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Hypersonic Vehicle Electrical Power Generation through Efficient Thermionic Conversion Devices

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Develop thermionic generation device technology that will take advantage of the extreme temperature within a hypersonic platform resulting in efficient thermionic conversion into electrical power while existing in a modular package to survive the harsh environment. 

DESCRIPTION: Thermoelectric generators (TEG) and Thermo-photovoltaics (TPV) are common candidates for energy harvesting studies, but the extreme internal temperature profile within a conceptual scramjet driven hypersonic vehicle presents a challenging design issue for TEG or TPV integration. Conceptual hypersonic aircraft are generally designed to be propelled by a non-rotating engine, such as a scramjet, prohibiting the use of conventional generators to draw electrical power from the engine. This fact, coupled with the extreme temperatures associated with hypersonic flight, has prompted studies of thermal-to-electric conversion of the excess heat energy on a hypersonic vehicle to provide electrical power to onboard systems. An alternative conversion technology that is especially attractive at higher temperatures is thermionic energy conversion. Thermionic devices have proven to provide high efficiency conversion from an extreme temperature source above 1800K and a reservoir “low” temperature in excess of 1000K. A thermionic energy conversion device consists of two metal electrodes separated by a narrow gap, where the high temperature emitter thermionically emits electrons into the gap and the collector absorbs them. This proposed program should address the application of modular thermionic conversion devices to convert internal heat within a hypersonic vehicle to electricity. The program should consider recent developments in micro-manufacturing and materials to reduce the interelectrode gap distance within the converter device and potentially eliminate the need for cesium vapor while suppressing the space charge effect. Device design should account for the harsh environment that includes high temperature and exposure to an oxidizing atmosphere and/or liquid fuel. Device operating temperatures should be explored between 1800K-2200K (emitter) and 800K-1200K (collector) with the potential for the temperature profile changing with time. The lifetime requirements could vary from a single use to a reusable system with 1 hour of power generation per use. The thermionic device power output goal would be 1-10 W/cm^2, which could be modularized to produce 10-100 kW of electrical power over 1 m^2 of surface area within the hypersonic vehicle. The work functions of the emitter and collector surfaces must be relatively low to develop a functional potential difference across the gap and draw electrical current through a load. The electron current through the gap can create a negative space charge which self-limits the current, so the negative space charge must be suppressed through device engineering. Significant engineering efforts were conducted in the 1960’s in the USA and Soviet Union to integrate thermionic conversion devices to space nuclear reactor or solar concentrator platforms for long duration operation. Typically, these devices included cesium vapor within a gap of ~0.1 mm to suppress the space charge and reduce the surface work functions. Thermionic conversion technology was demonstrated as feasible, but further development was curtailed for programmatic reasons. 

PHASE I: Design a thermionic conversion module that could operate for 1 hour within the high temperature and oxidizing environment of a hypersonic vehicle with a power output density >1W/cm^2. 

PHASE II: Fabricate and test thermionic conversion modules in simulated hypersonic vehicle operating conditions measuring power output and lifetime characteristics with the goal of 1 hour of operation at >1W/cm^2 . Deliver prototype module to AFRL/RQQE. 

PHASE III: Dual use commercialization: Explore military use applications of power generation for reusable hypersonic vehicles. Potential commercial applications could include direct conversion of fuel heat for remote electrical power with higher energy density than batteries. 

REFERENCES: 

1. Rasor, N.S., "Thermionic Energy Converter". In Chang, Sheldon S. L. "Fundamentals Handbook of Electrical and Computer Engineering." II. New York: Wiley. p. 668. ISBN 0-471-86213-4. (1983).; 2. Mahefkey, T., “Thermionics Quo Vadis?”, Washington, DC, USA: National Academy Press (2001).

KEYWORDS: Thermionic, Hypersonic, Direct Energy Conversion, Work Function 

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