Thermionics Quo Vadis?: An Assessment of the DTRA's Advanced Thermionics Research and Development Program (2001)

The consideration of the potential applicability of thermionic conversion to non-space, or terrestrial, applications can presently be divided into two broad categories, namely, commercial power production and special-purpose military applications. However, the attributes that make thermionics attractive for space power systems are not as compelling in terrestrial applications. Cost and long-life reliability considerations for terrestrial applications generally dominate decision making for terrestrial applications, whereas for space applications, mass, compactness, and efficiency tend to be the ruling criteria.

In the early 1960s, the American Gas Association and the U.S. Army started funding programs to develop fossil fuel powered thermionic converters. The material silicon carbide emerged as the preferred coating for emitters to protect them from air and combustion products. Because emitter materials, such as tungsten, had thermal expansion coefficients different from that of silicon carbide, cracking and separation problems were severe. These problems were not overcome until the 1980s. By that time, there was no funding available to demonstrate the practicality of fossil fuel powered thermionic devices.

Due to the Arab oil embargo and the increasing awareness of environmental issues related to energy production, terrestrial thermionic research and development received modest funding during the 1970s and 1980s. Since fossil fuel combustion temperatures can be much higher than allowable steam turbine operating temperatures, thermionic devices have the potential to increase overall power plant efficiency when used as a high temperature topping cycle. That is, the elevated combustion temperature of burning coal or other fossil fuels could heat the thermionic converters. The heat rejected by the thermionic devices could then be used to power the steam turbines. However, most of the power generating systems being constructed and ordered today are gas turbine systems, some using a combined cycle steam generation bottoming cycle with high efficiency. In this case, the exhaust of the gas turbine is used to generate steam. The potential benefits of thermionic topping cycles include reductions in waste heat, pollutants, and plant cooling water requirements. However, thermionic topping cycles have not been commercialized due to high capital and operating costs as well as the expense of developing initial prototypes. Some small market thermionic applications were explored in Europe, where fuel generally costs more than in the United States, but those efforts were abandoned because of unfavorable cost tradeoffs.

Finding: The committee could not identify any financially viable terrestrial applications that could make use of thermionic power conversion.

The military has a continuing need for improved transportable field power sources for remote sites. These systems have requirements ranging from a few watts to several hundred kilowatts in the case of futuristic airborne energy weapons. For lower power requirements, advanced battery concepts are being pursued. For mid and high power requirements, alterna-

tives to diesel-generator systems are under development. Thermionics can be used with combustion heat sources, but thermionic systems are less efficient than turbines or fuel cells.

Cost-effectiveness is also a significant issue. None of the military organizations responsible for the development of terrestrial military power sources are currently examining thermionics, presumably for the same reasons that the technology is not being pursued for commercial purposes, namely, the availability of other approaches that cost less and that are more efficient.

Finding: Thermionic technology is not being pursued for special-purpose military applications primarily because of the technology’s high cost and low efficiency.