Five years ago, an automotive company looking toward a green future might have considered replacing or augmenting traditional batteries with more efficient components such as ultracapacitors. At that time, such a consideration might have yielded disappointing results. Engineers might have noted that ultracapacitor performance wasn’t quite ready for automotive applications or that the price wasn’t attractive.
Both of those concerns are now outdated, and there is a growing adoption of this technology in the automotive industry.
The rechargeable battery has serious limitations. Batteries are heavy and large, and they offer a limited charging rate and potentially high maintenance. They also can suffer degraded performance at low temperatures. Alternatively, ultracapacitors, or electrochemical double-layer capacitors (EDLCs), provide high charge acceptance, high efficiency, cycle stability, and excellent low-temperature performance.
Ultracapacitors function well in weather as cold as -40°C (-40°F). Batteries, by comparison, do not operate well below 0°C (32°F). Ultracapacitors also are extremely safe and have a long life cycle—one that lasts throughout the life of the machines into which they are incorporated. This means they are maintenance-free, which ultimately results in cost savings. Finally, they are more efficient than batteries: up to 95% as compared to an average of about 70% for batteries in automotive applications.
Being 70% recyclable, ultracapacitors are also environmentally friendly. They contain no heavy metals detrimental to the environment.
And, ultracapacitors offer up to 10 times the power of batteries—an important benefit in vehicle acceleration.
Increasingly, ultracapacitors are besting batteries in terms of price. Over the past decade, their price has fallen by 99%. The cost of a 3000-F ultracapacitor 10 years ago was $5000; today, the component sells for $50. During the same period, battery price fell by 30 to 40%.
There are several reasons for this disparity, including the fact that the increase in ultracapacitor consumption has led to volume discounts, driving the cost of raw materials down. The increased demand has sparked more companies to produce related materials and machines, creating cost-reducing competition. As the price of this equipment falls, ultracapacitor manufacturers can add more machines and further rely upon automated equipment that is less expensive to deploy. At some point, it is likely that demand will outstrip capacity, pushing prices upward, but that is not yet the case. Furthermore, advanced quality control efforts have dramatically trimmed the waste and scrap created by traditional manufacturing processes.
Automotive manufacturers and general transportation companies have embraced the benefits of ultracapacitors in myriad applications. In hybrid buses, for example, ultracapacitors are used to improve fuel economy and reduce emissions. Some hybrid architectures are also especially attractive for large vehicles, such as urban transit buses and delivery trucks, in stop-and-go driving. Conventional buses and trucks of this type use huge amounts of fuel and produce high levels of toxic emissions because they have large, typically diesel engines that are constantly ramping up and down, which is the least efficient way to operate a power source.
Engine efficiency is increased with the use of a smaller engine mated to a generator and operated at constant, efficient rpm and power output levels. When vehicle power requirements temporarily increase, such as during acceleration or hill climbing, additional power is drawn from an onboard energy-storage system comprising batteries and/or ultracapacitors. During deceleration, regenerative braking recaptures energy while slowing down the vehicle and recharging the energy-storage system. At other times when vehicle power requirements are low, the generator can recharge the energy-storage system.
Michael Leiber of Ioxus wrote this article for SAE Magazines.