introduction

selecting a thermoelectric cooler can be a difficult process. their performance depends on many variables including operating temperature, supply voltage and current, the performance of the heat sink on the hot side, and the power dissipated on the cold side. typically a system doesn't operate at one specific condition. the surrounding environment may be changing and the power dissipated may vary. if it is important that the unit operates to a requirement during a transient condition, this introduces many more variables including the thermal capacitance and thermal time constant of the system. if it is important that the surface being cooled be isothermal, then temperature gradients over the area of the cooler become important.

a single thermoelectric cooler will not perform at optimum performance under all conditions. it is essential to select one that works best over the desired conditions. there may be one specific condition that drives the design. this usually occurs with the combination of the hottest ambient condition and the highest power dissipation. quite often, however, they may not occur at the same time. therefore having the ability to simulate the performance of your system, including the effects of the thermoelectric device over the entire operating range, is crucial to the selection process.

harvard thermal, the developer of the general purpose thermal modeling tool tas, has jointed their expertise with that of melcor, a world leader in the design and manufacturing of thermoelectric coolers (tec's), to incorporate the performance of tec's in their tas software. the fundamental equations defining the performance of tec's have been integrated into tas. the only process left to the user to select the melcor part number to be used, geometrically add the tec to the model, and define its operating voltage or current. all electrical and thermal properties are temperature dependent. the user can define voltage or current or have it controlled by the tas on/off or proportional thermostat. reversing the voltage or current, as in reality, will operate the tec in heating mode. the tas thermostat can automatically control operation to shift between heating and cooling mode to maintain the desired set point temperature.


test vs. tas predictions

a melcor (tec) was tested with the setup shown in figure 1. a melcor model #pt6-12-40 tec was attached to a finned aluminum heat sink, which was cooled by direct impingement from an axial fan. an aluminum block was attached to the other side of the tec. a resistive heater was attached to the top of the aluminum block. a thermocouple was attached between the fins at the base of the heat sink at the center of the tec. insulation was placed over the thermocouple to minimize the effect of the moving air. a second thermocouple was attached to the top of the aluminum block.

a variable dc power was used to supply power to the tec. a variac (variable transformer) was used to apply a voltage to the resistive heater. a steady state was performed with the setup. for each test, readings were recorded for the two thermocouples, the voltage and current to the tec and the voltage applied to the resistive heater.

a computer model was developed to compare the tec modeling capability of tas with the test data. the model included the tec, interfaces, aluminum block, aluminum heat sink, and convection to the ambient. inputs to the model included the tec data, voltage measured during each test, thermal properties of each material, and convection to the ambient. the model is shown in figure 2.

figure 1. te cooler test setup

figure 2. tas model

the steady state model and test were run with a resistive load of 3.8 watts. the voltage varied from test to test, but was held constant in each test. the results of the test and tas model compare extremely well. the steady state results are shown in table 1. the temperature differential between the test and tas was within 1⁰ c for every case except near the maximum voltage, where the tec is very inefficient. even in this case (test #5) the delta temperature results from the heat sink to the heater were within 4%.

table 1. steady state comparison - 3.8 watts dissipated by resistive heater

in the transient case, a more difficult scenario was simulated. a proportional thermostat was added to the model to maintain the resistor temperature at 25.5⁰c +/- 0.5 ⁰c. the proportional thermostat switched the tec to cooling mode when the resistor reached 26 ⁰c and switched it to heating mode when the resistor dropped to 25⁰c. the ambient temperature varied linearly from -20⁰c to +50⁰c over the course of 10 minutes (600 seconds). to add to the dynamics of the problem, the resistor power was varied with time. the simulation was started at a cold soaked of -20⁰c.

figure 3 shows a plot of the thermostat sense point, the heat sink, and the ambient temperature versus time. it also shows the resistor power versus time. it is interesting to note that although the tec is in heating mode at the beginning of the transient, the peltier effect is taking place for the first 25 seconds. as a result, the heat sink drops below ambient until a sufficient temperature differential occurs. at this point the heat sink increases in temperature.

it takes about 57 seconds for the unit to get to the desired temperature. increasing the voltage or current to the tec could reduce this. at about 440 seconds the unit transitions from heating to cooling mode. it is noted that even with the environment temperature and the heater power changing, the thermostat set point remains within the desired temperature.

figure 3. tas transient results with a proportional thermostat controlling the tec

simulation procedure

melcor has developed free software that is used to select a tec for your particular application. the software is called aztec and is available for download from their web site. using this software, the hot and cold side environmental conditions of the tec can be define at a specific operating point. the software will recommend one or more melcor part numbers that will fulfill the requirements at that condition.

the tas software comes with a database defining all melcor tecs. the library contains thermal and electrical material properties (including thermal capacitance for transient operation) and physical parameters for that device (geometry and number of couples). in the tas software, the part number recommended by aztec can be selected from this library. you then define the voltage or current to the device or add a thermostat if the voltage or current is to be thermostatically controlled.

defining a negative voltage or current will cause the tec to operate in heating mode. for the thermostat, if the low voltage or current factor is set to -1 and the high voltage or current factor is set to +1, then at temperatures below the set point, the tec will operate in heating mode and above the set point, the tec will operate in cooling model. this is illustrated in the example below. the tec is next geometrically modeled within tas.

background

thermoelectric coolers (tecs) are solid-state heat pumps used in applications where temperature stabilization, temperature cycling, or cooling below ambient are required. over the past several years there has been a growing interest in and use of thermoelectric devices. the use of these devices includes electro-optics applications, such as the cooling and stabilizing of laser diodes, ir detectors, cameras (charge coupled device), microprocessors, blood analyzers, and optical switches. operating requirements imposed on these devices are becoming more stringent.  the typical thermoelectric module is manufactured using two thin ceramic wafers with a series of p and n doped bismuth-telluride semiconductor material sandwiched between them. each of these is called a couple. the ceramic material on both sides of the thermoelectric adds rigidity and the necessary electrical insulation. the thermoelectric couples are electrically in series and thermally in parallel. a thermoelectric module may contain anywhere from one to several hundred couples. thermoelectrics can be used to heat and to cool, depending on the direction of the current. in an application requiring both heating and cooling, the design should focus on the cooling mode. using a thermoelectric in the heating mode is very efficient because all its internal heating and the load from the cold side is pumped to the hot side. this reduces the power needed to achieve the desired heating.

about david rosato: david rosato received a bsme from the university of massachusetts in 1977 and a msme from worcester polytechnic institute in 1981. dave worked as a thermal analyst for raytheon company for 15 years. while at raytheon dave developed a general-purpose thermal modeling tool. in 1994, he started harvard thermal to market the software. as a principal engineer, he left raytheon in 1996 to grow the business on a full time basis. today dave is president of harvard thermal, developer of the tas software and providers of thermal analysis services. he has over 20 years of thermal analysis experience mostly in military and commercial electronics.

about tim fleury tim manages the engineering services side of harvard thermal. prior to starting at hti in 2000, tim was a principal engineer at raytheon company in the analysis section. he has over 22 years of experience in the thermal analysis of military and commercial electronics.