Development of Micromachine Gas Turbines at Tohoku University

Development of micromachine gas turbines is underway at a group lead by Tohoku University in Japan. The goal is to develop a light-weight power source to replace the heavy batteries that may limit the progress of humanoid robots. Internal combustion engines have advantages of both high power density and high energy density, so it can be the best choice for the power source for mobile machines. The project is currently challenging to develop the smallest possible gas turbine of three-dimensional geometry to be fabricated by machining. The micro-combustor has already been completed the validation test, and experimental validation of the turbo-unit and the generator of the diameter 10mm is underway. Current major technical challenges are the development of the bearings to sustain the high-speed rotation of the rotors and the technologies to fabricate the bearings within the required tolerance. 1.0 INTRODUCTION 1.1 New Market for Portable Power Generation One of the technologies that is rapidly progressing to the level of practical use these days, is a humanoid robot. There are number of projects all over the world, from national team level to private company level, to develop variety of humanoid robots. Especially after Honda’s humanoid robot Asimo (Figure 1-1) debuted to the world, people realized the vast capability of humanoid robots, and research and development of robot has now become a boom. Isomura, K.; Tanaka, S.; Togo, S.-I.; Kanebako, H. (2005) Development of Micromachine Gas Turbines at Tohoku University. In Micro Gas Turbines (pp. 10-1 – 10-34). Educational Notes RTO-EN-AVT-131, Paper 10. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. TO-EN-AVT-131 10 1 Development of Micromachine Gas Turbines at Tohoku University 10 2 RTO-EN-AVT-131 Figure 1-1: Asimo of Honda. (from Asimo home page; http://www.honda.co.jp/ASIMO/) When many people start the research and the technology level increases, the system becomes more and more complicated and sophisticated, and hence, variety of applications and concepts appear. Toyota has recently revealed several types of their new robots that have different function from Asimo. They have a capability of playing trumpet with their controllable soft lip with a breath (Figure 1-2). National Institute of Advanced Industrial Science and Technology (AIST) and The University of Tokyo jointly succeeded in let HRP-2 robot to dance the traditional Japanese folk dance “Aizu Bandaisan” by simulating the dance master’s movement (Figure 1-3). This technology requires collateral movement of many servomotors at various joints at the same time. Hence, the power requirements for the advanced robots are increasing. Figure 1-2: Toyota’s Robots to Play Trumpet. (from Toyota’s home page; http://www.toyota.co.jp/jp/special/robot/index.html) Development of Micromachine Gas Turbines at Tohoku University RTO-EN-AVT-131 10 3 Figure 1-3: HRP-2 of AIST Dancing “Aizu Bandaisan” with a Dance Master. (http://www.aist.go.jp/aist_j/press_release/pr2005/pr20050112/pr20050112.html) Other than the autonomous humanoid robots, powered mechanisms to aid human motion are also under development. Figure 1-4 shows the DARPA’s concept sketch of future armour called exoskeleton that increases the loading and running capability of soldiers. Prototype of such machine is developed at U.C. Berkeley is called Berkeley Lower Extremity Exoskeleton (BLEEX), and is shown in Figure 1-5. The backpack of the man wearing the BLEEX is explained to be capable of carrying 120 pounds of loads, but it is easy to imagine that the power source to drive such a device long enough for the practical mission may weigh about the whole loading capacity. Figure 1-6 shows the people mover that Toyota is currently developing. It carries the whole weight of a man, so the power requirement would be even larger. Figure 1-4: DARPA’s Conceptual Sketch of Exoskeleton. (http://www.darpa.mil/dso) Development of Micromachine Gas Turbines at Tohoku University 10 4 RTO-EN-AVT-131 Figure 1-5: U.C. Berkeley’s BLEEX. (http://www.berkeley.edu/news/media/releases/2004/03/03_exo.shtml) Figure 1-6: Toyota’s People Mover. (http://www.toyota.co.jp/jp/special/robot/index.html) All of these machines are using batteries such as Nickel-Metal Hydride batteries and Lithium-ion batteries, for the power sources. Hence, the weight of the power source is very heavy, and the operational time is very limited. Asimo, which requires only 380W, can operate only 30 minutes after half a day of Development of Micromachine Gas Turbines at Tohoku University RTO-EN-AVT-131 10 5 recharging. The more complex robots’ operating time is even shorter. The reason for those robots using only batteries is simply that the number one technical issues to realize robots have been sophisticated control of the movement synchronized with related sensing and image processing. Almost all the researchers developing robots are in such fields, which is very different from the field of power generation, so they might have stuck to the existing reliable power source to reduce the technical challenges. It has already been very challenging to develop the autonomous walking robots. When the control technology is matured and the development comes to the phase of practical system in near future, they will certainly start looking for a better power source. One of such movement can be seen as the Palm Power initiative by DARPA that started in year 2001. The Palm Power initiative is a technology development program that will advance the technology as far as possible by demonstrating new approaches that will ultimately lead to complete system demonstrations. The requirement of the target systems shown in the Palm Power initiative is shown in Figure 1-17. The figure shows that the required energy densities are far beyond those available by batteries, and therefore a technology development program has been started. In the same year a study on micro-power sources has been conducted [1]. Po w er D en si ty , W /k g Energy Density, Wh/kg Figure 1-7: Variety of Mobile Power Requirements shown in DARPA Palm Power Project. (http://www.darpa.mil/dso/thrust/matdev/palmpower) Many people think fuel cells are the most promising power sources in future. However, not only that the development of fuel cell will take more time, but also, fuel cell is very heavy, and therefore, it is not a power source suitable for mobile machine as robots. The power character of various power sources is shown in Figure 1-8. The power density is the power ( watt = voltage x current ) divided by system weight, and the energy density ( watt hour = watt x operation time before recharging ) divided by system weight. Fuel cells have large energy density, but have low power density, while the character required for power sources for mobile machines is large power density. Note that the applications shown in Figure 1-7 require the power density higher than 200W/kg, while that of fuel cells shown in Figure 1-8 is an order smaller 20W/kg, at most. The efficiency of the fuel cell drops when the current density in the cell stack becomes large. This means that fuel cells need to be large to reduce the current density, or the application should not require large current, but require large voltage, to achieve high efficiency. Hence, fuel cells are good for driving IC based electronic equipments, but not good at driving electric motors in mobile machines that the weight of the system will penalize the performance. Development of Micromachine Gas Turbines at Tohoku University 10 6 RTO-EN-AVT-131 From Figure 1-8, the power sources capable of providing large power density are internal combustion engines and batteries. Since batteries have very low energy density, they cannot provide long operation time. Therefore, the best power source to solve the short duration time problem of batteries will be the internal combustion engines. Note that Hydrocarbon fuel commonly used for internal combustion engines, such as JP-8, contains 13,200 Wh of energy per kg. Hence power generations from hydrocarbon fuel have capability of further increasing the energy density to an order larger value by technology innovations and by combination and integration of multiple of generation methods into a system. 1 10 10