Ultracapacitors Enhance Electric-Drive Vehicle Performance

December 2010

To meet the performance expectations of today's drivers, automotive drivetrains must be able to provide sudden bursts of power for rapid acceleration. This high peak power demand challenges both battery electric and fuel cell drivetrains in several ways. First, peak power demands can reduce battery life. Second, engineers tend to oversize a vehicle's battery or fuel cell system to ensure it meets the peak demand, driving up costs, even when the need for peak power is infrequent. To address these challenges, alternative energy storage devices, such as ultracapacitors, could be incorporated into electric drivetrains to reduce the peak electrical load on batteries and fuel cells. An ultracapacitor, sometimes called a supercapacitor, is a large capacitor with high energy density and low internal resistance.

ITS-Davis Research Engineer Andrew Burke, who was awarded the first annual Leadership Award in Advancing Energy Storage Technologies by AES in October 2010, is one of the world's leading researchers in this field. Burke, Associate Project Scientist Hengbing Zhao, and Senior Development Engineer Marshall Miller have recently completed several computer simulation studies that evaluate ways to incorporate ultracapacitors into plug-in hybrid electric vehicle and fuel cell drivetrains.

In their paper “Simulations of Plug-in Hybrid Vehicles Using Advanced Lithium Batteries and Ultracapacitors on Various Driving Cycles,” Burke and Zhao present results of their study that produced simulation results for mid-size PHEVs with all-electric ranges of 25-90 km. For the simulations, the investigators used the ADVISOR program (ADvanced VehIcle SimulatOR). The simulations combined ultracapacitors with batteries that had high energy density (200-400 Wh/kg) and relatively low power capabilities.

The investigators simulated various driving cycles operating in both the charge-depleting and charge-sustaining modes. The use of ultracapacitors was shown to reduce peak currents from the batteries by a factor of 2 to 3. The hybrid electric drivetrains modeled included the Honda single-shaft, Toyota planetary, GM two-mode, and the VW/Borg Warner dual clutch transmission designs.

The battery-ultracapacitor combination was shown to provide good all-electric vehicle performance while greatly reducing stress on the batteries, significantly enhancing battery life.

In a second paper, “Fuel Cell Powered Vehicles Using Supercapacitors: Device Characteristics, Control Strategies, and Simulation Results,” Zhao and Burke again used the ADVISOR program to simulate hybridization of fuel cells with supercapacitors. Results showed that combining fuel cells with supercapacitors and load leveling electronic controls can significantly reduce the electrical and mechanical stress on fuel cells while increasing vehicle fuel economy by up to 28%. Alternative simulations that hybridized fuel cells with batteries instead of supercapacitors showed lower levels of improvement.

The fuel cell powered vehicle is one of the most attractive candidates for the future due to its high efficiency and fast refueling capability with hydrogen. However, its relatively poor dynamic response, high cost, and limited lifetime have impeded its widespread adoption. Fuel cells in conjunction with supercapacitors can create high power with fast dynamic response, making them well-suited for automotive applications. The best approach for hybridization of the fuel cell vehicles is to use supercapacitors with load-leveled control as it greatly mitigates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel cell durability.