The High Altitude Airship (HAA) has various application potential and mission scenarios that require onboard energy harvesting and power distribution systems. The power technology for HAA maneuverability and mission-oriented applications must come from its surroundings, e.g. solar power. The energy harvesting system considered for HAA is based on the advanced thermoelectric (ATE) materials being developed at NASA Langley Research Center.
The materials selected for ATE are silicon germanium (SiGe) and bismuth telluride (Bi2Te3), in multiple layers. The layered structure of the advanced TE materials is specifically engineered to provide maximum efficiency for the corresponding range of operational temperatures. For three layers of the advanced TE materials that operate at high, medium, and low temperatures, correspondingly in a tandem mode, the cascaded efficiency is estimated to be greater than 60 percent.
The first layer is built from the array of SiGe, while the second and third layers are respectively built from PbTe and Bi2Te3 as regenerative cycles. Such an arrangement allows effective energy harvesting from a heat source. First, solar flux is concentrated and heats up the first layer, which is built with high-temperature SiGe. The unused thermal energy from the first layer is subsequently used by the second layer, which is built with mid-temperature PbTe. The third layer of Bi2Te3 uses the unused energy from the second layer to maximize the conversion of the energy that is otherwise dumped away. In this fashion, the ATE devices become more effective than solar cells because the performance of solar cells is monolithically tied to band-gap energy structure, so that they only couple with certain spectral lines.