Electric propulsion is shaping the future of the automotive and the aerospace business. The question is no longer whether we will or will not drive and fly electrical vehicles, but when the transformation will take place. The automotive industry is already living the reality of electrical car mass production, with all major manufacturers and some outsiders jumping on the bandwagon, all of them hoping to carve out bigger slices of the market. The aerospace industry is lagging behind, for some good reasons.
There are challenges of all sorts yet to overcome before electric aircraft become a common sight. Items like electrified powerplant management, certification procedures, power electronics thermal control, electrical system integration, and even troubleshooting routines are still waiting for aviation-grade treatment.
On top of that, switching to a pure electric propelled aircraft is largely dependent on the electrical power source weight. Let’s assume for a moment that the electric airplane structure and components weigh the same as their counterparts used on aircraft equipped with an internal-combustion engine (ICE). This line of thought allows the comparison of the energy density of the electrochemical process (battery or fuel cell) against the energy obtained from fossil fuels (hydrocarbon) through combustion.
Currently, aviation gasoline (avgas) with an energy density 12,500 W·h/kg has a tremendous advantage over 250-W·h/kg lithium-ion batteries in terms of the energy-to-weight relationship. Avgas has at least 50 times more energy per unit of weight than current lithium-ion batteries. No wonder fossil fuel is the option of choice for aviation.
As an example, let’s consider an ICE and electric motor efficiency by comparing two existing products. The Cirrus SR 22 is an aircraft equipped with a 300-hp Continental, a typical aviation ICE that burns 20 gal/h at 75% power cruise settings. This engine converts only 25% of the fuel energy into mechanical energy, losing the remaining 75% as heat through the exhaust pipe, the cooling system, and friction in various forms. The piston engine is notorious for the inefficient job of converting chemical energy into mechanical energy.
Competing in the same 300-hp category, the Siemens SPD260 electrical motor installed on an Extra 300LE experimental aircraft has only 10% of the moving parts and weighs one quarter of the equivalent reciprocating engine. It is a 95% efficient AC induction motor with a variable-speed drive designed specifically for aviation application. Because the electric motor is far more efficient than the ICE, the energy density ratio drops to 13 from 50.
Even with this amazing improvement caused by the electrical motor efficiency, a lithium-ion battery capable of storing the same amount of energy as avgas would be 13 times heavier. Take the Cirrus SR22 mentioned above as an example. It carries fuel in tanks capable of accumulating 250 kg of avgas for a maximum flight time of 4.4 h, or 260 min. If the fuel tanks were replaced by 250 kg of Li-ion batteries the flight time would be limited to no more than 20 minutes.
Clearly, the inevitable transition to electric propulsion is not around the corner as electrical power storage has a long way to go before becoming a viable alternative, but there is good news on the horizon.
Samsung is said to already be providing Tesla with 300 W·h/kg Li-ion batteries. Pellion Technologies next-generation magnesium-ion may offer batteries with 50% higher energy density than current Li-ion, reaching 360 W·h/kg. Solid Energy with lithium metal, is claiming a new era in technology, with a battery capable of 450 W·h/kg. With several new suppliers trying to win a position on the future $30 billion market for lighter and more powerful batteries it is reasonable to expect further improvements.
Starting with 117 W·h/kg in 2008 the battery energy density doubled by 2015 reaching 250 W·h/kg, and will almost double again between 2015 and 2018 reaching 450 W·h/kg. At that pace, by middle 2020’s we may expect new batteries, most likely beyond lithium, and new fuel cells, capacitors, or a combination of them all reaching 1000 W·h/kg storage capacity.
That is when it gets interesting for aviation, as 1 kW·h/kg may be the necessary threshold upon which the electric aircraft can compete head-to-head with internal combustion engine propulsion.
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