The Hybrid Systems Research Team at GE Global Research, the company’s central technology development group, has demonstrated a dual battery system that it claims could reduce battery cost by up to 20% and help accelerate the electrification of buses, delivery trucks, off-highway vehicles, and other heavy-duty vehicles.
The dual battery system—pairing a high-energy-density sodium battery with a high-power lithium battery—was installed on a hybrid transit bus that recently completed a test-drive on roads in and around Albany, NY.
GE’s Hybrid Systems Team, which consists of about 25 scientists and engineers, believes a dual system that combines high power and energy storage capacity could achieve the optimal electric driving range and acceleration requirements at a more practical size scale and cost for larger vehicles. The research is being conducted as part of a $13 million research project with the Federal Transit Administration (FTA) and Northeast Advanced Vehicle Consortium, funded under the National Fuel Cell Bus Program.
The system has been in development by GE’s team for more than three years.
“With the cost of the battery remaining a principal hurdle, a dual battery system could bring these costs down and help accelerate the electric revolution for bus and delivery truck fleets representing hundreds of thousands of vehicles,” said Lembit Salasoo, Senior Electrical Engineer and Principal Investigator on the hybrid bus project at GE Global Research.
“With heavier vehicle platforms, both energy storage and power are a premium to deliver optimal vehicle performance, but the exact needs can vary based on a vehicle’s size and drive cycle,” Salasoo added. “The beauty of our dual battery system is that it can be scaled to deliver just the right combination of power and storage.”
GE researchers note that most types of batteries come with a trade-off between power and energy storage: lithium batteries, for example, provide sufficient power for acceleration but are not optimized to store energy for driving range, while sodium batteries store large amounts of energy but are less optimized for power. Thus, the reason they decided to combine both battery types in their solution.
In the hybrid transit bus demonstration, the lithium battery focused on acceleration and braking, and the sodium battery provided even power flow to extend range.
“We used a sodium metal halide battery for the energy battery and A123Systems’ lithium-ion nanophosphate battery for the power battery. The sodium battery we used was a similar technology to GE’s new NaMx sodium battery technology,” Todd Alhart, a spokesman for GE Global Research, told SAE Magazines. “For this proof-of-principle demo, we utilized a combination of 54-kW·h sodium batteries and 21-kW·h lithium batteries. In our next design, we will rebalance the two battery technologies based on performance optimizations.”
GE plans to use its own sodium battery as part of the next phase of the FTA project in 2011.
The key cost advantage of a dual system is that it enables the integration of less-expensive battery chemistries without having to increase the size of the battery. A single battery system would require a more costly scale-up in battery size to achieve the same result, the researchers claim.
“Both the sodium and lithium battery technologies need to have a suitable aspect ratio for the vehicle,” Alhart said in discussing packaging and other challenges with the system. “In our current mule bus demonstration, this is not an issue since the bus is configured for easy access to developmental systems from project to project. However, for a passenger-ready bus such as the one we will be developing next, a lower profile is generally useful. In addition, both sodium and lithium technologies require thermal management to dissipate losses and maintain appropriate operational temperature ranges.”
So where does the technology go from here?
“We will take more data on the operation of the technology development bus, including its performance with a hydrogen fuel cell, and use what we learn to update our design for a second bus to be as cost-effective as possible,” Alhart said. “The next step is to install the technology we have developed under this project on a new modern, advanced composites lightweight zero-emissions bus. The technology comprises advanced energy-management techniques including high-performance power electronics and more robust components for GE’s NaMx battery.”
GE is exploring partnership opportunities across the electric-vehicle value chain through its Licensing business to commercialize its dual battery technology.
“In this project, we are continuing to prove out the dual battery concept to understand its full potential,” Alhart added. “With more real-world results and experience, we will get a much better handle on the market opportunities and benefits our dual system can provide in the transportation sector.”
GE recognizes that in some applications, a single battery technology is still appropriate. “In particular, for the 4400-hp hybrid heavy-haul freight locomotive, GE’s NaMx technology is the best fit,” he claimed.