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Engineers from all over the world came to the $4^{\rm th}$ International Energy Conversion Engineering Conference (IECEC) to describe their new aerospace technologies and methods for reducing consumption of costly petroleum products. The American Institute of Aeronautics and Astronautics (AIAA) sponsored this conference, held in San Diego, California, June 26–29, 2006, and IEEE-AESS was one of the three organizations that participated in this IECEC. The General Chair was Robert J. Pinkerton from General Dynamics. Topics ranged from extracting energy from sewage and garbage, to generating electric power in spacecraft that travel beyond the borders of our solar system planets. Pertinent to Systems magazine readers were new developments like nanotechnology for efficient generation of power in aerospace vehicles.

The Stirling engine, invented in 1816 by Reverend Robert Stirling, has now become a key power source in aerospace applications where solar power is not available. These engines were used in the $19^{\rm th}$ century to propel ships; but simpler steam engines were subsequently adopted for propelling the huge $20^{\rm th}$ century ocean liners and battleships. More convenient fuel-oil burning boilers then replaced the coal-burning boilers that supplied steam for the early ships. Diesel engines are now common sources of propulsion as well as generating auxiliary power in ships. The latest Stirling engines convert heat to electricity with an efficiency of 50%. In contrast, thermoelectric converters and solar cells generate electric power with efficiencies that are typically less than 30%. A radioisotope-powered Stirling engine was designed for powering a spacecraft that will explore the outer regions of the solar system. It converts heat to electric power with a 40% efficiency. Long life is achieved by having no moving parts to rub on the cylinder walls. In sight is an engine that has an efficiency higher than the 63% efficiency of the best combined-cycle electric-utility plant.

Typical automobile engines convert gasoline energy into propulsion power with less than 25% efficiency. The t-Zero electric car, which was propelled with energy stored in lithium batteries, consumed the energy equal to that in one gallon of gasoline, in traveling 150 miles at a speed of 60 miles per hour.

Once General Motors manufactured 100 battery-powered sedans and leased them to Californians who were very pleased with their performance. Then for political reasons, General Motors cancelled the leases and smashed the cars in the desert, leaving the car renters unhappy. Now a manufacturer in China is preparing to export to California their new lithium-battery powered car that carries a cable for plugging into any available electric outlet for recharging the battery at stopovers during a long trip. In the United States, the development of efficient power for traveling is not encouraged as effectively as in many other nations.

Nanotechnology is a new aerospace-related field being advanced by over 4000 scientists and engineers all over the world. It deals with molecules, atoms, and even the neutrino, which has a diameter of $10^{-6}$ times a hydrogen atom's diameter. Stars in deep space produce neutrinos that travel all the way through the Earth with only trivial losses. Russian scientists have found nanotechnology-based methods for reducing the losses of current flowing through power transmission lines. The nanotech based 6-watt light-emitting diode, which delivers the light output of a 60-watt filament lamp, is already being manufactured in China for nighttime study lamps in the remote regions of China, Africa, and India. These lamps run on batteries that are recharged with sunlight during the daytime.

Generating nuclear power for a lunar colony of scientists was an important topic of presentations at this IECEC. Solar power is not practical for occupied structures on the Moon where night can last for half a month, so nuclear-reactor power sources have been evaluated. New power sources for manned spacecraft that travel to other planets in the solar system were evaluated. Authors from Japan described successes in extracting energy from garbage and even gases that are emitted by garbage dumps and sewage treatment plants. Hydrogen for fuel-cell electric-power generation can now be extracted from wood and other plant structures.

These developments are recorded in the Proceedings of the $4^{\rm th}$ IECEC that are available from the AIAA [1]. Summarized in the text that follows are the reported developments that pertain to our AESS engineering activity, as well as those that can solve the coming world's energy crisis. We also describe aerospace-pertinent developments in the fast developing field of nanotechnology.

SECTION 1

## STIRLING ENGINE DEVELOPMENT FOR GENERATING POWER ON THE MOON AND IN SPACECRAFT

The Carnot law limits the efficiency of a heat engine to the difference between its input temperature and exhaust temperature, divided by absolute temperature of its input. A gasoline burning automobile engine has an exhaust temperature that is much higher than the temperature of its muffler's output. An automobile engine's efficiency is typically around 20%. A combined-cycle steam power plant's exhaust temperature is the boiling point of water in its condenser, so efficiencies as high as 63% are achieved. This plant contains a gas turbine that delivers power, plus high-temperature exhaust that is used to boil water for a power-producing steam turbine. The motor in an electrically propelled car can have an efficiency of over 94%, so electric cars have delivered over 150 miles of travel with the electric-energy equivalent in one gallon of gasoline.

The Stirling cycle consists of an isothermal compression, a constant-volume heating, an isothermal expansion, and constant volume cooling. The basic Stirling engine has a hot space heater, regenerator, and a cold space, as shown in Figure 1. The gas-flow passages have no valves. The lower piston moves upward to compress the gas, which gets warmer in flowing through the heated recuperator. Further heat is added at the top of the power-producing piston, which is then pushed downward. The lower piston also moves downward. Then the top piston moves upward, pushing the warm gas through the regenerator and cooler. Then the lower piston moves upward, compressing the cooled gas to restart the power-producing cycle. Efficiencies of over 50% have been achieved when delivering maximum rated power output. A Stirling cycle engine that was built in 1958 had too many bearings for space use, as shown in Figure 2.

Fig. 1. Pistons on separate crankshafts compress cold air and extract energy from heated air in early Stirling engines
Fig. 2. Practical Stirling engines were so complicated they could not compete with steam engines in the 19th century when fuel was cheap

Application of Sterling engines to generation of power for spacecraft and planet stations required a thorough analysis for detecting possible deficiencies in their design. This needmotivated the NASA Glenn Research Center to create a program that elevates the Stirling industry's suite of design tools beyond the currently employed one-dimensional analysis level. R. Dysondescribed a virtual three-dimension full-fidelity converter, including structural, mechanical, electromagnetic, and thermal dynamics [2]. They incorporated the complete three-dimension geometry of modern Stirling converters, and coupled this with multi-physical capability to examine the effects of magnetic, pressure, gravity, spring, and inertial forces on startup behavior, plus material creep. Their output is overall system performance/efficiency/reliability. This approach had to be accurate and fast enough to be practical in use, and was challenging because of the large number of geometric details of the Stirling engine, and significant computer requirements.

This approach was applied to two leading Stirling converters. One was the Infinia Corp's Technology Demonstration Converter, and the other was Sunpower's Advanced Stirling Converter. After the entire converter is successfully modeled, design changes can be made quickly and the overall design can be completed. This level of optimization allows for mass-minimization geometry modification, and reduction of non-uniformities that are not possible with current one-dimension analysis tools. Their paper delivered the authors' most recent results from the first fully 3-dimension Stirling engine analysis-capability in the United States. It provided an approach for gas spring and flexure modeling that is required for modern whole-engine analysis. Preliminary one-dimensional Ultra Hi-Fi results showed the potential speedup and improvement of whole converter experiments that would be realized when the latest numerical techniques are utilized.

A unique component in a Stirling engine is its regenerator that captures and stores engine exhaust heat, and delivers it to the incoming gas before the gas is recompressed. For example, a radioisotope-heated Stirling engine for a deep space mission has an alternator in which permanent magnets slide back and forth inside the ac armature winding. This field magnet assembly is pushed and pulled by the Stirling engine's piston, to which it is solidly coupled (Figure 3). This moving assembly is supported by magnetic bearings, so there is no metal-to-metal contact, and no bearings that would require lubrication to avoid wear-out during multi-year space missions.

Fig. 3. Pistons in a spacecrtaft Stirling engine are positioned and moved by electromagnetic fields and so supported that there are no rubbing metal-to-metal touching surfaces that would wear out. Long operating lifetimes are predicted.

The porous-media screen in the Stirling engine regenerator is a brand-new product. Its long-life in frequent heating-and-cooling has not yet been confirmed. To generate 60 Hz power, the piston would have to move back and forth 60 times a second, and the regenerator's screen would go through a heat-and-cool cycle 60 times per second. R. Tew created a model for simulating the operation of this new regenerator component [3]. The design of the regenerator, which needs to last for many years of continuous operation on a spacecraft, becomes a critical component of the Stirling engine. S. Oriti, from NASA Glenn Research Center, described modifications and supporting hardware that were required to make a Stirling engine run for 9000 hours in a thermal-vacuum that simulated the space environment [4]. The engine being tested resembled an engine that would be used in space, but did not have the required low mass. Data that confirms a 5-year operating lifetime of a Stirling engine for space use will be required.

A Stirling radioisotope generator for powering Space Science missions is being developed by a team consisting of engineers from the US Department of Energy, Lockheed Martin, Infinia Corporation, and NASA Glenn Research Center. This Stirling cycle energy converter achieves higher efficiency than can be achieved by currently used alternatives. S. Oriti described a test at the Glen Research Center that demonstrated the functionality of the Sterling converter in a thermal vacuum environment. The tested converter did not yet have the low mass required for the application. Modifications to the supporting hardware are required to attain the desired operating parameters throughout the 9000 hours of operation. This team also studied the expected launch environment to which a Stirling radioisotope generator will be exposed in coming space science missions. The results described by D. Hill showed that this generator would be exposed to a random input spectrum that is significantly higher than that in historical power-system levels, and this will be a challenge for designers [5]. Analysis of prior work predicted that tailoring the compliance at the generator-spacecraft interface reduced the dynamic response of the system, thereby allowing higher launch load input levels and expanding the range of potential generator missions. The authors described the dynamic simulator design, the test setup and methodology, test article modes and frequencies, and dynamic responses, followed by post-test results. Post-test analysis included finite-element model tuning to match test frequencies, and random response analysis using the test input spectrum. Analytical results agreed with test results. At a typical power-output level, the assembly had an overall efficiency of 21%. The team confirmed that the Stirling SRG110 power system might be considered for a broad range of potential missions, including those with demanding launch environments.

SECTION 2

## POWER SOURCES FOR LUNAR COLONY AND EXPLORERS

The NASA lunar research program described by officials at the $3^{\rm rd}$ IECEC includes surface explorations by lunar pioneers. H. Brandhorst came from the Space Research Institute at Auburn University to describe the power requirements of lunar expeditions [6]. The NASA Vision for Exploration proposes sending astronauts to the Moon, starting with 4-Earth-day stays, and growing to stays that include 12 days of lunar nighttime. Expeditions will also search for water at the Moon's poles. As missions progress the power levels are expected to grow from a few kilowatts to 25 kW. Solar arrays will need to have high efficiency, light weight, high packaging density, and the ability to withstand broad temperature swings on the Moon. In craters, the temperature could be as low as 50° K. Brandhorst expected that, by the time the lunar expeditions start, a stretched-lens solar array on a square-rigger platform would be available. A specific solar-array power density of 300 W/kg and 300 W per square meter can be achieved, and operating voltages up to 600 volts will be practical. An 80 kW array could be shipped in a cubic-meter package. A robotic station in the bottom of a crater could be supplied power by an array of gallium arsenide solar cells to which a laser beam is directed.

A lunar colony is expected to grow into a crew of up to 50 persons engaged in mining and manufacturing. A. Juhaz proposed development of a second-generation gas-cooled fission reactor that supplies heated helium to two 5-MW closed-cycle gas turbines [7]. He developed a BRMAPS code that establishes system performance and mass details. In the future, additional plants could be added to form a power grid.

SECTION 3

## WEARABLE FREE-PISTON STIRLING ENGINE FOR SOLDIERS

A present-day foot soldier has to carry, in addition to Arms and ammunition, a variety of electronic and optic sensors, plus communication equipment that will enable him to survive in a modern battlefield. Batteries for powering this equipment become heavy, so J. Huth at Sunpower, Inc. described a Stirling-engine alternative being developed with Yale University and Precision Combustion, Inc. [8]. The 35-watt wearable free-piston Stirling power-source is based on the Sunpower EE-35 FPSE that was designed to use radioisotope heat to power NASA space probes during deep-space missions. Combustion of sprayed JP-8 fuel occurs on the surface of a special catalyst-mesh developed by Precision, Inc. The unit delivers 43 watts of electrical power with an overall efficiency of 21%. Its volume is 2.6 liters and its weight 1.7 kg, excluding its fuel tank. Its energy density is in the order of 1000 to 2000 watthours per kg. A compact integrated system was field tested in June 2006.

A key to efficient Stirling-Engine performance is the regenerator that recovers waste heat from the engine's exhaust and pre-heats the compressed intake air being delivered for final heating over the piston. F. Furutini came from Japan to describe the development and testing of a “mesh sheet” for collecting, storing, and releasing heat in the regenerator [9]. The testing was done in a 3-kW 2-piston type Stirling engine, NS03T. Engine performance was changed significantly with differences in the dimensions and shape of the mesh sheet. A mesh sheet based on flow dynamics has been designed.

SECTION 4

## ELECTRIC POWER FROM NON-PETROLEUM RESOURCES

In many nations, the cost of petroleum has risen to the point where effective development of alternatives are actively explored. For example, in China the energy generated at the 9-gigawatt power plant at Three Gorges Dam on the Yangtze River could propel every man, woman, and child in the nation a distance of 53 miles every day on electric bicycles. Distributing this power throughout China is not practical, so new nuclear power plants are being built. China's electric bicycle production is now over 4 million per year, and electric automobile production is growing fast.

In Japan, a modem chicken farm uses a lot of energy for lighting, ventilation, and refrigeration. The rising price of petroleum motivated the Tokyo Institute of Technology to develop a small-scale power generation system that consumes 100 kg/hour of chicken manure to generate 55 kW of electric power [10]. Their STAR-MEET system has an air-blown gasifier, a reformer that uses high-temperature gas and steam, gas-cleaning devices, and a Stirling engine with a generator. The generated gas has a low calorific value, fluctuates in heat value and flow rate, and includes a small amount of tar. This power system has attractive features of fuel flexibility, high electrical efficiency, and low emission. Tests showed a 28.3% efficiency.

SECTION 5

## NANOTECHNOLOGY ROUTES TO HIGH-EFFICIENCY CONVERSION OF FUEL TO POWER

Nanotechnology is a science that deals with nanoparticles that have dimensions in the order of one-ten-thousandth of the diameter of the nucleus of an atom. One millimeter equals one million nanometers. A human hair is 80,000 nanometers thick. Research in this field has already produced light-emitting-diode (LED) lamps that consume only 4 watts of electric power and deliver the same light output as a 60-watt incandescent lamp. Natural nanotechnology processes in muscles deliver mechanical power without the Carnot-cycle losses that our heat engines produce.

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