ATZ Worldwide EMagazine Volume 115 Issue 11 2013 [Doi 10.1007%2Fs38311-013-0128-1] Dr.-ing. Manuel Wehowski,Dr.-ing. Jürgen Grünwald… -- Thermoelectric Heat Pump for Lithium-ion

THERMOELECTRIC HEAT PUMP FOR LITHIUM-ION BATTERIESThe thermoelectric heat pump is an attractive alternative to conventional methods used

to control the temperature of lithium-ion batteries and other vehicle sub-assemblies. In

addition to ensuring efficient temperature control, Mahle Behrs design concept of these

heat exchangers operating independently of the refrigerant circuit includes optimisation

in terms of durability, cost, weight and package size.

40

DEVELOPMENT THERMAL MANAGEMENT

Thermal management

BASICS

Lithium-ion cells are used as energy storage devices in the traction batteries of electric and hybrid vehicles and also in auxiliary units of internal combustion engine vehicles. Since they heat up during use and in extreme climatic conditions, they have to be kept within a certain temperature range by means of temperature control systems in order to maintain their service life in the long term [1, 2].

Along with conventional temperature control methods using air, coolant, and refrigerant, the principle of the thermoelectric heat pump can be applied to the heating and cooling of lithium-ion batteries.

The thermoelectric heat pump (TEHP) is based on the Peltier effect, so that heat is pumped from a cold to a hot side. This heat pump effect is achieved using thermoelectric modules (TEM) comprising semiconductor elements electrically con-tacted in series and alternately p- and n-doped. These TEMs create a temperature gradient which, depending on the direc-tion and strength of current, can be used for heating or cooling purposes.

Other factors involved in thermoelectric modules along with the Peltier effect are the Joule heating and heat conduction. Whereas electrical resistance causes heating of the TEM (Joule heating), heat conduction in the TEM creates a heat flux from the hot to the cold side in the opposite direction to the active heat flux of the Peltier effect.

Eq. 1 shows the correlation between cooling power Q.

c, heat-ing power Q

.h, and electric power Pel with a good level of approxi-

mation in the temperature range of practical relevance. The relevant parameters, along with the material- and temperature-dependent Seebeck coefficient Se, are the electrical resistance R, the current intensity I, and the heat transfer from the cold to the hot side kA at the respective surface temperatures (cold side: Tc; hot side: Th). The temperature control efficiency is described by the coefficient of performance or COP (for cooling: COPc/for heating: COPh).

EQ. 1

Pel = Q.h Q

.c = (ThTc)*Se*I + I2*R where

Q.c = Tc*Se*I I2*R/2 k*A*T

Q.h = Th*Se*I + I2*R/2 k*A*T

COPc = Q.c / Pel

COPh = Q.h / Pel = COPc + 1

THERMOELECTRIC HEAT EXCHANGERS

The selection of suitable TEMs is decisive for effective tempera-ture control. Also, in view of the marked impact of the temper-ature difference on the COP, it is essential to ensure good heat transfer between the TEM and the hot or cold side in order to achieve efficient operation. To do this, it is advisable to provide a good thermal connection between TEM and heat source/sink, and, for fluid media, good heat transfer between the fluid and the fluid ducting.

When designing a heat exchanger as a thermoelectric heat pump, it is necessary not only to ensure the energy-efficient operation of the device but also to maintain a highly durable

AUTHORS

DR.-ING. MANUEL WEHOWSKI is Project Manager in the

Central Advanced Engineering of Mahle Behr GmbH & Co. KG

in Stuttgart (Germany).

DR.-ING. JRGEN GRNWALD is Project Manager in the

Central Advanced Engineering of Mahle Behr GmbH & Co. KG

in Stuttgart (Germany).

DIPL.-ING. CHRISTIAN HENEKA is Project Manager in the

Central Advanced Engineering of Mahle Behr GmbH & Co. KG

in Stuttgart (Germany).

DR. RER. NAT. DIRK NEUMEISTERis Head of Systems/Concepts

in the Central Advanced Engineering of Mahle Behr GmbH & Co. KG

in Stuttgart (Germany).

41 11I2013 Volume 115

Thermal management

component that is optimised in terms of cost, weight, and package size. Behr is currently developing thermoelectric heat exchangers for various heating and cooling applications. Here the TEMs are in thermal contact with at least one fluid (usually coolant or air) and/or in direct contact with the component requiring temperature control. The supply voltage is variable, but voltage levels of 12 to 60 V can be used to good effect.

The TEHP development process at Behr involves measuring both perfor-mance data of individual thermoelectric modules and heat transfer processes of bonding layers. These data are used for the representation of thermoelectric heat exchangers in the thermohydraulic simu-

lation tool Biss (Behr Integrated System Simulation) [3]. This tool is used to simu-late the thermoelectric behavior of heat exchangers with any kind of fluid duct-ing and solid connections integrated into an overall system including both steady-state and transient simulations, . Ther-mohydraulic measurements of heat exchangers are carried out on specifi-cally constructed thermoelectric test benches to validate the simulation model.

Two TEHP variants integrated into an overall system with a lithium-ion battery requiring temperature control are dis-cussed in the following by comparing them with one another and with a con-ventional chiller and a low-temperature radiator system.

THERMOELECTRIC TEMPERATURE CONTROL PLATE

shows the thermoelectric temperature control plate (TETP) in direct thermal contact with a battery to be cooled/heated. TEMs control the battery temper-ature to a level that can be lower than the ambient temperature. The hot waste heat side of the TEMs has to be cooled in turn via a cooling circuit (primary cir-cuit). A pump conveys the cooling medium through the TETP and the low-temperature radiator (LTR) for heat dissi-pation into the ambient air, and option-ally through further components requir-ing cooling. There is no direct coupling with the vehicle air conditioning circuit. A shorter coolant line length and a smaller number of valves are necessary compared to a chiller system. An espe-cially beneficial feature is the heat con-duction path between the TEMs and the battery. The minimised temperature dif-ference between the cold and hot side allows efficient temperature control. Intelligent, cost-effective measures are implemented to ensure uniform tempera-tures and heat flux densities over the entire surface area of the plate.

THERMOELECTRIC COOLANT CONDITIONER

shows a thermoelectric coolant condi-tioner (TECC) placed between two cool-ant circuits for the cooling and heating of a battery. In the primary circuit, the coolant conveyed by a first coolant pump is passed through an LTR and the pri-mary side of the coolant conditioner. In

Battery

el. Heating

Chiller

Condenser

LT-radiator

Tempering plate

TXV-SO

TXV-SO

Battery

HVAC-evaporator

TECC

Condenser

LT-radiator

TXV

Battery

Condenser

LT-radiator

TETP

TXV

Primary circuit

Secondary circuit Primary circuit

(a) (b) (c)

Tempering plate

HVAC-evaporator

HVAC-evaporator

Chiller cooling system (a); thermoelectric heat pump (coolant conditioner (b) / temperature control plate (c))

Thermohydraulic Biss-simulations

DEVELOPMENT THERMAL MANAGEMENT

42

the secondary circuit, a second pump conveys the coolant through the second-ary side of the TECC and a tempering plate which is in direct thermal contact with the battery. Further components requiring temperature control can be integrated into the primary circuit.

One of the advantages of the coolant conditioner is the spatial separation of the thermoelectric component from the battery. This means that no changes to the interface between battery and tem-pering plate are required, thus avoiding any need to increase the size of the tem-pering plate. Because of the decentral-ised positioning of the TECC in the avail-able package space, there is no need to adjust for different batteries. The modu-lar construction of the coolant condi-tioner allows the simple and economical accommodation of a wide range of power classes. It also eliminates the need for temperature homogenisation which exists where TEMs are placed directly on the tempering plate surface.

SYSTEM COMPARISON

shows transient simulation results of the maximum battery temperature TBat,max for the two heat pump systems, a chiller system, and an LTR system with a battery waste heat of Q

.Bat = 150 W,

respectively. In the latter system, the coolant is conveyed through an LTR and a cooling plate in the bottom-cooled bat-tery by a pump (bottom surface area: ABat = 550 cm2).

All simulations have an air mass flow of 8 kg/min flowing through an identical LTR (cross-sectional area Aq 6 dm2) with an air temperature of Tair = 35 C. In the coolant conditioner system, the primary circuit pump conveys a volume flow of 600 l/h compared with 120 l/h in the other cooling circuits. The simulations take account of the thermal inertia of the cool-ant and of realistic heat conduction paths for both the battery and the battery/TEM connection.

The simulations start at a homogeneous maximum permissible battery temperature of 30 C. At this threshold the battery cool-ing function of the chiller and TEHP vari-ants is switched on. Cooling of the LTR system only begins at TBat = Tair = 35 C.

In the LTR system, and with permanent cooling, the maximum battery tempera-ture remains steady at a temperature of 42 C, . The temperature difference of

7 K between Tair and TBat,max is based on the heat conduction path in the LTR system.

With the chiller system, battery cooling depends on the air conditioning of the vehicle. Since the temperature of the refrig-erant in the chiller is generally < 10 C, a large quantity of power is drawn off from the battery when the battery cooling sys-tem function is on. To prevent excessive battery cooling, an active control such as a two-step control is required. Because of chiller TXV switching, this can cause oscillations in the refrigerant circuit, . In contrast, with both heat pump sys-tems, the maximum battery temperature is maintained at approx. 30 C using an active current flow.

COEFFICIENT OF PERFORMANCE (COP)

shows simulated COP curves for both TEHPs for battery waste heat levels Q.

Bat = 100 W and 150 W. Varying the simulation parameters accepted as real-istic (for example the thermal heat con-duction path) has a direct impact on

the COP, so the COPs shown are to be regarded as guidelines only.

Lower battery waste heat levels (100 W) tend to result in a higher COP. When the battery cooling system is switched on, the COP decreases and progressively approaches a steady value. Steady-state COPs of up to 2 are achieved with waste heat levels of 150 W as compared with coefficients of around 2.5 with waste heat levels of 100 W.

The following relations also apply: : The lower the ambient air tempera-

ture, the higher the COP. Although high air temperatures mean low COPs even in extreme cases, sufficient cool-ing is still achieved.

: In general, COPs of > 1 are achieved in heating situations. The heating COP is increased by 1 compared to the cool-ing COP at the same temperature dif-ference and current flow.

: Optimum COP for TEHPs can be var-ied via the number of TEMs. Increas-ing the number of TEMs boosts the COP, but it also increases costs, weight, and package space.

20

25

30

35

40

45

0 200 400 600 800 1000 1200

LTR system

TEWP system

Chiller system

Max

imal

bat

tery

tem

pera

ture

[C

]

Time [s]

Q.

Bat = 150 WTair = 35 C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 200 400 600 800 1000 1200

100 W

150 W

COP simulations forcoolant conditioner& temp. control plateTair = 35 C

CO

Pco

ol [

-]

Time [s]

Maximum battery temperatures

Cooling COP ranges for TEHP with Tair = 35 C and Q.Bat = 100 W/150 W

11I2013 Volume 115 43

COMPARISON HEAT PUMP AND CHILLER SYSTEM

The independent and functionally spe-cific control of thermoelectric heat pumps using TEMs helps them to oper-ate efficiently. In chiller systems, the dependence between cabin and battery temperature control via the refrigerant circuit causes feedback in the other sub-system, especially when the two-step control is shifted. This can cause effects ranging from instability in the refriger-ant circuit to noticeable changes in the evaporator air outlet temperature in the vehicle cabin. The decoupling of temper-ature control tasks in TEHPs avoids con-trol adjustments and simplifies the design of the air conditioning circuit, for exam-

ple by eliminating the TXV shut-off function. Also, no additional capacity is required in the A/C compressor for bat-tery cooling purposes.

One of the major benefits of TEHP tech-nology is that both cooling and heating functions can be performed with a single component by reversing the TEM polarity. In contrast, with a chiller system, the heating function can only be provided by means of an auxiliary heating system.

SUMMARY

A thermoelectric heat pump is a single component for both heating and cooling batteries at very low and very high tem-peratures, with COPs of > 1 over wide operating ranges.

Basically, thermoelectric temperature control is feasible in all power categories. In view of the direct correlation between useful output power and the number of TEMs and cost, the use of a TEHP is appropriate for cooling and heating power levels in the range of < 1 kW.

REFERENCES [1] Stripf, M.; Wehowski, M.; Schmid, C.; Wiebelt, a.: Thermomanagement von hochleistungs-li-Ionen-

Batterien. In: aTZ 114 (2012), no. 1, pp. 52-56

[2] neumeister, D.; Wiebelt, a., and heckenberger, T.: Systemeinbindung einer lithium-Ionen-Batterie

in hybrid- und elektroautos. In: aTZ 112 (2010),

no. 4, pp. 250-255

[3] gneiting, r.; heckenberger, T., Sauer, C.: Virtual Thermal Management in Cars requirements and

Implementation. 6th FKFS Conference on Progress

in Vehicle aerodynamics and Thermal Management,

2007

DEVELOPMENT TherMal ManageMenT

44

11I2013 Volume 115 45

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