I’ve been thinking a lot lately about electric jets. With all the purported battery breakthroughs, and a discussion of what aviation might be like in 2030, 2040 or 2050.
For the battery baseline, we’ll go with Solid State Lithium metal batteries. These batteries are completely solid, they don’t have a liquid electrolyte that can oxidize and catch fire. They are currently under development, and are expected to go to smaller scale commercial production in the 2016-18 timeframe. Dyson has invested in one of the leaders in the space, Sakti3, with the hope that Sakti3 can produce cells above and beyond current Li-Ion cells. The experimental values for the weight and volume that have been developed so far are about 800Wh/kg and 1500Wh/l, compared to 250Wh/kg and 700Wh/l for the top of the line batteries today. I would expect these batteries to be widely available and inexpensive by the mid-2020s.
I first looked at a Boeing 737, and whether you could build a similar size and weight, but replace the fuel tanks with batteries, and the engines with ducted fans, and see what that would give you.
Boeing 737-800 weight of max fuel: 24,025 kg
Boeing 737-800 volume of max fuel: 29,660 liters
Replace 24,025 kg of fuel with 24,025 kg of batteries: 12,813 liters of batteries (43% of original fuel volume)
Energy storage: 19.2 MWh
Replace 29,660 liters of fuel with 29,660 liters of batteries: 48,198 kg of batteries (200% of original fuel weight)
Energy storage: 38.5 MWh
You can only realistically use the smallest value of the two, since both volume and weight are limiting factors, its about which one you hit first. In this case, its weight limited. So our new aircraft has 19.2MWh of batteries weighing 24,025kg and taking up 12,813 liters of volume (less than half of the volume the fuel used up, so there may be opportunities to redesign the aircraft and reduce its overall weight). The power these batteries could generate based on the cell weight is a maximum power of 12MW, estimating 500W/kg of cells. This estimate is in line with what today’s batteries can produce on a sustained basis.
So what kind of demands are going to be put on the battery to propel the aircraft from ground to flight? Initial take-off thrust required will be high. The Airbus E-Fan demonstrator had hub-motors in the landing gear to help get the aircraft up to speed. This reduced the load on the engines to propel the aircraft up to take-off speed. If the aircraft is running maximum throttle, then the two engines are producing about 242kN of total force. Based on the E-Fan’s substitution of ducted fans, they have 30kW = 0.75kN, or 40kW = 1kN, which means the engines would need about 10MW of power over the course of about 45 minutes to get up to cruising altitude (using 7,500 kWh of energy). Cruise thrust is about 40% of max thrust (depending on altitude, air density, etc.), so energy usage per hour of flight is 4,000 kWh, and energy usage during descent is 25% for about 30 minutes (1,250 kWh). To have a three hour flight (plus 45 minutes of reserves per FAA FAR 121), you’d need about 18,750 kWh of energy, just under our estimated capacity of 19.2 MWh.
Turns out no matter how you run the numbers on any sized airplane, you really only get about 2-3 hours of operating the aircraft, which is bad for aircraft that tend to fly 2-5 hours (think B737 and A320). But, its good for regional jets where the longest flights are only 2-3 hour. It would seem logical to have Regional Jets be the first type of aircraft using batteries for propulsion. In order to have electric aircraft fly longer routes, you’d need to improve engine efficiency (use less kW to generate 1kN in thrust), increase battery weight and volume characteristics (store more energy per unit mass or per unit volume), or figure out a way to put a highly efficient generator on the plane along with a fuel source (e.g. a 5MW turbine and natural gas to provide power during take-off and in case of emergencies).
There are other side effects to running an aircraft on electricity – you’ll end up redesigning the aircraft since you can ditch the fuel tanks; you’ll be flying slower, probably M0.7 instead of M0.78 or M0.8 that current jets fly at, which means that three hour flight wont go as far as it used to; better protection against lightning; more efficient interior use of energy; and more. The ranges from various airport hubs (700mi) show that it’s range wont be a big deal (map from Great Circle Mapper).