New power generation for high-altitude airships

A high-altitude airship (HAA)—essentially a lighter-than-air craft that can travel at roughly 20km above the earth's surface—could be used for many applications including atmospheric or military surveillance. Takeoffs, landings, and maneuvering, however, will require a light and efficient source of power. Nonetheless, stringent weight limitations, which are needed to allow high-altitude flight for long periods, do not permit the use of onboard fuel storage for power generation. Solar energy could be used instead. For this reason, the HAA requires lightweight systems that can harvest solar energy and distribute power for various applications during long-term missions. These could rely on an advanced thermoelectric (TE) generator—a device that converts heat to electricity—being developed by my research group at the NASA Langley Research Center (Hampton, VA).

My colleagues and I have developed an advanced TE system that consists of three layers: silicon germanium (SiGe), lead tellurium (PbTe), and bismuth telluride (Bi₂Te₃, see Figure 1). During the daytime, this system could provide direct solar-energy conversion to power the HAA. Nanomaterials under development at Langley make this system very efficient.¹ In addition, we have engineered the layered structure of the advanced TE materials to provide maximum efficiency across the required range of high, medium, and low operational temperatures. In addition, the TE generator works like a regenerative-cycle system, producing a cascade efficiency that is greater than 60%. In fact, this system generates a higher quantity of harvested energy than photovoltaic (PV) cells.

A proposed HAA of 152×60×24m would receive 9MW of total incident solar power (see Figure 2).² Using PV cells that are 20% efficient, the converted power would be less than 2MW. With advanced TE materials of the same efficiency, the converted power would be greater than 4MW, because the cascaded efficiency of three layers is approximately 49%. With our triply layered structure of advanced TE materials, the cascade efficiency approaches 66%. Losses caused by geometrical orientation, reflection, and transmittance, however, reduce the obtainable power to 3.84MW.

For night-time operation, this HAA would convert power transmitted from a ship-board or a ground-based microwave station and run a megawatt-class fuel-cell system. The onboard hydrogen fuel-cell system would generate 1MW. Considering a state-of-the-art power density of 0.5kW/kg, a megawatt-class, fuel-cell system would add only 2000kg to the HAA. The end product of the fuel-cell system is water. The water from nighttime fuel-cell operation is collected and recycled during the daytime by breaking it down into hydrogen and oxygen using the excess power from the triple-layer TE generated.

The power harvested can be used for various applications: propulsion of the HAA, operation of microwave-powered aerial vehicles (MPAVs) that are under development at the NASA Langley Research Center (see Figure 3),³ weather observation, remote surveillance and monitoring of air and water pollution, air- and maritime-traffic control, a telecommunication-relay station, a number of military applications, a power station for wireless-power transmission to rural areas or areas lacking electrical power, and so on. Wireless-power transmission to MPAVs broadens their roles for close-proximity measurements and surveillance. All of these applications require a continuous power source that will run for several hours in a sequential or pulsed mode. My colleagues and I are continuing theoretical and experimental studies of both the HAA power-budget plan based on the tandem mode of advanced TE materials and the new class of lightweight, microwave-powered MPAVs.