Range limitations of electric vehicles is deeply tied to thermal management issues

Image of Car Tachometer

The range of battery electric vehicles (BEV) is directly proportional to their battery capacity. With the current technology of battery cells on average a range of 6.6 kilometers per 1 kWh of battery capacity is achieved.

In practice the range of a BEV depends on the climate and environmental conditions. A large portion of the battery's power must be used for regulating the temperature in the passenger cabin. Under unfavourable conditions this can lead to a range reduction of up to 40%. For example, during a cold winter day the loss heat of a conventional internal combustion engine is no longer available as a "free" source of energy for heating the cabin.

Thermal analyses of cabin climatization have always been highly important for the layout of HVAC systems in cars. BEVs call for an even more extensive analysis covering the energy balance of the entire thermal system of the car. This necessarily includes the batteries themselves since running within the optimal operating temperature range extends life time and maintains maximum possible capacity. Furthermore, the study has to cover all measures for reducing battery power invested in passenger comfort rather than for the pure purpose of powering the car.

Thermal analysis with THESEUS‑FE helps to increase the range of electric vehicles

Temperature range of operation for the battery

The optimal temperature range for battery electric vehicles is between 20 and 30 °C. Higher temperatures reduce the life expectancy of the battery. Lower temperatures negatively impact the electrical capacity. For that reason alone, during summer a separate cooling mechanism has to be applied to the battery independent of the need for air-conditioning of the passenger cabin. During winter there is no heat loss from the engine available for heating. The heat for regulating the internal battery temperature to an optimal range has to be supplied by the battery itself.

Maximizing the range through intelligent climatization

All potential measures listed below can be tested and evaluated by simulations with THESEUS‑FE. Relying on simulations during the early stage of car development drastically reduces the costs for trying out some variations compared to real physical testing. Most importantly these virtual tests can be done even before any prototype of the car exists. The insight gained during this stage can save a lot of money before a wrong decision materializes in a real prototype.

Image of Modern Climatization Control Instrument in Car
Electric vehicles demand novel, economic climatization concepts

Measures for intelligent, range increasing climatization concepts include:

  • improved insulation of the cabin
  • application of materials with low thermal capacity to reach the desired cabin temperature faster and with less energy investment
  • zonal climatization, especially if the driver is the only person in the cabin
  • increased recirculating air operation of the HVAC system
  • pre-conditioning using auxiliary heating systems
  • electrically heated window panes which is more efficient than heating them up by warm air
  • fuel-powered heating systems (e.g. ethanol range extender)
  • seat heating to quickly increase local comfort levels
  • infrared radiators and electrically heated surfaces to support classical HVAC systems
  • use of modern glass materials which limit the exchange of thermal energy by radiation through the windows to the visible spectrum (e.g. infrared reflecting windows)
  • use of ventilation systems during parking driven by solar panels
  • infrared-reflecting surface coatings
  • optional shadowing measures to reduce solar energy input into the cabin

To cite an example, THESEUS‑FE allows for wavelength-dependent transmission and reflection properties of windows. Several variants of windows and coatings can be simulated with ease to rate their influence on the thermal budget of the car. Simulation results thus give immediate feedback about the effect of the HVAC systems power as well as the thermal comfort of passengers.

Case study: Thermal Simulations of a Volkswagen e-Golf Cabin Incorporating Human Thermal Comfort Models

Image of real Volkswagen e-Golf and Virtual Model as it was used for Simulation with THESEUS-FE
Volkswagen e-Golf - real model and cross section of simulation model showing individual component subdivision

As part of the publicly funded BMBF project, E-Komfort involved generating a highly detailed finite element model for simulating the climate control system of the passenger compartment of a Volkswagen e-Golf. A model originally used for crash simulations was used as a basis for developing a cabin model for interior climate control systems. Simulations were performed in an virtual environmental chamber under winter and summer load conditions relevant to vehicle design and range. This work also involved testing and calibrating special lamp models (for simulating sunlight in environmental chambers), and then using these models in the simulation.

Schematic demonstrating the various Environmental Conditions Acting on a Car Cabin with Passenger
Environmental thermal influence on car passenger cabin

The paper (available for download below) describes an initial validation study demonstrating a simulating technique capable of delivering realistic predictions of average and local cabin air temperatures. We illustrate how we extrapolated and calibrated a simplified cabin model (known as a rapid or generator model) from a highly detailed e-Golf simulation model. Doing so allowed us to reduce computing time from several days to a few seconds without compromising the quality of the simulated average temperature of the cabin air.

Image of CFD results of Car Cabin
Computational Fluid Dynamic Results for cabin climatization

Using THESEUS‑FE simulations, we were able to demonstrate that employing zonal climate control concepts can massively reduce the amount of energy that the climate control system consumes. Energy can also be conserved through the use of infrared emitters, which were simulated using suitable models and assessed in terms of their impact on local thermal comfort values. Under winter load conditions, e-vehicle range is significantly limited by the amount of energy consumed by the climate control system. As such, heating or cooling passengers only where needed and/or only to a comfortable temperature makes more sense than heating/cooling the entire passenger compartment. Against this backdrop, our simulation also incorporated a thermophysiological model and used the concept of equivalent temperature as a basis for assessing local thermal comfort. The final portion of the paper provides a discussion of motivation and strategies for coupled cabin simulations - in this case coupling THESEUS‑FE and OpenFOAM - as well as the corresponding validation work.

Related publications available as free download

Thermal simulation of a complete vehicle using manikin models [english, 3.3 MB]
S. Paulke, D. Köster, R. Hass, V. Bader, S. Menzel, A. Gubalke
P+Z Engineering, Volkswagen AG

17. SIMVEC 2014 | November 18-19, 2014 | Baden-Baden, Germany

Thermische Simulationen einer Volkswagen e-Golf-Fahrzeugkabine unter Einbezug von thermischen Menschmodellen [german, 3.1 MB]
S. Paulke, D. Köster, R. Hass, V. Bader, S. Menzel, A. Gubalke
P+Z Engineering, Volkswagen AG

17. SIMVEC 2014 | November 18-19, 2014 | Baden-Baden, Germany

Thermal Simulations of a Volkswagen e-Golf Cabin Incorporating Human Thermal Comfort Models [english, 3.3 MB]
S. Paulke, D. Köster, R. Hass, V. Bader, S. Menzel, A. Gubalke
P+Z Engineering, Volkswagen AG

17. SIMVEC 2014 | November 18-19, 2014 | Baden-Baden, Germany