For the foreseeable future, aircraft with batteries as the sole onboard energy carrier will only be suitable for short regional flights. And even then, significant improvements in battery performance are required, argue TU Delft’s Henri Werij and Marnix Wagemaker.
Aviation is facing one of its most difficult challenges ever: how to make mass air transport carbon neutral, even climate neutral. This industry is currently responsible for roughly 2.5 percent of the global man-caused CO2 emission, a number that’s bound to go up as we continue to fly more and more. We also have to take into account non-CO2 effects (due to NOx emission, contrails, high cirrus clouds), which are expected to increase the aviation-induced climate impact by a factor of 2-3. The continuous improvement of aircraft, however impressive by itself, can’t keep pace with the annual growth in passenger kilometers.
Tackling this huge challenge calls for a truly holistic approach, in which we address the energy carrier (getting rid of fossil fuel), the efficiency of aircraft and their propulsion systems and the flight path itself (shortening routes, optimizing flight altitudes to minimize cloud formation, continuous descent approaches). At first sight, one of the obvious choices is replacing fossil fuel with batteries, like we see happening in the automotive industry right now. The advantages are evident: sustainable electricity can be stored and retrieved with minimal losses, electric engines are extremely efficient, there’s no emission during flight and as the number of charging cycles increases, the environmental and climate impact related to battery production decreases. However, the situation for aircraft is a bit more complicated than for cars.
To understand the limitations of batteries for aviation, let’s first have a look at the amount of energy required to travel a certain distance once at cruise altitude. Flying horizontally at a constant speed, the lift L equals the weight W, while the propulsive force F equals and counteracts drag D. Since the energy E required equals the amount of work performed to travel a certain distance, which equals force times distance, we can derive a simple equation for the distance flown given a certain amount of energy stored in the battery (neglecting wind effects).
Independently of the size or weight of the aircraft, there are three relevant parameters to maximize distance: the overall energy density (energy in battery divided by total aircraft mass), the so-called glide ratio (L/D) and the efficiency η of the electrical and propulsion system, ie the percentage of the energy stored in the battery that’s transformed into work for propulsion. To start with the good news, the efficiency η for battery-powered aircraft is very large, close to 85 percent. But what about the other parameters?
The ratio L/D is a well-known number for glider pilots, since it equals the horizontal distance in meters covered per meter altitude lost while gliding in windless conditions. For a Cessna 172 general aviation aircraft, it’s about 10; for an albatross (one of the most efficient gliders among birds), it’s 20; for a normal sailplane, it’s somewhere around 30; for a high-performance competition sailplane, it’s around 50. A modern commercial airliner has a glide ratio of approximately 20. Clearly, a well-designed aerodynamic shape of the fuselage and wings is crucial; protruding parts like engines don’t contribute to increasing the glide ratio. For electric aircraft, a high L/D is even more important than for conventional aircraft. However, reaching sailplane-like shapes is difficult since a significant amount of passengers and cargo will have to be taken aboard.
The overall energy density is the true limitation of battery-powered aircraft. Currently, the best rechargeable battery cells have a gravimetric energy density of close to 400 Wh/kg (which, by the way, is larger than what’s currently being used in even the best electric cars). However, this isn’t the full story. A battery stack consists of individual cells, a mounting structure, a thermal system and electronics. The extra weight related to these components, according to NASA, lowers the energy density by an easy 30 percent. We also have to take into account other adverse factors: aging due to frequent charging/discharging, minimum and maximum state of charge and the impact of temperature and discharge speed (low temperature and high power are detrimental). Data for commercial battery systems is hard to find in open literature, but it’s very unlikely that the useable energy density of a battery stack exceeds 50 percent of the value claimed for individual cells. This implies that we should use a value of 200 Wh/kg = 720 kJ/kg for state-of-the-art systems.
To determine the energy density on aircraft level, we have to know which percentage of the airplane weight originates from the battery system. While landing, the total weight consists of three components: the empty operating weight (EOW), the weight of passengers and cargo, and the weight of the remaining fuel. Together, these shouldn’t exceed the maximum landing weight (MLW). In a conventional aircraft, the maximum takeoff weight (MTOW) is greater than the MLW as fuel is burnt during flight. In a battery-powered aircraft, however, the weight remains constant, implying that MTOW equals MLW. In a wide range of propeller aircraft being studied (ranging from two-seaters to 100-seaters), the EOW typically amounts to 60-65 percent of the MLW. Distributing the remaining weight evenly across battery system and passengers/cargo yields some 20 percent of MLW available for both. Electric engines weigh less than conventional engines, giving another few percent, so that we may assume that 25 percent of MLW being available for the battery system is a realistic number. We have to keep in mind that due to the fixed weight of the batteries, this leaves substantially (30-40 percent) less weight available for passengers plus cargo, compared to when using conventional fuel.
Assuming a fairly optimistic value of L/D = 27, the maximum distance would be 25 percent x 720 (kJ/kg) x 27 x 85 percent / 9,8 (m/s2) = 422 km. Depending on the velocity of the aircraft, this corresponds to 1-2 hours of flight. Unfortunately, the actual range of the aircraft would be much less, since we need energy for taxiing, instruments, air conditioning, heating and, most of all, the required reserve flight time of 30-45 minutes (to account for head wind, holding pattern, go-around and diversion to an alternative airport). This implies that the actual range will be substantially less than the 422 km calculated above, and likely more in the range of 250 km.
To improve the situation, a lot of research on rechargeable batteries is conducted worldwide. Increasing the energy density is one of the main goals. Here, introducing high-capacity anodes such as Si and Li-metal is being considered a promising route. However, the big challenge of high-capacity anodes is the reversibility: the relatively low number of charge-discharge cycles is one of the major bottlenecks to commercialization. This is far from advantageous for aviation applications.
Let’s consider the battery warranty of a leading supplier of electric cars. For their latest long-range model with a specified range of 580 km, this warranty accounts for 192,000 km and allows for a maximum capacity decrease of 30 percent. Assuming a full charge yields a range that’s 85 percent of the original 580 km, this corresponds to slightly less than 400 full charging cycles. A commercial electric aircraft that will (have to) be used for short regional flights is expected to be recharged several times a day (which also puts severe requirements to the maximum allowed charging power). This will easily lead to the equivalent of 1,000 full charging cycles per year. Needless to mention that a 30 percent capacity decrease, leading to an even larger range decrease (due to the reserve required) for such an aircraft, would be quite unacceptable. It’s therefore evident that in battery research, energy density and power density are important topics, but improving the number of charging cycles is also a key challenge.
For the foreseeable future, aircraft with batteries as the sole onboard energy carrier will only be suitable for short regional flights. This might be interesting for applications where air travel is not only faster but also more energy efficient and thus more environmentally friendly than road travel (think of Norway or other regions without a suitable road or rail infrastructure). Even for these cases, however, significant improvements in several aspects of battery performance are required.
Given the range limitation, purely battery-powered aircraft will play a very small role in lowering the climate impact of aviation. Instead, we’ll have to rely on other carbon-neutral energy carriers, such as hydrogen or synthetic fuels. Several serious developers have chosen the path of electric engines based on hydrogen in combination with fuel cells. Burning hydrogen in combustion jet engines is another option being considered. Whatever solution we go for (and there’s no single silver bullet), we’ll need huge amounts of sustainable electricity and aircraft that require as little energy as possible.
Main image credit: Royal NLR