Key Drivers for Aeronautic Batteries

Today and Future Electrically Powered Aircraft 15/03/2016
Publication REE REE 2016-1 Dossier MEA2015
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Key Drivers for Aeronautic Batteries


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DES AVIONS PLUS ÉLECTRIQUESMEA 2015 80 REE N°1/2016 Introduction Depending on the targeted class of electric aircraft, various power systems may be envisioned. While today battery perfor- mances are expected to initiate commercial developments for small full electric aircrafts, hybrid propulsion schemes are under investigation for larger aircrafts and at least regional ones [1]. Whatever their mission, either for Auxiliary Power Unit (APU) or propulsion, key drivers for aeronautic batteries are safety, energy to power density, cycling capabilities & certifi- cation. Especially “Power to Energy” ratio will differ from full electric to hybrid propulsion and thus will drive technology selection. High Energy: Status/Potential for lithium- ion and potential for lithium-sulphur and metal-air Today, Panasonic commercial NCR18650 and NCR18650A lithium-ion (Li-ion) types achieve respectively 620 Wh/l (NCR18650) and 675 Wh/l (NCR18650A) i.e. ~230-245 Wh/kg over 300 cycles. Panasonic high energy Li-ion cells (18 650) foreseen are nickel based positive electrode mate- rial based on LiNiO2 (Panasonic’s proprietary). The current design weight energy density is about 252 Wh/kg. Panasonic announcement of beginning silicon alloy anode battery (+ 30 % energy) volume production in 2012 was postponed in 2013. Those 260 Wh/kg cells are currently sampled at Panasonic customers. At larger capacity levels, in 2012, Envia, a US start-up, has demonstrated Li-ion accumulators between 378 & 400 Wh/kg from C/10 to C/3 in 45 Ah cells. Integrating sili- con-based negative with Li-rich positive, the 45 Ah cell ini- tially at 400 Wh/kg fades at 300 Wh/kg after four cycles. ENVIA cells were benchmarked by end-users but so far no advertisement regarding a possible industrial manufacturing or commercialization has been released up to now. Silicon- based negative major patent holders are end-users and fully integrated players (Sony, LG, Samsung, Panasonic), battery manufacturers (Sanyo, GS Yuasa, Hitachi, Hitachi Maxell), material manufacturers (Shin-Etsu Chemical, Sumitomo Metal Industries, Mitsui Mining Smelting, 3M) and institutio- nal players (Argonne National Laboratory, Chinese Academy of Science, AIST (JP), KAIST (KR)). Almost all Li-ion players (materials providers, cells manu- facturers, R&D groups…) worldwide are willing to participate to silicon technology success story. Consequently, the market may be poised for the entrance of a first wave of higher- energy Si-C cells, with various performances, in the 2015 timeframe. After Li-ion… Rechargeable lithium-sulphur (Li-S) batteries have recent- ly received ever-increasing attention due to their high theo- retical specific energy density, which is 3 to 4 times higher than that of Li-ion batteries based on intercalation reactions. Li–S batteries may represent a next-generation energy sto- rage system, particularly for large scale applications. The obs- tacles to realize this high energy density mainly include high internal resistance, self-discharge and rapid capacity fading Key Drivers for Aeronautic Batteries Today and Future Electrically Powered Aircraft Par Florence Fusalba, Jean Oriol & Eric Pinton CEA-Grenoble Selon la classe d'avions électriques visée, des systèmes électriques divers peuvent être envisagés. Tan- dis qu'aujourd'hui on s'attend à ce que les performances des batteries limitent leurs applications à des développements commerciaux pour les avions électriques légers, des programmes de propulsion hybride et élec- trique sont à l'étude pour les avions régionaux [1]. Le stockage d'énergie électrique pourrait fournir la puissance supplémentaire exigée pour le décollage. Indépendamment de leur mission, comme APU (unité de puissance auxiliaire) ou pour la propulsion, les critères des batteries pour l’aéronautique sont la sécurité, la densité d’énergie et de puissance, la durée de vie (en cyclage et calendaire) et bien sûr l’accès à la certification. Particulièrement le ratio puissance/énergie différera selon le degré d’électrification (tout électrique ou propulsion hybride) et condui- ra ainsi le choix des technologies. Le CEA travaille à adapter ces hypothèses technologiques dans un contexte long terme, à identifier les verrous technologiques et propose des solutions novatrices au niveau du système. Les technologies de batteries avec les performances d’aujourd'hui et celles attendues dans l'avenir sont discutées. Leur pré-dimensionnement est évalué pour des technologies sélectionnées en réponse à notre liste de critères. RÉSUMÉ REE N°1/2016 81 Key Drivers for Aeronautic Batteries on cycling. Competition is already very active in the field, led by several start-up and large companies: lithium metal protection and expanded graphite; Lithium-sulphide and in-situ protection of lithium; liquids silica tethered and confinement of polysulfides using inorganic materials. Sion Li-S battery benchmarked by Astrium GmbH (Airbus Defence and Space today) for High Altitude Pseudo-Satellites (HAPS) exhibits 350 Wh/kg with few cycles (less than 20) with volume specific energy of only 320 Wh/l [2]. 500 Wh/kg prototypes were designed over 4-5 cycles. - cal specific energy compared to all other batteries, because there is no need to store one of the reactants inside the battery, namely the cathode reactant oxygen which is sup- plied from the ambient air. Since this metal-air battery weight case of a Li-MnO2 battery, the cathode reactant‘s weight is 30 times higher than the weight of the metal anode. The lithium- air (Li-air) battery has a significantly higher specific energy theoretical specific energy of a zinc-air (Zn-air) is in the range of 1 350 Wh/kg, while that of Li-air is 11 140 Wh/kg. In contrast to other metal-air batteries, the Li-air battery is the only one which may lead to a practically rechargeable system even if not demonstrated today. At system level Li-ion has potential for gravimetric and volumetric energy density improvements by a factor of ~2 but requires high energy density negative (silicon based) (table 1). System level volume/mass analysis for Li-S indicates similar weight but substantial increased volume relative to advanced Li-ion. System level volume/mass analysis for Li-air indicates no gravimetric improvement and substantial volumetric penalty Figure 1: Current batteries technologies and expected 10-20 years developments [1]. Current Li-ion Optimistic Li-ion* Optimistic Li-Sulfur* Optimistic Li-Air* Specific Energy Density - Wh(total)/kg (cell) 250 530 550 710 Specific Energy Density - Wh(total)/kg(system) 150 290 300 280 Energy density - Wh(total)/liter(cell) 520 1050 620 760 Energy Density - Wh(total)/liter(system) 230 375 260 240 Table 1: General Motor (GM) high energy battery technologies cell and system comparison [3]. DES AVIONS PLUS ÉLECTRIQUESMEA 2015 82 REE N°1/2016 relative to Li-ion. Cost is likely higher for Li-Air systems versus advanced Li-ion. board batteries while increasing their payload capabilities (in terms of power and duration), energy batteries combined with a bank of supercapacitors may also be investigated as hybrid power source. Fuel cells An 80 kW fuel cell exhibits a power density at system level for automotive application without H2 storage, without power electronic (e.g. converters) and with no battery hybridization of ca. 659 W/kg (state of the art according to DOE [4]), the- refore 122 kg for 80 kW. The embedded energy is 145 kWh in case of use at 80 kW during one hour with an overall efficiency of 55 %. It cor- responds to a mass of 125 kg of storage system (under 350- 700 bar). The associated DC/DC converter mass is estima- ted at 14 kg [5]. In conclusion and for one hour of use, the full system mass is evaluated at 261 kg with a net power of 80 kW (306 W/kg) and 145 kWh (555 Wh/kg) embedded. These data can be compared with GM calculated bat- tery system specific energies (table 1). However, this spe- cific energy increases with increasing the autonomy factor (embedded H2) from ca. 555 Wh/kg to 950 Wh/kg for an autonomy of ca.1 h (145 kWh) to 5 h (725 kWh). Therefore, fuel cells are of interest for large autonomy or high energy to power ratio with a maximum energy density achievable of 1 500 Wh/kg and 825 Wh/l for H2 tank sto- rage up to 700 bars. High energy with sufficient power for Light Sport Aircrafts (LSA) Specific energy depends on discharge rate and tempera- ture. Thus values of specific energies to be compared must be measured in similar conditions (discharge rates, tempera- tures, life expectancy (cycle)) or, if not similar, comparable. If we take the case of Tesla batteries (very mediatized) which uses 18 650 cells (approximately 210 Wh/kg), the pack is 150 Wh/kg but with ~400 kg of embedded battery sys- tem to hold rates of 3-4 C discharge and 600 cycles (charge at max 1 time the nominal capacity (C)). Specification of pack targets 150 Wh/kg with 200 kg max of batteries (sys- tem level) for a rate of 3-4 C and 2-3 000 cycles (charge at 1 C-1.5 C). Depending on system form factor (1.2 to 1.4), cells of about 180-200 Wh/kg are requested. The notion of safety and aeronautical “qualification” is also missing in these data. These parameters often require the addition of elements of safety (if possible redundancy) to the detriment of the mass performances, in minima. A reliability of 10-7 is specified for the aeronautical applications. Figure 2: Compared specifications of Auxilliary Power Unit (APU) and electric propulsion (for light aircrafts). REE N°1/2016 83 Key Drivers for Aeronautic Batteries High Power with sufficient energy for helicopters hybridization Aircrafts hybridization for fuel saving embraces wide stra- tegies and thus various power to energy ratio, therefore seve- ral potential technologies. Hybridization in helicopters may deal with autorotation powering, fuel saving, engine starting in addition to power supply of the distribution network. Priority criteria are here high power density (W/kg) and safety with ca. 100 kW requested in 10-30 sec under ca. 200 cycles. Low self-di- scharge for the decrease of costly maintenance and low temperature capability are also needed. Mass limit drives the technology selection. Of particular interest are Li-ion cells, power sized, that retain low internal resistance from a fully charged to a fully di- scharged state. This feature allows more robust performance and greatly reduces concern about heat generation for high pulse rate applications. Regarding battery materials, the stra- tegy is then to select a “power sized” chemistry to embed more useful capacity (to avoid oversizing) while better ope- rating at enhanced temperature range. Supercapacitors offer potential higher power capability but with lower energy. Supercapacitors support very high charge and discharge currents, implying important issues of voltage drop and dissipation linked to the high ESR (electrical serial re- sistance) for classic banks of supercapacitors. When sufficient energy is requested, classic supercapacitors (from Maxwell, Nesscap…) do not display sufficient energy to cope with the power mission profile corresponding to the coming applica- tions requested for air hybridization use. On the other hand, the classic lithium-ion energy batteries do not display suffi- cient power and are not well adapted, particularly in terms of series resistance and charge rates capacity. Therefore hybrid supercapacitors which have a relatively higher energy density with still high power capability are expected to find their way as this technology is likely to offer access to higher energy levels, of course provided that those performances are not decreased at low temperature. Scientific and technology methodology To develop such innovative power modules, a set of seve- ral competencies should work together: technology in function of chemistry, energy/power perfor- mances, safety operating conditions, packaging and lifetime constraints and to size the storage system on specific air- craft power profiles; on dedicated packaging and power connections, to provide assembling consistent integration factor with safety and environmental constraints (thermal, mechanical…); - hardware and associated software), to provide performance data on innovative solutions; the preliminary design of the whole storage power module (battery, power connections, packaging and battery system Conclusions limited to small vehicles and hampered by rather short ranges and endurance. Neglecting costs, the current technology is suitable for small ultra-light aircraft, but not for commercial aviation (except for APU). In order to power larger aircraft, a dramatic improvement in battery technology is required. Comparing with today’s technology specific energy values, the mass specific energy density would have to be increased at least by a factor of 5 to become useful. More realistically, this factor would have to be in the order of 10 to attract com- mercial interest [1] for larger (regional) aircraft. Li-ion has potential for gravimetric and volumetric energy density improvements by a factor of ~2 but requires high energy density negative. Today system level volume/mass analysis for lithium-sulfur indicates similar weight but subs- tantial increased volume relative to advanced Li-ion. Today Li-S energy density performance is close to 300 Wh/kg but is expected to shortly reach 600 Wh/Kg (in 2-3 years according to Sion Power and Oxis Energy) with challenges to overcome as safety issue, internal resistance, self-discharge and rapid capacity fading on cycling. Cycling efficiency system level volume/mass analysis for Li-air indicates no gravimetric improvement and substan- tial volumetric penalty relative to Li-ion. Cost is likely to be higher for Li-air systems vs. advanced Li-ion. Also common questions for lithium metal batteries (Li-air, Li-S…) remain concerning operation at high C-rates and safety, at low tem- peratures. to power ratio. Moreover, the produced water can be reused required to make fuel cells attractive for aviation application. In particular, the power density of the electrical generator is expected to meet 850 W/kg and 650 W/l, and 1 200 Wh/kg and 1 000 Wh/l for the net system energy density in the next 10-20 years. Life time is also an important parameter to be enhanced with a target at 50 000 hours. DES AVIONS PLUS ÉLECTRIQUESMEA 2015 84 REE N°1/2016 Micro hybridization of aircraft like helicopters requires very high power (100 kW for tens of seconds) with medium energy to limit the system mass. Current energy storage tech- nologies are expected to respond this need. Large Aircraft Hybrid or Electric Propulsion (Regional types [1]) powering system design, modelling and instal- lation concepts request to be assessed using long-term assumptions (10-20 years) and investigating battery packs with very high energy contents (several hundreds of kWh). Thus, it is necessary to adapt technological assumptions to this long-term context, identify technological locks and propose innovative solutions at system level (low level of maturity is acceptable). This work has to be conducted complying with aircraft power sources regulatory issues, general and safety requirements under progress and docu- ment updates in order to ensure foreseen compliance with the future standards. Aircraft electrification will imply innovative energy distri- bution as for distributed propulsion systems to improve the propulsive efficiency but also possible distribution of energy systems into the aircraft (number of modules and packs) and space allocation to power auxiliaries (power batteries to start on the ground & serve as a backup for electronic flight systems) or doors opening (supercaps). Installation constraints (plug-in or « rackable » concept), connection of electronics, thermal, structural integration and handling of batteries by operators or automatic machines will have to be fully redefined, imagined. - cations where the “electrification” of functions previously powered hydraulically, like actuation, requires high voltage architectures. Lithium technologies are today under assess- ment for emergency systems, APU & main batteries, LSA, helicopters hybridization (autorotation powering, fuel saving, engine starting in addition to power supply of the distribution network…), unmanned aircraft (UAV). While perhaps the nearest term commercial opportunity for fuel cells systems in aviation is in small unmanned aircraft (UAVs) and uncritical embedded system, fuel cells are also assessed for APU or hybridization level when over than 500 Wh/kg embedded energy is requested, fuel cells become a necessity but pro- bably not sufficient. In addition, the fuel cell byproducts might be interesting in terms of overall efficiency such as heat valorization, Oxygen Depleted Air (ODA) production and water reuse. Anyway, multisources will be envisioned, hybri- dization (i.e. supercaps/batteries/fuel cells) but not only, also new electric sources distributions, harvesting, energy saving, energy management... progress to be made in materials, air- craft platforms, engines, converters, etc... and breakthroughs LES AUTEURS Florence Fusalba (Ph.D) is Energy Storage Program Man- ager at CEA. Jean Oriol is Industrial Partnership Manager at CEA with a specific expertise in fuel cells. Eric Pinton is Head of the fuel cells laboratory at CEA. REE N°1/2016 85 Key Drivers for Aeronautic Batteries being still waited in the field of energy generation, storage and conversion of course fitting the economical business model and safety concerns. References [1] “ElectricFlight–PotentialandLimitations“MartinHEPPERLE. German Aerospace Center. Institute of Aerodynamics and Flow Technology. [2] Dr. Jens Federhen, Astrium, 2nd Workshop “Lithium Sulphur Batteries” Dresden, Nov. 6/7, 2013. [3] General Motor (GM) beyond Li-Ion Berkeley, CA 2012. pdf [4] Hydrogen Storage Technologies Roadmap Fuel Cell Technical Team Roadmap - June 2013. roadmap_june2013.pdf [5] Optimizing the Weight of an Aircraft Power Supply System through a +/- 270 VDC Main Voltage Brombach, J. Lücken, A. Schröter, T., Schulz, D. Przeglad Elektrotechniczny 2012 | R. 88, nr 1a | p47-50.