Using Toshiba’s Battery Electrochemical Models to Make System-level Decisions Faster

Eliminate the Guess Work in Battery Modeling Selections 

System-level design engineers have a difficult task. They deal with challenging questions such as: What Lithium-ion cells work best for a particular application; How many cells should be placed in series and parallel; or What type of motor and drive technology should be used? They have to make these decisions with little information about the actual components and before any prototypes. Often, important decisions with long-lasting consequences are made with “rules of thumb” and shortcuts such as; let’s use the component-level data from the previous generation.

This blog will focus on two of these challenges:

• What Lithium-ion cells work best for a particular application?
• How many cells should be placed in series and parallel?

To help engineers answer these questions, we’ve partnered with Toshiba Corporation. Toshiba provides the SCiB™ Lithium-ion cells that use lithium titanium oxide (LTO) anodes for superior safety, long life, rapid charging, and excellent performance at low temperatures. Within the SCiB™ product lineup, Toshiba provides a full spectrum of cells ranging from high-power cells (2.9 Ah, 10 Ah), high-energy cells (20 Ah, 23 Ah), and combination cells (20 Ah-HP). 

For cell selection and battery sizing, a system-level engineer might start by using models and data from batteries used in previous generations of similar products. Additionally, if specification sheets for Lithium-ion cells are available, they can be used to calibrate electrochemical models. However, Toshiba, a long-time user of Gamma Technologies’ (GT) GT-AutoLion, wanted to provide more than just a specification sheet and provide their clients with encrypted GT-AutoLion models. 

Mission – Tugboat

Tugboats are key for the navigation of large and bulky vessels in the narrow water channels of typical ports. Our today’s mission is around a battery-electric tugboat that is responsible for the towage of large vessels and consist of a series of phases:

• Transit phase: transit from the tugboat pier to the calling vessel
• Towing phase: towage of the vessel from the dock and out of the port
• Return phase: returning to the tugboat pier and charging station

We will take the view of an electrical system engineer at a tugboat building company, going through the cell selection and pack sizing process with these models provided by Toshiba. The system we are modeling is a battery-electric propulsion tugboat operating in a port requiring idling, transit, and towing maneuvers for multiple large vessels in a single-day of operation.

To illustrate the mission, we’ve overlaid arrows over a Google Maps screenshot of the Hamburg harbor, along with the target speed vs. distance profile we’ve applied to the model (Figure 1).

Figure 1. Google Maps screenshot of the Hamburg harbor with the tugboat target speed vs. distance profile

This mission is repeated four times over the course of a 15 hour workday, meaning that the tugboat will have 3 hours and 45 minutes to accomplish the mission and recharge the battery before being sent on the next mission.

Tugboat Simulation Model

The system-level model of the tugboat consists of the following components:

• 42m boat hull with a displacement of 280t
• Propulsion:
  • Two 2.4 m azimuth thruster
  • Two 2.7 MW Permanent magnet synchronous machines
  • Maximum bollard pull of 58 tons
• Genset:
  • Diesel genset with 1000/1260 kW @ 50/60 Hz
• A battery that needs to be selected and sized (will be described in the text below)

These components are arranged according to the single-line diagram below (Figure 2). In this system design we assume a DC link between the components. In case of an AC link design, additional switchboards would need to be integrated. In the image, “ESS” represents the electrical storage system (battery), “Shore Supply” is the onshore power supply (for cold ironing and battery charging), and “Hotel Load” stands for the onboard electrical power consumers.

Figure 2. Single-line diagram of battery-electric tugboat’s ESS, shore supply and hotel load

The model, built in the simulation platform GT-SUITE, is shown below (Figure 3). Please note the yellow links represent electrical connections and the black links represent mechanical connections. Additional thermal components and connections can be integrated to include thermal dependencies in the model and the warm-up of components.

Figure 3. Battery-electric tugboat system model built in GT-SUITE

Please note, that the four bulk carriers displayed in the image will be towed individually throughout a 15-hour workday. Each bulk carrier is being modeled as a passive load weighing nearly 47,000 tons.

GT-AutoLion Model

The Toshiba-provided model of the SCiB cell is built using GT-AutoLion, which follows the principles of the pseudo two-dimensional model (P2D) for lithium-ion batteries. The P2D model is based on the work of and captures the electrochemical reactions occurring inside the cell to capture terminal current, terminal voltage, power, heat generation, and concentration gradients of Lithium throughout the cell. As shown in the figure below, the model discretizes the lithium-ion using the finite control volume approach (Figure 4). The cathode, anode, and separator are discretized in the “thickness” direction; additionally, in each control volume of the cathode and anode, a spherical representation of active materials is used and is discretized in the radial direction.

Figure 4. Lithium-ion battery finite control volume discretization

In addition to the P2D model, GT-AutoLion has built upon the original work from Doyle, Newman, and Fuller to also include capabilities to capture Li-ion degradation, swelling, and thermal runaway.

The performance of the cell model was calibrated by Toshiba between -30°C and 70°C, and the voltage results of putting the cell model through constant current discharge tests are shown below (Figure 5).

Figure 5. Toshiba battery cell model discharge test voltage results in temperatures between -30°C and 70°C

Integrated Model

As GT-AutoLion is available as a model template in GT-SUITE, the model integration into system-level is very simple. The physics-based battery model received from Toshiba is simply linked to the electrical domain, which allowed us to replace the existing electrical-equivalent battery model, as shown in the image below (Figure 6).

Figure 6. GT-SUITE and GT-AutoLion battery-electric tugboat system model integrating Toshiba’s battery models

The supervisory controls were defined to operate the tugboat in “battery-electric” mode until the battery state of charge falls below 20% and to switch on the genset to continue the maneuver in “diesel-electric” mode, while keeping the battery state of charge at almost constant level.

The results shown below are displayed for a single maneuver (one 3 hour and 45-minute interval) with a battery that was sized to have 250, 20 Ah SCiB™ cells placed in series and 170 placed in parallel (250S/170P) (Figure 7).

Figure 7. Simulation results of a single mission. The Toshiba 20 Ah SCiB™ cells are arranged in 250S/170P.

Battery Sizing Results

The next image compares results between two different battery sizes, 250S/170P is shown in light grey and 250S/142P is shown in black (Figure 8). Notice how the state of charge decreases slower in the battery with more parallel cells, which allows the tugboat to complete more of its mission before needing to turn the genset on. Ultimately, this will mean that as battery size increases, less fuel is required to accomplish a single mission.

Figure 8. Simulation results of a single mission comparing two different battery designs. The Toshiba 20 Ah SCiB™ cells are arranged in 250S/170P (light grey) and 250S/142P (black).