Introduction
During my bachelor thesis at the Institute of Automotive Technology at the Technical University Munich I created a thermal simulation of the transport of lithium-ion battery cells, battery systems and electric vehicles. On this webpage I want to share an overview of my thesis and some of the results.
The goal of the project was to gain a better understandig of the
expected temperature during transport of lithium-ion batteries.
Storing lithium-ion batteries outside of an optimal temperature
interval can accelerate the ageing process of the cells. Due to ageing
the cells loose parts of their capacity.
Lithium-ion batteries are needed for an evergrowing list of different
products. Focus of the project was the production of battery cells and
systems for electric vehicles. Because the majority of battery cells
is currently being produced in Asia, but assembled into battery packs
and installed in electric vehicles in production plants worldwide,
long transports are expected. Those transports are typically carried
out by land and sea. Therefore different types of transport and
packaging needed to be evaluated.
Methodology
To investigate the temperature during transport, a simulation approach
was chosen. This allowed to quickly analyze a large amount of
different transports.
I used the following tools for the project:
- Python for collecting historical temperature data and controlling the simulation
- OpenFOAM for thermal CFD simulation running on a Linux machine
- Visualization of simulations results with Python and Paraview
The thermal model was implemented with the CFD software OpenFOAM. To
generate the input data, control the thermal simulation in OpenFOAM
and to format and visualize the output data, a Python program was
implemented.
As input of the simulation following variables were considerd:
- general data of the transport, i.e., vehicle, start date and time, stops
- thermal properties and geometry of the cargo, packaging and carrier
- transport route
- weather data along transport route
The transport route could either be defined by a set of coordinates or via the internal routing service of the institute. The weather data was gatherd with a self written tool, that downloads the historical temperature data from the closest available weather station. This data is available in the integrated-surface-database (ISD) of NOAA.
The OpenFOAM thermal model considerd following thermal effects:
- conduction through the walls of the carrier and inside the batteries
- convection on the outside and inside of the carrier
- solar radiation on the outside of the carrier
It was assumed that the transport carrier is always in full contact with the environment, i.e., the carrier is never shaded by other objects and always affected by airstream proportional to the traveling speed. This was done to reveal the most extreme conditions.
Results
One of the most interesting analysis was a transport from a cell
manufacturing plant in Poland to a car manufacturing plant in Germany.
Prior to my work, the institute conducted an experiment, where the
temperature inside of the truck during this transport was measured. In
figure 1 you can see the measured and the simulation data. The
simulation was able to fit the measured data well. The plot also shows
the ambient temperature that was retrieved from historical weather
data.
Figure 2 shows a visualization of the transport route and simulation.
The literature indicated that especially on long transports with
shipping containers the highest temperatures should be expected.
Therefore another focus was the investigation of this kind of
transport.
For the definition of possible routes, sensor data of sea freight
container transport by
DB Schenker
were used. Figure 3 shows the simulated temperature inside of the
container aswell as the ambient temperature. Figure 4 shows the route
of the transport.
The results show that temperatures up to 55°C on an transport of this
kind can be expected. The figure indicates that the temperature in the
container mostly follows the ambient temperature but especially in the
daytime an elevation due to solar radiation exists. Notably the
highest temperatures are reached when the container is stationary,
i.e. no forced convection due to an airstream is present.