Fast-charging latent heat storage system for fully-electric city buses

Emission-free heating of fully-electric vehicles is currently only possible with a significant reduction in range. In order to solve this problem, the Fraunhofer IVI developed a fast-charging latent heat storage system in the course of the Heat2Go project. The energy is thus no longer supplied by the traction batteries, but by wayside infrastructure. Battery buses that operate by the principle of opportunity charging are now able to absorb all the energy required for a cycle within a few minutes at the terminal stop and without straining the battery during heating mode.

In order to provide the required temperature in the passenger compartment of electric buses in winter and to meet the heating requirements of the driver’s workplace, a considerable amount of heating power must be provided. Due to the low engine waste heat of fully-electric buses, it is necessary to provide the heating power by means of an additional heat source.

Overview of previous heating systems of electric buses:

• Fuel powered heater – these do not reduce the range of the electric bus, but they do generate CO₂ emissions.
• Electrical heating systems – these are potentially emission-free, but have a negative effect on the range of the bus.

The idea of the project is based on the concept of so-called opportunity charging for public buses. Using this method the battery of the vehicles is recharged at (terminal) stops and on tour if required. The decisive advantage of opportunity charging is the possibility to reduce the battery capacity, as theoretically it only has to cover the consumption between two charging intervals. The charging infrastructure can usually provide greater charging power than required by the vehicles. This charging capacity can therefore be used for other tasks (e. g. charging the heat storage). As a result, the provision of energy for heating can be shifted from the traction battery to the charging infrastructure.

Advantages of charging thermal storage heating at the charging station parallel to the traction battery:

• No strain on the battery while driving and heating; longer battery life
• No reduced range in heating mode
• Heating energy provided without energy loss of the battery
• No wear and tear of the storage unit in contrast to the battery
• Locally without CO2 emissions

The technological challenge of the project was the design and implementation of the fast-charging heat storage modules, which together should absorb the energy for 60 minutes of heating within a charging time of 6 minutes. Moreover, the heat output must take place at a sufficient temperature level to heat the interior according to demand. A phase change material (paraffin) acts as storage medium for the required heat, storing heat during its melting processes and emitting heat during its solidification (60 to 80°C).

Advantages of latent heat storage:

• Heat storage capacity in phase change is greater than with sensitive heat storage with 100 k temperature stroke; Higher volumetric energy density
• High heat storage capacity at low temperature stroke
• Heat emission at constant temperature

The heat storage was integrated into the institute’s own test vehicle EDDA bus (Electromobility Demonstration Docking Application) as a central storage unit in order to keep the integration effort as low as possible. The area behind the driver’s workplace was chosen as the location of installation. Due to the modular design of the heat storage unit, a vehicle-integrated installation without reducing the number of seats is also possible. The convectors already installed in the vehicle were used for heat distribution. The hydraulic connection lines of the convectors to the heat storage were installed in the passenger compartment in order to keep the heat loss to the environment as low as possible.

With the energy storage management created in the project, an optimal charge control was developed for a homogeneous and complete charging of the modules. Furthermore, the system can be recharged from the battery when the heat storage is empty, thus ensuring continuous heating operation.

For the demonstration and evaluation of the heat storage system, extensive series of measurements were carried out in the climate chamber of the project partner Konvekta. For example, door openings and the associated air exchange were also taken into account. The conducted tests could prove the basic functionality of the heat storage system, whereby the required temperature of 18 °C in the interior could always be provided. In addition, a test application on the road underpinned the results of the tests in the climate chamber.

The tests in the climate chamber demonstrated the following system properties:

• Design capacity of 13.5 kWh
• Loading rate at the charging station of 7.6 h-1
• Average discharge rate 1.0 h^-1
  
• Peak heating capacity for heating up to 18 kW

• AURORA Konrad G. Schulz GmbH & Co. KG
• Konvekta AG
• Fraunhofer Institute for Transportation and Infrastructure Systems IVI