Total Thermal Management Of Battery Electric Vehicles (BEVs)

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NREL/CP-5400-71288. Posted with permission. Presented at SAE CO2 Reduction for Transportation Systems Conference, 6-8 June 2018, Turin, Italy.2018-37-0026Published 30 May 2018Total Thermal Management of Battery ElectricVehicles (BEVs)Sourav Chowdhury, Lindsey Leitzel, Mark Zima, and Mark Santacesaria Mahle Behr Troy Inc.Gene Titov, Jason Lustbader, John Rugh, and Jon Winkler National Renewable Energy LaboratoryAamir Khawaja and Murali Govindarajalu FCA US LLCCitation: Chowdhury, S., Leitzel, L., Zima, M., Santacesaria, M. et al., “Total Thermal Management of Battery Electric Vehicles (BEVs),”SAE Technical Paper 2018-37-0026, 2018, doi:10.4271/2018-37-0026.AbstractThe key hurdles to achieving wide consumer acceptanceof battery electric vehicles (BEVs) are weather-dependent drive range, higher cost, and limited battery life.These translate into a strong need to reduce a significantenergy drain and resulting drive range loss due to auxiliaryelectrical loads the predominant of which is the cabin thermalmanagement load. Studies have shown that thermal subsystem loads can reduce the drive range by as much as 45%under ambient temperatures below 10 C. Often, cabinheating relies purely on positive temperature coefficient (PTC)resistive heating, contributing to a significant range loss.Reducing this range loss may improve consumer acceptanceof BEVs. The authors present a unified thermal managementsystem (UTEMPRA) that satisfies diverse thermal and designneeds of the auxiliary loads in BEVs. Demonstrated on a 2015Fiat 500e BEV, this system integrates a semi-hermeticIntroductionIn recent years, the global automotive industry has focusedon developing efficient, affordable, long range, batterypowered passenger vehicles that will compete with andultimately replace their fossil-fuel-powered counterparts.While battery electric vehicle (BEV) architecture andsupporting infrastructure are maturing, hybrid and plug-inhybrid electric vehicles have immediate appeal even thoughthey retain the dependence on fossil fuels. Further adoptionof battery-powered vehicles will require lowering the costof batteries, enabling fast charging, ease of access to charginglocations, and reliable longer range. It is also important thatrange does not significantly vary due to auxiliary loads suchas heating and cooling of the cabin and vehicle components,similar to passenger experience with traditional internalcombustion engine (ICE)-powered vehicles. In ICE vehicles,the auxiliary loads represent a small fraction of the fuel usesince a significant fraction of energy is lost as waste heat. InBEVs, due to a highly efficient conversion ratio (batteryenergy to traction), waste heat energy is very low and so the 2018 National Renewable Energy Laboratory.refrigeration loop with a coolant network and serves threefunctions: (1) heating and/or cooling vehicle traction components (battery, power electronics, and motor) (2) heating andcooling of the cabin, and (3) waste energy harvesting andre-use. The modes of operation allow a heat pump and airconditioning system to function without reversing the refrigeration cycle to improve thermal efficiency. The refrigerationloop consists of an electric compressor, a thermal expansionvalve, a coolant-cooled condenser, and a chiller, the latter twoexchanging heat with hot and cold coolant streams that maybe directed to various components of the thermal system. Thecoolant-based heat distribution is adaptable and saves significant amounts of refrigerant per vehicle. Also, a coolant-basedsystem reduces refrigerant emissions by requiring fewer refrigerant pipe joints. The authors present bench-level test dataand simulation analysis and describe a preliminary controlscheme for this system.auxiliary loads account for a much larger fraction of energyuse; therefore, BEVs require auxiliary systems to be moreefficient. Specifically, heating in an ICE vehicle is virtuallyfree due to being able to use waste heat from the engine. InBEVs, heating competes with traction power and can heavilydrain the battery in cold weather conditions. A survey ofthe BEV architectures in recent years indicates that theindustry has been experimenting with combinations ofdifferent thermal management concepts: pre-conditioningof the cabin; air-, coolant- and refrigerant-cooled batteries;heat pumping; collection and re-use of waste heat; etc. Someof these technologies can be combined to increase efficiencywhile lowering the cost and complexity of implementation.This study used a 2015 Fiat 500e BEV (Figure 1). Typicalof this generation of BEVs, this vehicle has three thermal loops:1. Cabin air conditioning loop2. Battery heating/cooling loop3. Power Electronics and Electric Motor (PEEM)cooling loop.

2TOTAL THERMAL MANAGEMENT OF BATTERY ELECTRIC VEHICLES (BEVs)A cursory analysis reveals that while the three subsystems are somewhat separate and independent in operation,lending themselves to a straightforward method of control,the electric heating of the air for HVAC management represents a significant drain on the battery energy, while the wasteheat of the battery and PEEM are not used.FIGURE 1 2015 Fiat 500e BEV with a 24-kWhlithium-ion battery SAE InternationalThe UTEMPRA SystemThis vehicle has a standard vapor compression loop forcabin air cooling and providing active cooling to the tractionbattery via a refrigerant-to-coolant heat exchanger (batterychiller). The vapor compression loop uses R-134a refrigerantand includes an electric compressor, a standard refrigerantto-air evaporator, and standard thermal expansion valves(TXVs). Heating the cabin air is achieved using a 5-kWpositive temperature coefficient (PTC)-based electric airheater located in the heating, ventilating and air conditioning(HVAC) module.In addition to being actively cooled by a chiller, thebattery is also cooled by coolant circulating in a loop betweenthe battery and a dedicated front-end radiator receivingforced ambient air flow. The loop has a 6 kW PTC coolantheater for heating of the battery. Figure 2 shows the schematicof the thermal loops in this vehicle. Testing has confirmedthat loss of range of this vehicle is 45% at -10 C compared torange at 22 C.Three thermal sub-systems of Fiat 500e BEV SAE InternationalFIGURE 3 UTEMPRA’s compact refrigerant sub-systemrunning between hot and cold coolant streams SAE InternationalFIGURE 2With its unique flexibility in design and integration of thecoolant architecture, the Unitary Thermal EnergyManagement for Propulsion Range Augmentation(UTEMPRA) system unifies the thermal management systemsof BEVs and may be thought of as a natural evolution of thevarious types of thermal management architectures in theBEVs to date. It comprises a semi-hermetic refrigeration loop[1] and a coolant network for thermal energy distribution andwaste energy harvesting. The refrigeration loop, shown inFigure 3, consists of an electric compressor, a TXV, a coolantcooled condenser, and a chiller. The condenser and the chillerserve the same purpose as the condenser and the evaporatorin a traditional refrigeration loop. Instead of exchanging heatwith air, these heat exchangers exchange heat with circulatingcoolant and therefore act as sources of hot and coldcoolant streams.A version of the UTEMPRA coolant network thataddresses the same thermal functions present in the Fiat500e is shown in Figure 4. This design uses two coolantpumps and valve manifolds that help distribute thermalenergy to the vehicle HVAC system and other thermal loadssuch as the battery, PEEM, etc. In the cooling mode, the coldcoolant is conveyed from the chiller to the HVAC Cooler forcabin cooling and dehumidification. When needed, inparallel with the cooler, this same coolant stream can be splitand partly routed to cool the battery. A front-end heatexchanger (FEX), sitting in the vehicle front typicallyoccupied by a radiator in ICE cars, rejects the heat from the 2018 National Renewable Energy Laboratory.

TOTAL THERMAL MANAGEMENT OF BATTERY ELECTRIC VEHICLES (BEVs)hot coolant coming from the condenser to the air outside.In heating mode, the hot coolant from the condenser isrouted to an HVAC heater for cabin heating, while FEXreceives coolant colder than ambient air and thereforeabsorbs heat from it. In parallel with the HVAC heater, thiscoolant can be routed to the battery to maintain its temperature within the limits. The rapidity of cabin warm-up (HPmode) is tolerably less than that for the baseline vehicle asthe latter has HVAC air directly heater where as in this casethe intermediate f luid (i.e. coolant) is heated first. Thisrapidity is similar to that for ICE vehicles in which engineheating takes time also.Further, since the PEEM produces waste heat and thusalways needs cooling, the coolant, in parallel with thecondenser, is routed to the PEEM to pick up this heat andthen deliver it to the HVAC heater, thereby recycling thewaste energy and improving the BEV range. Also, in thismode, the cold coolant stream is routed to the FEX to absorbenergy from the ambient air. Therefore, the cooling mode issimilar to the standard air conditioning operation while theheating mode operates as a heat pump. The heat exchangers,pumps, the compressor, and TXV are sized to meet theneeded thermal capacity requirements of the Fiat500e components.The UTEMPRA system replaces the separate condenser,battery radiator, and PEEM radiator of the Fiat 500e with asingle heat exchanger, thereby increasing its capacity andeffectiveness due to higher availability of ram air pressure.Further, it eliminates the need for separate refrigerationand/or coolant loops for the battery and PEEM cooling. Italso eliminates the need for an electric air heater. Togetherthese eliminations and consolidations reduce the totalrefrigerant charge, pumping power, overall system mass,and cost. In contrast with the baseline vehicle system, onefeature of UTEMPRA is that the rapidity of cabin warm-up(HP mode) is less than that for the baseline vehicle as thelatter has HVAC air directly heated by PTC air heater whereas in this case the intermediate fluid (i.e. coolant) is heatedfirst. This rapidity is tolerable and is similar to that for ICEvehicles in which engine heating takes time also. AddedPTC Coolant heater power during the first few mins willreduce the warm-up time without significantly alteringthe range. 2018 National Renewable Energy Laboratory.Multi-Mode Flow ControllerThe multi-mode flow controller (MMFC) is the novel component that enables a practical implementation of the UTEMPRAsystem. It is separated into hot and cold halves that direct therespective coolant streams to the different heat sources andsinks. The separate locations of these valve systems willprevent parasitic heat loss or gain. Each half comprises severalvalves whose bodies are integrated and consolidated to reducemass and cost while saving precious under-hood packagingspace. In the present scenario, coolant flow in the hot coolantloop is directed by a hot MMFC with eight valves which areof on/off type. The eight valves are configured such thatcoolant can be routed to the HVAC heater, FEX, or the batteryseparately or jointly. Further, the function of cooling thebattery with coolant circulating through the FEX is enabledby a pair of bypass valves. The Fiat 500e BEV also has anoption for battery temperature equilibration by flow of coolantout of and immediately into the battery to keep all the cellsof the battery within a narrow band of temperature. Thisfunction is enabled by another bypass valve in the hot MMFC.Similarly, the cold MMFC comprises six on/off valves anddirects cold coolant to the HVAC cooler, the battery, or theFEX, based on the mode of operation. Figure 5 shows a modelof the cold MMFC prototype.In the absence of the MMFC, design, packaging, cost,and installation complexity of such a large number of valveswould have made the UTEMPRA system not viable. Hence,this component is a major enabler for this technology.Location of Coolant PTCHeaterOf the two thermally self-regulating PTC heaters in the Fiat500e’s original system, one was a direct air heater, and theother was a coolant heater. The UTEMPRA system has eliminated the direct air heater but retained the coolant heater.There are two choices of location for the PTC coolant heater:in the hot loop or in the cold loop. Each of these locations hasits advantages and disadvantages. The hot loop locationenables fast heat discharge to the cabin and the battery sinceit only needs the coolant in the hot loop to be heated. However,FIGURE 5 SAE International SAE InternationalFIGURE 4 Example configuration of the UTEMPRA coolantnetwork addressing the same functions as in the Fiat500e system3Model of the cold MMFC prototype

4TOTAL THERMAL MANAGEMENT OF BATTERY ELECTRIC VEHICLES (BEVs)this also means that the total available heat is limited to only6 kW and may compromise the heating capacity compared tothe original system. A newer generation of PTC coolant heatercan be designed with higher capacity, thereby addressing thisissue. The cold loop location, on the other hand, increases thedelay in sending heat to the cabin and battery. In addition,the refrigerant loop components will need to be heated. Incontrast to the hot loop location, the cold loop locationincreases the total heating capacity as the power from thecompression work is now available as an additional heat source.Bench Test RigA configuration of the UTEMPRA system was built usingprototype and production-level components to support abench test program conducted at the US National RenewableEnergy Laboratory. The objective of the test program was tomeasure the performance of the prototype system and gatherinformation to support system controls development. The testapparatus, described in a previous work [2] and shown inFigure 6, is a hardware-in-the-loop system that imposesthermal loads on the UTEMPRA system and measures theresulting energy consumption and thermal performance. Thebench test apparatus consisted of two separate air ducts, acabin air simulator, and an outdoor air simulator. The benchtest apparatus had two electrical resistance coolant heaters,one to simulate the heat from the vehicle PEEM and the otherto simulate the heat from the hot-soaked energy storagesystem (i.e. battery). Key changes to the test apparatusdescribed in [2] include the addition of a humidifier andmoving the PEEM heater to the cold loop for heating tests.The UTEMPRA system and test apparatus were controlledusing software proportional-integral-derivative controllers aswell as simple thermostats programmed into the LabVIEWdata acquisition and control program. There were two sets ofcontrols: the test apparatus controls used to stabilize thethermal inputs into the system such as inlet air temperatures,and the UTEMPRA system controls that mimic automaticvehicle climate control. To impose realistic BEV loads on thethermal system, the test bench incorporated a vehiclepowertrain model, thermal and efficiency PEEM and energystorage system models, and a thermal cabin model.UTEMPRA bench setup SAE InternationalFIGURE 6Simulation ModelDescriptionNREL’s “Quasi-Transient” CoolSim modeling method wasemployed for both the refrigerant and coolant circuits fordeveloping the UTEMPRA system model. The details of thesolution method are discussed in [3] and details of the coolantloop modeling approach can be found in [4]. Both refrigerantand coolant circuits are represented by 0-dimensional (0-D)volumes connected with 1-D pipes, valves, or orifices. The 1-Dlines provide flow rate due to the pressure differential betweenthe inlet and outlet and are used to represent passes in heatexchangers as well as lines connecting components. The 0-Dvolume blocks represent physical volumes such as heatexchanger headers and also serve the purpose ofconnecting lines.The 1-D pipe block assumes a constant coolant massflow rate along its length. The flow rate then becomes a simulation state variable. At each time step, the coolant pressuredifferential across each line is compared to the pressuredifference between the 0-D volumes that they connect. Anumerical method is applied to continuously adjust thecoolant or refrigerant mass flow rate in each of the lines. Thegoal of this method is to match the pressure change in theline to the pressure difference between the volumes that theline connects. Ideally, sub-iterations would be continueduntil convergence is reached at each time step of the solutionto ensure a diminishing difference between the pressuredrop in the line and the pressure difference between theconnected junctions. This would result in a steady-statesolution corresponding to the instantaneous values ofboundary conditions at each simulation time step (hencethe name “Quasi-Transient”). To speed up the solution,however, only a single iteration is done in each time step.This was found to be an acceptably accurate approach whenthe computational time-step is relatively small compared tothe system-level thermal response characteristic time. Inthis case, the solution converges fast enough to accountfor transients.To further speed up simulations by increasing thesolution time step, the notion of artificial bulk modulus wasintroduced. This allows for changing the relationship betweenpressure and density and thus the system “stiffness.” Bysetting the artificial bulk modules smaller than the true bulkmodulus of liquids, the numerical stiffness in the coolant andliquid portions of the refrigerant networks can be reduced.This quasi-transient solution method results in lost accuracyfor fast transients (on the order of seconds) such as pumpcycling. For steady-state conditions, however, the conservation of mass and energy for each volume and each of the 1-Dpipes in the model is ensured. The UTEMPRA system, beingelectrically operated and controlled, does not typically exhibitsharp changes except for closing/opening the valves. Shortduration transients resulting from valve operation are oflesser importance from the overall system performancestandpoint and are replaced with instantaneous changes. Allother transients in the system such as those resulting fromthe compressor RPM control are adequately represented bythe method. 2018 National Renewable Energy Laboratory.

TOTAL THERMAL MANAGEMENT OF BATTERY ELECTRIC VEHICLES (BEVs)ResultsFIGURE 7 Capacity and temperature response whenUTEMPRA system received no PTC heat (a) or 2.0-kW PTCheat at steady rate (b)FIGURE 8 Estimates of energy savings and enhancedrange with UTEMPRA SAE InternationalTests were conducted at two nearly extreme ambient temperatures, hot (43.3 C) and cold ( 6 C), to generate systemresponses that were used as calibration inputs for MATLAB/Simulink models of the UTEMPRA system. The calibratedsimulation model was then used to predict the system behaviorat various conditions.Figure 7 indicates the simulation results of a heat pumpcase at modeled for 10 C ambient. Figure 7(a) is the casewhen PTC heat is not provided to supplement the heat fromheat pump while Figure 7(b) shows the results with 2-kWsteady PTC heat added to supplement the amount of heatpulled by the heat pump from the ambient air. A 0.75-kW heatfrom the PEEM is assumed in both cases.The time to reach the cabin target set point temperatureof 22 C is significantly affected by PTC heating. Figure 8(a)shows how the electrical power of the PTC heater affects thetime required for the UTEMPRA vehicle to attain the cabinaverage temperature of 22 C starting from a soak temperatureof 10 C with the vehicle traveling at 40 km/hr constantspeed. If an average of 0.5-kW PTC heat supplements the heatpulled by heat pump, the time to reach the cabin set point willbe the same as observed in the original vehicle test withroughly half of the original electric energy consumption.Figure 8(b) shows an initial estimate of range benefit due tothe UTEMPRA system: 15.5% at 10 C assuming a steadyspeed of 4

be directed to various components of the thermal system. The coolant-based heat distribution is adaptable and saves signifi - cant amounts of refrigerant per vehicle. Also, a coolant-based system reduces refrigerant emissions by requiring fewer refrig - erant pipe joints. The authors present bench-level test data and simulation analysis and describe a preliminary control scheme for this system .

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