Robert K. Sievers

Modeling of Remote Condensing AMTEC Cells

The Alkali Metal Thermal to Electric Converter (AMTEC) is a thermally regenerated sodium concentration cell that converts heat directly into electricity without moving parts. The high efficiency of AMTEC cells is useful for power generation in space and terrestrial applications (Ivanenok et al. 1993a, 1993b). One of the advanced features proposed in current high efficiency AMTEC cell designs is remote condensing. Remote condensing occurs when the condensing surface of the cell is thermally isolated from the high temperature β“ ‐alumina solid electrolyte (BASE) tube. The parasitic heat losses are significantly reduced, thereby improving the cell efficiency. However, this configuration also increases the local Na vapor pressure (sodium concentration) on the cathode side of the BASE tube, and thus lowers the BASE tubes power output. The balance of these opposing effects is very important in optimizing system designs. This paper derives the equations necessary to calculate the vapor flow pressure drop, and c...

Small capillary pumped AMTEC systems

Alkali Metal Thermoelectric Converter (AMTEC) systems offer significant potential advantages for space power. Recent experiments have shown that electromagnetic pumps can operate with a negative priming head and so may be suitable for space applications in microgravity (Hunt et al. 1992). Capillary pumped cells offer an alternative approach to microgravity compatibility. We have designed, built, and operated capillary pumped AMTEC cells in various orientations with respect to gravity in order to provide a presumptive demonstration of zero‐G capability (Sievers et al. 1992). We report lifetime and performance data for these capillary pumped AMTEC cells. Progress on other issues relating to space flight testing of AMTEC systems is also discussed.

AMTEC/SHE for space nuclear power applications

The Alkali Metal Thermoelectric Converter (AMTEC) is a high efficiency device for the direct conversion of heat to electricity. Applications for this converter range from space power to remote terrestrial missions. Significant progress has been made on both component and converter cell development, with particular interest focused here on high efficiency and zero‐G operation. Experiments were performed that demonstrate that electromagnetic pumping will operate without a gravity induced head, allowing pumped cell design to operate in zero‐G. A cell has also been developed that will operate in zero‐G without an EM pump. Wick return cells, without pumps, were also tested and found to be feasible for zero‐G operation. Design calculations, using test validated models, indicate that these cells can have efficiencies greater than 25%. These results will strongly influence state of the art and advanced design development.

AMTEC Module Test Program

The Alkali Metal Thermal to Electric Converter (AMTEC) is a thermally regenerated sodium concentration cell that statically converts heat directly into electricity. The high efficiency of AMTEC will be useful for power generation in space and terrestrial applications. A series of 6 geometrically similar cells with identical wick structures has been fabricated by Advanced Modular Power Systems (AMPS). Three electrode/current collector designs have been included in the testing matrix. Two cells of each design were built. One cell in each set had a thermocouple installed at the electrode/current collector interface to measure the temperature of the beta‐alumina solid electrolyte (BASE) tube. This thermocouple can also be used to measure the voltage at the cathode since its sheath is electrically isolated from ground. The BASE tubes used to build the cells were all from the same production lot. This testing program represents a unique opportunity to comparatively evaluate the performance of geometrically iden...

Small AMTEC systems as battery substitutes

Alkali Metal Thermal to Electric Converter (AMTEC) technology is highly scalable and converters can be designed to provide fuel based electric power at levels ranging from a few watts to several kilowatts. AMTEC is a static, modular, heat to electricity conversion technology with the potential for high efficiency and compact size using heat source temperatures readily achievable even with small combustors. AMTEC systems are currently under development for a variety of spacecraft and satellite applications related to potential NASA and Air Force missions requiring power levels in the 75 watt to multi-kilowatt power range. While AMTEC converters can operate with any heat source delivering heat at 900 K to 1200 K, the small AMTEC systems considered here, are suitable for system integration with small, combustion heat sources. In this paper we describe concept designs for small combustion fired, self-contained AMTEC systems whose size, operating duration and mass make them superior choices for applications no...

AMTEC power systems for remote site applications Conference on alternative powere from space; Conference

The Department of Defense, US Forest Service and the University of Alaska operate more than 200 remote sites in the Arctic. Most of these sites are now operated on fuel‐burning thermoelectric converters with an efficiency of less than 4%. The cost of supplying even moderate electric power requirements to remote environmental, treaty monitoring and communication sites is strongly dependent on the fuel delivery requirements. In the Arctic where solar input is frequently unreliable, the problem is particularly severe. On average, these sites need only about 60 watts of electrical power but will burn over 2,200 kg of propane per year for continuous operation. At these power levels, Alkali Metal Thermal to Electric Converter (AMTEC) systems with their projected 20% to 25% thermal to electric conversion efficiency can provide power for these remote sites with potential annual logistics cost savings (primarily in reduced fuel supply costs) reaching tens of millions of dollars.

Enhancement of AMTEC electrodes and current collectors

An improved electrode deposition technique has been developed for a Alkali Metal Thermal to Electric Converter (AMTEC). The innovative Sodium Modulated Electrode (SME) deposition technique has been developed which selectively deposits the electrode on inactive Na sites and adjacent to active Na sites on the electrolyte surface. This program has demonstrated SME processing feasibility and achieved electrode performance enhancement. Power density was improved by 51 to 56% at 973 K and 19 to 26% at 1073 K at the start of electrode testing. Na+ has been conducted through the beta’’‐alumina solid electrolyte (BASE) during the deposition process. Electrode deposition has been a random process, covering both active and inactive sites on the BASE. This random process did not optimize electrode placement or provide pore openings at the Na active sites to permit Na+ easy access to electrons and a low resistance path for Na atoms to move to the condenser. Both Mo and TiN electrodes were evaluated. It has been demons...

Preliminary evaluation of a space AMTEC power conversion system

An original evaluation of a space solar energy source coupled with Alkali Metal Thermoelectric Conversion (AMTEC) is presented here. This study indicates that an AMTEC system would have 30 percent of the mass of a photovoltaic system and 70 percent of the mass of a Stirling cycle system at the 35‐kWe level of power generation modules typical of the baseline for the U.S. Space Station. The operating temperatures and Sodium heat pipe components for solar receiver/TES hardware (currently being developed by NASA) integrate well with AMTEC power conversion. AMTEC is therefore an attractive alternative specifically for space solar power generation.