National Renewable Energy Laboratory

Energy Storage

Modeling and Simulation

Modeling and simulating advanced energy storage (ES) systems in vehicles helps researchers find the best ways to enhance overall vehicle performance. NREL's ES team evaluates energy storage devices, such as batteries and ultracapacitors, for advanced vehicles—and the interactions between the two—without having to test these devices and the vehicles themselves. This is accomplished by constructing computerized representations of ES devices and vehicles and simulating their performance under different conditions.

NREL uses standard test procedures developed by the U.S. Advanced Battery Consortium (USABC) to generate performance data and develop validated thermal and performance models. Researchers use these data in the ADVISOR™ vehicle simulation software and include them in NREL's public library. Collaborating with DOE and USABC, NREL is involved in the following projects to model and simulate ES devices:

• Improves models, validates developers' battery/ultracapacitor models, and conducts studies on the trade-offs between performance, life cycle, and cost for fuel cell, hybrid electric, and electric vehicles (FCVs, HEVs, and EVs).
• Collaborates with USABC and FreedomCAR technical teams to enhance ES test manuals and procedures, define energy storage requirements, and quantify the specifications and benefits of technical targets for energy storage technologies.
• Develops a "discretized" battery pack model that captures the individual behavior of each module in the pack rather than treating the pack as a single large module. The model—which allows researchers to evaluate the effects of pack imbalances in temperature, state of charge, and resistance—was created in Saber and is co-simulated with ADVISOR™ for use in vehicle simulations.
• Uses ADVISOR™ and industry battery pack models to investigate how varying battery temperatures, resistances, states of charge, and other parameters affect the fuel economy and performance of advanced vehicles. This enables researchers to determine how combining ES technologies can benefit the performance and fuel economy of a vehicle while maintaining or reducing the cost of the system.

NREL's energy storage experts also use other validated simulation tools to quantify the advantages and disadvantages of optimized hybrid ES solutions (batteries and ultracapacitors) for advanced vehicles. For example, to evaluate the benefits of combining ultracapacitors with batteries, researchers tested a hybrid pack of ultracapacitors and batteries in the energy storage laboratory, shown in the picture below. Next, they developed a MATLAB/Simulink model and validated the modeled results with laboratory results, depicted in the graph below.

Then they added the ultracapacitor/battery model to ADVISOR™ to determine the optimum cost and life solutions for various vehicle configurations. The BATMODEL analyzes test data from the laboratory and builds energy storage models suitable for vehicle simulation in ADVISOR. The resulting models are either internal resistance (Rint) models or Resistor/Capacitor (RC) models, depending on user selections and test data available. BATMODEL is also capable of comparing test data to model predictions for validation purposes.

In addition to modeling and simulation research, NREL experts conduct thermal management of ES devices from a systems perspective.