Cesar A. Luongo

Next Generation More-Electric Aircraft: A Potential Application for HTS Superconductors

Sustainability in the aviation industry calls for aircraft that are significantly quieter and more fuel efficient than todays fleet. Achieving this will require revolutionary new concepts, in particular, electric propulsion. Superconducting machines offer the only viable path to achieve the power densities needed in airborne applications. This paper outlines the main issues involved in using superconductors for aeropropulsion. We review our investigation of the feasibility of superconducting electric propulsion, which integrate for the first time, the multiple disciplines and areas of expertise needed to design electric aircraft. It is shown that superconductivity is clearly the enabling technology for the more efficient turbo-electric aircraft of the future.

HTS machines as enabling technology for all-electric airborne vehicles

Environmental protection has now become paramount as evidence mounts to support the thesis of human activity-driven global warming. A global reduction of the emissions of pollutants into the atmosphere is therefore needed and new technologies have to be considered. A large part of the emissions come from transportation vehicles, including cars, trucks and airplanes, due to the nature of their combustion-based propulsion systems. Our team has been working for several years on the development of high power density superconducting motors for aircraft propulsion and fuel cell based power systems for aircraft. This paper investigates the feasibility of all-electric aircraft based on currently available technology. Electric propulsion would require the development of high power density electric propulsion motors, generators, power management and distribution systems. The requirements in terms of weight and volume of these components cannot be achieved with conventional technologies; however, the use of superconductors associated with hydrogen-based power plants makes possible the design of a reasonably light power system and would therefore enable the development of all-electric aero-vehicles. A system sizing has been performed both for actuators and for primary propulsion. Many advantages would come from electrical propulsion such as better controllability of the propulsion, higher efficiency, higher availability and less maintenance needs. Superconducting machines may very well be the enabling technology for all-electric aircraft development.

High power density superconducting motor for all-electric aircraft propulsion

NASA conducts and funds research to advance the state of the art in aeronautics, including improvements in aircraft design leading to enhanced performance in areas such as noise, emissions, and safety. A particular initiative involves development of an all-electric aircraft requiring significant improvements in certain technologies. NASA has started a new project with one of the objectives being the development of enabling technologies for an all-electric aircraft. Electrical aeropropulsion requires the design of more compact and efficient electrical motors. In order to increase the power density, the weight/size must be minimized and the air gap flux density must increase significantly: the use of superconducting materials is an obvious choice. Existing HTS motors are proof-of-principle demonstrators and exhibit power densities lower than 1 HP/lb, which is too low to be considered in mobile systems. This paper deals with a preliminary electromagnetic design of a 200 HP high temperature superconducting motor optimized in terms of power density. The presented configuration is a synchronous motor with a nonconventional topology enhanced by HTS bulk material. The design targets the Cessna 172 propulsion requirements that are 200 HP at 2700 RPM.

Design of HTS Axial Flux Motor for Aircraft Propulsion

Development of all-electric aircraft would enable more efficient, quieter and environmentally friendly vehicles and would contribute to the global reduction of greenhouse gas emissions. However, conventional electric motors do not achieve a power density high enough to be considered in airborne applications. Bulk high temperature superconducting (HTS) materials, such as YBCO pellets, have the capacity of trapping magnetic flux thus behaving as permanent magnets. Experimental data show that one single domain YBCO pellets could trap up to 17 T at 29 K, which enables the design of very high power density motors that could be used in aircraft propulsion. We designed a superconducting motor based on an axial flux configuration and composed of six YBCO plates magnetized by a superconducting coil wound on the outside of the motor. The six-pole homopolar machine uses a conventional air-gap resistive armature. Axial-flux configuration allows several rotors and stators to be stacked together and therefore enables the use of one or several conventional permanent magnet rotors to generate minimum safety torque in case of loss of superconductivity. All-electric aircraft are expected to be powered by fuel cells or turbo-generators fed with pure hydrogen cryogenically stored that would provide the motor with a convenient cooling system at 20 K. This paper presents the design and simulated performance of the motor for an application in aircraft propulsion.

HTS motors in aircraft propulsion: design considerations

Current high temperature superconducting (HTS) wires exhibit high current densities enabling their use in electrical rotating machinery. The possibility of designing high power density superconducting motors operating at reasonable temperatures allows for new applications in mobile systems in which size and weight represent key design parameters. Thus, all-electric aircrafts represent a promising application for HTS motors. The design of such a complex system as an aircraft consists of a multi-variable optimization that requires computer models and advanced design procedures. This paper presents a specific sizing model of superconducting propulsion motors to be used in aircraft design. The model also takes into account the cooling system. The requirements for this application are presented in terms of power and dynamics as well as a load profile corresponding to a typical mission. We discuss the design implications of using a superconducting motor on an aircraft as well as the integration of the electrical propulsion in the aircraft, and the scaling laws derived from physics-based modeling of HTS motors.

A 100 MJ SMES demonstration at FSU-CAPS

The Center for Advanced Power Systems (CAPS) at Florida State University (FSU) was recently established to pursue research and education in power engineering. Development and demonstration of superconducting technologies is one of the cornerstones of the CAPS program. Important aspects of the program are the test of superconducting equipment at power levels up to 5 MW, and the creation of a reconfigurable network that will support pulsed power testing. A 100 MJ SMES system is being completed at BWX Technologies for integration to the CAPS test facility, to allow pulsed power operation of the testbed. The SMES coil, scheduled for completion in 2003, is based on cable-in-conduit technology and NbTi superconductor. The full system (including cryostat and power converter) will be integrated at CAPS and be operational in late 2004.

Three-Dimensional Micrometer-Scale Modeling of Quenching in High-Aspect-Ratio

YBa2Cu3O7-δ coated conductors have very slow normal-zone propagation velocity, which renders quench detection and protection very difficult. To develop effective quench detection methods, it is paramount to study the underlying behavior that drives quench propagation at the micrometer-scale level. Toward this end, numerical mixed-dimensional models, composed of multiple high-aspect-ratio thin layers, are developed. The high-aspect-ratio modeling issues are tackled by approximating the thin layers either as a 2-D surface or as an analytical contact resistance interior boundary condition, which also acts as a coupling bridge between the 2-D and 3-D behaviors. The tape models take into account the thermal and electrical physics of each layer in actual conductor dimensions and are implemented using commercial finite-element analysis software. In the first part of this two-part paper, the mixed-dimensional models are introduced and then computationally and experimentally validated. Validations are gauged by comparisons in normal-zone propagation velocity and in the time-dependent voltage and temperature profiles. Results show that the mixed-dimensional models can not only effectively address the high-aspect-ratio modeling issues of thin films but also accurately and efficiently reproduce physical quench phenomena in a coated conductor.

\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}

A high temperature superconducting (HTS) motor has been designed to power a general aviation aircraft. The propulsion requirements of the Cessna 172 have been chosen as baseline for the study: 200 HP at 2700 RPM. The designed motor is based on flux trapping in bulk YBCO plates and concentration of the flux generated by Bi-2223 coils and an ironless air-cooled resistive armature. The eight-pole machine would exhibit high power density comparable to that of small gas turbines around 4 HP/lb. Details of this HTS motor concept have been presented in a previous paper. However, the scaling up of such a configuration is not straightforward, as single domain YBCO elements cannot exceed a few centimeters in diameter. This paper presents the design of a motor based on the same configuration but with a much higher power rating, in the range of several MW, to power high altitude long endurance (HALE) aircraft or small jets. Due to the size limitation of the YBCO plates, two solutions can be used to increase the power: the radius and the number of poles can be increased, or the motor can be lengthened to accommodate more coil-plate pairs. The motor is able to reach more than 2 T in the air gap thus leading to high power density. The design optimization is done with respect to several objectives as a trade-off between amount of superconductor, efficiency, weight and volume. The cooling system is assumed to be provided by liquid hydrogen available onboard the aircraft as fuel for the fuel cells or turbo-generators.

Coated Conductor Tapes—Part I: Model Development and Validation

Environmental preservation has now become paramount as natural resources are rapidly consumed. A global reduction of the emissions of pollutants in the atmosphere is therefore needed and new technologies have to be considered. A large part of the emissions rejected in the atmosphere comes from transportation vehicles, including cars, trucks or airplanes, due to the nature of their combustion-based propulsion systems. To contribute to solving the problem, our team has been working for several years on the development of high power density superconducting motors for aircraft propulsion. This paper investigates the feasibility of all-electric aircraft based on currently available technology through the presentation of several designs of machines. Electric propulsion would require the development of high power density electric propulsion motors and generators. The requirements in terms of weight and volume of these components cannot be achieved with conventional technologies; however, the use of superconductors associated with hydrogen based power plants makes possible the design of power components reasonably light and would therefore enable the development of all-electric aero-vehicles. Many advantages would come from electrical propulsion such as better controllability of the propulsion, higher efficiency, higher availability and less maintenance needs. This paper shows that high temperature superconducting machines are mature enough to be used in aircraft.

Scaling Up of HTS Motor Based on Trapped Flux and Flux Concentration for Large Aircraft Propulsion

Due to their intrinsic thermal properties that lead to very slow heat diffusion, coated conductors are very difficult to protect against thermal instabilities and quench. This is particularly critical as the device increases in size and stored energy. Many experiments showing low quench propagation speeds and surface voltage distributions have identified the problem but, as of today, no models are available to perform high fidelity simulation of the quench process in a single tape, or in a full device. YBCO tapes are very promising conductors that would allow the development of high power density power devices such as generators and motors operating at liquid nitrogen temperature. The Department of Defense through the Air Force Research Laboratory launched a new project aiming at simulating quench propagation in YBCO tapes. The first step of the project deals with the understanding of the current diffusion in the different layers forming the tape. Experimental data on normal propagation zone (NZP) velocities and fault current limiters using this material show that the quench is most likely a 3D phenomenon involving many non trivial physical phenomena. Experimental data will help to understand the physics of the quench and develop a local electromagnetic/thermal model of YBCO tapes. This paper presents the preliminary results of this work including a discussion of the physics of current sharing between layers. We will evaluate different commercial codes and present a simplified 3D homogenized model applicable to a racetrack coil.