Energy Sources for Electric Aircraft

Electrical vertical take-off and landing aircraft (eVTOL) are quite common these days. They are found everywhere from small drones to large air-taxis. The reason for this is quite simple: the brushless electric motor is powerful and small. It can change the RPM of a rotor (propeller) quickly, is relatively quiet and widely available. For this reason, most designers of modern VTOL aircraft chose to go the electrical route. The issues really begin with the energy source - in other words, where does the electricity come from? Fortunately, there is some choice in terms of an energy source, but as always, every source has its pros and cons.

To understand this concept better, we introduce a physical value - the energy density of the power source. This value combines both the energy density of the fuel itself together with mass the storage and any auxiliary equipment (like a generator or a fuel cell) that is needed to create the actual electric power. Let's look at the four fundamental types of electric power sources starting from the least energy dense to the most energy dense: solar, battery, liquid-fuel, and nuclear.

1- Solar: The energy density of solar panels is very low - about 0.01MJ/kg. The value is barely high enough to make ultra-light solar aircraft a reality. Unfortunately, it is nowhere close enough to get vertical take-off and landing aircraft into the air as the power requirement is higher for such vehicles. Even if the efficiency of solar panels goes up from 30% to 99%, it is a safe bet that solar panels will never be used to power eVTOL aircraft.

2- Battery: Batteries are the most common source of energy for electrical aircraft, including eVTOL. Their energy density tops out at about 1MJ/kg and the DOE (Department of Energy) expects this value to double in about a decade. In the grand scheme, this value is still pretty low, but their ease of use makes them a popular choice for both drones and air-taxis.

3- Liquid Fuel: Liquid fuels, on their own, have fairly high values for energy density, 40MJ/kg for Diesel, and 120MJ/kg for Hydrogen. However, when we add on the storage mass of the tanks as well as the mass of the generation equipment (micro turbine, piston-engine or fuel cell), the actual source energy density falls into the range of 2MJ/kg to 20MJ/kg. Still, manufacturers can expect at least a two-fold increase in aircraft endurance by switching from a battery eVTOL to a hybrid-electric variant.

4- Nuclear: Nuclear fuel such as Plutonium has an energy density of millions of MJ/kg. However, in practice, the reactor, containment vessel and generation equipment greatly diminish this value. The most practical reactor type is the Radioisotope Thermoelectric Generator (RTG). This device (currently in use by the Mars Curiosity and Perseverance rovers) has no moving parts and generates electricity directly from the heat of the radioactive fuel. Unfortunately, it has a source energy density of 20MJ/kg, which is comparable to chemical fueled power source. Due to its radioactive nature, we will not see nuclear reactors powering eVTOL aircraft on earth anytime soon, but there may still be applications for space exploration.

All in all, manufacturers of eVTOL aircraft are stuck with batteries or hybrid-electric power sources for the foreseeable future. However, there is one more purely theoretical source that may one day be exploited - fusion energy. Fusion-fuel (such as Deuterium and Helium-3) has a comparable energy density to nuclear fuel (such as Plutonium) but without all the harmful radiation. Moreover, high energy plasma released from a fusion reaction could be directly converted to electricity to power an eVTOL aircraft. Of course, we must approach this with a level of skepticism, as humans haven't even figured out how to build a stadium-sized reactor, let alone a reactor small enough to fit onboard an aircraft. But if one day we do realize this technology, it could offer a clean, ultra-high energy density power source for all electric aircraft - fingers crossed this technology comes to fruition in our lifetime.