Is there a Case for Short Range Hybrid-Electric Flight?

Introduction

If you follow the latest newest about electric flight, usually the coverage is focused around topics like test flights of airplanes where the electric drive train replaced the former combustion engine for demonstration purposes. If we leave aside eVTOLs, most companies and start-ups in the field (Faradair, Eviation, VoltAero, Bye Aerospace, just to name a few) seem to focus on aircraft with < 20 PAX.

Apart from the recent stop of the E-Fan X program, beautiful images of concept designs and senior executive statements setting the short-range electrification horizon to the very far future (> 2040), there seem not to be a lot of movement or excitement about hybridization or electrification of the short-range segment.

Thus, one could raise the question if a short-range hybrid-electric airplane does not provide any meaningful business case. In this article I would like to share some thoughts about this issue.

The idea behind hybrid-electric in a nutshell

Starting from a very high level perspective, the idea behind hybrid-electric is to trade drive train weight for new degrees of freedom in the aircraft design that allows to build aircraft with lower flight drag and therefore lower fuel consumption and emission.

That’s right, a hybrid-electric drive train will in most cases be heavier (and less efficient) than its non-hybrid counterpart during cruise flight. Comparing both drive train architectures (Fig. 1), you see that in the hybrid-electric case, you keep the gas-turbine, the fan (and the gearbox) but add a whole bunch of components, like the electric machines, the inverters and the power distribution system. No matter, how efficient and light you can make these electric components, physics dictates that their efficiency is lower than 1 and the mass larger than 0. Therefore, just looking at the drive train the whole idea of hybridization does not seem very clever at first.

Figure 1: A conventional turbo drive train (top) and a serial-electric propulsion drive train (bottom). In the second case, generator(s), rectifier(s), inverter(s), motor(s) and a power distribution center (that includes thing like switches, cables and protection devices) are added to the system. These additional components make the system heavier and less efficient in the hybrid-electric case.

However, electric motors inherit a nice feature that I would call power scalability. Take a look at Fig. 2.: Suppose that Pref is the power requirement for a chosen aircraft. If it is realized by two large gas-turbine engines, their efficiency would be around 40 %. If four or six engines are used, the efficiency already drops but is still relatively high (above 30%). Thinking about more engines, the efficiency starts to go down drastically. In contrast this is not the case for an electric motor which allow to provide the required power at nearly the same efficiency regardless the number of motors used to provide this power.

If you look at modern passenger jets you will notice how this fact drives the airplane design. Both the A350 and the B787 have only two engines installed (compared to the A340 and the B747 which have four engines) as turbofans with sufficient thrust and reliability became available in the last 20 years. in hybrid-electric drive train, the energy and thrust generation can be separated spatially. one large gas-turbine, can power multiple electrical thrust generating motors of arbitrary number and power, located at positions in the aircraft where they fit best aerodynamically.

The logical question to ask is “Can these new degrees of freedom in aircraft design offered by a hybridization of the drivetrain be used in such a way that they overcompensate the additional mass and the reduced efficiency?”

There have been numerous design studies devoted to this topic for electrified passenger aircraft in the past, starting from the late 2000s. I have included a few selected concepts in the next section, that allow to get a general understanding what the ideas behind the design of hybrid-electric passenger jets are. If you are interested in a detailed review of the topic, I recommend reading reference [1] (from 2015) or [2], that was recently published (April 2020).

Figure 2: The efficiency of an electric motor (red) and gas-turbine (left) for various number of torque generating devices. The total required power (Pref) is distributed between the consequent number of engines. Data taken from [3].

Selected Design studies

A selection of most prominent design studies from US and Europe is shown in Fig. 3 and Fig. 4. While a detailed discussion each of the concepts is not the purpose of this article, I would like to highlight key aspects that the designs share in order to achieve a better aerodynamics performance.

One key feature that most designs share is a high aspect ratio wing, i.e. a long and “thin” wing as it is typically known from gliders that can have lift-to-drag ratio of up to 50 while commercial short-range jets hardly exceed 20. As in most hybrid configurations, the gas turbine is not placed under the wing, its structural loading is reduced and therefore it can be built rather longer and thinner. If the propulsors are still placed on the wing, they can either be distributed more equally over the wing (examples f) and g)), which means rather many smaller propulsor than one large propulsor in one place.

Such a propulsor configuration is often referred to as “distributed electric propulsion” (DEP). Examples are b), f) and g), where b) is an exotic configuration called blended wing body where the propulsors are placed at the back of the aircraft on top of the fuselage. The benefit of DEP is additional lift that is generated by the airflow of the propulsors over the wing area. Consequently, a lower wing area is required for take-off which in turn reduces the drag (and therefore the SFC) during cruise flight. While the idea to increase lift by blowing exhaust air over the wing is not new and was used for instance in the An-72 design back in the 70s in order to build an aircraft that can start on a very short runway (< 500 m), many electric motors that are distributed over the majority of the wing would take this concept to its extreme.

Furthermore, DEP offers a couple of nice “side effects”: If many propulsors O(10) are used, than in case of a one engine inoperative event, only a small part of the total thrust is lost. Thus, the drive train requires significantly less oversizing than in the conventional case with two engines, where more than twice the power is installed that is actually needed for minimum requirements for take-off and climb. Also, the flight control of the airplane can fully or partially be realized by differential thrust. This would allow to reduce the tail size (and consequently the drag losses associated with it) and/or redesign (or completely omit) some parts of the flaps-slats system that could make the aircraft more silent.

A last feature that many designs share is called boundary layer ingestion (BLI). In this case the idea is to reduce the drag by affecting the flow around the fuselage of the aircraft, either by an arrangement of fans (like in example b) — the fans are above the fuselage, it is actually combination of DEP and BLI) or by having one large fan that has almost the size of the fuselage (examples c), d) and h)).

So what can we gain from these more or less radical airframe design changes? It is interesting to observe that depending on the study, there is very wide spread of claims regarding the reduction in energy demand and emissions. For instance, the result of the NASA N3X study (example b) is a gain of up to 67 % in energy demand and the SUGAR Freeze concludes that a reduction of 56 % in fuel burn (which is proportional to the energy demand but neglects the electric energy) and 70 % in CO2 emissions is possible. In contrast to this the DLR study (example g) concludes that only 5 % fuel burn reduction are achievable. The BHL study (example c) sees only 8 % fuel burn reduction possible.

Where does this discrepancy come from? It’s important to read the fine print. Most of the US studies in the 2010s compared their results with today’s state-of-the-art aircraft (in order to see if the AIAA and NASA N+3 goals could be met) while the German studies compared their results to the best configuration that can be achieved by gradually improving the current designs. Indeed, if the DLR or BHL design is compared to current state-of-the-art, one comes to the conclusion that improvements in energy demand between 40 % to 55 %. To my personal taste, the second approach is a better metric for the real benefit of hybrid-electric designs. Thus, I will restrain myself to the studies that compare their results against the best “conventional future design”.

These studies claim that values between 3 % to 10 % in energy reduction are feasible, if the corresponding technology requirements are met. What are these requirements? Electric drives with power densities around 10 kW/kg and battery energy densities ranging from 400 to 750 Wh/kg. Challenging numbers but not undoable (at least for the electric drives).

Figure 3: Rendered images of selected passenger airliner design studies: a) E-Thrust (Airbus Group) b) N3X (NASA) c) Centerline (Bauhaus Luftfahrt) d) VoltAir (Airbus Group) e) Ce-Liner (Bauhaus Luftfahrt) f) ECO-150 (Wright Electric) g) Regional Jet (DLR) h) SUGAR Freeze (Boeing). Image courtesy of the corresponding institutions names in parenthesis. Remark: d) and e) are not hybrid-electric but fully electric concepts, that are included here for the sake of completeness.

It could be about operations

Designs that promise SFC gains between 3% to 10% on the paper, might end up having gains of only 2 % to 4 % after solving all the “small, little” problems that come along with making the design a certified, customer-friendly product. Thus, why bothering with a completely new degree of complexity?

Some years ago, I had the opportunity to listen to a presentation to by a senior Airbus executive, who talked about the typical way that airlines operate their A320s and B737s: While an A320 is designed for an operating range of up to 4000 nm, airlines tend to use them for ranges below 1000 nm most of the time as can be seen in Fig. 4 was created based on data from openflights.org. It is interesting to analyze what effect this way of operations would have on a fleet of hybrid-electric short-range passenger jets.

Figure 4: Distribution of flight distances flown with passenger jets. Image taken from [4]. Data from openflights.org

In order to understand what benefits hybridization might have, let’s have a look on Fig. 5. Here the power demand during a typical short-range flight mission is illustrated schematically. In a hybrid-electric drive-architecture, the battery will normally be used only for Taxi (e.g. in the case of green-taxing [5]) and to boost the take-off as its energy density would limit the flight range if used during cruise [6].

The total energy demand of the mission depends on three things:

· The efficiency of the turbine and the electric drive train

· The ratio between battery delivered power and turbine delivered power

· The duration of the cruise phase

The more power can be taken from the battery during take-off (and climb) and the shorter the cruise phase is in relative terms, the better a hybrid-electric drive train will be compared to a non-hybridized design.

A short example (power demand and flight duration data was taken from [10]): For mission of 1500 nm a hypothetically, a hybrid-electric single aisle aircraft will require 6.5 MWh of electric energy for take-off (and climb), 15.0 MWh equivalent of fuel for take-off (and climb) and 105 MWh equivalent of fuel for the cruise. Thus, 126.5 MWh in total. In comparison, a non-hybrid version would require 30.0 MWh during take-off since it does not profit from a higher efficiency. The ratio of energy demand between a hybrid and non-hybrid mission is 96.7 %. The contributions of the taxing and landing phases are not considered for the moment but could bear further opportunities for SFC reduction in for the hybrid configuration.

Figure 5: Schematic power demand during a typical short-range aircraft mission. The duration of the flight phases and the power demand are indicative and not quantitatively accurate.

We can plot this ratio (of energy demand) for different travel distances, which is shown in Fig. 6 (left) for realistic and optimistic assumptions. The key insight here is: For distances over 2000 miles the gains are marginal while below 1000 miles (where most flights happen) the gains can go over 20 % (if we belief that the electric drive trains can be further optimized for higher efficiency and the take-off/cruise power demand ratio is larger than 1-to-1 [7]).

Now, the interesting question is what happens if when combine Fig. 4 and Fig. 6, which basically means to calculate the possible gains average over the typical operations of a fleet. For pessimistic assumption the result is roughly 92 %, for optimistic assumption the result is roughly 84 %. It’s not a 50 % effect but 15 % on a fleet level is significant. Nevertheless, keep in mind that an important premise for this result is that a hybrid-electric short-range aircraft has indeed at least the same (or a slightly better) SFC than its non-hybrid competitor for the design mission.

Another quantity which might be even more interesting in connection with electrification are the emissions. A similar analysis as for the total energy demand, can done for the emissions. The result is shown in Fig. 6 (right), again for similar optimistic and pessimistic assumption as before. You can notice that the assumptions have a much lower impact as in the case of total energy demand. This can be attributed to the fact that I assumed that no emissions “are emitted” by the battery. Such assumption is probably true from the view of the airline (as it does not produce any local emissions) but of course in a more GHG focused analysis this would have to be taken into account. The fleet average is about 82 % to 84 % depending on the assumption.

Figure 6: Ratio of total energy demand (left) and total emissions (right) for a hybrid to a non-hybrid configuration (y-axis) for different mission ranges (x-axis).

It’s interesting to look at this number from a total cost of ownership perspective. Roughly 40 % to 50 % of total costs of ownership [8] are fuel costs. What about emissions? Recently, the German government passed the so-called “Klimapaket” with the intention to reduce emissions further. As a consequence, the price for a ton of CO2 rose by roughly 40 % to price level which his similar to that of Sweden (140 €/ton). At this price, payments for CO2 emission are about 30 % of the price of fuel. Most experts commented that this is still a relatively moderate price level that will have little impact on the behavior of customers and companies. Thus, with increasing pressure on governments and the aviation industry from the public, it is likely that by the time that a new short-range aircraft could come into service, the costs for emission could be on the same level as for fuel; hence, accounting for 60 % to 70 % of total cost of ownership.

Reducing this cost block by up to 20 % (the combined effect of better aircraft performance and reduced energy demand and emissions on a fleet level) could be a differentiator in competitiveness between airlines which are under strong price pressure.

Conclusions

Starting from the beginning of the millennium various design studies in Europa and US have been done to look into an electric or hybrid-electric short-range passenger jet. Depending on the study, SFC reductions between 3 % and 10 % are predicted compared to a conventional aircraft at the same technology level.

While this does not seem to be overwhelming, an additional SFC reduction in the range of 8 % to 15 % could be gained on fleet level due to the way most airlines operate their aircraft. This SFC reduction would be accompanied by an emission reduction of up to 20 %.

Taking all three effects together and assuming that prices for CO2 emissions will continue to rise (The next stage could come with the EU green deal). the reduction in total cost of ownership of a short-range fleet could be substantial. In contrast to “traditional” airlines (like British Airways) which might include electric commuters or regional jets to their fleet for very short distances, for a budget airline (like Ryanair) that saves a large part of its operating cost by flying one type of aircraft, the savings connected to a hybrid-electric fleet could significantly increase its profit margin (and/or its price competitiveness).

Surely, the development of a hybrid-electric short-range passenger jet would require substantial budgets, probably in the same range as was required, for instance for the A380 — about €12B. Is this too much for 20 % SFC and emission reduction? A good comparison might be the PW1000G program. United Technologies invested over $10B to develop a geared turbofan that reduced SFC by roughly 15 %. Thus, price-performance ratio is within industry standards.

Of course, in times of Corona, after the stop of the A380 and with all the troubles with the 737 MAX, it will be hard for Airbus and Boeing to find the deep pockets needed for such an endeavor. Is the risk too high? Personally, I am sure that the drive system can be designed, built and certified at the necessary power densities. It is certainly challenging but we don’t have to rewrite the laws of physics to do so [9,10]. Furthermore, on the way to first flight the innovation and outcome of such a development will certainly be much more than “just” a hybrid-electric passenger jet

The latest news posted on Aeronews about the French government wanting to support a new A320 program sounds encouraging. Hopefully, Airbus will decide to make the big leap happen!

References

[1] C. Pornet and A.T. Isikveren: “Conceptual design of hybrid-electric transport aircraft” in Progress in Aerospace Sciences 79 (2019), pp.115–135

[2] S. Sahoo, X. Zhao and K. Kyprianidis: “A Review of Concepts, Benefits, and Challenges for Future Electrical Propulsion-Based Aircraft” in Aerospace 7 (2020), 44

[3] Georgi Atanasov: “Energy Efficient Hybrid Propulsion Concept for Twin Turboprop Aircraft” presented at the e2Flight Symposium 2019

[4] “US Domestic Flight Lengths” on hastydata.wordpress.org (2015)

[5] M. Lukic, S. Nuzzo et al.: “State of the Art of Electric Taxiing Systems” in International Conference on Electrical Systems for Aircraft, Railway, Ship Propulsion and Road Vehicles (ESARS) and International Transportation Electrification Conference (ITEC) 2018

[6] A. Gnadt, R. Speth, J.S. Sabnis and S. Barrett: “Technical and environmental assessment of all-electric 180-passenger commercial aircraft” in Progress in Aerospace Sciences 105 (2019), pp. 1–30

[7] A.H. Epstein: ”Aeropropulsion for commercial aviation in the twenty-first century and research directions needed” in AIAA Journal 52 (2014) 5, pp. 901–911

[8] Steve Saxon and Mathieu Weber: “A better approach to airline costs” from McKinzey Consulting (July 2017)

[9] F. Berg, J. Palmer, P. Miller and G. Dodds: “HTS system and component targets for a distributed aircraft propulsion system” in IEEE Transactions on Applied Superconductivity 27 (2017) 4, pp. 1–7

[10] M. Boll, B. Stefan, M. Corduan, M. Filipenko et al: “A Holistic System Approach for Short Range Passenger Aircraft with Cryogenic Propulsion System,” in Superconductor Science and Technology 33 (2020), 4

[11] “A320 successor among ambitions outlined in French aid scheme” on flighglobal.com (June 2020)