Solid-state cooling and power generation have long been sought after asa solution for challenging thermal-management and energy problems.Thus, to address some of these issues, thermoelectric modules have been available for decades.
The core component of a thermoelectric module is a thermocouple,which consists of two dissimilar semiconductors (referred to as p-typeand n-type todescribe dissimilar electrical conduction mechanisms inthe two materials) connected together by a metal plate.
Electrical connections at the top complete an electric circuit. Thermoelectric cooling (TEC)occurswhen current is supplied, in which case the thermocouple cools on oneside and heats on the other by the Peltier effect.
Thermoelectric generation (TEG)occurs when the couple is put in a thermal gradient (i.e. the top ishotter than the bottom), in which case, the device generates current,converting heat energy into electrical power by the Seebeck effect (Figure 1, below ).
|Figure1: TEC occurs when current is supplied (left). TEG occurs when thecouple is put in a thermal gradient, in which case, the devicegenerates current, converting heat energy into electrical power by theSeebeck effect (right).|
In practical applications, a large number of p- and n-type pelletsare cut from boules of thermoelectric material, and thermocouples areassembled together (electrically in series, thermally in parallel) toform a TEC or a TEG. Conventional modules are referred to as “bulk”modules because of their size and fabrication approach (Figure 2, below ).
|Figure2: Conventional modules are referred to as “bulk” modules because oftheir size and fabrication approach.|
Such devices found early (and continued) use for power generation inaerospace applications, and for cooling and temperature control ininstrumentation, telecommunications and other specialty applications.
While thermoelectric modules deliver the benefit of solid-stateoperation, they suffer from many disadvantages. They are generallyinefficient, fragile and big.
The very size and discrete nature of conventional thermoelectricmodules severely limit their implementation.
More recently, a significant amount of development has been focusedon thin-film thermoelectric devices.Thin-film thermoelectric materials can be grown by conventionalsemiconductor deposition methods, and devices can be fabricated usingconventional semiconductor micro-fabrication techniques.
The resulting devices are much smaller than conventional offeringsand are suitable for direct integration into modern manufacturingmethods (Figure 3 below ).
|Figure3: A thin-film thermoelectric module measures 3.5mm x 3.0mm x0.1mm in size.|
The thin-film TEC is about 6x smaller in its x-y dimensions andabout18x smaller in the z dimension. Thus, on a volume basis, thethin-film TEC is about 110x smaller in size.
Thin-film vs. bulk
While it is interesting to compare thin-film and bulk thermoelectricmodules on the basis of size, it is much more useful to compare them onthe basis of performance. The most common measure of the performance ofa thermoelectric module is its load line.
|Figure4: (a) The thin-film TEC pumps almost 4x the power of the bulk device,although it holds off only about 60 percent of the temperaturedifference; (b) In the cooling mode, thin-film thermoelectric devicesdeliver good power-density pumping capability (Q/area) combined with agood temperature gradient.|
A load line is generated at fixed operating current and specifiedreference temperature by plotting the temperature difference ( Delta T)that the device can achieve between its top and bottom plate. As afunction of the power (Q), it can pump against the temperaturegradient.
Figure 4a above shows theload lines of the thin film and bulk devices, both measured at areference temperature of 25 degrees Centigrade. In this case, thecharacteristic load line for a module is shown at its maximum operatingcurrent (Imax ).
Under Imax conditions, the Delta T at zero Q is referredto as DeltaTmax, and the power pumped at zero Delta T is referred to as Qmax .
Neither Delta Tmax nor Qmax is a practicaloperating condition for the device. However, both define theperformance envelope of a device and are often used as a basis ofcomparison.
The performance comparison shown in Figure4a above is interesting – the thin-film TEC pumps almost 4x thepower of the bulk device, although it holds off only about 60 percentof the temperature difference.
Yet, if we consider dimensional differences, the intrinsicperformance of the thin-film TEC is quite amazing. The thin-film TECholds off a maximum of 40°C (Delta Tmax ) across thethickness of a piece of paper.
And it pumps a maximum of almost 16W (Qmax ) through thearea of a piece of confetti. The respective response time of the bulkand thin-film devices is not shown. The thermal response time of bulkdevices is in seconds (e.g. to achieve a cooling target), while theresponse time of thin- film TECs is in milliseconds.
Figure 4a shows the measuredload lines of the bulk and thin- filmTECs, recast to account for dimensional performance differences.
In this case, Delta T is divided by the thickness over which it isachieved, and Q is divided by the area over which it is achieved. Thevertical axis represents the temperature gradient that the device canhold off across the thickness, and the horizontal axis shows the powerdensity that the device can pump over its area. The complete bulk-thinfilm comparison is summarized in Table1 below .
|Table1: Thermal response time of bulk devices is in seconds, while that ofthin-film TECs is in milliseconds|
The comparison in Figure 4bearlier illustrates a fundamentally new operating regime offeredby thin-film TECs.
In the cooling mode, thin-film thermoelectric devices deliverunprecedented power-density pumping capability (Q/area) combined withan extraordinary temperature gradient (Delta T/H). Similarly, in powergenerating mode, these devices achieve unmatched performancecharacteristics.
Nevertheless, to take advantage of the characteristics shown in Figure4b , the thin-film devices need to be put into the right thermalanddimensional form-factor environment.
Due to their small size, the devices can be integrated directly intoa semiconductor or optoelectronic package to achieve localized coolingand temperature control hat. The devices are also small enough and pumpenough power to enable new applications in power generation, medicaldevices and instrumentation.
Finally, the concept of integrated, localized cooling and powergeneration is now available to make temperature control or energydelivery a complementary function in an electric circuit.