Most of us don’t invest much time in learning about things that don’t directly impact our lives or hold a significant interest to us. This can present a problem when new technology makes familiar spaces feel strange. You hear terms that don’t fit into anything you know. The good news is that you don’t need an advanced degree to become familiar with — even comfortable with — the concepts and the language of a new technology. Electric aircraft offer a great example. Let me introduce you to the basics of this fast-developing arena.
Electric and combustion propulsion systems do the same thing: they transform potential energy into the kinetic energy needed to fly. Now let’s talk terms. The watt is a unit of power that signifies a selected amount of energy transfer over a specific amount of time. One watt equals one joule of energy transferred per second. In most uses, we talk about kilowatts (kW) which is 1,000 watts. In transportation, power, (particularly engine power), is usually expressed in horsepower (hp). Since both are derived units, they can be easily converted (1 hp = 0.7457 kW). You can apply either unit to any engine. A 100 hp engine would roughly equate to a 75 kW engine regardless of power source.
The next unit to consider is the kilowatt hour (kWh), which you know as the major component of your electric bill. One kWh is the energy of one kW (flowing) for an hour. The kWh is the unit of measure for an electric vehicle’s “fuel tank,” making it the counterpart of the gallon or liter for an internal combustion engine (ICE). A 10 kWh battery could supply 1 kW for 10 hours or 10 kW for one hour assuming the motor and systems attached to it could draw that amount of power. This hypothetical system also doesn’t account for transformation losses in the system, but we’ll talk about efficiency later.
Reactions of Another Kind
Electrification trades one chemical reaction for another. Like most tradeoffs, there are pros and cons. On the positive side, baseline efficiency is better than ICE. Electric motors are generally 75–95% efficient. With ICE engines usually in the 30–40% range, airplane engines aren’t exactly on the leading edge of efficiency. So, electric motors could possibly triple the per unit efficiency of “fuel,” with no direct emissions. Electric motors are also lighter and mechanically simpler with fewer moving parts. Slam, dunk. Electric is the way to go, right?
Not so fast. All methods of transportation use chemistry to transform potential energy into kinetic energy. This points toward the primary challenge of electrification, specific energy.
We discussed specific energy in detail in a previous article, but here’s the recap. Batteries store far less energy per unit of either volume or mass than fossil fuels. Avgas (100LL) has a specific energy of about 12 kWh per kilogram (KG). The best batteries currently available (in terms of specific energy) are lithium ion (Li-ion) batteries with a specific energy around 0.25 kWh/KG. Even assuming 100% motor efficiency, the battery would manage only 0.25 kWh/KG while an ICE with 25% efficiency would net 3 kWh/KG — about 12 times more energy. In energy intensive operations like aviation, that matters.
The Darker Side of Green
Another challenge to electrification is the use of certain resources. Most of the cobalt needed for batteries comes from the Democratic Republic of the Congo, where conflict and regulatory structure raise ethical and environment issues. Lithium (Li), a key component of a Li-ion battery, must be processed from compounds. Hard rock mining has been the dominant source for Li but processes now enable extraction from salt brine deposits primarily in South America. Researchers are working on methods to extract Li from seawater (where it occurs naturally at 0.1 parts per million), but right now Li ore mining involves the kind of environmental impacts associated with open pit and mountain top removal mining. Brine extraction has less impact but can consume massive amounts of water — a problem in very arid regions.
Does that mean that Li-ion batteries are bad? Not exactly. All forms of energy have negative externalities. Thinking of batteries as a “zero impact” solution to environmental concerns is incorrect, but batteries clearly have a role to play in our energy future. How that evolves will depend on how technology advances.
While electric motors are simpler and less maintenance intensive than their ICE counterparts, batteries are far more expensive and complicated. The actual chargers for most electric vehicles (EVs) are built into the vehicle, enabling installation of “chargers” in homes and public places without compatibility issues. Regardless of where the actual charger resides, there are different levels of charging. How this will be applied to aircraft remains to be seen, but the EV world offers a few choices. Level 1 alternating current (AC) charging (120V/up to 16 amps (A)) is a standard household electrical outlet. Level 2 AC charging is 240V (usually about 50A) and is typically similar to an electric clothes dryer. The higher the voltage and amperage of the circuit, the more electricity it can provide to the vehicle. A typical Level 1 charger can only supply less than 2 kW, while a Level 2 charger typically provides around 7–11 kW. Using basic math, a 20 kWh battery would take about 10 hours to charge on Level 1 but only 2–3 hours on Level 2. Losses in charging make it a tad more complicated, though: Level 2 is close to 90% efficient while Level 1 is less than 84%.
For aviation, Level 1 might seem too slow, but its low cost and ready availability are advantages. Some hangars already have electricity, so adding a 120V outlet isn’t a big deal. For an airplane that spends most of its time in the hangar, extra charging speed probably isn’t worth the cost. Even a large spec 92 kWh battery would be charged in about three days from empty off of a standard outlet. You could fly, roll your airplane back in the hangar, plug it in, and go home. When you return a few days later, your airplane is “refueled” for less than $20 of electricity. But what about those times when you don’t have time?
If time is an issue, you might need DC Fast Charging. Often called Level 3 charging, DC Fast Charging is a very different technology. The electrical grid provides power in alternating current (AC), but batteries store it in direct current (DC). AC works very well for most applications. To store that energy in a battery, though, you need an inverter to transform it into DC. DC Fast Charging eliminates that step by going directly into the battery and at a much higher voltage and amperages. DC Fast Charging can provide over 300 kW of charge if your vehicle can accept that much. While still not quite as fast as a visit from the fuel truck, it’s getting much closer.
The downside is that it creates a lot of heat and requires heavy gauge cables. It’s also hard on the battery to be charged that rapidly. So using DC Fast Charging a lot could potentially reduce battery performance and life faster over time than less aggressive charging. It’s also important to remember that regardless of the level, charging varies with conditions and state of charge (SOC). This is why you often see charging times listed at 5–80% rather than 0–100%. Charging will slow dramatically above 80% in most applications. Level 1 and 2 will be less impacted by virtue of their lower base charging rate.
The Sky Ahead
So does an electric future lie ahead? Will we see electric aircraft become a factor in GA?
I believe the answer is yes, but it’s not going to be a quick revolution or even possibly a complete one.
Because electrification — at least for now — is harder for bigger and more powerful aircraft, we are likely to see GA lead the way. The initial training market looks to be the most ripe for conversion. Electric aircraft now available or soon to be available can cover most required tasks. They can benefit from significantly reduced fuel costs and the reduced noise signature from switching. The picture for general purpose GA use is a little more challenging but could become a reality in the coming years. This is especially true if you operate from a hot or high airport. Since the electric motor doesn’t depend on atmospheric oxygen, density altitude won’t compromise EV motor performance in the same way it affects ICE.
It’s an exciting time to be in aviation, and it will be fun to watch the electric airplane fleet develop.
🛩️ “Ride the Lightning, Aviation’s Electric Future?” FAA Safety Briefing — Nov/Dec 2018
James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.