Aviation : can hydrogen live up to the hype ?

On 21^st Sep­tem­ber last year, Air­bus unvei­led three concepts for the world’s first zero-emis­sion com­mer­cial air­craft, which could enter ser­vice by 2035 ¹. While eve­ry plane is dif­ferent in size, desi­gn and range, they all use the same pri­ma­ry power source – hydro­gen. Air­bus, the world’s big­gest plane manu­fac­tu­rer in 2019, believes that it “holds excep­tio­nal pro­mise as a clean avia­tion fuel and is like­ly to be a solu­tion for aeros­pace and many other indus­tries to meet [their] cli­mate-neu­tral tar­gets.” What’s more, the French govern­ment has com­mit­ted to pou­ring €7bn into this up and coming ener­gy source bet­ween now and 2030 ².

As it hap­pens, Air­bus is not the only one loo­king into hydro­gen. After all, it is the only fuel source aside from kero­sene that seems to be sui­table for com­mer­cial avia­tion. While Boeing is not convin­ced of its short-term poten­tial, some smal­ler com­pa­nies have been explo­ring the tech­no­lo­gy. On 24^th Sep­tem­ber, start-up ZeroA­via per­for­med a trial flight with a hydro­gen-fuel-cell-powe­red, six-sea­ter plane. And Israe­li air-taxi deve­lo­per Urban Aero­nau­tics has been wor­king with Cali­for­nian start-up HyPo­int to create a ver­sion of their City­Hawk VTOL air­craft that is powe­red by a hydro­gen fuel cell.

There are two ways that hydro­gen can be used. First, the conven­tio­nal method is to burn it in an engine. This tech­no­lo­gy is alrea­dy used by the space indus­try to launch rockets. Second, is to use hydro­gen to pro­duce elec­tri­ci­ty via a fuel cell. Howe­ver, this tech­no­lo­gy is still very expen­sive. In any case, both methods still face some tech­no­lo­gi­cal hurdles.

The main dif­fi­cul­ty comes from fuel sto­rage. “Hydro­gen gas is very low in den­si­ty,” John­ny Des­champs, lec­tu­rer and resear­cher in the Che­mis­try and Pro­cesses Unit at the ENSTA Paris (IP Paris), and an expert in hydro­gen sto­rage, notes. “Even when com­pres­sed at 350 bars, you still need sub­stan­tial volumes – a 210 litre-tank for 5kg of hydro­gen (or a 125-litre tank at 700 bars).”

The second solu­tion is to store it in liquid form, “but it takes a lot of ener­gy to lique­fy hydro­gen, as it requires a tem­pe­ra­ture of minus 253°C. What’s more, cryo­ge­nic tank insu­la­tion isn’t abso­lute, so a per­cen­tage of the liquid hydro­gen will keep eva­po­ra­ting and need to be ven­ted – which is a ris­ky pro­po­si­tion.” Light, durable mate­rials the­re­fore need to be deve­lo­ped for the tanks.

Fuel tanks also have to be cylin­dri­cal or sphe­ri­cal. This requires a com­pre­hen­sive review of the shape of the plane, as cur­rent desi­gns store fuel in the wings. With all these limi­ta­tions, the idea of using hydro­gen to power a plane for a long-haul flight with seve­ral hun­dred pas­sen­gers seems unli­ke­ly at present. Ins­tead, com­pa­nies are loo­king more at air­craft with a few dozen seats for domes­tic flights.

Although hydro­gen engines give off fewer CO₂ emis­sions, that doesn’t mean that it is squea­ky clean. Bur­ning it at high tem­pe­ra­tures results in pol­lu­tion in the form of nitro­gen oxides (NO_x), and water vapour, which forms contrails and cir­rus clouds that are also bad for the climate.

Hydro­gen can be pro­du­ced from water (H₂O) using elec­tro­ly­sis, or from hydro­car­bons like methane (CH₄). Even though elec­tro­ly­sis is a clean pro­cess, this tech­no­lo­gy can­not yet be rol­led out on an indus­trial scale. “We can make hydro­gen by elec­tro­ly­sing water, espe­cial­ly when the elec­tri­ci­ty is being pro­du­ced in a stable way, such as in a power plant,” Samuel Says­set, lead tech­ni­cal consul­tant at Engie, says. “But if we want ‘green’ hydro­gen made with rene­wables, which are an inter­mit­tent power source, we need to deve­lop new tech­no­lo­gies. Plus, ~95% of hydro­gen is cur­rent­ly made from petrol, gas or coal, which emits CO₂.”

There is also the all-impor­tant ques­tion of cost, which varies depen­ding on the source. Cur­rent­ly, hydro­gen made from methane costs €1.5–2 per kilo­gram, whe­reas hydro­gen made by elec­tro­ly­sis costs four to ten times more. It is one of the rea­sons for the invest­ment from the French govern­ment : a large pro­po­si­tion of the €7bn is aimed at fin­ding solu­tions to make the price of hydro­gen more reasonable. 

There are also other costs invol­ved – hydro­gen planes would require signi­fi­cant invest­ment for fuel pro­duc­tion, trans­port and sto­rage infra­struc­ture, new air­craft and mate­rials deve­lop­ment and, above all, for the hea­vy tanks. It remains to be seen if a hydro­gen-powe­red plane would be eco­no­mi­cal­ly viable in the long term.

Never­the­less, the avia­tion indus­try seems to be inter­es­ted in hydro­gen, with manu­fac­tu­rers like Air­bus, and the French govern­ment com­mit­ting to invest in this area. But we still do not know how the many unans­we­red ques­tions rela­ting to hydro­gen planes will be resol­ved. Which engine manu­fac­tu­rers will be invol­ved ? What will be the engines’ fuel consump­tion and effi­cien­cy ? Will they be powe­red by inter­nal com­bus­tion or fuel cells ? How will the fuel be sto­red ? Hydro­gen has yet to demons­trate that it has a future in aviation.