Technical article: Induction heating reduces power cost

Advanced Battery Technology, Nov 2000 by Fireman, Jerry

Induction heating has shown the ability to reduce the cost of producing an innovative new power source. The Alkali Metal Thermoelectric Converter (AMTEC) is a silent, efficient, compact way to create electricity from any 700 deg C to 900 deg C heat source, including electric, gas, liquid, solar and a variety of others.

One of the main obstacles to wider use of this new power source is the high labor and energy cost involved in sintering Beta"-Alumina, the sodium ion-conducting solid electrolyte that is a critical component of the new generators. A pilot program at Northwestern University, Evanston, Illinois, has demonstrated that an induction-coupled plasma, using a radio frequency generator produced by Lepel Corporation, Edgewood, New York, can dramatically reduce these costs by speeding up the sintering process and eliminating the tedious manual process of encapsulating the tubes.

"Our studies show that this new induction-heating-based process can reduce the cost of making the tubes from tens of dollars now to an estimated 12 cents each," said Dr. Lynn Johnson, Walter P. Murphy Professor of Materials Science and Engineering at Northwestern.

Reliability, efficiency, portability and quiet operation are qualities that make AMTEC a very attractive technology for spacecraft power, auxiliary and remote power, portable power, and self-powered appliances, to name but a few potential applications. Based solely on an electro-chemical process and a closed cell design, AMTEC has no moving parts and therefore no noise or vibration, high reliability, and virtually maintenance-free operation. The cells are also very small and light relative to their power output. For example, a cell that produces 4 to 6 watts of power is currently 4 inches long by 1.5 inches across and weighs about 4 ounces. The cells are also modular, allowing for more powerful systems to be made simply by connecting smaller cells together.

Advanced Modular Power Systems (AMPS), Ann Arbor, Michigan, was formed to commercialize this technology after its discovery in a Ford Motor Company research laboratory. AMPS has received a contract to design and manufacture the AMTEC power systems for NASA's next generation of interplanetary robotic spacecraft.

Unique Ceramic Material

At the heart of the new power cells lies a ceramic material, Beta"-Alumina Solid Electrolyte (BASE), which has some unique properties. The most notable is its excellent conduction of sodium ions, but poor conduction of electrons. The BASE ceramic is in the form of individual tubes inside the AMTEC cell. Electrodes and current collectors are placed on the inside and outside surfaces of the BASE tubes. Liquid sodium, the working fluid in the cell, is heated from one end of the tube and turns into sodium vapor.

The heating of the sodium inside the BASE tube creates a greater pressure inside than outside the tube. This pressure differential defines a potential energy. The expansion of sodium to realize that energy can only occur through the BASE material when the sodium is in the ion form. Sodium atoms are stripped of an electron, becoming sodium ions that can then easily pass through the tubes to allow the pressure differential to be converted into work. The flow of electrons produces electrical power. When a load is placed on the system, it draws current produced by stripped sodium electrons that flow from the inner electrode, through the load, to the outer electrode. The electrons are finally reunited with the sodium ions on the outer surface of the BASE tubes. The sodium atoms are then condensed into liquid form, wicked back to the hot end of the cell where they evaporate, and the sodium begins to flow through the cell all over again.

Portable Power Systems

While the AMTEC has been validated for use in relatively high-cost, low-volume applications such as the NASA probes, the goal of everyone involved is to reduce its cost to the level where it can be used as a cost-effective terrestrial power system. Potential applications include use at remote sites, cogeneration units, self-powered furnaces, rechargeable batteries, and auxiliary power supplies for cross-country trucks, among others. AMTEC generators could be used to build self-powered furnaces that would save an estimated $50 per year in electricity cost and continue running through power outages. Approximately 5 million residential furnaces are sold annually and, if the cost of AMTEC systems can be brought low enough, a significant fraction of these furnaces can become candidates for energy-saving AMTEC installations.

Combustion-heated AMTEC units could also provide 2.5kW power supplies for recreational vehicles. In longhaul trucks diesel-fueled units could be used to keep engines warm while providing cab air-conditioning and electrical power during stand-down periods. The Department of Energy estimates that this application could save $2,400 per truck annually compared to the cost of idling the vehicle's engine when the truck is not traveling. Generators based on this technology could also provide much lighter portable power systems for the military than existing alternatives.