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Hydrogen engines: an essential component in low-carbon transport

Gaëtan Monnier
Director of the IFPEN Mobility Results Centre and the Carnot IFPEN Energy Transport Institute
Luis Le Moyne
Professor at the Institut Supérieur de l'Automobile et des Transports (University of Bourgogne)
Cédric Philibert
Associate researcher at the French Institute of International Relations (IFRI) and the Australian National University (ANU)
François Kalaydjian
Director of the Economics & Intelligence Division and Hydrogen Coordinator at IFP Energies nouvelles (IFPEN)
Key takeaways
  • Burning hydrogen or its derivatives in engines could make it possible to decarbonise trucks, boats and planes.
  • This method has two main advantages: few technological adjustments and an affordable price.
  • For heavy trucks, the hydrogen engine seems to be as viable a solution as fuel cells.
  • It is rather the hydrogen derivatives (synthetic fuel and ammonia) that should be put forward for aviation and maritime transport.
  • The future of these sectors will not be mono-technological because they are much more difficult to decarbonise than mass transportation.

The pos­si­bil­i­ty of burn­ing hydro­gen, or deriv­a­tives such as ammo­nia, in engines is receiv­ing renewed inter­est. These engines could be one of the solu­tions to the urgent need to decar­bonize trucks, ships, and aircraft.

Engine man­u­fac­tur­er Cum­mins is test­ing a hydro­gen-pow­ered truck engine, Renault Trucks is devel­op­ing its own with the French Petro­le­um Insti­tute for New Ener­gies (IFPEN), and Air­bus has announced a hydro­gen-pow­ered air­craft for 2035, which could be dri­ven by a mod­i­fied gas tur­bine. All these announce­ments sug­gest that the good old com­bus­tion engine is not dead, and that by decar­bon­is­ing its fuel, it even has a role to play in achiev­ing the cli­mate neu­tral­i­ty objec­tives for long-dis­tance trans­port by 2050.

Burning hydrogen directly in combustion engines

Hydro­gen, an ener­gy-dense mol­e­cule, can be pro­duced in a “clean” way by elec­trol­y­sis of water, with ener­gy from renew­able or nuclear sources. Elec­tric bat­ter­ies, which are now the main way to reduce the car­bon foot­print of pri­vate cars, lack auton­o­my in the heavy vehi­cle seg­ment: “bat­ter­ies can pow­er bus­es or city deliv­ery vans that recov­er ener­gy when brak­ing and can be recharged fre­quent­ly. But not a heavy vehi­cle, which even with 350 kW super­charg­ers would have to spend more than an hour recharg­ing every 300 km,” says Gaé­tan Mon­nier, direc­tor of the IFPEN Trans­port Results Cen­ter. As part of its 2020 “hydro­gen strat­e­gy for a cli­mate-neu­tral Europe”, the Euro­pean Union has val­i­dat­ed the industry’s goal of run­ning 100,000 of the 3 mil­lion trucks in Europe on low-car­bon hydro­gen by 2030. “Ini­tial­ly, the objec­tive was to con­sume this hydro­gen in fuel cells, a sys­tem that pro­duces elec­tric­i­ty to pow­er an elec­tric motor. But the idea of burn­ing hydro­gen direct­ly in ther­mal engines has been gain­ing inter­est for a few years among researchers and man­u­fac­tur­ers,” says the specialist.

Its a low-cost solu­tion for reduc­ing the car­bon foot­print of transportation.

It has sev­er­al advan­tages. First, burn­ing hydro­gen in a com­bus­tion engine only requires adjust­ments: “we need to inte­grate met­als capa­ble of with­stand­ing high­er tem­per­a­tures and an injec­tion sys­tem adapt­ed to this high­ly volatile fuel, and review the con­trol of com­bus­tion, whose char­ac­ter­is­tics are very dif­fer­ent from those of diesel… But these are by no means tech­no­log­i­cal break­throughs,” says Luis Le Moyne, direc­tor of ESAT. Sec­ond­ly, this solu­tion would allow man­u­fac­tur­ers to keep their pro­duc­tion line and thus not increase their prices too much. “It’s a low-cost solu­tion for decar­boniz­ing trans­porta­tion,” he concludes.

By com­par­i­son, fuel cells are not yet man­u­fac­tured on a large scale and con­tain plat­inum, a rare met­al… which con­sid­er­ably increas­es the pur­chase cost of the vehi­cle. “As their life span is cur­rent­ly lim­it­ed, it is also nec­es­sary to plan for a replace­ment of the fuel cell dur­ing the vehi­cle’s life cycle”, adds Gaé­tan Mon­nier. How­ev­er, fuel cells can offer bet­ter ener­gy effi­cien­cy (up to 65%) than the hydro­gen engine (up to 45%). Which solu­tion will win? At the moment it’s not clear: “The hydro­gen engine appears to be less expen­sive to invest in, but poten­tial­ly slight­ly more fuel-inten­sive over the vehi­cle’s life cycle. Both could there­fore have their eco­nom­ic rel­e­vance depend­ing on the intend­ed use of the vehi­cle,” sum­maris­es the researcher.

Is hydrogen too volatile for aviation?

When it comes to air trav­el, there is once again a debate over the use of hydro­gen ver­sus gas tur­bines. Air­bus is cur­rent­ly test­ing both solu­tions to get a hydro­gen-pow­ered air­craft off the ground by 2035. How­ev­er, “the chal­lenge is not so much in propul­sion as in the stor­age of hydro­gen on board, which will have to be in its liq­uid state, its most com­pact form,” points out Cédric Phillib­ert, for­mer ana­lyst at the Inter­na­tion­al Ener­gy Agency (IEA). Even so, hydro­gen takes up four times more space than kerosene and must be kept at the extreme­ly low tem­per­a­ture of ‑253°C. This has one sig­nif­i­cant con­se­quence: the liq­uid hydro­gen tank, which is spher­i­cal or cylin­dri­cal in shape – to lim­it heat loss as much as pos­si­ble – can­not be housed in the air­craft’s wings… which means it must be placed in the fuse­lage and the air­craft must be com­plete­ly rein­vent­ed! Man­u­fac­tur­ers are there­fore study­ing the pos­si­bil­i­ty of burn­ing a more “prac­ti­cal” deriv­a­tive of hydro­gen in the cur­rent tur­bines: syn­thet­ic fuel.

This fuel, a strict decar­bonised equiv­a­lent of today’s kerosene, can be made from decar­bonized hydro­gen and CO2 cap­tured from the atmos­phere, using a process called Fis­ch­er-Trop­sch. Even though the process is extreme­ly fuel-inef­fi­cient, with a well-to-wheel effi­cien­cy of 15% com­pared to 30% for hydro­gen, there is a strong argu­ment to be made: “it does not require the rein­ven­tion of air­craft or air­port infra­struc­ture,” says the expert. Syn­thet­ic fuels are much eas­i­er to trans­port than the high­ly volatile hydro­gen. “We could there­fore import it from coun­tries with strong renew­able ener­gy poten­tial, capa­ble of mas­sive­ly pro­duc­ing hydro­gen on site,” he con­tin­ues. This is exact­ly the plan of Porsche, which has start­ed build­ing an eFu­el plant in Chile in 2021.

Maritime transport: ammonia takes to the sea

Sim­i­lar think­ing is going on in the mar­itime trans­port sec­tor. In the field of long-dis­tance ships, the com­bus­tion of hydro­gen, which was once envis­aged, now seems to have been aban­doned in favour of a close cousin, ammo­nia. Ammo­nia, with the for­mu­la NH3, can be pro­duced in a neu­tral way from decar­bonized hydro­gen and nitro­gen (N2) using the Haber-Bosch process. With one advan­tage: “port infra­struc­tures are already designed to han­dle this gas, which is liq­uid at ‑33.5°C and is used in the man­u­fac­ture of indus­tri­al fer­til­iz­ers,” explains François Kalay­d­jian, Direc­tor of Eco­nom­ics and Mon­i­tor­ing at IFPEN. But here again, the bat­tle has not yet been won. Ammo­nia is a tox­ic gas that needs to be han­dled with care. Anoth­er deriv­a­tive of decar­bonised hydro­gen and CO2, methanol, is there­fore in the run­ning to burn in the mas­sive two-stroke engines of car­go ships.

All forms of tech­nol­o­gy will be need­ed to achieve real­is­tic decar­bon­i­sa­tion of transportation.

Hydro­gen engines or fuel cells; syn­thet­ic fuels, ammo­nia, methanol… many options are on the table to decar­bonize all long-dis­tance trans­port. “Unlike the con­sumer car sec­tor, where the elec­tric bat­tery will become the dom­i­nant tech­nol­o­gy, the future of this sec­tor will not be ‘mono-tech­no­log­i­cal’. Decar­bon­i­sa­tion is more dif­fi­cult, and uses are more diverse,” sum­ma­rizes Gaé­tan Mon­nier. There­fore, while the future does not belong entire­ly to the hydro­gen engine and its deriv­a­tives, « it is part of the mix of solu­tions that will enable us to meet the challenge.

This con­clu­sion is con­trary to that of envi­ron­men­tal asso­ci­a­tions, such as Trans­port & Envi­ron­ment, which advo­cate the full elec­tri­fi­ca­tion of road trans­port. The Euro­pean Com­mis­sion is expect­ed to set­tle this debate in ear­ly 2023. It must review the def­i­n­i­tion of “zero emis­sion vehi­cles”, which until now has been lim­it­ed to an exhaust emis­sion lev­el of less than one gram of CO2 per kilo­me­tre. “If this cri­te­ri­on is main­tained, it is unclear whether inter­nal com­bus­tion engines using car­bon-free fuels such as hydro­gen or ammo­nia will qual­i­fy, since these engines still burn a small amount of oil. But the tiny CO2 emis­sions that result are dif­fi­cult to quan­ti­fy,” says Gaé­tan Mon­nier. “Aban­don­ing the prin­ci­ple of tech­no­log­i­cal neu­tral­i­ty would be harm­ful, because all forms of tech­nol­o­gy will be need­ed to achieve a real­is­tic decar­bon­i­sa­tion of trans­port “, con­cludes François Kalaydjian.

Hugo Leroux

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