Studies on an Energy-Efficient Air Conditioning of Hybrid and Ele

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2012

Studies on an Energy-Efficient Air Conditioning ofHybrid and Electric VehiclesJoerg [email protected]

Rico Baumgart

Christoph Danzer

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Aurich, Joerg; Baumgart, Rico; and Danzer, Christoph, "Studies on an Energy-Efficient Air Conditioning of Hybrid and ElectricVehicles" (2012). International Refrigeration and Air Conditioning Conference. Paper 1301.http://docs.lib.purdue.edu/iracc/1301

2442, Page 1

International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

Studies on Energy-Efficient Air Conditioning

of Hybrid and Electric Vehicles

Joerg AURICH1*, Rico BAUMGART,

Christoph DANZER, Thomas VON UNWERTH

1TU Chemnitz, Professorship of Advanced Powertrains,

Chemnitz, Germany

[email protected]

+49 (0)371 531 38763

* Corresponding Author

ABSTRACT

Energy-efficient air conditioning of passenger cells is an ever-increasing challenge in the development of electric

vehicles because the electric heating in particular reduces the cruising range significantly. For this reason, a simula-

tion model has been developed at Chemnitz University of Technology, which simulates the whole air conditioning

system including the passenger cell and the complete powertrain in electric cars. With the help of this model, differ-

rent optimization approaches on the cruising range have been analyzed and evaluated.

This paper first illustrates how much the cruising range of an exemplary electric vehicle is reduced by using the

electric heating under different wintery weather conditions. Afterwards, the exploitation of the waste heat produced

by the powertrain components (electric motor and power electronics) will be explained. Finally, it shall be described

to what extent this exploitation increases the cruising range.

1. INTRODUCTION

The development of electric cars has been expedited for several years. The air conditioning of these new vehicle

types, however, is fairly challenging because there is no waste heat produced by the internal combustion engine to

be exploited. Furthermore, the energy capacity of the battery is highly limited. The climatisation of the passenger

cell thus reduces the cruising range dramatically, which makes energy-efficient temperature regulation of electric

cars an important research subject.

Various optimization measures for heating electric vehicles were analyzed at Chemnitz University of Technology in

order to maximize the cruising range. This research resulted in a simulation model that represents the heating and

the cooling system as well as the powertrain of electric vehicles.

The simulation model for the air conditioning system and the powertrain will be explained in the following. After

that, it will be exemplified by means of an exemplary electric car how much the cruising range might be reduced by

running the electric heating under different wintery weather conditions. As a conclusion, it will be described to what

extent the cruising range is influenced by exploiting the waste power of the powertrain components (engine and

power electronics).

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

2. SIMULATION MODEL

For the simulation and optimization of air conditioning systems in passenger cars, geometry and process based mo-

dels for

the compressor,

the heat exchangers (condenser and evaporator),

the thermostatic expansion valve and

the passenger cell

have been developed at Chemnitz University of Technology. These models were combined to one main model,

which can analyze and optimize the processes in the air conditioning system for any weather and driving condition

(Baumgart 2010).

In addition, another simulation model for the powertrain of electric vehicles has been developed, that consists of the

following submodels:

permanent-magnet synchronous machine with vector current regulation

inverter and converter

battery

gearbox

wheels and vehicle

Among others, the power losses produced by the powertrain components can be calculated with these simulation

models.

The models for the air conditioning system, the passenger cell and the powertrain have also been combined to inves-

tigate the effect of different optimization approaches on the cruising range.

To determine the effects of waste heat exploitation, an additional simulation model for the cooling circuit of the

powertrain components has been created.

In the investigations explained below, an exemplary medium-sized electric vehicle was used. Table 1 details the

most important parameters of said vehicle.

Table 1: Vehicle and gearbox parameters

Vehicle parameter Value Gearbox Value

Class Medium-sized Type Stepped transmission

Weight 1,500 kg Number of gears 2

Frontal surface area 2.00 m2 Synchronization Active through engine

Drag coefficient 0.30 Gear shift strategy Efficiency-optimized

Air density 1.21 kg/m3 Gear ratio speed 1 15.6

Wheel diameter 0.625 m Gear ratio speed 2 6.5

Rolling resistance coefficient 0.009 Final drive ratio 4.5

The simulated car was driven by a permanent-magnet synchronous machine with a maximum continuous output of

80 kW and a maximum rotational speed of 10,000 rpm, which allows a pleasant driving behavior for this car type. A

two-speed transmission connects the engine with the wheel axle.

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

The car was equipped with a battery featuring the following attributes:

type: lithium-ion

capacity: 10 kWh

nominal voltage: 300 V

maximum power output: 60 kW (temporary peak power up to 100 kW)

Other assumed conditions were an unlimited energy recuperation, an efficiency-optimized gear shift strategy and a

basic electric power demand of 250 W, e.g. for lighting and other electric devices.

The investigations were based on a driving cycle that was specifically created for Chemnitz and its environs. The

most important data can be seen in Figure 1.

Figure 1: Chemnitz driving cycle

This cycle includes three parts (urban, extra-urban, highway), that are about the same size, and also a section of stag-

nant and slow-moving traffic. The overall distance is about 23 km and the duration is 39 min, which results in an

average speed of about 45 km/h.

The simulation explored how often the car could run through the cycle until the useable battery capacity was

exhausted. The result was used to calculate the cruising range.

The applied weather conditions were taken from the Test Reference Year of the German Weather Service

(Christoffer et. al. 2004).

N

Chemnitz

A4

A72

Chemnitz driving cycle Value

Distance [km] 22.88

Duration [min] 39.00

Average speed [km/h] 44.54

Average driving power [kW] 5.79

Average energy recuperation [kW] 2.38

Ratio driving power/

energy recuperation2.13

0

20

40

60

80

100

120

140

0 10 20 30 40Time [min]

Sp

eed

[k

m/h

]

0 10 20 30 40

140

120

100

80

60

40

20

0 0

100

200

300

400

0 10 20 30 40

Dir

ect

ion

F

[ ]

aF

W O

N

S

400

300

200

100

0

Time [min]

0 10 20 30 40

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

In all simulations, the driving cycle began with an hour of parking to reach the respective start temperature for each

component and the air in the passenger cell.

3. RESULTS

The simulations have been executed for two different days under wintery weather conditions and were based on the

following key values:

the 6th February of the Test Reference Year with an average temperature of -11 C in the investigation period

the 4th April of the Test Reference Year with an average temperature of 5 C in the investigation period

Figure 2a presents the ambient temperature profile for the 6th

February of the Test Reference Year and the investiga-

tion period that started at 8 a.m. with an hour of parking. It can also be seen that the solar radiation is very low with

a maximum value of 40 W/m2.

Figure 2: Heating of the vehicle on the 6th

February

Figure 2b shows the behavior of the cabin temperature (black graph), which rises only slightly during the parking

period. However, it increases significantly during the ride because of the heating process. The initial temperature of

the air blown into the passenger cell is 55 C (grey graph). Already after six minutes the indoor temperature reaches

20 C (black graph) and a comfortable 23.5 C after 15 minutes. From then on, the inlet air temperature can be re-

duced without affecting the cabin temperature.

The following figure illustrates the required heat output for various climatisation scenarios.

In the first scenario, it was assumed that the temperature of the passenger cell can only be controlled by using an

electric heating, that can be powered by the high-voltage power supply of the battery (Figure 3a). It was also

assumed that the heating is run in fresh air mode. Figure 3a (grey graph) shows the resulting heat output require-

ment. In the first phase of the driving cycle, the electric heating works at its maximum capacity. This value was

exemplarily assumed to be 6 kW. The heat output decreases gradually because the comfort temperature is reached

after a few minutes and thus the required air inlet temperature falls (cf. Figure 2).

-15

-10

-5

0

5

0

20

40

60

80

0 4 8 12 16 20 24

-20

-10

0

10

20

30

40

50

60

0 20 40 60 80 100

6th February Test Reference Year, region 13, start: 8.00 a.m., Chemnitz driving cycle, fresh air mode

Am

bie

nt

tem

per

atu

re[

C]

Time of day [h]

So

lar

rad

iati

on

[W/m

2]

Driving cycle

Inlet temperature

Cabin temperaturea) b)

Tem

pera

ture

[ C

]

Cycle time [min]

Parking Cycle

Diffuse radiation

Direct radiation

Total radiation

Ambient temperature

2442, Page 5

International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

Figure 3: Heat output for various climatisation scenarios

In addition, Figure 3a shows the heat flux from the powertrain components, namely the electric motor, the inverter

and the converter (thin black graph). As can be seen, the waste heat increases significantly at the beginning of the

heating period. It was assumed that the waste heat of the inverter and the converter can be completely transmitted

into the cooling water. The stator and the cooling jacket of the electric motor need to be warmed up first, which

delays said transfer. After the cooling water is warmed up in the powertrain components, it flows through a heat ex-

changer and releases heat into the air. The so pre-heated air streams through the electric heating and is afterwards

blown into the passenger cell. Thus the required power of the electric heating can be reduced by more than 500 W.

At some operating points the exploitable power losses are more than 1 kW, especially in the highway driving cycle

between the 75th

and 80th

minute (cf. Figure 3a).

Contrary to Figure 3a, 3b is based on the assumption that the recirculated air mode is used. As expected, this leads to

a significant reduction of the required heat output. The maximum value is hence only about 3.5 kW. Moreover, the

heat output can be decreased much faster than in fresh air mode.

0

1

2

3

4

5

6

7

60 70 80 90 100

Cycle time [min]

Po

wer

/hea

tfl

ux

[kW

]

Power demand of the electric heating

6th February and 4th April, Test Reference Year, region 13, start: 8.00 a.m., Chemnitz driving cycle

6th February, fresh air mode

Power demand of the electric heating with exploitation of waste heat

Waste heat flux of powertrain components

Cycle time [min]

Po

wer

/hea

tfl

ux

[kW

]

Cycle time [min]

Po

wer

/hea

tfl

ux

[kW

]

Cycle time [min]

Po

wer

/hea

tfl

ux

[kW

]

6th February, recirculated air mode

6th February, recirculated air mode + heat insulation 4th April, recirculated air mode

a) b)

c) d)

0

1

2

3

4

5

6

7

60 70 80 90 100

0

1

2

3

4

5

6

7

60 70 80 90 100

0

1

2

3

4

5

6

7

60 70 80 90 100

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

As a next step, the thermal conductivity of all car body elements, especially the door cards, the windows, the head-

liner and the vehicle floor, was reduced by 50 %. In the simulation it was assumed that the cabin was heated with

recirculated air. Thereby, as shown in Figure 3c, the required heat output can be slightly reduced compared to the

simulation with an uninsulated car body.

Figure 3d presents the simulation results for the environmental conditions on the 4th

April. The heat output is con-

siderably lower because the average ambient temperature is clearly higher than on the 6th

February. It is even pos-

sible to heat the passenger cell completely with the waste heat of the powertrain components at some operation

points, for instance on the 4th

April in the highway cycle. As a result, the electric heating can be deactivated in these

phases.

The reduction of the cruising range that results from the above-mentioned measures is shown in Figure 4.

The cruising range of the electric vehicle is 85.59 km in the Chemnitz driving cycle when the cabin is heated with-

out using the electric energy in the battery, e.g. with the help of a fuel heater, or not heated at all.

When the electric heating is used, however, the cruising range for the 6th

February is reduced by 50.03 km (58.5%)

in fresh air mode. On 4th

April it is decreased by 31.10 km (36.3 %), as can be seen in Figure 4, modification A.

The heat output and consequently the reduction of the cruising range can be lowered significantly by using recircula-

ted air (Figure 4, modification B). For the 6th

February the range reduction is only 33.70 km, which is equivalent to

an extra cruising range of 53.2 % compared to the simulation in fresh air mode.

When the waste heat of the powertrain components is additionally exploited, the cruising range loss can be reduced

from 50.03 km to 46.92 km (8.7 %) in fresh air mode (Figure 4, modification C). In recirculation mode the reduction

can even be decreased from 33.70 km to 26.82 km compared to the exclusively electric heating (Figure 4, modifica-

tion D), which is an increase of 13.3 %.

By applying a suitable car body insulation and using the waste heat efficiently, the cruising range loss can be limited

to only 25.60 km in recirculation air mode. This is an increase of 2.1 % compared to the simulation with an uninsu-

lated car body and a rise of 15.6 % compared to the simulation with the electric heating without exploiting the waste

heat (Figure 4, modification E).

Figure 4: Cruising range with different heating modifications

Modification

A

B

C

D

E

6th February

4th April

Red

uct

ion

of

the

cru

isin

gra

ng

e[k

m]

= Electric heating + fresh air mode

= Electric heating + recirculated air mode

= Electric heating + waste heat exploitation + fresh air mode

= Electric heating + waste heat exploitation + recirculated air mode

= Electric heating + waste heat exploitation + recirculated air mode + car body insulation

Cruising range without any heating: 85.59 km (Chemnitz driving cycle)

0

10

20

30

40

50

60

70

A B C D E

50

.03

km

(58

.5 %

)

31

.10

km

(36

.3 %

)

33

.70

km

(39

.4 %

)

18

.92

km

(22

.1 %

)

46

.92

km

(54

.8 %

)

23

.46

km

(27

.4 %

)

26

.82

km

(31

.1 %

)

7.6

4 k

m(8

.9 %

)

25

.60

km

(29

.9 %

)

6.7

6 k

m(7

.9 %

)

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International Refrigeration and Air Conditioning Conference at Purdue, July 16-19, 2012

On a mild day like the 4th

April, with an ambient temperature of 5 C, the simulation results are similar. The requi-

red heat output, however, is considerably lower due to warmer environmental conditions. Consequently, the achiev-

able cruising range is clearly wider than on the 6th

February.

4. CONCLUSIONS AND OUTLOOK

The investigations have demonstrated that purposeful waste heat exploitation increases the cruising range signifi-

cantly because the required power demand of the electric heating is reduced. An additional extension of the cruising

range can be realized by insulating the car body.

Furthermore, the waste heat can be exploited under summery weather conditions to reheat the air cooled down by

the air conditioning. In todays cars the air conditioning system regulates the evaporator outlet air temperature in a

way that keeps it from exceeding a certain value, for example 10 C, to avoid a musty smell. The temperature of the

air introduced into the passenger cell must often be higher, though. This is why the cooled air is reheated first and is

afterwards blown into the passenger cell.

While with conventional cars the waste heat of an internal combustion engine can be used for this purpose, with

electric vehicles the required heat output needs to come from the battery. To limit this additional power consump-

tion, the waste heat of the powertrain components could be used.

REFERENCES

Baumgart, R., 2010, Reduzierung des Kraftstoffverbrauches durch Optimierung von Pkw-Klimaanlagen, Verlag

Wissenschaftliche Scripten, Chemnitz, Germany, 230 p.

Christoffer, J.; Deutschlnder, T.; Webs, M.; Deutscher Wetterdienst, 2004, Testreferenzjahre von Deutschland fr

mittlere und extreme Wetterverhltnisse. Selbstverlag des Deutschen Wetterdienstes, Offenbach, Germany

ACKNOWLEDGEMENT

The authors are very grateful to

Theresa Klinner and

Diana Lohse

for supporting this work.