The U.S. military uses large amounts of fuel during deployments and battlefield operations. In fact, 70% of the gross tonnage transported when the Army deploys is fuel. The U.S. Army pays $3.2 billion per year to maintain active-duty and reserve personnel to transport fuel that costs the Army another $0.2 billion per year to purchase. Consequently, the U.S. military has a strong need to develop technologies that increase fuel efficiency and minimize fuel requirements along the logistics trail and in all battlefield operations. This project explored the use of advanced thermoelectric (TE) power generators for reducing fuel delivery demand in theater. TE power generators by definition consist of a system of solid-state TE conversion modules coupled with advanced micro-heat-exchanger technologies that are subsequently arrayed in concentric, stacked rings to facilitate energy recovery and conversion.
The objective of this project was to develop a lightweight, small form-factor, soldier-portable advanced TE system prototype to recover and convert waste heat from various types of deployed equipment (e.g., diesel generators/engines, incinerators, vehicles, and potentially mobile kitchens), with the ultimate goal of obtaining additional power for soldier battery charging, advanced capacitor charging, and other battlefield power applications. The project sought to achieve power conversion efficiencies of greater than 10% (double that of current TE conversion efficiencies) in a system with approximately 1.6-kW power output for a spectrum of battlefield power application.
The technical approach employed microchannel technology, a unique “power panel” approach to heat exchange/TE system integration, and newly characterized LAST (lead-antimony-silver-telluride) and LASTT (lead-antimony-silver-tin-telluride) TE materials segmented with bismuth telluride TE materials in designing a segmented-element TE power module and system. This project researched system integration challenges of designing a compact TE system prototype consisting of alternating layers of thin, microchannel heat exchangers (hot and cold) sandwiching thin, segmented-element TE power generators. The TE properties, structural properties, and thermal fatigue behavior of hot-pressed and sintered (HPS) LAST and LASTT materials were characterized, and the first segmented-element TE modules using LAST/LASTT materials were fabricated and tested.
The p-type LASTT materials exhibited figure of merit, or ZT (a measure of thermodynamic efficiency, in which an increase in ZT indicates increased thermodynamic efficiency), values of 1.0 at 700 K, slightly lower than the 1.2 at 700 K goal for these materials. The p-type LASTT power factors were improved during the project to about 17 μW/cm-K2 at 600-700 K, but also fell short of the expectations of 20-22 μW/cm-K2 at 600-700 K. Further work is needed to increase p-type LASTT power factors.
The n-type LAST materials exhibited ZT values of 1.0 at 700 K and fell short of the efficiency goal of 1.5 at 700 K. Although n-type LAST material power factors were improved significantly to 16-26 μW/cm-K2 at 700 K, the thermal conductivity of these materials remained too high to achieve the n-type ZT goal. Additional work is needed in developing annealing techniques to reliably reduce the thermal conductivity of these materials.
Progress was made in characterizing the thermal fatigue of the HPS LAST and LASTT materials. Thermoelectric, thermal, and structural analyses conducted for this project ultimately led to these LAST and LASTT materials being successfully segmented with bismuth telluride and electrically interconnected with diffusion barrier materials and copper strapping within operating TE modules. The TE module targeted design efficiency of 9% was not achieved. This was primarily a result of extraneous internal interface resistances, which while successfully reduced with each new module build, were still too high.
A compact TE system design was developed to produce 1.4-1.5 kW of electrical energy using these new TE modules (with 9% conversion efficiency) from the exhaust waste heat of 60-kW Tactical Quiet Generators. This was slightly below the project goal of 1.6 kW and 10% conversion efficiency. The system design incorporated high-performance hot-side microchannel heat exchangers designed to provide a heat flux of 5.6-12 W/cm2 to the TE modules. The system design also incorporated high-performance cold-side microchannel heat exchangers capable of absorbing a heat flux of 11 W/cm2 in cooling the TE system. Useful, flexible, and modular TE system designs were developed for both 30-kW and 60-kW Tactical Quiet Generators.
The use of thermoelectric generators (TEG) to capture waste heat for generating additional electricity could provide the military increased energy efficiency on a dispersed battlefield. This technology has the potential to provide additional electrical power per gallon of JP-8 fuel consumed by diesel generator power systems (gensets) and reduce the logistics needed to transport fuel to and around the battlefield, thereby reducing overall fuel costs and personnel risk during operations. The use of waste heat could also reduce the thermal signature of gensets and incinerators, thereby improving soldier and system survivability. This technology also could potentially be applied as an effective battery and capacitor charging system to help satisfy the large and growing power requirements for battlefield equipment.