Document
Technical Challenges and Barriers
Affecting Turbo - electric and Hybrid
Electric Aircraft Propulsion
Dr. Ajay Misra
Deputy Director, Research and Engineering
NASA Glenn Research Center
Keynote presentation at ENERGYTECH 2017 conference October 31 – November 2, Cleveland, OH NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE
Why Electric
• Improve fuel efficiency • Lower emissions • Reduced noise • Low operating cost • Efficiency of electrical components significantly higher than IC engines or gas turbine engines NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE
All Electric Aircraft
Power Converter NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 3
Battery Capability Required for All Electric Urban Mobility Aircraft
2 – 3 VTOL passenger Rotorcraft Normal aircraft 2 – 3 passenger 4 passenger Range, Miles 4 passenger 2 – 3 passenger Current commercial battery state - of - the - art: 170 Wh/kg 400 Wh/kg 300 Wh/kg 150 Wh/kg Battery Specific Energy, Wh/kg (Pack Level) NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 4
All Electric Commuter Aircraft
Studies by Happerle (German Aerospace Center) Dornier 328 turboprop 28 passengers Baseline range – 750 miles Speed for electric – 140 – 200 mph Considering 30 min of reserve Range, Miles 180 360 (Modified aircraft) (Modified aircraft) (Modified aircraft) (Baseline) Battery Specific Energy, Wh/kg (Pack Level) NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 5
Series Hybrid Electric Aircraft
• Gas turbine/IC engine used either to charge batteries or provide auxiliary power to drive the motor Gas • Battery used during takeoff and Fuel Turbine part or all of cruise • Gas turbine/IC engine used Electric primarily during part of cruise Motor Generator • Gas turbine/IC engine can continuously run at the most efficient point Power Battery • Gas turbine is sized much smaller Converter Challenge: • Efficiency of small engines relatively low (on the order of 30% ) Range Extender compared to large gas turbine engines NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 6
Parallel Hybrid Electric Aircraft
• Degree of hybridization can be adjusted depending on the mission • One option is for gas turbine to always operate at its peak Gas Fuel efficiency point and to use Turbine battery when power required is larger than power delivered by the gas turbine (e.g., during takeoff) Challenge: Power Electric • Increased complexity of the Battery Converter Motor mechanical coupling • Increase in control complexity as power flow has to be regulated and blended from two power sources NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 7
Battery Requirement for Commuter and Regional Hybrid
Electric Aircraft
48 passenger turboprop Hybrid electric, 70 Passengers • For 600 nm, battery pack with specific energy (Based on Isikveren et.al., ISABE greater than 500 Wh/kg for total energy to be 2015 Paper) less than conventional propulsion Range for 15% block fuel reduction • Battery specific energy must be at least 600 Wh/kg for operating fuel/energy cost parity with advanced conventional propulsion Assuming pack specific RUAG Dornier – DO228NG (Juretzko) energy = 60% of cell Range, Nautical Miles specific energy • 20 passenger hybrid electric aircraft 300 400 500 600 700 800 900 Battery Pack Specific Energy (Wh/kg) • 335 – 350 miles range with battery specific energy of 150 Wh/kg NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 8
Single Isle 737 - Class Aircraft
Pompet et.al. (Germany): Boeing SUGAR Volt • Single aisle 180 passengers, 3300 n • Parallel hybrid, ~ 150 PAX miles base line • 1 - 5 MW, 3 - 5 kW/kg, 93% • 13 % block fuel reduction for 1500 efficient electric machines Wh/kg battery pack • 60% efficiency improvement over 2005 baseline aircraft if a • 6% block fuel reduction for 1000 renewable grid is assumed Wh/kg battery pack Battery requirement: 750 Wh/kg • May not be any benefit for battery pack with less than 1000 Wh/kg Lent (UTRC): specific energy • Single aisle 737 class, geared turbofan • Electric motor used during takeoff along with gas turbine engine Single - aisle 737 class hybrid electric • Addition of turbogeneartor during aircraft will require 1000 Wh/kg or higher takeoff more effective battery specific energy • Batteries greater than 1000 Wh/kg required to be competitive with turbogenerator NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 9
Hybrid Electric Helicopters
Multi Mission Baseline (no electric) Light Utility Hybrid – 290 Wh/kg Hybrid – 523 Wh/kg Range, Miles Medium Utility Light Utility Multi - Mission Medium Utility (Sikorsky S - (Bell 206 L4) (Airbus EC 300 C) 175) NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 10
Battery Chemistry Possibilities
NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 11
Reduction in Cell - to - Pack Battery Specific Energy
Typically 40 – 50 % decrease Reduction in Specific Energy From Cell to Pack: • Thermal management • Battery management system • Safety features • Packing Specific Energy (Wh/kg) Cell Battery Pack NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 12
Limits on Useable Specific Energy
Based on current packaging and integration technologies Mg-ion Li-O Li-NMC622 Tesla Model S Li-S Gr-NMC622 Useable Energy Density (Wh/L) J. Electrochem. Soc., 162 (6), A982 (2015), Energy Environ. Sci.,2014, 7, 1555-1563 Nissan Leaf 0 100 200 300 400 500 600 Useable Specific Energy (Wh/kg) NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 13
Notional Progression of Battery Capability
> 500 Wh/kg, Li – oxygen, Beyond Li chemistries 400 – 500 Wh/kg Li metal anode, sulfur cathode 300 – 400 Wh/kg Li metal anode, advanced cathode SOA – 250 Wh/kg at 300 – 350 Wh/kg Si anode, advanced cathode cell level 5 Years 10 Years 15 Years NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 14
Projected Advances in Battery Technology
Rate of increase in specific energy is typically on the order of 5 – 8% per year Specific energy loss from cell to pack is typically 50 to 60% Assuming 8% increase per year at cell level 15 % loss from cell to pack (current) Innovation required in: • New chemistries and materials for cells Cell • Pack design and 32 % loss from cell to integration pack Specific Energy (Wh/kg) 23 24 25 18 19 20 21 22 26 28 29 17 27 2017 2030 Year NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 15
Dependency of Evolution of Electrified Aircraft on Battery Advances
Notional timeline based on optimistic 2 - 3 passenger 30 passenger projections VTOL, 100 300 miles miles More system analysis required to identify 4 passenger 4 passenger requirements VTOL, 100 - 2 - 3 VTOL, 60 120 miles passenger , miles All Electric 200 miles ???
??? ??? Timeline Today 2030 2028 150 - 170 400 300 500 600 700 Wh/kg Wh/kg Wh/kg Wh/kg Wh/kg Wh/kg Wh/kg Wh/kg 70 passenger 800 miles (15% block 20 – 30 20 passenger Single aisle 70 passenger fuel reduction) passenger 300 miles 737 class 1000 miles > 400 miles (15% block Light utility (?)
fuel reduction) helicopter Hybrid Electric NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 16
Multifunctional Structures With Energy Storage Capability
Battery Pack Electric motor Replace battery with multifunctional structural element Batteries with some load bearing capability or structure with energy storage capability ????
NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 17
Application of Fuel Cells
X - 57 FUELEAP System Using Solid Oxide Fuel Cell: Power output: 120kW (161hp) max continuous, 158kW (209hp) peak • Specific power: 314 W/kg (0.19 hp/lb) • Efficiency: 62% (10k ft, std day) Solid oxide fuel cells • High efficiency • Low power density at system level • Potential range extender for small hybrid electric aircraft • Durability, thermal cycling PEM fuel cell: • Needs hydrogen NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 18
Turboelectric Aircraft
Benefits of Turboelectric Propulsion: • Enables new aircraft configurations • Decoupling of speeds of turbine and fan Challenge: • Multiple fans can be driven by one gas • Development turbine, providing high propulsive of distortion tolerant fan efficiency due to higher bypass ratio (BPR) • Can enable boundary layer ingestion Boundary Layer Ingestion Testing at capability NASA GRC NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 19
Advanced Single Aisle Turboelectric Concept
Single - aisle Turboelectric Aircraft with Aft Boundary Layer Ingestion (STARC – ABL) • Conventional single aisle tube - and - wing configuration • Twin underwing mounted turbine engines with attached generators on fan shaft • Ducted, electrically driven, boundary layer ingesting tailcone propulsor • Projected 7 – 12 % fuel burn savings for 1300 nm mission NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 20
Distributed Electric Propulsion
• Dozen, small electric motors that accelerate airflow over the wing – provides more lift at low speed – enables takeoff in normal runway • Cruise – two electrically driven propellers mounted on tip of each wing X – 57: Distributed Electric Propulsion Demonstrator • 9 passenger plane, battery powered with turbine range extender • Much more efficient, cost effective and quiet than comparable aircraft • Increase use of small and medium US 9 - Passenger Concept airports and decrease emissions NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 21
Propulsion - Aircraft Integration for Electrified Aircraft
Need analytical tools to assess benefits of propulsion airframe integration on electrified propulsion aircraft configurations NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 22
High Power Density Electric Motors
Large gas turbine engine NASA research (power density at electromagnetic level), 1 – 3 MW, >96 % efficiency Key Technologies for Increasing Power Density: Various claims (100 – • Higher conductivity materials 200 kW) • Insulation materials with high thermal conductivity Siemens (200 kW) • Better magnetic materials • Better packing density System level, 95 % efficiency • Advanced topology Power Density, kW/kg • Thermal management • Lightweight structures Current electric vehicles • High speed operation Current industrial NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 23
High Power Density Power Converters
• High power density power converters needed • Need 2 - 3X increase in power density of MW scale converters • Goal: • 19 kW/kg plus 99 % efficiency with non - cryogenic cooling • 26 kW/kg plus >99 % efficiency with cryogenic cooling • Approach – Wide bandgap semiconductors (SiC and GaN) Advanced magnetic materials to handle the higher switching speeds afforded by new SiC and GaN semiconductors.
NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 24
Large Turboelectric Aircraft With Superconducting Motors
Large Aircraft with Superconducting Motor Cu stator coils Superconductor rotor coils Current research: Development of fully superconducting, MW scale motor High ac losses major challenge for superconducting stator Power densities greater than 20 kW/kg achievable NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 25
Thermal Management Challenges
• For a 5 - 10 MW system, 100s of kW heat generated • Heat from multiple sources – power electronics, motors, batteries • Integration of heat rejection from multiple sources • Low grade heat difficult to handle • Increasing use of composites lowers the heat rejection capability of the system • Lightweight and compact thermal management system required • Integrated thermal management approach at the aircraft level required NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 26
Power Transmission
• Transmission of MWs of power will require large diameter Cu with severe weight penalty • High voltages (on the order of 2000 V or more) will be required for the current generation of power cables (Cu, Al) – Advanced insulation materials required – Thermal management of the power cable system • New materials with higher electrical conductivity than Cu required – Carbon nanotubes show promise – Superconducting materials possibility – will require cryogenic cooling of transmission cables NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 27
Development of Integrated System
NASA Electric • Integration of multiple components Aircraft Testbed and optimization of performance of (NEAT ) for testing integrated system a challenge multi MW level power system – Optimum power extraction from turbine – Coupling of generator, motor, and fan – Optimized energy management • Control system with energy coming from multiple sources • Integrated testing required to address system and sub - system level integration challenges NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 28
Notional Progression of Electrified Aircraft
• 4 - seater all • Large aircraft with Increasing electric urban superconducting range commuting motors and hydrogen • 9 - 10 seater all fuel ??
• 20 - 30 passenger electric and commuter all hybrid electric electric aircraft range extender • 50 - 100 passenger • 20 - 30 passenger regional hybrid • 4 - passenger all commuter hybrid electric aircraft electric urban electric aircraft with full range commuter • 50 passenger • 9 - 10 passenger regional hybrid commuter all electric aircraft Increasing electric and with limited hybrid electric range range range extender 5 Years 10 Years 15 Years 20 Years + Today Turboelectric single aisle with innovative architecture NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 29
Concluding Thoughts
• Electrified Aircraft is a reality – NOT “IF”, IT IS “WHEN” • Progression of all electric and hybrid electric aircraft is a strong function of advances in battery technology • Turboelectric aircraft with innovative propulsion architecture and integrated airframe - propulsion system is an attractive option for large single aisle aircraft • Advances in many component technologies required – 3 to 5 times increase in power density of electric motors – 3X increase in power density of power converters – 3 - 5X decrease in weight of power cables for MW level power transmission • Integration of technologies and demonstration of integrated technologies at sub - system and system level are required NASA GRC • RESEARCH AND ENGINEERING DIRECTORATE 30