygen batteries will become more prevalent. “It’s really a game-changing technology,” Nelson says. “You would be looking at increasing the amount of energy that can be stored in a battery by a factor of 10.” The most daunting challenge with lithium-oxygen batteries is their cycle life. At the moment, batteries using this technology only work for one charge.
“The days of burning fossil fuels or wasting energy by allowing an internal combustion engine to run when it’s not producing energy are limited,” says Burke. “Everyone agrees energy is abundant, be it solar or wind, but we just need to know how to store it effectively and put it to use when we need it.”