By Bjorn Fehrm
July 1, 2021, © Leeham News: In our Friday Corners, we analyze the development challenges of aircraft. We will launch a concrete project Friday where we intend to develop a 19 seat airliner. To make it interesting, it will be a Green aircraft. We focus on the Certification issues in the Corner series.
To complement it, we here look at the operating cost of a battery-based electric airliner, as there are costs that are often not presented to the public in the marketing of these alternatives. The operational costs for the huge batteries are too often forgotten.
The marketing videos or pamphlets that present battery-based airliners say these are suitable short haul airliners as their operating costs are lower. They motivate this with the low cost of grid electrical energy combined with low maintenance costs for the electric propulsion system for the aircraft.
The energy as such is cheaper, but the maintenance costs are not. The failing link for the maintenance costs is the cost of swapping batteries when these reach the end of their useful life on the aircraft.
Grid electricity in the US, if you are an industrial consumer, costs between six and seven cents per kWh. The current price of Jet A1 fuel for 19 seat Turboprops like the Beech 1900 in Figure 2 is $565 per 1,000kg of fuel. It sets the kWh price at five cents per kWh (each kg of Jet A1 holds 12kWh of energy).
But the efficiencies of the engines for the market’s 19 seater Turboprops are very low. The engines of the Beech 1900 (Figure 2) or Jetstream 31 (Figure 3) were designed in the 1960s, and they are today 60 years old in their architecture.
We have analyzed them with the professional tool Gasturb. Their thermal efficiency (the amount of energy they convert to shaft hp to drive the propeller) on the typical 19 seater mission is 27%. This is compared with a modern turbofan at 50% to 55% efficiency.
An electric motor for electric aircraft has a typical efficiency of 94%. Combined with its control electronics (the inverter, 98% efficiency), it has a system efficiency of 92%.
It gives an energy cost per kWh of 17.4 dollar cents for the Turboprop aircraft and 7.1 dollar cents for the electrical aircraft (both engines drive the propellers, which are assumed to be equally efficient in the two cases).
The above assumes both aircraft have the same design. It will not be the case as battery energy weighs considerably more than the energy stored in Jet fuel. Jet fuel holds 12kWh per kg, whereas certifiable battery systems are today at around 0.160 kWh per kg.
I write certifiable battery systems as a Li-Ion battery is dangerous if not carefully monitored and protected with temperature, volt, and current sensors monitoring the condition of each battery cell through a battery management system.
As a typical battery system in this class has over 300,000 cells, it needs half a million protection sensors and circuits. The certification rule also states that the battery shall not catch fire if cells burn (through thermal runaway). To stop it, additional hardware protection around each cell is necessary. In the end, a certifiable battery weighs around 50% more than its cells.
To cater to the development of batteries and give the battery-based aircraft its best shot, we put the battery-specific energy at 0.250 kWh/kg for our analysis, an improvement of over 50%. This can represent the density of battery systems later in this decade. It renders the battery system 48 times heavier than Jet fuel for the same energy content.
A 19 seater Turboprop aircraft on the nominal 200nm maximum range mission of our battery-based aircraft consumes 320kg Jet fuel. Should the battery energy fueled aircraft have the same efficiency (which it will not because of the heavy batteries, we come to this later), it needs a flight energy battery of 4,500kg.
In addition, a commercial airliner must plan with ample energy reserves. The turboprop carries 308kg reserve fuel on the trip, covering the mandatory IFR reserves of 5% route contingency, the flight to a 100nm alternate and circling there for 30 minutes before landing. This means our battery aircraft needs an additional 4,300kg of battery as reserves.
In total, the electric aircraft, with a maximum range of 200nm, has batteries on board of 8,800kg. It’s more than the empty weight of comparable Turboprops. The heaviest, the Beech 1900D, has an empty weight of 4,900kg.
We will analyze how to get an aircraft to fly with these enormous battery weights in next week’s article. For this first article, we assume that the electric aircraft consumes the same energy to fly the 200nm as the Turboprops.
If this is the case, the jet fuel cost for the flight is $181, and energy from the grid for the batteries $67. So has a battery-based aircraft the chance to be cheaper in operation just as the promoters of this kind of flight say? An electric motor is cheaper to maintain than the gas turbine of the turboprop. This is correct, but it’s not the whole picture. There’s a dominant cost for the electric aircraft that is often not quoted in the marketing figures.
As we know from our cell phones, batteries don’t last forever. Each charging and discharging of a Li-Ion battery, whether for a cell phone or an aircraft, taxes the battery’s life.
If the charging and discharging go to 100%, the life is around 1,000 cycles. Batteries are down to about 80% of their capacity at this point, and their delivery of energy is impaired as the internal resistance has increased.
The 20% reduction of capacity means we have to size the battery 20% larger from the start to cater for this deterioration. This also helps with the longevity of the batteries as we don’t need to charge them to 100% until the end of their active life on aircraft.
Another factor that helps with battery life is the flight reserves that are left for the majority of missions. For batteries to last long, you shall not charge them fully nor empty them entirely.
How long will such a battery last? We have projects that quote 1,500 cycles, but it could be longer. To understand the variability, we test the battery costs with different assumptions.
The cost of replacing such a battery can be projected to reach around $400 to $500 per kWh mid-decade. A battery for our aircraft, if we size it for an 80% charge, weighs 11,000kg and costs between $1m to $1,3m, Figure 4.
If we assume we can keep the batteries on the aircraft for 1,500 flights, it puts the energy cost at $67 for this maximum range flight, and the battery costs at $678 to $848. If the batteries last 3,000 flights the battery costs reduce to between $339 and $424.
So, what is the maintenance cost of the engines of the above turboprops? Around $200 per flight (gas turbine maintenance costs are like batteries, flight cycle rather than flight hour based).
Could this mean battery-based aircraft could reach the operating costs of gas turbine aircraft if we can get the batteries to last 5,000 flights? As we will see next week, No.
We have projected above an aircraft with batteries that weigh 450kg (the average of the fuel mass for the flight) when we are now at 11 tonnes for the batteries. Does it stop there?
No, once we have wrapped a 19 seater airliner around 11 tonnes of batteries plus 1.9 tonnes of passengers with bags, we have a much larger aircraft that consumes more energy than we assumed here. Where does it end up? This we analyze next week.