Question

10.8D According to researchers, advances in nanomaterial
fabrication are leading to development of tiny thermoelectric
modules that could be used in various applications, including
integrating nanoscale cooling devices within the uniforms of
firefighters, emergency workers, and military personnel;
embedding thermoelectric modules in facades of a building;
and using thermoelectric modules to recover waste heat in
automobiles. Research two applications for this technology
proposed within the past five years. Investigate the technical
readiness and economic feasibility for each concept. Report your findings in an executive summary.

Answer 1

Applications of THERMO ELECTRIC MODULES

Thermo Electric Modules can be used for Waste heat recovery and shape-engineerable thermoelectric painting

1. Waste Heat Recovery

A typical automobile converts about one-quarter of the combusted fuel energy to useful work, while the remaining energy escapes as waste heat through the radiator and engine exhaust. The American manufacturing industry as a whole does a better job of utilizing energy, but still roughly one-third of the energy consumed escapes as heat to the atmosphere or to cooling systems.Waste heat recovery recaptures this lost heat for conversion into electrical power. As researchers attempt to address this task, TE conversion continues to emerge as a prime candidate for waste heat recovery applications. Not all waste heat streams are good candidates for thermoelectric waste heat recovery. Since thermoelectric efficiency is a function of temperature difference, most low-grade waste heat applications, at temperatures below 200°C, are not good candidates for waste heat recovery. The conversion efficiencies will be too low to provide adequate payback (e.g., three years) for the thermoelectrics, associated heat exchangers and other hardware.Higher energy waste streams, such as automobile engine exhaust, have larger temperature differences and yield higher conversion efficiencies. Higher temperatures require higher temperature thermoelectric materials with higher figures of merit (ZTs) over the temperature range of interest. Lead tellurides, skutterudites, clathrates, magnesium silicides and others are all candidate materials for these applications.Thermoelectric (TE) generators convert heat directly into electricity when a temperature gradient is applied across the junctions of two dissimilar metals. These devices have the potential to increase the fuel economy of conventional vehicles by recapturing a portion of the waste heat from the engine exhaust and generating electricity to power a vehicle’s accessory loads. At present, device efficiencies are low (~5%); however, thin-film and quantum well technologies offer the possibility of higher efficiency in the future (~10 % to 15%). Four vehicle platforms are considered: a midsize car, a midsize sport utility vehicle, a Class 4 truck, and a Class 8 truck. A simple vehicle and engine waste heat model shows that the Class 8 truck presents the least challenging requirements for TE system efficiency, mass, and cost. This is because Class 8 trucks have a relatively large amount of exhaust waste heat, have low mass sensitivity, and travel a high number of miles per year, all of which help to maximize fuel savings and economic benefits. A driving and duty cycle analysis for the Class 8 truck elucidates trade-offs in system sizing and shows the strong sensitivity of waste heat, and thus TE system electrical output, to vehicle speed and driving cycle. It is not feasible for a TE system to replace the alternator, as too little waste heat is available during city driving and/or idling. Together with a typical alternator, a TE system could enable the electrification of 8% to 15% of a Class 8 truck’s accessories, providing 2% to 3% fuel savings. Additional electrification would require a larger alternator and battery to augment the TE system so that adequate electrical power is available during low-speed driving and idling. Achieving an economic payback in three years dictates that the TE system cost less than roughly $450/kW, requiring an almost tenfold reduction from today’s costs. Such a cost reduction might be enabled in the future by thin-film devices that use expensive TE junction materials more efficiently.

2. Shape-engineerable thermoelectric painting

The thermoelectric (TE) effect has attracted considerable attention from various research areas, as its ability to directly convert between thermal and electrical energy offers a unique solution to sustainable power generation from waste heat sources. The overall power generating performance of solid-state TE devices largely depends on the characteristics of the TE materials itself. This means that the efficiency can be estimated from a dimensionless figure-of-merit inherent in the materials: ZT=(S2?T/?), where S, ?, ? and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature, respectively. TE materials chipped into devices are mostly prepared in the form of cube or cuboid blocks from a TE ingot by means of a top–down dicing process. A potential problem of this conventional procedure is a relatively high production cost due to the energy intensive processing for ingots such as zone-melting or hot-pressing as well as the post-processing due to shape control. The latter, in fact, suffers from another problem that attempts to realize any complicated shape other than a cube are technically impossible within the context of mass production.On the other hand, in the real-world applications, the minimization of heat loss due to incomplete contact between the surface of the heat source and the TE module is no less important than the figure-of-merit of materials. It is noted that the majority of heat sources for TE generators has irregular shapes, where the conventional planar-structured TE devices composed of cubic blocks should fail in achieving a desirable contact .

One readily available solution to settling down the aforementioned issues would be to secure a way to maximizing the flexibility in the shape and dimension control of TE materials during the forming stage, where the well-established printing technology would best-serve the purpose. However, this printing-based technology has faced at least two major challenges. One is the poor functional properties due to the unavoidable organic-conducting binders in the inks for electrical interconnection among TE particles at the expense of TE properties. Although the properties can be enhanced by high temperature processing instead of using organic binders, such enhancement is quite limited; for example, the state of the art utilizing a printing technique for TE materials deposited on a glass fabric achieved ZT values of 0.35 (n-type) and 0.27 (p-type)23 which are at most 20–40% of the commonly reported values from the conventional processing even with the sintering temperature of as high as 530?°C close to the melting point of the TE materials. The other is the limited choice of substrates, that is, the usual printing technique forces one to deposit TE materials only on a flat surface, though the targeted heat sources where TE modules are attached are generally curved.

As a solution to these challenges, we present the development of high-performance shape-engineerable TE painting via the molecule-level sintering effect. To this end, we utilized the molecular Sb2Te3-based chalcogenidometalate (ChaM) for n-type BiTeSe and p-type BiSbTe TE particles, which are arguably known as the best TE materials at near room temperature. The Sb2Te3 ChaM turned out to promote the sintering process effectively even at as low as 350?°C without any remnant secondary phase, the presence of which jeopardizes the expected TE properties. With the processing optimized, we have achieved the ZT values of 1.21 for p-type and 0.67 for n-type TE materials that are comparable to the bulk values2 and three times higher than the best values among the printed TE materials reported in the literature. To show the feasibility of the currently proposed technology, we fabricated TE generators through painting TE paints on flat, curved and large-sized hemispherical substrates, demonstrating that it is the most effective means of heat energy collection from any heat sources with exceedingly high output power density of 4.0?mW?cm?2, which is the best value among the reported printed TE generators.