The Department of Defense (DoD) and its mobile forces must address the logistical challenges associated with fuel transportation for electrical power generation. Deployed force mobility as well as the cost of supplying energy to those forces improves significantly with increases in on-site, efficient electrical power generation. Moreover, the resulting reduction in required liquid fuel deliveries via manned convoys lowers risks to these personnel. Thermoelectric (TE) power generators have the potential to help generate this on-site electrical power by recovering waste heat and converting it to useful electric power, thereby increasing the efficiency of existing power generators. The objective of this project was to demonstrate the efficient conversion of waste heat using newly developed TE technology in military equipment known to produce waste heat, thus enabling the transition of these TE technologies to deployed forces for fuel and energy savings.
TE technology concepts based on thin-film superlattice and thin substrate bulk materials were used to accommodate the large range of temperatures expected in the various waste heat recovery applications. Given the large ranges of waste heat temperature in generator exhaust gases, the TE technology design changed at different temperature thresholds. Standard superlattice TE devices were designed to accommodate temperature changes greater than 150°C. Thin substrate bulk TE devices based on lead-tellurium (PbTe) and other TE alloys were developed to accommodate temperature changes greater than 400°C. Segmented TE devices that incorporate high-temperature silicon- germanium (SiGe) alloys were developed to accommodate temperature changes greater than 600°C. The research team incorporated all three stages via a segmented bulk/superlattice cascade structure (developed with support from the Defense Advanced Research Projects Agency and the Office of Naval Research) that enables higher TE conversion efficiencies and that is compact and lightweight.
The team harvested the waste heat of exhaust gas from an air cooled diesel engine in the 3kW Tactical Quiet Generator (TQG). The TE device was integrated with heat exchangers directly attached to a modified muffler assembly and served as a replacement for muffler assembly on the 3kW TQG set. Integration and initial testing was carried out and a proof-of-concept demonstration was conducted at the U.S. Army Communications-Electronics Research, Development, and Engineering Center (CERDEC) facilities. Although three phases of testing were proposed, with progressively higher power output, the project ended at Phase 2 because of a failure to meet the Go/No-Go criteria.
The lab-scale performance of the individual components, defined as the TE converter and the heat exchangers, performed near the Phase 1 targets of thermal efficiency (11% actual vs. 14% target). However, the integration of all the components on the generator led to lower than expected performance in the generator environment (5.3% thermal efficiency), due to the much smaller temperature difference (ΔT) and the transfer of that ΔT to the TE device. Issues encountered in the integration of the components led to parasitic drops of ΔT in interfaces due, in part, to the mechanical coupling of hot and cold heat exchangers with the TE device as a stressed member. The configuration evaluated did not allow for repeatable and quantifiable pressure to be put on the devices, which led to some degradation of ΔT across the device. The maximum ΔT observed at a generator load of 3kW was only 215 K, which is approximately 2/3 of the design goal ΔT of 325K. Given that TE power output is proportional to the square of ΔT, the observed TE device efficiency was less than half of the expected value. Phase 2 testing at Fort Belvoir produced only ~1.5 Watts of power, well below the Phase 1 goal of 50 Watts. As a result, this project was terminated at Phase 2. However, valuable information was compiled during the course of the project. All TE work performed previously under CERDEC programs was burner-fed; whereas, this is the first waste heat generator of its kind integrated with an Army TQG. The research team has proposed additional engineering solutions to improve the performance with a more optimized design.
The application of TE-based, solid-state, waste-heat energy converters to military logistical equipment has the potential to lower the fuel burden on armed forces by producing more efficient on-site power via cogeneration with diesel generators. However, the implementation of TE devices to utilize waste heat from generators requires additional technology development before it can be deployed in the field. The results of this project indicate that TE waste heat recovery is dependent on system integration and heat exchanger performance. Performance is also driven by the amount and the quality of waste heat in the exhaust stream.