Electromobility Thermal management

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Thermal management in vehicles with electric drive system

Up until now, thermodynamics special -ists at Porsche were responsible for effi-ciently conducting waste heat awayfrom the combustion engine, ensuringthermal stability and functionality withan optimized cooling system and reduc -ing fuel consumption at all operatingpoints using sophisticated thermal man -agement strategies. They also had toensure that components, auxiliary equip-ment and the transmission were cooled,ventilated and protected from high tem-peratures.

Electric vehicles have completely chang -ed the core tasks required. Along withcontrolling the temperature in the trac-tion battery and cooling the power elec-

tronics, electric motor and range exten-der, integrating these different coolingsystems and the efficient climate con-trol in the vehicle interior while alsosaving energy is now the main focus.

Because traction batteries in electricvehicles still have a limited capacity incomparison to conventional cars, elec-tric vehicles have a significantly smallerrange. This means that it is of centralim por tance to ensure that all oper atingfunctions are reliable, while also main-taining comfort and the energy effi -ciency of all technical systems. This lim -ited range can be even more drasticallyre duced by auxiliary loads such as heatingand climate control.

Cooling the battery, power electro-nics and the electric motor

From a thermal point of view, there arethree main aspects to take into accountwhen using lithium-ion batteries in elec-tric vehicles. At temperatures below zerodegrees Celsius (32 degrees Fahrenheit),the performance and therefore the rangedrop significantly due to slower chemi-cal reactions taking place in the battery.At temperatures above 30 degrees Cel-sius (86 degrees Fahrenheit) the batterydeteriorates exponentially, while extremetemperatures of above 40 degrees Cel-sius (104 degrees Fahrenheit) can leadto serious and irreversible damage inthe battery.

The ideal temperature for a lithium-iontraction battery is approximately thesame temperature that is ideal for hu-man beings. In order to achieve themaximum power output and a long life -span for the battery, it must be keptwithin an ideal temperature range ofbetween 20 and 30 degrees Celsius (68and 86 degrees Fahrenheit). To achievethis, it must be ensured that the batterydoes not exceed a maximum tempera-ture limit, while heat must also be distrib -uted as evenly as possible in the batterycells.

The thermal influence on a battery,such as fluctuations in the battery’sown temperature caused by resistanceinside the battery, external tempera tures,sunlight and waste heat, must thereforebe monitored and controlled by appro-priate thermal control systems, i.e. bymeans of heating or cooling as neces-sary. This ensures proper functioning

Transforming the drive train into an electric drive sets new

and exciting challenges for the thermodynamics specialists

at Porsche Engineering.

Porsche Engineering Magazine 1/2011

Power electronicsand electric drive Passenger cabin

Coolingand

heating systems

Traction battery

Synergy effects

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in the temperature range of –20 to+45 degrees Celsius (–4 to +113 de-grees Fahrenheit) as required by cus -tomers.

Depending on the type of electricallypowered vehicle (for ex ample mild hy- b rids, full hybrids, plug-in hybrids, com-pletely electric vehicles), requirementsprofile, battery type, cell chemistry andcell geometry, various cooling agents andmethods can be used. These methodsinclude air-cool ing, cooling with cool antsor refrigerants, and direct cooling orsecon dary cooling.

The latter involves cooling the batteryusing an external low-temperature cooler.Only if necessary, e.g. at high outdoortemperatures, an additional heat ex-changer (chiller) is used to transfer thelow temperature of the evaporating re-frigerant from the climate circuit to thebattery cooling circuit.

Along with the traction battery, it isimportant to take the temperature ofother components in an electric vehi-cle into account, such as electric mo-

development using previously identi-fied performance data and driving pro-file requirements.

The graphic below shows an example ofsome of the results from a thermal sim -ulation of a battery cooling processfor the New European Driving Cycle(NEDC). Based on the calculation ofthe power loss, the increase in the tem-perature of components as well as thecool ing requirement can be defined.Furthermore, the energy requirementsfor var ious cooling systems can be calcu-lated and the optimum design for lowenergy consumption in the entire vehiclecan be selected.

Thermodynamic engineers at PorscheEngineering develop and optimize thevehicle’s entire cooling system, indi -vidual cooling and heating circuits orcomponents according to customer re-quirements. An example of this processis the cooling plate from the traction

tor(s), power electronics and range ex-tenders.

As these systems all operate at differ -ent temperature levels and must becooled to different degrees (see graphicon the left), the aim is always to createsynergy effects by harmonizing all thevarious cooling and heating circuits(see graphic on opposite page). By har-monizing the temperature levels, thenumber of different cooling circuits canbe minimized, which in turn results inlower weight and costs, as well as morespace in the vehicle.

The engineers at Porsche Engineeringselect the cooling agents and methodsas early as during the first design con-cept phase. For this purpose, PorscheEngineering has developed a thermalsimulation tool and validated it in tests.This can be used to work out and definethe optimum cooling and heating sys -tem design at an early stage of vehicle

Porsche Engineering Magazine 1/2011

Electromobility Thermal management

Time

—— T Targets —— T Coolant —— P Loss —— Speed

Temperature levels

fi

fi

fi

fi

Electric motor

Power electronics

Battery

Combustion engine

140

120

100

80

60

40

20

T [°C]

fiAMBIENT TEMPERATURE

Simulation results for a vehicle in the NEDC

Calculations show that these kinds ofheating systems can actually absorb justas much power as driving itself and cantherefore reduce the range by up to 50percent. Porsche Engineering has devel -oped new and innovative approaches torelieve customers from the need to pickwhether to drive or freeze. Integratedauxiliary heating systems based on re-newable fuels or combinations of heatpumps, latent heat-storage units and air-and water-based auxiliary heaters can bedesigned and built according to the vehi-cle model and the requirements profile.

The Porsche engineers and designers al-ways have the whole vehicle in mind,which means that secondary methodsare also taken into account and testedfor efficiency and feasibility in the op-timization of thermal management.The use of innovative (insulation) ma-terials and designs to insulate the inte-rior and components can therefore leadto targeted results in order to promoteheat transfer or specifically insulate it.

Conclusion: challenges and solutions

Electrically powered vehicles no longerrequire conventional cooling as in com-

36 Porsche Engineering Magazine 1/2011

bustion engines, or they require it only toa limited extent. With the lack of enginewaste heat and the option of poweringaccessories using the combustion en -g ine, new solutions must be developedfrom the viewpoint of both thermal andenergy requirements.

This leads to complex thermal mana ge -ment systems at various temperaturelevels for cooling the battery, the elec-tric motor, the power electronics andany range extender. Climate control inthe vehicle interior is also becomingmore complex.

The specialized engineers at PorscheEngineering meet these challenges withsolid thermal management experienceand expertise from classic sports cardesign and construction as well as ex-perience from electromobility projects.

Using all available development toolsfrom simulation to calculation, from con -ventional component testing in the tes t -ing facility to the testing of the ent irevehicle on the test track, Porsche Engi-neering develops components, mod ulesand entire systems for the thermal pro-tection and optimization of vehicles andproducts of all kinds.

battery in the Boxster E, as it can beseen above. Based on the analysis re-sults from the thermal model describedabove, the cooling plate was designedgeometrically and optimized using com -putational fluid dynamics (CFD). The re-sult is a highly efficient and lightweightheat exchanger, optimally tailored andadapted to the battery pack, with lowpressure losses, high cooling perfor-mance and a very even distribution oftemperature.

Vehicle interior climate control

Another challenge in the design of elec-trically powered vehicles is the provi-sion of heating and cooling systems forpassenger comfort, as well as consider -ing aspects relevant to safety such askeeping the windows free of condensa-tion or ice (defrost function). The climatecontrol systems customers are familiarwith and expect from conventional vehi -c les require an energy input of up to 3 kWon hot and humid days, and up to 7 kWin the cold winter months for heating thevehicle. As there is no waste heat fromthe combustion engine, this energy re-quirement must be met by the tractionbattery, e.g. using a high-voltage PTCauxiliary heater.

Electromobility Thermal management

Boxster E battery cooling plate

Temperature