Electric Aircraft - The Weight Challenge

My general view on the decarbonisation of all sectors is that anything that can be plugged in or connected to a clean generator should be. Batteries are an alternative for mobile or isolated applications, but only if they can truly replace the role of liquid or gaseous energy stores.

The principal fundamental challenges for batteries are their specific energy, both gravimetric (kWh /kg) and volumetric (kWh/l) and specific power (kW/kg).

Li-ion battery gravimetric specific energy is some 30 times worse than most fossil fuels, i.e. a given amount of stored energy requires 30 times more weight. The poor volumetric specific energy compounds this value with lithium-ion batteries requiring 50-100 times more volume. The basic vehicle weight will also increase if it requires a larger vehicle to accommodate the battery.

Finally, there is specific power. People often talk about electric vehicle range and battery capacity. However, the maximum required battery power discharge can also size the battery. Usually, the two requirements are connected. The equivalent limit for fossil fuels is the fuel flow rate, e.g. pumping power, rather than the fuel tank capacity.

However, many researchers correctly partially counter these disadvantages with the electric drive train being about twice as efficient as fossil-fuel combustion. But they rarely talk about vehicle weight effects.

CARS

I really like the following chart from 'Transport and Environment' as it describes the situation for alternative car fuels very nicely, concluding that electric cars are the most efficient by far. I hope that Transport and Environment do not disapprove of my use of their image

Small ground vehicles are ideal for electric power systems. The fossil fuel onboard a car represents about 2% of the vehicle mass, i.e. a 2% fuel/energy mass fraction. Adding enough batteries to meet the car's power demand (a customer choice as the range is an outcome) adds a modest weight to the car. Compare the all-electric VW ID4's 1965kg-2224kg weight range to the petrol/diesel VW Passat at 1454-1754kg - the ID4 is about 30% heavier, is taller and still has less internal volume.

The chart does not mention vehicle weight as it is of secondary importance for cars cruising speed on long journeys. When cruising at a constant speed on a level multi-lane highway, the aerodynamic drag dominates power requirement with a weak, if any, dependency on vehicle weight. Weight is more important during acceleration so its effects increase when frequently stopping and starting. The ID4 drag is probably slightly higher than the Passat's - they have very similar reported drag coefficients but the taller ID4 presumably has a larger reference area, i.e. higher drag for the same coefficient.

AIRCRAFT

Much of the same logic applies to aircraft, although aircraft weight is of much greater importance due to the lift-dependent drag accounting for about 40-50% of the total aerodynamic drag. Further, additional weight also requires faster cruising speeds to maintain optimum aerodynamic efficiency increasing the profile drag.

Further, more weight requires a bigger, heavier wing to maintain the desired landing speed and take-off runway length with more powerful propulsion needed to maintain take-off acceleration rates at the higher weights. These factors feed back into the design and further increase aircraft drag and weight, generally making the aircraft even heavier.

Hence, let's assume that doubling an aircraft's weight doubles the propulsive energy per mile travelled due to a constant L/D. If W doubles, L doubles and so must D for constant L/D.

The larger wing might also improve the L/D (better wetted aspect ratio) a little if the aircraft reduces its indicated airspeed, but will increase wetted area (more drag). Flight times could be recovered by flying higher but only if the flight is long enough to get up and down to the higher altitude.

However, let's stay with a basic 1:1 ratio on % weight increase vs % energy increase, while recognising it could be significantly less, or more, than this. Refining it requires considerable work.

These reasons explain why electric aircraft opportunities currently concentrate on small, short-range aircraft where the fossil fuel aircraft's fuel mass fraction is about 5% or less - a bit higher than cars but still very low.

ELECTRIC AIRCRAFT WEIGHT

The Eviation Alice is a transport aircraft with a similar 9-seat cabin to the fossil-fuelled PC-12. However, the Alice's current reported MTOW is ~60% higher than the PC-12's MTOW, increasing to almost double the PC-12's TOW when fuelled to match the Alice's maximum range. At this shorter range, the PC-12 fuel mass ratio is <5%. The Alice's MTOW is pre-certification and could well increase during flight-testing.

Suppose we assume that doubling aircraft weight doubles the propulsive energy required to transport the aircraft. In that case, the Eviation Alice needs twice as much propulsive energy as a SAF or kerosene-fuelled PC-12. The Alice could claim lower drag than the PC-12, but there is little stopping a brand new kerosene/SAF-fuelled achieving something similar, e.g. the Otto Celera 1000L ( larger version of the 500L).

Moving up in aircraft size, I recently read about the Venturi Echelon 01 aircraft (now branded' Maeve'). It addresses the 44 pax market, similar to the ATR42, but with a shorter 550km range capability.

Its reported MTOW is 45 tonnes!

That is about 2.5 times more than the ATR42's MTOW, nearer 3 times the ATR42 TOW to deliver the Maeve's 550km mission. Its wingspan is similar to 150-200 seat A320 and 737 aircraft making airport integration a consideration. Using the assumption of 1:1 weight delta and energy delta, The Maeve will require about 2.5-3 times the propulsive energy to move the same payload with a similar L/D.

The 19-seat Heart ES-19 sits between the Alice and Maeve aircraft. It probably requires about 5t of battery (based on current Li-ion specific powers to supply 4 x 400kW motors and secondary power and cover system losses). Once you add 2 tonnes of payload and the airframe and system mass, its MTOW is likely to exceed 10 tonnes, i.e. approximately doubling the existing 19-seat Dornier 228 or DHC-6 TOWs when fuelled for the ES-19 range.

The electric motors might enable some improvements to the 1:1 weight: propulsive energy ratio through more efficient off-design energy usage (taxi and descent) that might be significant on short missions. However, this is probably in the noise relative to the effect of battery attributes.

AIRCRAFT EFFICIENCY COMPARISON

Hence, I have attempted to recreate the 'Transport & Environment' car chart for aircraft novel propulsion systems. NOTE: it is a first cut, and many of the inputs are assumptions subject to change, based on technology and on the viewpoint of whoever creates it.

In an attempt to be objective, I read-across many of their assumptions for the various process efficiencies (I am not an expert in many of them). I added a battery discharge inefficiency (a minor effect). I also improved the P2L SAF engine efficiency as gas turbines are more efficient than car piston engines. All options also received an 85% propeller efficiency as they are less efficient than shaft driven wheels.

The inclusion of a decimal place does not indicate the accuracy and the analysis - it is approximate.

The most significant change was to factor in the 'aircraft weight per passenger' term. It essentially represents the energy required to carry additional weight relative to the SAF aircraft. It mainly affects the electrical aircraft whose MTOW is assumed to be double the SAF aircraft's MTOW. Hence, half of the electrical energy is required to move the extra aircraft mass rather than payload.

The hydrogen aircraft both include a smaller 10% 'aircraft weight per pax' penalty representing the extra H2 tank mass – differences for fuel cells and combustion are assumed small, at least for now. Moving to 20% would clearly make the hydrogen results a little worse. Note: the short ranges effectively mean that the hydrogen and SAF aircraft weights vary very little during the flight.

As range increases, the hydrogen, and especially SAF aircraft, will gain from reducing aircraft weight through the flight. This chart naturally favours the electric aircraft because it assumes a range that the electric aircraft can fly. Repeating it at, say, 500nm with IFR reserves would eliminate the electric aircraft using current battery technology, leaving the hydrogen and SAF aircraft only, although the hydrogen tank mass and volume may also be limiting

I also replaced the '100% Renewable Energy' feedstock with '100% Low Carbon Energy' as all solutions require vast quantities of clean energy, probably necessitating nuclear power, rather than wind turbines and PV.

Future Propulsion Improvements

Improving battery technology well beyond the current Li-ion standard is often cited as the path to future electric aircraft improvements and expanding market share. However, it only enhances the overall electric aircraft efficiency in a new aircraft design when holding the payload/range capability constant, i.e. substantially reducing the weight and decreasing the wing area - or delivering drag reductions not available to other fuels.

Replacing 5 tonnes of old battery with 5 tonnes of new battery technology in an existing aircraft unquestionably improves the aircraft's payload/range. Still, it will not affect the weight inefficiency element as the TOW for the original short missions remain the same.

It will be an interesting choice for future electric aircraft designs. I suspect the initial focus for new battery technology will be range increases to provide broader market coverage.

Meanwhile, the other alternatives are not standing still and will also improve, e.g. some electrolysis techniques claim efficiencies significantly above 80%, and hydrogen tank masses are also reducing as the need for them increases. Hence, their overall efficiencies should improve over time. These hydrogen and SAF options also incur less penalty as range lengthens, especially the former - tank mass and volume constrain hydrogen range capability.

These fuels have less challenge achieving increased ranges, especially SAF. Note: if the energy efficiency comparison were made at 500nm, the results would exclude the option using electric battery technology as it cannot achieve the required range. The hydrogen might also struggle due to tank mass and volume, but it is straightforward for the SAF.

I also have performed a similar analysis for 'Series-Hybrid Electric', and the results are not … compelling. They must enable considerable aircraft efficiency improvements, unavailable to the other power trains, to overcome their additional energy conversion losses.

CONCLUSIONS

The overall result is that electric aircraft using current technology assumptions remain the most efficient compared to two hydrogen and SAF P2L options. They are twice as efficient as the others but with a much-reduced margin relative to that observed for cars.

However, if the electric-battery aircraft weight is 3x the P2L SAF alternative, i.e. the 'Maeve', then its overall efficiency drops to just 20%, i.e. not much better than the hydrogen or SAF options!

The Alice and Maeve data points and a RAWAvCon model of the Heart ES-19 suggest the weight penalty increases proportionately with aircraft size and range, i.e. the baseline fuel mass ratio. The battery mass becomes increasingly dominant in the total aircraft mass, i.e. the battery is increasingly sized to carry itself (range and take-off power).

Moving forward, it is likely that near-term battery technology improvements will focus on increasing range to cover more of the sub-regional and regional markets rather than improving overall efficiency. If so, the electric aircraft weight inefficiency will remain while hydrogen and SAF fuel production improvements improve their overall efficiencies, closing the margin to electric propulsion.

It will be intriguing to see what comes to pass or whether other options gain traction.