Transitioning to renewable, green-energy fuel sources is an active, ongoing goal in virtually every industry. While the generation of low-carbon- footprint energy—from nuclear, natural gas, wind, and solar—might someday be adequate to meet our real-time energy requirements, the storage of such energy for future use is still a major hurdle limiting the wholesale shift to renewable power.
be harvested from 1 kilogram of an energy source. For kerosene—the fuel of choice for rockets and aircraft—the energy density is 43 MJ/Kg (Mega Joules per kilogram). The “energy density” of the lithium ion battery in the Tesla, on the other hand, is about 1 MJ/kg—or over 40 times heavier than jet fuel for the same output of work. And yet the battery on the Model 3, for all its weight and low “energy
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density” is perfectly suited to meet that vehicle’s “concept of operation” i.e. short to medium length trips, at high speed, with a robust payload. The difference of course is the Model S is not flying but driving to its destination. A plane flies when the “lift” it generates equals the all-in weight of the aircraft. Lift of course can be achieved in various ways, including aerodynamically shaped wings, copter rotors, jet propulsion and so on. However it is achieved, the heavier the aircraft, the more lift is required to get it off the ground and accelerate it on its way. So, whenever we increase the weight of an aircraft (unlike a car) with a lower- density power source (such as a battery) we must increase the plane’s mechanism of lift accordingly. By way of reference, about 20% of the all-in weight of a commercial aircraft is its fuel. Given their relative energy densities, the transition to battery power from kerosene in an Airbus A320 would result in an additional 260,000 Kg of weight—or more than four times the total weight of the aircraft itself. The Falcon9—to return to our ridiculous comparison—
In aviation—an industry that universally relies on kerosene as its primary energy source—this dilemma is profound due to the basic thermodynamics of combustible fossil fuels versus battery-stored electrical power. Take, for example, the thermodynamics of the Tesla Model S all-electric battery-powered car versus the SpaceX Falcon 9 rocket. The high- performance 85 kWh Model S battery pack weighs in at 1,200 lbs. and delivers a range in excess of 300 miles for the 5,000 lb. vehicle. The Falcon9, on the other hand, burns 147 tons of rocket fuel to lift and vector its 50,000 lb. payload into low earth orbit. Apples and oranges, you say, as the scale of the missions and the “concepts of operation” are so different. True, but that in fact is the point. Different types of fuel can be measured for efficiency using a simple rule called “energy density.” Energy density is the measure of the energy that can
ENERGY DENSITY COMPARISON SpaceX Falcon 9: 43 MJ/Kg Tesla Model S: 1 MJ/kg
photo: NASA
photo: raneko via Wikimedia Commons
QwikConnect • July 2021
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