The traveling range of electric vehicles must increase signiﬁcantly for this drive system to take oﬀ in future. Controlling the temperature of traction battery and other electric components as well as passenger compartment air-conditioning have a major inﬂuence on the energy needed. For this reason, IAV is working on innovative and eﬃcient thermal management systems to ensure optimum operating temperatures and a comfortable climate for vehicle occupants.
In the same way as human beings, vehicle components also have a feel-good temperature: electric vehicle batteries should operate at between 15 and 35 degrees Celsius, with their optimum operating temperature in the region of 25 degrees Celsius. Any ambient conditions varying widely from these will have a negative impact on the performance and life of the energy storage system. Electric motors feel at home in a temperature range of between 30 and 80 degrees Celsius, their optimum coolant temperature being 60 degrees Celsius which is also the temperature that should be provided for the power electronics. And last but not least: the occupants of an electric vehicle also have expectations – they don’t want to shiver in winter nor do they want to mop their brows at the height of summer either. All of this costs a lot of energy: “After the drive system, thermal management is the second largest energy consumer in the electric car”, explains Dr. Ralf Tröger, head of the IAV’s Fluid Dynamics and Thermodynamics Computation department. “At worst, its traveling range can fall by 50 percent if a lot of energy goes into heating e-components and passenger compartment while battery performance is reduced.”
Innovative overall concepts for the cooling and air-conditioning circuits in e-vehicles
This results in challenging demands placed on thermal management in electric vehicles. “A large part of available energy goes on air-conditioning for the passenger compartment and battery as well as on controlling the temperature of electric motor and power electronics”, Tröger says. “This is why IAV’s Powertrain Systems Development and Vehicle Integrated Systems sections are working together on developing innovative overall concepts for the cooling and air-conditioning circuits in electric vehicles. With the objectives involved being so multi-faceted, the layout of cooling and air-conditioning layouts is increasingly merging, resulting in higher circuit complexity and necessitating greater control and calibration input.”
This explains the need for two separate water circuits in electric vehicles – one at low temperature (for the battery) and a second one at a higher temperature (for the electric motor and power electronics). Complex control strategies are responsible for taking components as quickly as possible to their comfort temperature without overheating later on while driving.
All-embracing simulation of thermal management
In designing components and optimized control strategies, IAV champions simulating all aspects of thermal management as the basis for quickly evaluating and optimizing new ideas and strategies at an early stage of the development process. “To do this, we need to replicate individual components in terms of their geometry and physical operating principles as accurately as possible while also describing the overall system with an adequate degree of detailing” Tröger summarizes. At system level, 1-D simulation is used, whereas some components can only be described with sufficient accuracy using 3-D simulation – such as the cooling plates which, wherever possible, have the purpose of maintaining the battery’s cells at the same temperature. Besides commercial tools, like GTSUITE, AMESim and StarCCM+, IAV’s specialists use MATLAB-based tools developed inhouse. “Even when we use commercial tools, we supplement them with our own user routines – particularly if the existing model libraries can’t meet all of our demands in spite of being exactly calibrated.”
In-house model libraries for simulation
This being so, IAV has developed its own model libraries for simulating thermal management in oil-cooled electric motors and traction batteries. This also applies to the entire passenger compartment as well as to all of the elements of the refrigerant circuit (compressors and condensers) for which physical models are in some cases also available. “We can generate detailed simulations of all interfaces leading from the cooling and heating components into the vehicle interior as well as into the refrigerant circuit” says Jan Ackermann, Senior Vice President for Energy and Concepts at IAV. “This guarantees maximum comfort in the passenger compartment.” Proprietary processes also help to handle data from growing model diversity. A high degree of automation simplifies the work of engineers in complex parameter studies.
To optimize thermal management in e-vehicles, IAV also places an emphasis on cooperation between different departments in powertrain and vehicle development. Together, they work on developing an interdisciplinary approach that examines the flows of energy in the powertrain and passenger compartment simultaneously and under the aspect of their correlations. This makes it possible to develop requirement specifications, functions and operating strategies for component and vehicle at early stages in the engineering process. The aim here is to provide on-demand thermal management that offers the optimum combination of operating reliability and comfort. The availability of vehicle power is ensured by the fact that all components reliably operate in their optimum temperature window. Our integrated approach reaching from system development to designing individual components means we can act as an excellent link between the OEMs and their suppliers”, Tröger explains.
This has already paid dividends in numerous customer projects. “Implementing optimization measures, we have in some cases managed to increase traveling range by 15 to 20 percent”, Tröger reports. “And we are also working on improving our toolbox further – for instance, by adding real-time models that provide a link with the test bench or HiL environment. In future, we will be able to use them for examining the repercussions of the thermal management system on the hardware or control system before a finished vehicle exists.”