Ph.D. Thesis Defense by Paul E. Hausgen
Thursday, November 16, 2000

(Dr. James G. Hartley , advisor)

"A Thermal Analysis of an Alkali Metal Thermal to Electric Converter with Geometrically Designed Interior Surfaces Exhibiting Directionally Dependent Radiative Properties"

Abstract

The Alkali Metal Thermal to Electric Converter (AMTEC), conceived in 1962,  is a static, direct thermal to electric energy conversion technology.  As with any energy conversion technology, the thermal efficiency of the AMTEC is of great importance.  Various studies have concluded that the AMTEC could potentially achieve a very high thermal efficiency.  To obtain high thermal efficiency, parasitic heat losses within the AMTEC must be reduced.  The parasitic heat losses involve heat transfer from the high temperature region of the converter to the low temperature region of the converter that is not directly involved in the energy conversion process.  Any attempt to reduce the parasitic heat losses of the AMTEC must involve the effective control of the radiation heat transfer within the AMTEC.  The goal of this investigation was to identify geometrically designed surfaces (grooves) that could be used in the interior of the AMTEC enclosure to control the thermal radiation exchange in a manner that increased power output and efficiency by reducing parasitic heat losses.  Asymmetric wall grooves were investigated as possible geometrically designed surfaces that could meet this goal.

The investigation of the asymmetric wall grooves’ ability to increase AMTEC performance included both theoretical and experimental efforts.  The theoretical analysis was divided into four thrusts:  (1) effective groove radiation properties, (2) re-radiating cylindrical wall model, (3) experimental device model, and (4) operational AMTEC device model.  The experimental analysis involved fabricating and testing two non-operational prototypical AMTEC cylindrical devices, which included a device with a flat wall and a device with an asymmetrically grooved wall.  A comparison of the experimental results from the two devices demonstrated the ability of the asymmetrically grooved wall to reduce axial heat transfer, which is necessary to increase AMTEC efficiency.

Both the theoretical and experimental efforts yielded results that support the conclusion that asymmetric wall grooves, with proper opening angles and surface specularity, can significantly affect the interior thermal radiation exchange in a cylindrical AMTEC device in a manner that increases both power output and efficiency compared to a flat wall design.  Theoretical modeling of an operational AMTEC cylindrical device showed that the asymmetrically grooved wall could increase (compared to a flat wall design) the conversion efficiency by 2.5 percentage points (~15.9% increase) under certain conditions by reducing unwanted axial heat transfer (parasitic heat transfer) and by increasing BASE tube temperature.  Results from the experimental investigation showed that the asymmetrically grooved wall reduced the axial heat transfer rate in a non-operational prototypical AMTEC cylindrical device by approximately 48.6% for a heat input temperature of 1100 K and a heat rejection temperature of 600 K.