NREL/GM Project Finds Ultracaps Perform as Well or Better Than NiMH Pack for Energy Storage in Mild Hybrid Vehicle
|NREL simulation results showed similar fuel consumption vs. energy window trends for various drive cycles, with approximately half of the fuel savings being provided by a window ≤ 50Wh. Source: Gonder et al. Click to enlarge.|
A National Renewable Energy Laboratory (NREL) project, funded by GM, found that an ultracapacitor-based energy storage system (ESS) in a Saturn VUE BAS (belt alternator starter) mild hybrid performed at least as well or better than the NiMH battery system in the production hybrid. Jeffrey Gonder, a senior engineer at NREL, presented the results at the SAE 2010 Hybrid Vehicle Technologies Symposium in San Diego.
The project grew out of analysis NREL performed for the United States Advanced Battery Consortium (USABC) suggesting that a hybrid vehicle can achieve significant fuel savings with a high-power, but low-energy on-board storage system. Approximately half of fuel savings on various drive cycles were achieved with an “energy window”—the in-use min-max energy required for a particular cycle—of ≤ 50Wh.
Significant fuel savings could be achieved with a 150 Wh high power ESS, the earlier study found, with with fuel savings tapering off at energy windows > 200 Wh.
Reasons for the large total nominal energy in present production HEVs, the study concluded, included:
- Infrequent drive cycle use (e.g., long up/downhill grades)
- Achieving longer cycle life from reduced SOC swings
- Energy comes along with sizing for power requirements (particularly at cold temperatures)
- Power dominates cost in HEV (high P/E ratio) batteries
That earlier study concluded that ultracapacitors should be considered as a low-energy, high-power ESS and can bring potential benefits such as long cycle and calendar life; good temperature performance; and low long-term projected costs.
For the Saturn VUE study, NREL selected off-the-shelf Maxwell 48 V, 165 F modules (each with around 35 Wh usable). NREL tested single and dual (in parallel) module configurations, providing a lower (35 Wh) and upper (70 Wh) boundary to the 50 Wh window.
Vehicle testing took place using the stock as well as the one- and two-ultracapacitor module configurations, both on-road and in a climate-controlled chassis dynamometer test cell. The ultracap modules were paired with a spare Energy Storage Control Module (ESCM); the stock NiMH pack remained in the vehicle, and the testers could toggle between it and the ultracaps.
The Saturn VUE BAS hybrid has a 25% higher window-sticker fuel economy than its conventional counterpart; all three configurations showed comparable results on the FTP, Highway and US06 cycles.
|Dyno testing, FTP Cycle. Click to enlarge.||Dyno testing, Highway and US06 Cycles. Click to enlarge.|
Areas for future exploration, Gonder said, could include support for a larger motor and/or a smaller ultracap module; study of more cycles; and use of a different vehicle platform.
Gonder, J.; Pesaran, A.; Lustbader, J.; Tataria, H. (2009). Fuel Economy and Performance of Mild Hybrids with Ultracapacitors: Simulations and Vehicle Test Results (Presentation).