Energy and thermal management, air conditioning, and waste heat utilization

This model is used to determine the energy consumption of the drivetrain as well as the cooling circuit components under various ambient and operating ditions.. The radiator shutter also

Energy and Thermal Management,

Air-Conditioning,

and Waste

Heat Utilization

Christine Junior

Oliver Dingel Editors

2nd ETA Conference, November 22–23,

2018, Berlin, Germany

Tai ngay!!! Ban co the xoa dong chu nay!!!

and Waste Heat Utilization

Chemnitz, Germany

ISBN 978-3-030-00818-5 ISBN 978-3-030-00819-2 (eBook)

https://doi.org/10.1007/978-3-030-00819-2

Library of Congress Control Number: 2018960428

© Springer Nature Switzerland AG 2019

This work is subject to copyright All rights are reserved by the Publisher, whether the whole or part

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This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

The efficient and intelligent use of energy resources is of key importance to ourfuture in transport, industrial, and building services As a result, the sparing use andthe exploitation of as-yet-unused energy resources are attaining ever greaterimportance In order to draw on existing potential and also generate new ideas, allthe relevant energy and heat flows will need to be considered This means thatenergy and thermal management, air-conditioning, and waste heat utilization aretoday analyzed across the board in the search for solutions.

However, the development of cross-sectoral solutions and ideas is not affectedonly by physics but also lasting influenced by underlying frameworks Due to thedemands of society and policymakers, the requirements concerning the efficientutilization of energy are subject to constant change In addition, the wealth oftechnically feasible solutions is generating increasing complexity within thedevelopment process Thus, interdisciplinary and cross-sectoral solutions are chal-lenged by new constraints which are impacting future concepts and components.But how do sustainable solutions and innovations in energy and thermal man-agement, air-conditioning, and waste heat utilization need to be structured for thischanging environment of the future?

ETA 2018 offers answers to this question and shares the latest research results,innovative technologies, and best practices Be inspired by approaches, technicalsolutions, and possibilities for an energy-efficient future!

Christine JuniorNovember 2018

Oliver Dingel

v

Energy and Thermal Management

Choice of Energetically Optimal Operating Points in Thermal

Management of Electric Drivetrain Components 3Carsten Wulff, Patrick Manns, David Hemkemeyer, Daniel Perak,

Klaus Wolff, and Stefan Pischinger

Higher Cruising Range Through Smart Thermal Management

in Electric Vehicles– Interaction Between Air Conditioning

and Cooling System Components in the Overall Network 15Daniel Moller, Jörg Aurich, and Ronny Mehnert

Auxiliary Heating, Cooling and Power Generation in Vehicles

Based on Stirling Engine Technology 30Hans-Detlev Kühl

Experimental Investigation on Effect of Fuel Property on Emissions

and Performance of a Light-Duty Diesel Engine 40

M Thamaraikannan, P L Rupesh, K Raja, and K Manideep

Conception and First Functional Tests of a Novel Piston-Type Steam

Expansion Engine for the Use in Stationary WHR Systems 49Michael Lang, Christian Bechter, Sebastian Schurl,

and Roland Kirchberger

Thermal High Performance Storages for Use in Vehicle Applications 66Werner Kraft, Veronika Jilg, Mirko Klein Altstedde, Tim Lanz,

Peter Vetter, and Daniel Schwarz

Determination of the Cooling Medium Composition in an Indirect

Cooling System 80Alexander Herzog, Carolina Pelka, Rudolf Weiss, and Frank Skorupa

vii

Air Conditioning

Approach for the Transient Thermal Modeling of a Vehicle Cabin 101David Klemm, Wolfgang Rößner, Nils Widdecke,

and Jochen Wiedemann

Personalized Air-Conditioning in Electric Vehicles Using Sensor

Fusion and Model Predictive Control 119Henning Metzmacher, Daniel Wölki, Carolin Schmidt,

and Christoph van Treeck

Simply Cozy - Adaptive Controlling for an Individualized

Climate Comfort 130Martin Noltemeyer, Lanbin Qiu, Christine Susanne Junior,

Thomas Wysocki, Johannes Ritter, and Jan Ackermann

Waste Heat Recovery

Waste Heat Recovery Potential on Heavy Duty Long Haul

Trucks– A Comparison 141Thomas Reiche, Francesco Galuppo, and Nicolas Espinosa

Combining Low- and High-Temperature Heat Sources in a Heavy

Duty Diesel Engine for Maximum Waste Heat Recovery

Using Rankine and Flash Cycles 154Jelmer Rijpkema, Karin Munch, and Sven B Andersson

Simulative Investigation of the Influence of a Rankine Cycle Based

Waste Heat Utilization System on Fuel Consumption

and Emissions for Heavy Duty Utility Vehicles 172Kangyi Yang, Michael Grill, and Michael Bargende

Requirements for Battery Enclosures - Design Considerations

and Practical Examples 194Jobst H Kerspe and Michael Fischer

Design of a Thermoelectric Generator for Heavy-Duty Vehicles:

Approach Based on WHVC and Real Driving Vehicle

Boundary Conditions 206Lars Heber, Julian Schwab, and Horst E Friedrich

Author Index 223

Energy and Thermal Management

Points in Thermal Management of Electric

Drivetrain Components

Carsten Wulff1(&), Patrick Manns2, David Hemkemeyer2,

Daniel Perak2, Klaus Wolff2, and Stefan Pischinger1

1 RWTH Aachen University, Institute for Combustion Engines,

Forckenbeckstr 4, 52074 Aachen, Germanywulff@vka.rwth-aachen.de

2

FEV Europe GmbH, Neuenhofstr 181, 52078 Aachen, Germany

Abstract Increasing the efficiency of electric vehicles is a development focus

in the automotive industry in order to reach the range targets set by customerrequirements Thermal management can have a positive effect on the systemefficiency of electric vehicles In this contribution, a simulation model of thedrivetrain and cooling system of an electric vehicle has been build up The aim

is to investigate the influence of the cooling system control and resultingcomponent temperatures on the drivetrain efficiency Thus, energetically optimaltarget temperatures for inverter and motor can be identified and implemented inthe cooling system control

This approach goes beyond the state of the art control strategy of keeping thetemperatures under the component protection threshold Related research sug-gests that the component efficiency of inverter and motor can be increased byreducing their operation temperature The simulation results in this article showthat choosing target temperatures for inverter and motor below the components’safety limit can have a small, positive impact on the system efficiency of theelectric vehicle

As the model is yet to be validated, these results implicate that the optimalcomponent target temperatures for inverter and motor regarding system effi-ciency are below the protective limit As a next step, the model will be validatedwith comprehensive component and vehicle measurement data in order to give aquantitative statement on the possible benefits of optimized thermal manage-ment control

Keywords: Electric vehicles
Thermal management
Optimal control

1 Introduction

Vehicle range shows to be a major contributor to the consumer acceptance of batteryelectric vehicles As the battery capacity installed into a vehicle is limited by cost- aswell as weight-considerations, one development focus for electric vehicles lies in theimprovement of the system efficiency [1] Thermal management is seen as a consid-erable factor in the system efficiency of battery electric vehicles [2]

© Springer Nature Switzerland AG 2019

C Junior and O Dingel (Eds.): ETA 2018, Energy and Thermal Management,

Air-Conditioning, and Waste Heat Utilization, pp 3 –14, 2019.

https://doi.org/10.1007/978-3-030-00819-2_1

This paper aims to investigate the effects of the cooling of electric drivetraincomponents on the system efficiency of a battery electric vehicle To this end, asimulation model is developed which simulates the energy flows within the electricdrivetrain of an A-Segment BEV.

The model includes map-based models for an inverter as well as motor andtransmission, which simulate the effects of component temperatures onto their effi-ciency The simulation model features comprehensive models for the cooling system aswell as the vehicle longitudinal dynamics in order to simulate the system energyconsumption This model is used to determine the energy consumption of the drivetrain

as well as the cooling circuit components under various ambient and operating ditions Finally, an analysis of these results is conducted tofind energetically optimaloperating points and control strategies for the cooling system of battery electricvehicles

con-2 Simulation Model

The simulation model is composed of three main parts:

1 The drivetrain model, which consists of a simplified longitudinal dynamics modelfor the calculation of the loads for the drivetrain, and map-based models for thetransmission, electric motor and inverter

2 The cooling circuit model, which consists of physical models for the coolant tubes

as well as degas-bottle and map-based models for the coolant pump and radiator

3 The map-based underhood-model, which thermally links the other submodels bycalculating the relative air speeds and ambient temperatures for all othercomponents

The model has been implemented in Matlab Simulink The following sectionsprovide a detailed description of these submodels

2.1 Drivetrain Model

The drivetrain is modeled as an inverse model in which the desired vehicle speed fromthe drive pattern acts as an input to a signal path Along this path the required powerdemand in order to follow the drive pattern is calculated (see Fig.1)

Within the drivetrain model, the model control provides the desired speed andgradient to the vehicle model In the vehicle model, the drive resistance resulting fromthe given drive pattern is being calculated with a simple longitudinal dynamics mode[3,4] The resulting wheel torque and speed are propagated to the transmission model.The transmission model calculates the resulting motor speed with thefinal drive ratioand uses an efficiency map to calculate the required motor torque This efficiency mapuses wheel torque and transmission oil temperature as inputs The consecutive motorand inverter model also use efficiency maps to calculate the resulting power demandfor the given drive pattern These efficiency maps use the component temperatures as

an additional dimension

2.2 Cooling Circuit and Underhood Model

The cooling circuit model consists of physical models for the coolant tubes as well asthe degas bottle The models for the radiator and the coolant pump are map-based (seeFig.2) For a given pump speed the pump model calculates the volume flow in thecooling circuit based in the resulting pressure drop of the cooling circuit Motor as well

as inverter are part of the cooling circuit model, with physical hydraulic models for thecalculation of the pressure drop [5]

Fig 1 Signalflow in the inverse model of the drivetrain

Fig 2 Integration of drivetrain and cooling circuit into underhood model

The underhood model consists of a single air volume, which represents the thermalmass of the engine compartment air within the vehicle The inlet airflow is calculatedwith a map depending on vehicle and fan speed This airflow is zero when the radiatorshutter is closed The radiator shutter also changes the drag resistance coefficient withinthe vehicle model depending on its state.

2.3 Energy Flows Within the Model

As this model is designed to simulate the influence of the cooling system on the electricdrivetrain components, in addition to the electric and mechanical energy flows allrelevant thermal energyflows are modeled This enables a more precise prediction ofthe temperatures of the electric drivetrain components

The thermal energy flows that have been included comprise all heat transfermechanisms The radiation losses towards the engine compartment are modeledphysically based on the components’ temperature, surface area and emissivity Con-ductive heat transfer is considered between the transmission and motor, as those arephysically joined in the reference vehicle Conductive heat transfer is also consideredwithin the thermal networks that model each components’ thermal behavior Theamount of conductive heat transfer is determined by the temperature difference betweenthe thermal masses andfixed thermal resistances

Three thermal masses are considered for the motor, the component housing, coolantwithin the component and abstract inner thermal mass to simulate the relevant tem-peratures for the component efficiency maps For the motor, the temperature of theinner mass represents the stator temperature For the inverter, the thermal masses of thehousing is combined with the inner thermal masses The resulting temperature of theinverter’s thermal mass aims to simulate the temperature of the power electronics

A separate thermal mass for the coolant is also part of the inverter model For thegearbox, only two thermal masses are considered These are the combination of thegears and housing and the oil

The convective heat transfers considered are those between the components and theengine compartment air as well as the heat transfer to the coolant circulating betweenthe drivetrain components For these physical models, the heat transfer classes asdescribed in [6] are used Also, the heat losses from the coolant tube surfaces to theengine compartment are considered For the heat transfer via the radiator, a map-basedapproach is used, while the pump and degas-bottle are considered as adiabatic.2.4 Model Parametrization

As the main aim of this article is to investigate the effects of the cooling circuit on thesystem efficiency, the parametrization of the drivetrain components’ efficiency maps iscrucial The efficiency maps for the components within this model are not onlydependent on speed and torque, but also on the component temperature This enables asimulation of the temperature-dependent behavior of the drivetrain As the efficiencymaps that are provided by the manufacturers do not reflect the component temperature,several assumptions have to be made in order to model the behavior of the drivetrain

components The process of generating these temperature-dependent efficiency mapsshall be explained in the following chapters.

Inverter Efficiency Map The inverter efficiency map provided by the manufacturerhas been measured at a constant coolant temperature Information concerning the actualtemperature of the different inverter components at the time of measurement is notavailable Therefore, the temperature of the IGBTs and diodes have to be estimated inorder to separate the influences of the load and the device temperature within the

efficiency map It is assumed, that the efficiency map has been measured in stationaryconditions and that all power losses within the inverter are dissipated by the coolant.Furthermore, it is assumed that the temperature of the IGBTs and diodes TInverter isequal to the derating temperature of the device TDeratewhen it is operated at maximumcontinuous load In peak load conditions, the junction and diode temperatures areassumed as being equal to the derating temperature For operating points below themaximum continuous inverter load, the junction and diode temperatures are assumed to

be proportional to the inverter power loss in that point At no load, these temperaturesare assumed to be equal to the coolant temperature TCoolant The inverter temperaturesare calculated using (1)

TInverter¼TCoolantþ Tð DerateTCoolantÞ

For the determination of the temperature-dependant losses of the inverter, theapproach developed by Feix et al [7] is used According to this approach, the switchinglosses as well as the conduction losses can be calculated by using correlations For theconduction losses, Feix et al [7] provide Eq (2)

For the switching losses Feix et al [7] provide another Eq (3)

For the determination of the temperature-dependent maps, the device-specificparameters need to be known To this end, a regression analysis is done with the knowntemperatures, currents and losses from the given efficiency map for the prior calculatedtemperatures in the given map (4)

PLoss¼aESWð ÞfT SWþ bPonð ÞT ð4Þwhere PLoss is the power loss in a given point of the efficiency map, a and b asweighing factors and the constant switching frequency fSW As a result of the regressionanalysis, a set of parameters is created which can be used for the generation of thetemperature-dependent efficiency map for the inverter

Motor Efficiency Map For the motor efficiency map, assumptions have to be made aswell due to limited information at hand It is assumed, that the temperature-dependency

of the motor losses are mainly linked to the copper losses Therefore, the dependency of friction losses and iron losses within the motor is neglected [8] Thecopper losses of the motor are assumed to be solely linked to the known phaseresistance of the motor (5)

temperature-PLoss ;copper¼ R Tð ÞðI2

qþ I2

where the copper losses of the Motor PLoss;copperare a result of the linearly dependent phase resistance R Tð Þ and the two components of the phase current Iqand Id[8]

temperature-In order to calculate a temperature-dependent efficiency map, the losses in theknown motor efficiency map need to be linked to respective temperatures Theapproach applied here is analog to the one applied to the inverter With the knowntemperatures and currents for the efficiency map of the motor, the copper losses can becalculated within the given efficiency map When deduced from the total losses, a map

of constant residue losses remains, which is not assumed to be temperature-dependent.The full efficiency map for the motor is then calculated by adding the temperature-dependent copper losses according to (5) to the map of residue losses for differenttemperatures, thus adding the third dimension to the efficiency map

Further Parametrization The further parametrization of the model is being done byusing maps and parameters as provided by the manufacturers of the components withinthe reference vehicle For a comprehensive overview of the vehicle specifications,please refer to the Annex

3 Simulation Approach

As this contribution aims to evaluate the influence of the cooling system on thedrivetrain efficiency of an electric vehicle, the choice of the control strategy for thecooling system is crucial for this investigation Also, the choice of boundary conditionsfor the simulation strongly influences the results The control strategy as well as thechoice of boundary conditions are subject of the following chapters

3.1 Cooling System Control Strategy

The control strategy for the cooling system applied within the model aims to control thetemperatures of the inner thermal mass of the electric motor and the inverter Thetemperatures of the inner thermal mass determine the efficiency of the componenttogether with the load point in the efficiency map Therefore, the aim of the coolingcircuit controller is to keep the component temperatures below a desired target tem-perature at minimum power consumption

The actuators, which need to be controlled, are the coolant pump, the vehicle fanand the radiator shutter While all component temperatures are well below the desiredtarget temperature, the radiator shutter is closed and coolant pump and fan switched off.When the component temperature reaches within 5 °C of the set target, the radiatorshutter is opened, enabling an airflow through the engine compartment This affects thedrag resistance in the vehicle model When the component temperature reaches thedesired target temperature, the coolant pump is switched on The pump speed iscontrolled by a PI-controller depending on the deviation of the component temperaturefrom the set target If the deviation increases even if the pump has reached full speed,the vehicle fan is engaged and also controlled with a PI-controller depending on thecomponent temperature This control strategy is engaged when either of the compo-nents reaches its target temperature, with the maximum of both temperature deviationsbeing the input for the controllers

3.2 Boundary Conditions

The boundary conditions for the simulations carried out in this investigation refer to thechoice of driving cycle, ambient temperatures, start temperatures of the components aswell as the target temperatures set for the control of the cooling system The WLTPClass 3 is an industry standard in the evaluation of the power consumption of bothconventional and electric vehicles [3] This representative driving cycle is chosen forthe evaluation of the drivetrain power consumption, as effects are evaluated on a systemlevel Ambient temperatures of 20 °C and 40 °C are chosen to be evaluated in order tocompare normal and higher load conditions of the cooling system Also, the startingtemperatures of the components are varied in order to evaluate the effect of the com-ponents’ thermal mass on the load for the cooling system Finally, the target temper-atures for the inverter are varied in a range between 60 °C and 130 °C and for the

Table 1 Boundary conditions for the simulationsDrive

cycle

Ambient

temperature/°C

Component starttemperature/°C

Inverter targettemperature/°C

Motor targettemperature/°CWLTP

motor in a range between 60 °C and 140 °C Table1 provides an overview over thedifferent boundary conditions set for the simulations.

With this set of boundary conditions, 360 simulations have been carried out in total.The following chapter gives an overview over the mainfindings that can be deducedfrom the simulation results

60 °C This Scenario reflects an operational scenario for the cooling system with higherload For the same ambient and starting temperature conditions, the drivetrain efficiency

is also shown for a simulation with the target temperatures for the cooling system set tothe energetically optimal temperatures of 90 °C for the inverter and 100 °C for themotor

The results show that the drivetrain efficiency actually increases for higher ponent temperatures, which is mainly due to the increased gearbox efficiency at higheroil temperatures Furthermore, it can be seen that the drivetrain efficiency does not

com-Fig 3 Comparison of electric drivetrain efficiency in WLTP Class 3 for different ambientconditions and component target temperatures

change significantly depending on the target component temperatures set for thecooling system control With the optimum component temperatures, the energyrequired for the completion of the drive cycle can be reduced by merely 0,05% Thismeans that the energy required for the coolant pump as well as the compensation of theadditional drag by opening the radiator shutter almost completely outweighs theinverter efficiency gains by reducing the component temperature.

The main contributor to this phenomenon seems to be the fact that the coolingsystem actually is not required to be active for the most part of the drive cycle Figure4gives an overview of the component temperatures in 20 °C ambient and componentstarting temperature conditions It can be seen that the maximum component temper-atures are not reached for neither inverter nor motor

A similar behavior can be seen for higher ambient and component starting peratures (see Fig.5) The inverter reaches its maximum temperature only towards theend of the WLTP at about 1600 s, which means that the cooling system does not need

tem-to be activated before that

If the cooling system is controlled in such a way that the minimum energy isrequired to complete the drive cycle (see Fig.6), it needs to be activated much earlierand therefore also requires energy for conditioning much earlier This almost outweighsthe efficiency gains for the inverter which thus can be achieved

In conclusion, it can be said that the control of the cooling system can have apositive effect on the drivetrain efficiency At the same time, the effect shown in thesimulations within this contribution are almost negligible

Fig 4 Component temperatures and system efficiency for WLTP class 3 at 20 °C ambient andcomponent starting temperature with inactive cooling system

5 Conclusion and Outlook

For this article, a simulation model of the drivetrain and related cooling system of aClass A BEV has been developed The model simulates the energyflows within thedrivetrain and cooling system, while the effect of the temperatures of the electricdrivetrain components is also modeled Due to limited data availability, a theoretical

Fig 5 Component temperatures and system efficiency for WLTP class 3 at 40 °C ambient and

60 °C component starting temperature with component maximum temperatures as target for thecooling system

Fig 6 Component temperatures and system efficiency for WLTP class 3 at 40 °C ambient and

60 °C component starting temperature with optimum target temperatures for the cooling system

approach has been chosen to model the temperature-dependent efficiency of electricmotor and inverter.

The simulations show a positive impact of a reduced target temperature for theinverter on the system energy consumption, although this effect is very small Nev-ertheless, the results found justify a deeper investigation of this matter

Thus far, the simulation model has not been validated Therefore, an assertion thatthese simulation results reflect the vehicle behavior in reality cannot be given In thenear future, the simulation model will be validated against vehicle measurements andmay require to be recalibrated Also, the thermal behavior of the inverter and motor will

be measured on a test bench The efficiency maps used within the model can therefore

be updated in the future to reflect the thermal behavior of the electric drivetraincomponents more accurately The simulations can therefore be revisited with anupdated parameter set in order to confirm or refute the findings made so far and allow aquantitative statement on the actual impact of the cooling system on electric drivetrain

efficiency

Acknowledgements Funded by the Deutsche Forschungsgemeinschaft (DFG)– GRK1856

Appendix

See Table2

Table 2 Simulation parameters

Modeled motor/gearbox Brusa DTSO1-096

Modeled coolant pump Pierburg CWA 100

Drag coefficient delta if radiator shutter is shut 0,018

Rolling resistance coefficient 0,0108

Coolant tube length between components 1 m

Inner coolant tube diameter 0,02 m

Constant LV supply voltage 12 V

Constant HV supply voltage 380 V

1 Kampker, A., Vallée, D., Schnettler, A (eds.): Elektromobilität Grundlagen einer ftstechnologie 1st edn Springer Vieweg, Heidelberg (2013).https://doi.org/10.1007/978-3-642-31986-0

Zukun-2 Hemkemeyer, D.: Thermomanagement im elektrischen Personenkraftwagen unter Nutzungder Abwärme des Antriebs PhD-thesis, Aachen (2017)

3 Pischinger, S., Seiffert, U (eds.): Vieweg Handbuch Kraftfahrzeugtechnik 8th edn SpringerVieweg, Wiesbaden (2016).https://doi.org/10.1007/978-3-658-09528-4

4 Eckstein, L.: Längsdynamik von Kraftfahrzeugen 4th edn Forschungsgesellschaft fahrwesen, Aachen (2011)

Kraft-5 Serghides, T.K.: Estimate Friction Factor Accurately Chem Eng 91, 63–64 (1984)

6 VDI Heat Atlas 2nd edn Springer, Heidelberg (2010)

7 Feix, G., Dieckerhoff, S., Allmeling, J., Schonberger, J.: Simple methods to calculate IGBTand diode conduction and switching losses In: 13th European Conference on PowerElectronics and Applications 2009, pp 1–8 Barcelona (2009)

8 Schützhold, J., Hofmann, W.: Analysis of the temperature dependence of losses in electricalmachines In: IEEE Energy Conversion Congress and Exposition, pp 3159–3165 Denver(2013)

Thermal Management in Electric Vehicles –

Interaction Between Air Conditioning

and Cooling System Components in the Overall

Network

Daniel Moller(&), Jörg Aurich, and Ronny Mehnert

IAV Gmbh, Auerstraße 54, 09366 Stollberg, Germany

daniel.moller@iav.de

1 Introduction

Reducing the power demand and thus increasing the cruising range of electric vehicles

is a significant challenge currently faced in the automotive industry It is well-knownthat a part by no means insignificant of the energy available is used for air conditioning

in vehicle interiors and batteries as well as for ensuring temperature control for theother electrical components For this reason, electric vehicle development today isfocusing very much on developing efficient and innovative thermal managementsystems

IAV is presently developing novel overall concepts for cooling and air conditioningcircuits in electric vehicles An essential challenge posed by this development espe-cially concerns the temperature limits which are sometimes very tight, and need to beobserved in order to ensure maximum service life and optimum performance capa-bilities for the components It is also necessary to ensure both a comfortable climate inthe vehicle interior and the longest possible cruising range

These complex objectives lead to an increasing amalgamation of the problems to besolved in cooling and air conditioning design which also entails more complex circuits

as well as higher efforts for control and coordination The holistic simulation of thermalmanagement allows quickly evaluating and optimising new ideas and strategies forthermal management already at an early stage in development The point is to map theindividual components as precisely as possible both in terms of their geometries andphysical operating principles while also describing the overall system at an adequatelevel of detail

The present article will first look at the requirements to be met by thermal agement in electric vehicles, and compare them with the needs of conventional pas-senger cars powered by internal combustion engines The development of the thermalmanagement system itself will be examined with reference to the IAV developmentprocess as shown in Fig.1 This examination will look at the example of an electricmachine, and focus on virtual development ranging from concept preparation, tosystem development and then to component development This will be followed by an

man-© Springer Nature Switzerland AG 2019

C Junior and O Dingel (Eds.): ETA 2018, Energy and Thermal Management,

Air-Conditioning, and Waste Heat Utilization, pp 15 –29, 2019.

https://doi.org/10.1007/978-3-030-00819-2_2

analysis and evaluation of various measures taken to ensure thermal management fordifferent driving cycles and environmental boundary conditions with regard to energy

efficiency (function) and comfort Based on these examinations, conclusions will bedrawn, and an outlook on further steps of development will be presented

2 Thermal Management Requirements

Apart from the vehicle cabin, a vehicle’s thermal management is basically limited tothe internal combustion engine in conventional vehicles To ensure this, a coolantcircuit provides for aflow around cylinders and combustion chambers, and for properheat dissipation in these areas Other heat sources include an engine oil cooler and,depending on the vehicle’s outfit, e.g., EGS cooling or transmission oil cooling Notemperature control is provided for the energy reservoir (fuel tank) Apart from dis-sipating heat and observing the temperature limits, the focus for internal combustionengines is on heating up the cooling medium as quickly as possible to an amount ofbetween 85 °C and 105 °C and the engine oil to between 85 °C and 110 °C Quicklyheating up the engine oil helps to reduce engine friction and thus ensures lower fuelconsumption When the engine is heated up, this also allows smart control of thetemperature inside the passenger compartment To achieve this, the coolantflow rate atthe cylinder head and crankshaft frame is actively controlled Sufficient heat sinks incurrent vehicle configurations are provided by the radiators located in a vehicle’s frontsection and by the interior heat exchanger which also allows heating the vehicleinterior Fuel-burning auxiliary heaters or a PTC heating element situated in the airfloware used for very large vehicles, or when higher requirements have to be met foroccupant comfort [1]

An air conditioning system ensures vehicle interior cooling In vehicles powered byinternal combustion engines, the refrigerant compressor is normally driven by theinternal combustion engine via a belt drive The system usually has an evaporatorinstalled in the air conditioner, a thermostatic expansion valve for expanding thecoolant, and a capacitor which provides a heat sink at the vehicle front end

Fig 1 Process for developing a thermal management system

In vehicles powered by electric batteries, temperature control is not only needed forthe drive motor but also for other components like power electronics, battery charger,and for the high-voltage battery used as an energy store The levels of temperaturediffer very much among these components, and require different cooling circuits orcooling concepts In this context, the optimum working envelope of a typical lithium-ion battery is between 15 °C and 35 °C which is below the common ambient tem-perature under midsummer ambient conditions Both the power requirements whiledriving and the battery charge process pose great challenges in this regard The fastercharging takes place, the higher is the electric charge current, and the higher is the heatgenerated inside the battery In today’s quick-charge stations, a high-voltage batterycan be charged with an electric power of between 40 kW and 150 kW within a limitedperiod of time The waste heat this generates in the battery also needs to be dissipatedwhen the vehicle is at standstill, while a maximum cell temperature of 50 °C must not

be exceeded But drive motor and power electronics can be operated at a coolanttemperature of up to 60 °C which allows implementing cooling by means of radiators

In many cases however, very tight temperature limits and the heat, which is alsogenerated during vehicle standstill, often do not allow such cooling using ambient airalone This requires the battery to be actively cooled by an air conditioning system.Electrically powered compressors are mostly used in the air-conditioning systems

of electric vehicles This is necessary, since no belt drive is usually available Apartfrom this, an electric drive makes it possible to control the compressor speed and thusthe refrigerant mass flow independently of the traction motor’s speed or operatingstatus This additional benefit can also be used, e.g., for stationary air-conditioning, and

at least partially outweighs the drawbacks caused by the efficiency chain from thebattery via the additional compressor drive In addition to providing air conditioning inthe passenger compartment, an air conditioning system is also used for active batterycooling in many vehicle concepts This is done via a chiller which is integrated into thebattery’s coolant circuit This requires trading off optimum comfort for occupantsagainst an optimum operating temperature for the battery which means that prioritieswill have to be defined in case of extreme ambient conditions under certain circum-stances In addition, a system having two refrigerant evaporators also involves par-ticular requirements to be met by control Contrary to the systems employed until now,electronically controlled expansion devices are frequently used for expanding thecoolant, and allowing to specifically control coolant distribution to the two evaporators.Another challenge for thermal management in electric vehicles is created by theconsiderably lower amount of heat input into the cooling system from the drivelinecomponents as compared to internal combustion engines This is especially due to thehigh efficiency values of the components, but leads to additional requirements for airconditioning in the passenger compartment, when the ambient temperature is low.Directly using the waste heat generated, e.g by the battery is not possible in general asits level of temperature is relatively low For this reason, PTC elements are often used

as additional electrical heaters for the passenger compartment They can be integratedboth directly into the air pathway leading to the cabin, and in the cooling circuitupstream of the interior heat exchanger But, as they have high power consumption, the

use of PTC air heaters or PTC water heaters leads to a very strong reduction in thevehicle’s cruising range Using a heat pump provides an interesting option in this case.Depending on ambient conditions, this pump can use the waste heat generated by thedriveline components or heat from the environment The heat pump’s coefficient ofperformance, i.e., the ratio of capacitor power and compressor drive power, helps tosave a considerable amount of energy and thus extend the vehicle’s cruising range.

3 Concept Preparation in the Development of Thermal

Management

When beginning the development of a thermal management system for a vehiclepowered by an electric battery, the initial part is to prepare a system concept and toclearly define the fundamental boundary conditions The first step during this process is

to indicate the place where the vehicle will possibly be used This has a direct influence

on the minimum and maximum ambient temperatures for designing and dimensioningthe thermal management circuit While the maximum ambient temperature is a decisivefactor for designing a cooling system for the electrical machine, power electronics,battery and cabin, the minimum ambient temperature defines the requirements to bemet by passenger compartment heating and by possible auxiliary heating for the bat-tery Using a heat pump for controlling the cabin temperature with a refrigerant likeR134a or R1234yf will, for instance, only be possible up to a specific ambient tem-perature At very low temperatures, only the waste heat from the powertrain compo-nents and the PTC element can be used for heating Another important item is to clearly

define the requirements in terms of performance and cruising range as well as the spaceavailable for installation in the corresponding vehicle These items are significant forselecting a suitable high-voltage battery and the corresponding electrical machine.When drawing up a suitable cooling and air conditioning concept, the essentialpoint is to make reliable statements on the performance capability of the correspondingthermal management system under the boundary conditions as defined above at a stage

as early as possible during development To allow this, an analysis of the performancedata and afirst choice of the corresponding system components from the IAV modeldatabase are made after having clearly defined the basic requirements In addition toreference to the company’s own database, possible manufacturers or suppliers can also

be consulted for making this choice Subsequently, these components are modelledwithin the scope of a 1D-CFD system simulation, and examined with regard to thefirstpossible loading cases Apart from the corresponding performance capabilities, e.g., ofheat exchangers, the main focus of attention concerning concept preparation for ther-mal management also is on the knowledge available about the heat generated by thevarious driveline components like battery, electrical machine, and power electronics,depending on the power required The pressure loss characteristic also is decisive whenselecting coolant pumps

4 System Development

Based on concept development, the objective pursued in system development is toperform detailed 1D-CFD analyses in order to determine the boundary conditions to beconsidered during further subsystem and component development, and to comparedifferent concepts for thermal management with one another and evaluate them already

at an early stage of development Thermal management circuits like the one shown inFig.2 are created within the scope of system development This circuit has a coolingcircuit for the high-voltage battery, a low-temperature circuit, and a refrigerant circuitprovided for the combined operation of air conditioning systems and heat pumps

In the battery circuit, the coolant pump delivers thefluid to the chiller where it iscooled down as required by using the A/C circuit After this, heating may take place ifnecessary in the PTC water heating element, and the coolant is conducted to the battery

or to the interior heat exchanger (HWT) via a valve This is followed by cooling down

in the battery radiator which can be circumvented by a bypass Finally, the coolingfluid

is returned to the pump

The low-temperature circuit includes the power electronics and the electricalmachine as heat sources, and the low-temperature radiator as a heat sink located at thevehicle’s front end If necessary, the latter can be circumvented by a bypass

But it is also possible to combine the two cooling circuits with one another In thiscase, the coolant is conducted from the battery or HWT to the LT circuit and from thelatter to the power electronics and to the electrical machine as heat sources It is then

Fig 2 Cooling and air conditioning circuit

returned to the battery circuit This enables, e.g., the waste heat from the drivelinecomponents to be utilised for heating the interior space.

The refrigerant circuit used for this analysis allows the combined operation of airconditioning system and heat pump using CO2 (R744) as a natural refrigerant Anessential component of the system is the refrigerant compressor which is designed as anelectrically driven scroll compressor in this setup From the compressor, the refrigerantflows via a four-way valve to the gas cooler at the front end of the vehicle, and then viaanother four-way valve on to the expansion devices After this, the refrigerant isevaporated in the evaporator and in the chiller before being conducted back to thecompressor The four-way valves make it possible to switch the functions of evaporatorand gas cooler In the heat pump mode, this allows utilising the chiller and/or heatexchanger at the front end as heat sources, while the heat exchanger in the air con-ditioner serves as a heat sink The control of the refrigerant massflow via evaporatorand chiller is ensured by means of electrical expansion valves in this case

To simulate cooling when preparing this article, the AMESim simulation tool wasused for simulating the cooling system and vehicle cabin (Fig.3) A simplifiedoccupant cell model was used during this, and parameterised for the correspondingambient temperature with the help of a considerably more complex geometric andphysical model from IAV [2]

Matlab and IAV’s own model database [3] were used to perform the correspondingsimulations for the air conditioning system This simulation relies on physically basedand extensively validated models for different compressor types, heat exchangers and

Fig 3 Coupled simulation environment

expansion devices Coupling the individual componentsfinally allows describing theentire refrigerant circuit.

The coupling of the cooling and air conditioning plant simulation was performed byusing multidimensional characteristic maps (N-D lookup tables) consisting of quasi-stationary simulation results for the operating points relevant for the air conditioningsystem This procedure allows a simulation time far below real time, while producingsufficiently precise simulation results even in case of highly dynamic driving cycles.Due to the very small inertia in the air conditioning system, the transient behaviour canalso be mapped with good accuracy in this way

5 Component Development

After concept preparation and system development, reference will now be made to anexample of an electrical machine in order to outline fluid-engineering and thermalcomponent development The overriding development goals are:

• The guarantee of thermal component safety, and

• The optimisation of the efficiency of the electrical machine

Thermal component safety relates, in particular, to compliance with the maximumtemperatures for windings and specifically for insulating varnishes and permanentmagnets so as to prevent any electrical insulation failure or demagnetisation Thisapplies both to steady and transient operating conditions, and requires a cooling con-cept which is dimensioned accordingly 3D-CFD programs e.g., STAR-CCM + areused for this

A typical 3D-CFD model comprises both the solid modelling of machine nents, andfluid modelling for coolants and air sections Due to the complex geometry,simplification is indispensable Windings, for instance, are modelled as homogeneousmaterial with anisotropic thermal conductivity taking into consideration the degree offilling, and insulations are mapped as thermal resistors taking account of the entrappedair encountered Even when applying such geometric simplifications, the CFD com-putation grids obtained require relatively intensive calculations (50 to 100 millioncells)

subcompo-The heat inputs into the corresponding active components are taken care of bythree-dimensional electromagnetic simulations Figure4 shows this process, or, cor-respondingly such a co-simulation using the JMAG and STAR-CCM + software tools.The electromagneticfield quantities are mapped in detail in the three-dimensionalJMAG simulation As a result, Joule’s loss values or the corresponding three-dimensional fields of the loss enthalpies are transferred to the corresponding CFDmodel areas The representation of the heat transfer processes in the CFD simulationwill then be close to reality The resulting temperature distribution will be transferredback to the 3D-JMAG model This makes it possible to capture local effects orinteractions between thermal and electromagnetic effects

The optimisation of coolant guidance is carried out as part of the CFD simulation.Both conventional methods of optimisation (e.g., based on parametric geometrymodels), and new approaches (e.g., the Adjoint procedure) are used for this purpose.Particular emphasis is placed on mapping electrical insulations or the thermalcontacts related to them These thermal contacts sometimes have a considerable impact

on the thermal conductivity of the electrical insulation Figure5shows a correspondinganalysis

The analysis of a heat path leading from a stator winding to a package coolant isshown with reference to an example on the left-hand side It indicates the dominancewhich is exerted by the thermal resistance of the electrical insulation both as compared

to the heat conductance processes under way inside the components, and with regard toconvective heat transfer at the cooling jacket Accordingly, appropriate care must betaken when defining the thermal contact resistance during CFD modelling To allow atleast an estimate of the entrapped air possibly encountered, this also requires consid-eration to be given to the manufacturing and assembly procedures among other factors.Regarding the design layout, this means that the specific design of the electricalinsulation presents a high potential for ensuring the thermal safety of the winding Thisrefers, on the one hand, to the manufacturing or assembly procedures (minimisation ofentrapped air) and, on the other hand, to the selection of the material (layer thickness,thermal properties) To illustrate this, the result of a corresponding potential analysis isprovided as an example on the right-hand side of Fig.5 Optimising thermal resistancehelps bringing about a relevant improvement in transient behaviour as the statorwinding will reach its critical temperature at a later time which makes it possible toconsiderably extend the time of possible transient full-load operation

Fig 4 Detailed analysis of the thermal behaviour of electrical machine components byJMAG/StarCCM + co-simulation

A compact design and increasing requirements on performance create higherthermal loads and thus the need for applying more efficient cooling concepts Oil-spraycooling provides an effective means for reducing component temperatures in thiscontext Figure6 shows a comparison with reference to the IAV DrivePac [4] as anexample.

The time characteristics applicable to the rotor or stator temperatures are plotted inthe diagram each without or with activated oil-spray cooling This clearly shows thepotential offered by oil-spray cooling which allows to constantly observe the maximumallowable winding temperature of 150 °C

Introducing oil-spray cooling involves a series of questions to be solved in terms offluid engineering When oil is supplied through the shaft, a high rotational speed willlead to significant centrifugal forces which, in turn, cause a marked static pressuredistribution or negative pressure values This may lead to the appearance of multiphasefluid flow effects, and involve the outgassing of air dissolved in the oil, or even the intake

of air, or impairments for the oilflow into the lateral rotor spaces Other items of interestinclude the oilflow or the characteristics shown by the two-phase flow in the lateral rotorspaces The essential points are to ensure oil transfer to the components to be cooled overthe entire speed range, bring about sufficient oil wetting on the corresponding surfaces,guarantee the safe return of oil, and prevent oil from entering the air gap

For the corresponding CAE-based development, it is preferable to apply a phase approach to simulation which allows mapping or forecasting all multiphaseflowregimes that may occur, i.e., jet penetration and jet breakup, wall wetting and wallfilmformation, pooling, bulkflow, etc The state-of-the-art approach to this is the volume-of-fluid method But, as it turns out, quite a high numerical effort is required formapping the individual phenomena mentioned For this reason, IAV is currently

multi-Fig 5 Heat path analyses and thermal optimisation of an electrical machine

evaluating alternative approaches to multiphase flow simulation with regard to thequality of its results, the ability to provide forecasts, manageability, and efficiency interms of computing.

Conjugate heat transfer simulations would be an ideal means to be applied tothermal analyses But they are ruled outfirst because of the computation grid size asmentioned before, and, in particular, for the different timescales involved

Figure7presents the procedure which is currently applied by IAV for evaluatingheat transmission on the oil-treated surfaces

Fig 6 (a) component temperatures without/with oil cooling, (b) design implementation

Fig 7 Determination of convective heat transmission for oil-spray cooling

Atfirst, the fluid section of a lateral rotor space is examined within the scope of ananalogous model, and the oil spread is determined by means of transient multiphaseflow simulation The volume-of-fluid method combined with a rotating mesh approachturned out to be suitable here This calculation covers the time it takes to obtain asufficiently realistic map of wall wetting In a second step, the resulting heat-transfercoefficients are transferred to the solid model of the machine in order to determine theindividual component temperatures which are then used as input parameters for 1Dsystem analysis and concept evaluation.

Similar to the detailed simulation of the thermal processes in the electric motorshown, the other relevant components are mapped in the system as required This isdone especially when the individual components are also included in the subject matter

of development Commercially available components will not normally be examined insuch detail but rather mapped in system simulation by referring to their known char-acteristic quantities and parameters

6 Concept Evaluation

For a conventional internal combustion engine, the initial assurance of cooling formance is generally obtained at maximum power and under steady boundary con-ditions But it is not possible to apply this procedure to an electric vehicle as themaximum heat generated by an electrical machine and by a high-voltage battery islimited in time This means for instance that, as already mentioned, the drive motor cansometimes not provide its maximum power unless for a short period of time as itsmaximum admissible winding temperature is limited For a high-voltage battery, themaximum heat will be generated in case of strong discharge or during rapid charging.These two strains are limited in time due to the battery capacity For a battery, a steady-state consideration would, for example, lead to a considerable oversizing of the chiller.This circumstance makes it useful to evaluate thermal management systems withreference to various transient driving cycles and ambient conditions as described in thisarticle in order to derive possible activities for optimisation The basic vehicle used as

per-an example for this was a mid-size electric vehicle

Figure8presents the velocity profiles of the driving cycles used For cooling design, acycle having a high load requirement and an ambient temperature of TAmb = 40 °C isselected (motorway cycle) This cycle is comprised of a motorway drive repeated twiceand a subsequent fast-charging phase The charge power is up to 150 kW This is fol-lowed by two additional motorway sections (Fig.8a) The evaluation of the heat-upperformance of the cabin is primarily done during an urban cycle at an ambient tem-perature of TAmb=−10 °C and with lower performance requirements (Fig.8b) Allexaminations were performed at an air recirculation rate of 50% for cabin air conditioning.The objective of cooling design is to observe the temperature limits which applyboth to individual component parts and in the overall system The main focus ofattention is on the maximum admissible component temperature for the stator windings

of the electrical machine (150 °C), and on the cell temperature of the high-voltagebattery (50 °C) while also guaranteeing comfortable conditions for vehicle occupants

(24 °C) The considerable difference between the temperature levels in the battery and

in the electric machine makes it necessary to decouple the two circuits for this loadingcase Due to the high ambient temperature, no radiator can be used for cooling the high-voltage battery, and the chiller is activated during the entire cycle

Figures9a and b show the temperature variation of electrical machine or batteryduring the driving cycle involving high load requirements The system design selectedallowed to observe the temperature limits for the two components The cell temperature

of the high-voltage battery clearly indicates that the fast charging process will stitute the greatest challenge for cooling This is partly due to the high-power loss of thebattery On the other hand, the low air massflow rate also creates unfavourable con-ditions for cooling at the capacitor during the standing phase Figure9c shows thetemperature in the passenger compartment during the driving cycle During the coolingphase at the beginning of the cycle, this temperature decreases considerably and canthen also be kept constant during the charging phase During the charging period, thecompressor is operated at a very high rotational speed and it is the only way to bothobserve the maximum admissible battery temperature and cabin comfort This becomesevident in Fig.9d showing the compressor drive power which, due to the considerablepower required for air conditioning the battery, is considerably higher during thecharging phase than during the driving phases

con-For evaluating cabin heating using the current concept at an ambient temperature of

TAmb=−10 °C, three different heating variants were compared with one another in afirst simulation Variants 1 and 2 each use only one of the heating componentsavailable (PTC heater or heat pump), while both the heat pump and the PTC elementwere combined in variant 3

Although operating the heat pump with the CO2 refrigerant used here is alsopossible at very low temperatures, the thermal output at the internal gas cooler is notsufficient for achieving a comfortable temperature of 24 °C during the urban cycle(Fig.10a, light blue line) The heat source used for heat pump operation is the chiller inthe battery circuit Using only the PTC element with a maximum heating output of

5 kW would not yet enable guaranteeing cabin comfort either (Fig.10a, green line)

A comfortable temperature of 24 °C can already be reached after about 150 s onlywhen combining the two heating elements (Fig.10c, dark blue line)

Fig 8 Driving cycles analysed for an electric vehicle: (a) motorway drive with intermediate fastcharging; (b) urban drive

To increase efficiency when heating the passenger compartment in winter, it ispossible to couple the battery cooling circuit either directly or indirectly with the low-temperature circuit In contrast to the direct interconnection considered here, a heatexchanger would be used for combining the two cooling circuits with one another incase of indirect coupling Both cases allow utilising the waste heat generated byelectrical machine and power electronics for controlling the temperature in the cabinand thus reducing the load on PTC element or AC system As it causes a considerabledelay in heat development due to the system’s rising thermal inertia at an initialambient temperature of T =−10 °C, the coupling of the circuits does not take place

Fig 9 Temperatures: (a) electrical machine (stator winding), (b) battery, (c) passengercompartment; (d) compressor drive power during motorway cycle with rapid charge

Fig 10 Cabin temperature and power values (heating case during urban cycle)

unless the coolant temperature downstream of the electrical machine exceeds the inlettemperature at the chiller without coupling The results of this analysis are shown inFig.11.

As the circuits are disconnected at the beginning of the cycle, the heating-up curves

of the cabin temperatures are identical for both variants Figure11also indicates thatthe direct connection of the two cooling circuits for the urban cycle with low loadrequirements only leads to a slight reduction in the output required for air conditioningfrom heat pump and PTC heater towards the end of the cycle This is due to the verylow heat input of the electrical components of the driveline

When analysing the aggregate power consumption required for air conditioningduring the urban cycle (Fig.12a), it also becomes evident that the energy savingsthrough coupled circuits of 0.1 kWh are almost negligible Considering the motorwaycycle and its significantly higher power requirements, however, a significant energysaving of about 5 kWh can be seen due to the strong heat development in the low-

Fig 11 Cabin temperature and power values (heating case), coupled circuits

Fig 12 Aggregate power consumption for air conditioning in urban cycle (a) and motorwaycycle (b)

temperature cycle If these are now related to the average consumption during themotorway cycle of 30.1 kWh/(100 km), a range increase of 16.6 km can be assumedfor this cycle.

AC simulation takes place via multidimensional characteristic maps These maps aregenerated using Matlab and IAV’s internal model database

By using this method, it was ensured that the maximum admissible componenttemperatures of the individual components were not exceeded and that a comfortabletemperature in the passenger compartment was maintained for the thermal managementsystem described at an ambient temperature of TAmb= 40 °C For heating the cabin at

TAmb= −10 °C, the combined use of both the auxiliary heater and the heat pump isrequired Depending on the driving cycle, significant energy savings of up to 5 kWhcan be achieved by coupling the battery cooling circuit and the low-temperaturecooling circuit

It should be noted that the precise design of the overall system and individualcomponents requires the constant verification and comparison of the simulation resultsbased on component, system and overall vehicle measurements to be performed assoon as corresponding systems are available Only a close integration of calculationand test can guarantee an efficient development process

Berücksich-3 Baumgart, R.: Reduzierung des Kraftstoffverbrauches durch Optimierung von Klimaanlagen Dissertation Technische Universität Chemnitz, Verlag WissenschaftlicheScripten (2010)

Pkw-4 Schneider, E., Fickel, F., Cebulski, B., Liebold, J.: Hochintegrativ undflexibel – ElektrischeAntriebseinheit für E-Fahrzeuge Automobiltechnische Zeitschrift – ATZ, May 2011

Generation in Vehicles Based on Stirling

to achieve a satisfactory performance in any of these The experimental machinerealized within this project may be operated as a Stirling engine, as a thermallyactuated Vuilleumier cycle heat pump or in a so-called hybrid mode featuringboth a mechanical power production and a heat pump effect The originalobjective was to develop a convertible domestic energy supply system that may

be adjusted to varying heat and power demands, but particularly the hybrid cyclealso appears to be well-suited for auxiliary heating cooling and power genera-tion in vehicular applications, possibly even without the option of switching todifferent modes The thermodynamic operating principle of this cycle, potentialdesign options as well as performance predictions are presented and discussed.Keywords: Stirling cycle
Auxiliary power unit
Heating
Air conditioning

1 Introduction

The so-called regenerative cycles, including the Stirling cycle as their presumably mostwell-known representative, may be used for a variety of heat transformation applica-tions, ranging from prime movers– such as the Stirling engine – to heat pumps andrefrigerators [1] In the latter case, the inverted Stirling cycle is once again among themost well-known and favored operating principles particularly for cryogenic applica-tions and has been widely used e.g for air liquefaction purposes [2,3] or at even lowercryogenic temperatures [4–6] In recent decades, it has, however, also been investigated

as an alternative to conventional vapor compression refrigerators, inter alia because ofthe environmentally benign workingfluid (typically helium) [7–9]

The same applies to the Vuilleumier cycle, which was also used for cryocvoolersoriginally, but has gained increasing interest as a thermally driven heat pump fordomestic applications during recent decades [10–13] Contrarily to the Stirling cycle, itfeatures three characteristic temperature levels and thus requires two regenerators forthe thermal separation of these In fact, the regenerators are the most crucial compo-nents in these cycles (and the origin of their name), because they allow for an almost

© Springer Nature Switzerland AG 2019

C Junior and O Dingel (Eds.): ETA 2018, Energy and Thermal Management,

Air-Conditioning, and Waste Heat Utilization, pp 30 –39, 2019.

https://doi.org/10.1007/978-3-030-00819-2_3

reversible periodic displacement of the working gas between the different temperaturelevels without any major net enthalpy transfer down the temperature gradient Inpractice, such a non-zero enthalpy transfer constitutes the major part of the so-calledthermal regenerator loss Particularly in refrigerators or heat pumps, highly efficientregenerators are indispensable for a successful operation.

In parallel to these developments, the Stirling engine is increasingly applied as an

efficient and durable micro-CHP system for a decentralized domestic heat and powersupply, particularly because of its multi-fuel capabilities including low-grade fuelsfrom renewable sources [14,15] Free piston Stirling engines form the core component

of several commercially available micro-CHP units [16] However, the typically led operation of decentralized CHP systems implies that most of the generated elec-tricity must be fed into the grid regardless of the current demand situation Therefore,the achievable earnings are moderate only, impairing the profitability of such plants

heat-On the other hand, the versatility of regenerative cycles offers the option of ceiving machines that may be toggled between several cycles and thus be operated indifferent modes to comply with varying demand situations Within a research projectfunded by the German Research Foundation (DFG), such a machine was devised andrealized experimentally, applying a similarity-based scaling procedure in order toreduce both the manufacturing and the operating expenses [17–19] The machine mayfirstly be operated as a c-type Stirling engine converting the hot-end heat input to amaximum of mechanical (and finally electrical) power, whereas the waste heat isrejected at a“warm” temperature still sufficient for domestic heating and warm watersupply, i.e., this mode corresponds to conventional CHP operation Secondly, themachine may be operated as a thermally driven Vuilleumier heat pump, which does notexchange any mechanical work in the ideal case, but absorbs heat at a“cold”, typicallynear-ambient temperature level and lifts it to the warm temperature of the heatingcircuit in addition to the hot end heat input, which is of course also released at the samelevel Thirdly, the machine may be operated in a so-called “hybrid” mode featuringboth a somewhat reduced mechanical power output and a heat pump effect

con-Thus, the heat-to-power ratio may be adjusted according to the current demands bychoosing the most profitable of three different modes The experimental resultsobtained in the aforementioned project confirmed that in any these modes, a goodthermodynamic performance and thus a far better exergetic utilization of the fuel than

by any conventional heating system may be achieved Furthermore, a simulation of apotential practical operating scenario based on statistical data [20] revealed that par-ticularly the “hybrid” mode is of special interest because it features the maximumoverall energy conversion rate, power density and efficiency of all Therefore, thismode– or rather the underlying thermodynamic cycle – may also be of interest in otherapplicationfields with a simultaneous demand for electrical power and a heat pump orrefrigeration capacity, e.g the automotive sector Here, such a machine may be used as

an engine-independent air conditioning and battery charging system for trucks, orpossibly as a battery-independent heating and air conditioning system with an addi-tional range extending capacity for all-electric vehicles If such a system is operatedwith fuel from renewable sources, such a solution would also be acceptable underenvironmental aspects This contribution will therefore concentrate on the thermody-namic features of this particular cycle as well as potential design options

2 Thermodynamic Fundamentals

2.1 Regenerator Layout and Operating Principles

Regenerative cycles are generally realized by piston-cylinder arrangements, which aremaintained more or less isothermal at the characteristic temperature levels These areinterconnected by ducts typically containing a regenerator to ensure the aforemen-tioned, almost reversible temperature changes of any working gas displaced from onetemperature level to another, since this is essential for a good thermodynamic per-formance of the cycle Regenerators are usually realized by highly dispersed, mostlymetallic structures– the so-called “matrix” – featuring both a high heat capacity and alarge specific heat exchange surface A, whereas their void volume Vr should be assmall as possible because of its negative effect on the pressure amplitude and thus thepower density of the cycle The combination of the latter two demands is equivalent tothat for a minimum hydraulic diameter, which is defined as dh= 4 Vr/A This explainsthe need for a highly disperse structure However, some minimum hydraulic diameter is

of course required to keep theflow losses at an acceptable level

In steady-state operation, an almost linear axial temperature profile is established inthe regenerator matrix, and due to the typically excellent heat transfer conditions, thegas is continuously heated or cooled depending on the direction offlow at a marginaltemperature difference Thus, any gas passing through the regenerator down the tem-perature gradient will almost be cooled down to the cold end matrix temperature, and

on its way back up, it will recollect the heat previously stored on the matrix and thus beheated almost to its original, hot-end temperature Therefore, its enthalpy will also berestored to almost the original value, and this is why the net enthalpy transfer through aregenerator may approach zero for a complete steady-state cycle in the limiting case of

an infinite heat transfer coefficient However, even in this case, there will still be lossesdue to thermal conduction via both the matrix and the surrounding walls So, mini-mizing the aforementioned flow losses by liberally increasing the cross section andreducing the length instead is not an option, and in the end, a good regenerator design isalways a compromise between several conflicting demands

2.2 Basic Analysis of Regenerative Cycles

Although isothermal conditions in the cylinder volumes are theoretically desirable, thelatter may rather be characterized as almost adiabatic under virtually any practicalconditions Instead, the required heat exchange is usually achieved by inserting addi-tional heat exchangers in the connecting ducts, i.e., in between the correspondingcylinder volume and the regenerator However, the design and layout of these heatexchangers will not be considered in detail within the scope of this contribution Theschematic illustrations of the Vuilleumier and the hybrid cycle in Fig.1therefore onlycomprise the cylinder volumes at the three characteristic temperature levels, theregenerators in the connecting ducts and a simplified crank mechanism

To analyze the fundamental characteristics of these cycles, it is sufficient to sider an energy balance for a section that is maintained at one of these temperaturelevels, i.e., a single cylinder volume or a combination of two directly coupled volumes,

con-possibly including any adjacent heat exchangers From the thermodynamic point ofview, this is an open, unsteady system, since the pressure p, the volume Vi(i = h, w, c)and– in the non-isothermal case – also the gas temperature Tiare functions of time (orcrank angle, respectively) Furthermore, the system features one or more open con-nections j to a regenerator outside the system, where a differential mass dmjmay enter(dmj> 0) or leave (dmj< 0) the system during a time increment dt The differentialchange of the internal energy Ui of the system is then given by

dUi = dQi p dViþ Rjh T
j

if dissipative effects by friction at the seals etc are ignored The specific enthalpy h isknown to be a plain function of the temperature in case of an ideal gas Within thiscontext, it is of no importance whether the differential amount of heat dQiis directlytransferred via the cylinder wall or by adjacent heat exchangers, since these areincluded in the system boundary However, it is important to note that the temperature

Tjof a differential mass dmjcrossing the boundary is not necessarily equal to Tiin thegeneral case of a non-isothermal system Integration of Eq.1over the cycle yields

“indicated” amount of heat Qi exchanged per cycle by the considered section in thiscase therefore corresponds to the loop area of a p,V plot for the corresponding volume

Viaccording to Eq 3, similarly to the indicated work delivered by a heat engine thatcorresponds to the p,V area for the total system volume Vtot Figure2 displays suchplots for both the Vuilleumier and the hybrid cycle These are based on experimentaldata obtained from the aforementioned laboratory-scale machine under nominaloperating conditions, as summarized in Table1 in addition to the major design data.2.3 Operating Principles of the Vuilleumier Cycle and the Hybrid Cycle

As illustrated in Fig.1, the Vuilleumier cycle is characterized by two double-actingpistons that are bypassed by connecting ducts containing the regenerators and aretherefore commonly referred to as displacers, since they only move the gas back and

forth between either the“hot” or the “cold” volume on the one side and the sponding“warm” volume on their reverse side If the connecting rods to the crankmechanism are ignored, the volume changes are the same on both sides, but 180° outphase, of course So, the overall cycle volume is essentially constant, and there is novolumetric compression or expansion Instead, pressure changes in the Vuilleumiercycle are induced by thermal compression effects only, i.e., by the displacement ofparts of the working gas via the regenerators, where it is either heated or cooled.Therefore, the pressure changes in the Vuilleumier cycle are comparatively small,

corre-as confirmed by Fig.2 However, due to the phase-shifted motion of the displacers, itturns out that positive,“clockwise” loop areas are obtained for both the hot and the coldcylinder volume, indicating a heat input to the cycle according to Eq.3 Contrarily, thecounter-clockwise loop obtained for the warm volume indicates a heat rejection at thistemperature, which is equal to the sum of the hot and the cold loop area for energyconservation reasons, if the overall cycle volume is actually constant However, acloser look reveals that the displacer rods, which are of course guided through seals toseparate the large void volume of the crank case from the active cycle volume, actuallycause a minor fluctuation of the latter, and thus a small, positive p,V loop area isgenerated, indicating an amount of work that is delivered by the cycle

Evidently, the magnitude of this work may be adjusted via the cross sections of thedisplacer rods, preferably by the cold one, which is highlighted in yellow in Fig.1 Asfar as known, this opportunity to generate some auxiliary mechanical power has so farbeen utilized in any reported design of a Vuilleumier heat pump to ensure a self-sustained operation without an external supply of mechanical or electrical power.Now, if it is possible to generate the power required to compensatefluid flow andmechanical friction losses by adequately increasing the cross section of the cold dis-placer rod, a further increase will consequently generate excess power, i.e., the systembegins to operate as an engine besides acting as a heat pump Remarkably, the indicatedrefrigeration capacity– as visualized by the “cold” p,V loop area – is unaffected bysuch a modification, since it results from the interaction between the cold volumevariation and the phase-shifted thermal compression effect by the hot displacer, both ofwhich remain virtually unchanged On the other hand, enlarging the cold displacer rodgenerates an additional volumetric compression effect, which adds to the thermalcompression by the cold displacer Now, the overall pressure change caused by the colddisplacer motion interacts with the variation of the hot volume, resulting in an increasedheat demand and a consequently enlarged hot p,V loop area (as well as an increasedheat rejection via the warm cycle section, of course) Thus, the power density of thecycle is increased, since a Stirling engine is gradually superimposed to the Vuilleumier

Table 1 Major design data and operating conditions of the laboratory-scale machine [17]Hot temperature Th 500 °C Cylinder swept volumes 218,5 cm3

Warm temperature Tw 30 °C Cylinder bore 80 mm

Cold temperature Tc 10 °C Piston/Displacer stroke 43,5 mm

Rotational speed n 383 min−1 Hot displacer rod diameter 9 mm

Mean pressurep 38.3 bar Cold displacer rod diameter 25 mm