Modeling and Sizing a Thermoelectric Cooler

Within a Thermal AnalyzerJane Baumann

C&R Technologies, Inc.Littleton, Colorado

Thermoelectric Device

Thermoelectric coolers are solid-state devices capable of generating electrical power from a temperature

gradient - Seebeck effect or converting electrical energy into a temperature gradient -

Peltier effect The ability to use TECs to heat as well as cool makes

them suitable for applications requiring temperature control of a device over a specified temperature range

Although these devices have been around for years, they are gaining popularity in the aerospace industry for providing temperature control within optical systems and loop heat pipe temperature control

Thermoelectric Cooler

A typical thermoelectric module is composed of P-type and N-type elements between ceramic substrates typically Bismuth Telluride several couples connected

electrically in series and thermally in parallel

When current is applied to the device, heat is moved from the cold side to the hot side where the heat is typically removed by a conduction or a cooling loop

Modeling TECs

Historically, modeling of a TEC device was left up to the analyst Hand calculations were performed external to a model using

sizing charts Simplified modeling in SINDA/FLUINT using a heater node,

user defined array lookups, or user defined logic New methods for TEC modeling

Built into SINDA/FLUINT and Thermal Desktop Steady state and transient simulations Enable sizing studies and parametric runs Proportional or thermostatic control options

SINDA/FLUINT Methods

SINDA/FLUINT definition User defines cold side/hot side nodes Define conductors between the cold and hot sides Define arrays containing above node and conductor IDs Define an array of areas associated with conductors Define input mode (power, current, or voltage) Define aspect ratio (area/thickness ration of a couple) Define number of couples

Thermal Desktop simplifies the input Define surfaces for substrate Define TEC contact between the surfaces Define input mode, aspect ratio, and number of couples

SINDA/FLUINT Methods

SINDA/FLUINT calculates cooling capacity and electrical power required

Heat pumped at cold side

Maximum temperature differential

Can define several independent TEC devices Can stack multiple devices for additional cooling

capacity or create multistage coolers

An Example

Sample Application* Device to be cooled is 1.525 inch x 1.525 inch Estimated heat load of 22 watts Maximum ambient temperature of 25°C Device needs to be maintained 5+2°C Convection heat sink with a thermal resistance of

0.15°C/watt

* An Introduction to Thermoelectric Coolers, Sara Godfrey, Melcor Corporation

Thermal Desktop Model

Device to be cooledCeramic substrates

Heat sink

Model development Use surfaces or

solids for ceramic substrates, device and mounting plate

Convection off mounting plate at 0.15°C/watt

Create TEC contact between substrates

Optional inputs Can model core fill

in TEC if desired

TEC Input

Simple user interface Provide input mode

Current Voltage Power

Aspect ratio Number of couples Select cold side Select hot side

Optional inputs Generate conductors Temperature control Non-bismuth telluride

devices

TEC Sizing Study

Key input parameters for cooler definition Maximum heat load of device to be cooled, 22 watts Maximum allowable temperature of device being cooled, 7C Maximum environment for cooling hot side, 25C

Thermal Desktop can handle complex thermal/fluid connections Minimum current, voltage, or power, 4 amps (max 6 amps)

Key output parameters Temperature of hot and colds substrates Aspect ratio Number of couples Optimum input current, voltage and power

TEC Sizing Study

Setup design sweeps on key parameters Aspect ratio range

Looking at TEC specifications, aspect ratios range from 0.1 to 0.4 cm

Number of couples Single stage coolers typically have between 17 to 127 couples

Preliminary selection of the TEC device Run a parametric on input current

Aspect Ratio/No. of Couples

Design point of 5C for device 120-130 couples 127 couples

standard Wish to minimize

aspect ratio to reduce heat path through the device Aspect ratio less

than .25

Design Point

Aspect ratio=0.1

Aspect ratio=0.15

Aspect ratio=0.2

Aspect ratio=0.3Aspect ratio=0.25

TEC Selection

Need to be able to generate a 40C temperature differential DTmax > 40C

Design Point

Maximum cooling capacity Qmax between 35-55 watts

TEC Selection

The Melcor CP1.4-127-06L meets the requirements and footprint required for our example

CP1.4-127-06L specifications Number of couples = 127 Geometry factor (aspect ratio) = 0.118 cm Imax = 6.0 amps Qmax = 51.4 watts Vmax = 15.4 volts Tmax = 67°C

Steady State Results

Parametric Sweep Input Current

Design point

Transient Simulation

Applied a time varying environment

Set proportional control in on cold side at 5+2C

Transient Response

For this sample we can demonstrate full control of the TEC device when exposed to the defined environment (3C<cooler.T115<7C)

Conclusion

New capabilities have been added to existing software tools allowing the steady state and transient modeling of thermoelectric devices

Access to built-in parametric and optimization methods in SINDA/FLUINT aid in design sizing and device selection

New features in Thermal Desktop allow the device to reside in an overall system model for system-level steady state and transient modeling

SINDA/FLUINT methods have been validated against published examples and sizing tools provided by TEC suppliers