Effective Management of MVA-range electric Power in Aircraft enabled by high Tc superconducting systems

03/02/2015
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Effective Management of MVA-range electric Power in Aircraft  enabled by high Tc superconducting systems

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application/pdf Effective Management of MVA-range electric Power in Aircraft enabled by high Tc superconducting systems Sergey Samoilenkov, Vladislav Kalitka, Alexander Molodyk, Konstantin Kovalev, Dmitry Dezhin, Roman Ilyasov

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Effective Management of MVA-range electric Power in Aircraft enabled by high Tc superconducting systems Sergey Samoilenkov, Vladislav Kalitka, Alexander Molodyk (1), Konstantin Kovalev, Dmitry Dezhin, Roman Ilyasov (2) 1: SuperOx, Nauchnyi proezd, 20-2, 117246 Moscow, Russia, email: ssv@superox.ru 2: Moscow Aviation Insititute, email: klink@mail.ru Abstract High Tc superconductors (HTS) are new materials providing significant potential for building energy equipment with high specific power. This is extremely important for new generation of aircrafts with high fraction of use of electric power. We demonstrate that MW-level superconducting equipment for generation, transmission and utilizing of electricity on board significantly outperformes conventional analogues, saving more than 740 kg per each MW of energy. The advantages of HTS systems, such as motor, generator and cable, are considerably higher if these systems are implemented simultaneously, because one cooling system can be used for the total system. Introduction The demand to increase airplane fuel efficiency makes more-electrical airplane (MEA) almost inevitable. In June 2014, the technology of electrically powered airplane was placed by Thomson-Reuters among 10 potentially most important technological breakthroughs to be made until 2025 [1]. This trend is also reflected by Strategic Research and Innovation Agenda of ACARE [2]. The electric power of modern aircrafts already increased up to 1 MW. Typical values of power for fully electrical airplane would be few MW to few tens MW, depending on range. The operational voltage on board is low (i.e., 100s of Volts). It is difficult to increase voltage due to safety reasons and lack of appropriate contactor technology [3]. High voltage equipment is also bulky and heavy-weight. High Tc superconductors (HTS) are the most advanced materials for transport of electrical current. State-of-the-art materials such as second generation HTS wires provide the current density, which is 500 higher than that of copper or aluminium conductors. Even if one takes into consideration cooling (HTS operate at 77K or below) and insulation infrastructure, the HTS equipment is lighter, smaller and more efficient than conventional solutions. Here we present our view of perspectives of HTS systems for use inside of MEA in the mid-term (2020) future. Power cables HTS power cables are more compact and significantly lighter than conventional solutions. According to calculations based on current performance characteristics and mid-term (2020) perspective, the HTS infrastructure necessary to transmit 800 kW of power at AC 230 V via a distance of 50 m would weight 450 kg today and 170 kg after performance/weight optimization by 2020. The aluminum cable of the same power weights 650 kg, and the copper one weights 1700 kg. For the transmission of 10 MW of AC power at 230 V over the same distance, the weight reduction for the perspective mid-term HTS system is estimated to be 6 tons for aluminium and 19 tons for copper (i.e., 0.6- 1.9 tons/MW). It can be concluded that HTS wires provide essential performance benefits for MEA, being probably the only viable solution for high power electrical airplanes. Thomson-Reuters research [1] and SRIA [2] clearly refer to superconducting technology in this respect. It is important to notice that the HTS power cable technology is well-developed, that is demonstrated by a number of pilot projects already built for electrical grids worldwide [4-7]. Even though civil grid companies are extremely concervative and have very stringent requirements for safety, these developments took place at extremely quick pace (Fig.1). All HTS systems demonstrate transmission of power by a factor of 3-5 exceeding conventional systems. Fig. 1: AC HTS 3-phase cable lines built worldwide. The size of the marker is proportional to the cable length (AmpaCity = 1 km). The power cable for airplane is supposed to operate at 100-400 V and power of 1 MW. The prototype of airborne AC HTS power cable system can be built already today. As can be seen from Fig.1, that the typical power of the AC cable line operating at 0.1-0.4 kV would be just above 1 MW. In addition it has to be noted that HTS cable extremely well suits for DC power transmission due to lower losses. In this case, the system is expected to be lighter than AC HTS version by further 20-30%. HTS rotating machines Owing to unsurpassed current density, 2G HTS wires allow to make efficient high-torque and high-power rotating machines, both generators and motors. Some studies already demonstrated feasibility of MVA- range rotating machines with specific power of above 20 kW/kg [8]. Together with necessary cooling system, the specific power can estimated to be over 15 kW/kg. The typical value for state-of-the-art conventional rotating machines are 2-3 kW/kg with only one version (Launchpoint 5 kW motor) reaching outstanding 7.81 kW/kg value. It is difficult to say, if similar values can be attained by conventional approach for power exceeding 1 MVA. If we consider the airplane with 10 MW of installed electric power and take into account only motors, the weight reduction provided by HTS equipment would be in the range of 0.7-3 tons. For the hybrid type of aircraft, generator equipment would be needed additionally, providing additional weight reduction of similar amount. Thus, the total effect that can be achieved by implementation of the HTS rotating machines can exceed 0.14-0.6 tons per MW. Challenges: AC-losses in 2G HTS wires AC-losses are a serious challenge for AC HTS cables and rotating machines in electric airplane, since the airborne electrical grid often operates at high frequency (up to 850 Hz). Hence AC-losses are high. The losses impose load on cooling system, challenging as a result the total system weight. There are ways to reduce AC-losses in 2G HTS wires by a factor of 10 or more, which are mainly filamentization and transposition [9]. It is regarded that the level of AC-losses should be reduced to below 10 W/Am at 500Hz, that is equivalent to HTS filament with the width of 10 microns [8]. Additional work is needed, especially for constructing and testing of prototypes of lightweight HTS rotating machines. Cryogenic system The weight of the cooling equipment is an important factor determining the performance of the superconducting system. Today’s systems are not optimized in this respect, with the specific cooling power of about 2W@77K/kg. This value, being already in acceptable range, can be greatly reduced by using lightweight construction materials such as aluminium, titanium and composites. It is important to keep the operating temperature range as high as possible, since the specific cooling power of cryocoolers decrease dramatically with the temperature decrease. This provides essential advantage for high Tc superconductors with the operating range above 70K in comparison to MgB2 (20K) or low temperature superconductors (4K). If one employs on board simultaneously HTS power cable and rotating machines, significant amount of cryocooling power can be saved. Fig. 2: Efficiency of available cooling systems vs. temperature. Conclusions The use of high Tc superconducting equipment for electric power generation, transmission and utilization on board of electrical airplane provides weight reduction of 0.74-2.5 ton per each MW of installed electric power. The HTS option is considered by strategic international R&D programs. Prototypes are urgently needed to demonstrate these advantages in practice. References 1 http://sciencewatch.com/sites/sw/files/m/ pdf/World-2025.pdf 2 http://www.acare4europe.org/sria 3 Rajashekara, in Power Electronics for Renewable Energy Systems, Transportation and Industrial Applications, Wiley VCH, 2014. 4 Volkov et al., Phys. C, 482 (2012) 87. 5 Kopylov et al., J. Phys. Conf. Ser. 507 (2014) 032047 6 Maguire et al., IEEE Trans. Appl. Supercond., 21 (2011) 961 7 Stemmle et al., CIRED2012, DOI 10.1049/cp.2013.0905 8 C.A.Luongo et al., IEEE Trans. Appl. Supercond. 19 (2009) 1055. 9 Goldacker et al., Supercond. Sci. Technol., 27 (2014) 093001