JX Crystals

Design and thermal modeling of a portable fuel fired cylindrical TPV Battery Replacement

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Courtesy of JX Crystals

ABSTRACT: A novel portable fliel fired cylindrical themiophotovoltaic batteiy charger is described. It uses an array of GaSb TPV cells along with a novel Omega recuperator design and a novel IR emitter design. Computational fluid dynamic (CFD) calculations are presented for this TPV cylinder design showing the potential for an overall fuel to electricity conversion efficiency of 10.9%. The estimated weight of the TPV cylinder is 200 em and its volume is 900 cc. Fuel volume is arbitrary, but a comparable 900 cc of fiiel will weigh 540 gm. The specific energy in a hydrocarbon fuel is 12.900 Wli/kg. resulting in 6970 W-hr of energy in the 900 cc tank. The weight of the TPV cylinder and the fuel cylinder combined is thus 740 gm. Given a TPV conversion efficiency of 10%. the converted energy available from the fliel will be 697 W-hr. The specific energy for this TPV system will then be 697 W-hr/0.74 kg = 942 W-hr/kg. A lithium ion rechargeable batteiy weighing 1.1 kg has a specific energy of 145 W-hr/kg. .. The TPV power system described here is lighter, has 6.5 times higher specific energy, operates 7 times longer, and is easily refueled. Keywords: GaSb. Thermophotovoltaics. Batteries.

The first high efficiency low bandgap infrared (IR) sensitive photovoltaic cell the GaSb cell, was invented and demonstrated by Fraas and Avery in 1989 [1.2.3]. This cell responds out to wavelengths of 1.8 microns and enables the use of man made hydrocarbon fuel sourced IR radiators operating at temperatures up to 1700 C for the fabrication of themiophotovoltaic (TPV) DC electric generators. The invention of this cell inspired a renewed surge in TPV system development starting in 1990.

The work on TPV at JX Crystals (JXC) from 1990 up to 2007 is summarized in the TPV-7 conference proceedings [4]. The development of a complete TPV system has required more than just the IR cell. An important additional area of development has included spectral control [5]. It is important to tailor the IR spectrum from the radiant heat source such that the photon wavelengths arriving at the cell fall within the convertible wavelength band for the cell. Longer wavelength radiation falling outside the cell conversion band will simply heat the cell. For this purpose. JXC has developed and patented Cobalt and Nickel doped ceramic matched IR emitters [6.7].

The TPV work at JXC up until recently has focused primarily on hydrocarbon fired combined heat and power systems for the home. Two such examples are the TPV Midnight Sun Stove [8] and the integration of TPV into a standard heating furnace [9].

This paper and a companion paper describe a more immediate TPV application: a portable fuel fired cylindrical TPV battery charger or battery substitute. It uses an array of GaSb TPV cells along with a spectrally matched IR emitter and a novel Omega recuperator. Currently, the military carries heavy batteries. The portable cylindrical TPV unit described here is potentially lighter, runs longer, and is more easily recharged (refueled) when compared with a battery. This paper describes the overall TPV cylinder design along with the novel recuperator design and modeled thermal performance. The companion paper [10] will focus on the matched emitter design and the spectral control topic.

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