Trends in Electric Vehicle Fast Charging
Ultra-fast charging methodology, ultra-fast charging station architectures, and improved battery technology are some promising trends for fast-charging electric vehicles.
EVs hold the potential for decarbonizing the transportation sector. But the crucial impediment to the adoption of electrified transportation is the charging time taken by an EV. A fuel-based vehicle takes only 15 minutes or less to refuel. Therefore, to encourage the use of EVs, the charging time is expected to match that of fuel-based vehicles.
EV charging. Image used courtesy of Pixabay
Fast charging is key to alleviating range anxiety issues associated with EVs. Various trends are observed in the fast charging of EVs, such as ultra-fast charging, higher battery capacity, and architectures for ultra-fast charging stations.
Ultra-Fast Charging is the Future
Figure 1 shows that the energy demand for EVs is set to go up drastically in the near future. The energy demand is considered in three emerging economies that will boost the adoption of EVs. The illustration in Figure 2 throws light on the importance of fast charging in the coming years. Fast charging is on the roadmap of every emerging economy to boost EV usage. The level 2 and DC fast charging will witness a surge in its share of the total energy demand as time passes.
Figure 1. Total energy demand by EVs. Image used courtesy of IEEE Open Journal of Power Electronics
Figure 2. Energy demand by charging mode of EVs. Image used courtesy of IEEE Open Journal of Power Electronics
The DC fast chargers are rated at 50 kW, marching towards ultra-fast charging. Most drivers prefer to charge the EV battery within 15 minutes which is very challenging from a technological perspective. Some commercially available EV models that capture such fast charging technology are Mini Copper SE, BMW i3, Hyundai Kona, Tesla Model 3, and Tesla Model S. The battery capacity ranges from 25 to 95 kWh, with a range starting from 180 km to 515 km.
Better Battery Technology
As EVs continue to evolve, their battery capacity is expected to increase. The advancement in power electronics technology alone cannot achieve ultra-fast charging. The current battery technology limitations also restrict how fast we can charge an EV. Energy capacity is one of the battery parameters to look out for in long-range EVs. Lithium-ion batteries are the most suitable for EVs of the many existing battery technologies globally. The lithium-ion batteries have a higher power and energy density compared to its counterpart. These features help in removing the EV range anxiety problems of the masses.
The material composition of electrodes used in the lithium battery is a key factor in deciding the energy density of the battery technology. Lithium-ion batteries can be charged in 15 minutes with an energy density of 150 Wh/kg or more with the latest state-of-the-art materials. CATL company has utilized graphite and lithium nickel manganese cobalt oxide (NMC) as an anode and cathode, allowing the battery energy density of 215 Wh/kg. Kokam is another battery manufacturing company with the same anode and cathode composition, offering a battery energy density of 152 Wh/kg. Enevate has utilized Si and NMC as anode/cathode compositions for an energy density of 350 Wh/kg.
Thermal management is another serious issue with battery management. As the energy and power density of the battery increases, it is crucial to look out for proper thermal management practices. Overheating is a fundamental thermal management problem in an electric vehicle. The battery packaging is not designed properly to avoid cooling loss in the battery pack. Such an event leads to events where the battery catches fire, and the whole EV is engulfed in flame. EV battery performance is also affected by cold temperatures. Lithium-ion batteries tend to perform slowly in charging and discharging batteries when placed at very low temperatures. Hence, in such a case, a nominal battery temperature is necessary through mild heating without overheating the battery pack. A third issue prominent in the thermal management of EV batteries is thermal runaway conditions. It is a phenomenon where an increase in temperature aids in further heating up batteries if the temperature is not regulated properly.
Ultra-Fast Charging Station Architectures
The ultra-fast charging station needs to employ its unique architecture to enable fast charging of different EVs connected to it. These stations are most expected along long-distance highways where regular charging is necessary for EVs. Tesla's fast charging stations consist of nearly 10 to 12 direct current fast chargers, each one bearing a capacity of 150 kW. Therefore, a typical fast charging station has to be rated at 1.5 to 1.8 MW capacity. It is advisable to draw the power from a medium voltage grid for such high power. The low-voltage grid cannot handle such high power, and it might also put an additional burden on the transformers used.
Figure 3 shows a conventional AC distribution network-based ultra-fast charging station. This architecture utilizes multiple AC to DC converters dedicated to each charging point. The AC distribution network is, at present, the most mature architecture, and it is easily viable commercially.
Figure 3. An AC distribution network-based ultra-fast charging station. Image used courtesy of IEEE Open Journal of Power Electronics
Figure 4 shows the DC distribution network-based ultra-fast charging station. This architecture is currently being researched for its efficiency and commercial viability. This architecture utilizes a simpler architecture with a reduced number of conversion stages. A single AC-to-DC converter is used, after which the DC power is distributed to all the charging points. A promising future ahead for the DC-based network configuration is the usage of Solid State Transformer in the initial conversion stage to replace the combination of medium voltage grid and AC to DC conversion stage. It helps improve the overall system's efficiency with a battery control mechanism due to the use of power electronics technology.
Figure 4. A DC distribution network-based ultra-fast charging station. Image used courtesy of IEEE Open Journal of Power Electronics
Figure 5 offers greater insight into the different power electronics converter topologies of an AC distribution network-based ultra-fast charging station. It consists of AC-to-DC and DC-to-DC power conversions. The AC-to-DC conversion is also called the power factor correction stage, the first of the two stages of fast-charging power conversion. Three topologies commonly used for this stage are the Vienna rectifier, conventional 2-level voltage source rectifier, and multi-pulse rectifier. The common features of all these topologies are simplicity in design, higher reliability, and their ability to draw input currents with low harmonics distortion. The second stage is the DC-to-DC conversion stage, where three commonly used topologies are the Half-Bridge LLC, interleaved buck converter, and dual active bridge converter. These topologies can provide galvanic isolation between the EV and grid to enhance the reliability of the whole charging station.
Figure 5. Different power electronics topologies for DC fast charging. Image used courtesy of IEEE Open Journal of Power Electronics
A modular structure of power electronics converters in the DC fast charging is useful in many ways. It helps distribute the voltage and current stress equally among the different modules. Each module can cater to the unique voltage level demand. Therefore, modularity allows for different voltage and power handling capacities within an ultra-fast charging station. As the modules are spaced at a reasonable distance from one another, it allows for proper cooling of each module. In the future, the power handling capacity of an ultra-fast charging station can be increased or decreased by adding or deleting the individual modules.
Key Takeaways of Electric Vehicle Fast Charging
Electrified transportation is witnessing some trends in fast charging. The article has highlighted and briefly explained some important trends. Some of the takeaways of the article are as follows.
- Ultra-fast charging is the need of the hour to facilitate charging the EV battery in the least possible time possible.
- Battery technology will play a pivotal role where the main challenge is increasing the battery energy and power capacity.
- Increasing the battery capacity also needs to address critical thermal management issues. Overheating, cold climatic environments, and thermal runaway conditions are key points in thermal management.
- AC and DC distribution-based ultra-fast charging stations are the two architectures that have a high potential to cater to the demands of fast charging. AC-based architecture is mature enough and is the ideal starting point.
- However, DC-based architecture is gaining popularity due to the emergence of Solid State Transformers, which can better control and simpler conversion stages.
This post is based on an IEEE Open Journal of Power Electronics research article.