Hydrogen engines, a technological challenge
In the ecological transition process, hydrogen is a promising energy carrier that is fundamentally sustainable, even in agriculture. However, challenges remain regarding the energy efficiency of its production and use, as well as safety in its management
The European Union has been pushing hard for some time now to replace fossil fuels in the automotive sector with other renewable energy sources. On the other hand, electrification is advancing at a rapid pace, although with some critical issues, such as the non-optimal energy density of batteries required to provide satisfactory operational autonomy of vehicles, and an infrastructural network of charging stations that is still numerically too scarce. Among the various technologies currently practicable, the use of hydrogen opens up interesting prospects, also for the options available for its production (see box).
The uses of hydrogen. With appropriate modifications, hydrogen can technically power a vehicle equipped with an internal combustion engine originally designed to run on diesel or petrol. The alternative option involves the use of one or more electric motors, in which a set of fuel cells use hydrogen to produce electrical energy. The main advantage of hydrogen as an energy carrier is that when used in a fuel cell or an internal combustion engine, in terms of emissions it produces mostly only water vapor, without releasing any CO2 into the atmosphere. However, it should be noted that the combustion of hydrogen in internal combustion engines is only slightly efficient, and it has to be taken into account that emissions into the atmosphere do not only include water vapor, but also a certain amount of nitrogen oxides (NOX), due to a small part of the lubricating oil fumes which are not intercepted by the recovery system and therefore disperses into the atmosphere.
To create a truly green future with hydrogen, it is necessary to consider that powering an agricultural tractor (or any other vehicle) with this gas presents considerable difficulties, due to the need to considerably increase its density, a result that can be achieved either by increasing the pressure and/or decreasing the temperature.
The energy balance. The numerous steps required to produce (and then use) hydrogen result in a rather low overall efficiency. Water electrolysis, one of the most widely considered green options, already has an efficiency of 60%, and its subsequent use in fuel cells is characterized by a similar value. The balance of the two transformations brings the result down to 36%, and we must still take into account the efficiency of the electric motor to generate mechanical energy which, although showing an excellent 90%, brings the final result to an unsatisfactory 32%. That is, a good two-thirds of the electricity initially produced is not available for useful purposes. It is no coincidence that even in the automotive sector, notoriously characterized by sales volumes and investments enormously higher than those typical of the tractor sector, there are still few examples of hydrogen-powered vehicles.
The focus on tractors. Several leading manufacturers in the tractor market have already presented models (mostly prototypes), powered by hydrogen, either with an electric motor powered by a fuel cell or with a modified endothermic engine. In the first case, probably the most widely recognized example at the moment is the Helios by Fendt, which has created two experimental prototypes as part of the H2AgrarProject. The vehicle is equipped with 5 hydrogen tanks located on the roof of the tractor, for a total capacity of 21 kg of gas, stored at approximately 700 bar. All of this powers a 100 kW fuel cell capable of generating electrical energy to power a maximum 100 kW engine. The system has a built in 25 kWh battery to accumulate any excess energy generated by the fuel cell, in order to compensate for any load peaks.
Conversely, the H2-Dual Power from New Holland is based on a T5.140 AutoCommand, powered by a 4500 cm³ FPT Nef Stage 5 engine, designed to also run on hydrogen in a mixture of up to 65% with regular diesel. Although it does not reduce the use of regular liquid hydrocarbons to zero, this solution allows CO2 emissions to be kept between 45 and 65%, and also slightly reduces NOx emissions. In fact, it reproduces the same solution that has long been adopted with mixed diesel+methane fuel in the automotive sector, both on commercial vehicles and on cars. In this case, the hydrogen is stored at 350 bar in 5 containers each with a capacity of 11.5 kg, and also positioned above the cab.
Advanced innovation. The quest for renewable and sustainable energy sources aligns closely with the evolution toward the automation of agricultural tractors. Notably, among several experimental projects, the Chinese prototype ET504-H stands out. This electric vehicle, powered by hydrogen fuel cells, was unveiled 4 years ago in Luoyang, China. It is an autonomous robotic tractor without a driver's place, capable of operating both autonomously and via remote control. Equipped with a central drive motor and auxiliary motors for lifting and steering, its electric drive can generate up to 50 Hp, allowing the vehicle to travel at speeds of up to 30 km/h.
In Europe, Exxact Robotics recently introduced the Traxx Concept H2, a hydrogen-powered straddle vineyard robot tractor. It features a fuel cell combined with a small battery pack, providing a total power output of 35 kW. The hydrogen is stored in two pressurized tanks with a total capacity of 9 kg, which, according to the manufacturer, allows the robot to operate for an entire day without refueling. The adoption of hydrogen enables the elimination of a heavy battery pack (necessary for fully electric operation), thereby reducing the machine's weight by 25%. This reduction helps mitigate soil compaction, a significant concern in vineyards.
The colors of hydrogen
Depending on the initial energy source and the processes adopted for its production, it has become common practice to classify hydrogen with certain colors, with varying designations. In general, however, these include: gray hydrogen: extracted from fossil fuels, particularly methane, through “steam reforming”, which however generates large quantities of CO2, normally released into the atmosphere. Today, it is estimated that 95 to 99% of hydrogen is obtained in this manner. According to the World Economic Forum, globally in recent years about 6% of natural gas and 2% of coal have been used for hydrogen production. The resulting CO2 emissions amount to approximately 830 million t/year; blue hydrogen: produced in the same way as gray hydrogen, but the resulting CO2 waste is captured and stored, yet with a significantly higher energy expenditure, thereby significantly lowering the efficiency of the process; green hydrogen: obtained from water through electrolysis, taking care to use electricity from renewable sources (e.g. solar energy, wind energy, etc.). The reaction takes place in electrolytic cells, in which two electrodes (the anode and the cathode) break down the water molecule, releasing hydrogen and oxygen, with an efficiency of approximately 60%. This is a significantly lower value than that which would be obtained by storing the same electrical energy used in common lithium or lead batteries, which achieve an efficiency of over 90%. Green hydrogen has the advantage of being totally “clean”, but currently accounts for less than 1% of global production; purple hydrogen: again obtained through electrolysis, but using the energy produced by nuclear power plants, which also have almost zero CO2 emissions.
For the agricultural sector, there is also another very interesting option to be considered, namely hydrogen obtained from biomass gasification, which typically contain approximately 50% carbon, 6% hydrogen, and 41% oxygen. However, this is a complex process, with a rather low production-energy balance. If significant process improvements can be achieved in the near future, it would open up opportunities for the extremely promising market of the agricultural sector.
Details on the alternatives for using hydrogen
Fuel cell. Similar to the well-known “battery”, the fuel cell is made up of a positive and a negative pole, and allows the electron contained in a hydrogen atom to be "isolated", thereby generating the positive ion H+, which combines with oxygen, producing water. The electron directly powers the electric motor, or is stored in a buffer battery, to be used later.
The fuel cell is then connected to an electric-drive motor, which however does not always use all the energy produced; the excess can be stored in a battery pack for use by the vehicle. It might have a lower capacity than that of a fully-electric car, as it acts as an "energy tank" under less demanding driving conditions, and then release it when greater performance is required.
Modified internal combustion engine. The architecture of this engine is traditional, with cylinders, pistons, valves, injectors, etc. This is undoubtedly a simplification of the starting base, because it involves adapting components that have ultimately evolved over more than a century, although it should not be forgotten that by its nature the internal combustion engine is less efficient than an electric one. The hydrogen combustion reactions in an internal combustion engine occur at high temperatures, and still emit nitrogen oxides and CO2, although in quantities significantly lower than those emitted by petrol and
Safety in hydrogen management
One of the most sensitive issues on the subject undoubtedly concerns storage cylinders, because the gas in them is compressed to hundreds of bars of pressure. Thus, the UNR134 standard contains “Uniform provisions for the approval of motor vehicles and their components with regard to the safety performance of hydrogen vehicles”, in which some of their technical characteristics are defined which provide an adequate safety margin in relation to progressive degradation phenomena, which may lead to fatigue failure.
In other words, it is the definition of the limit of a material to be "used" for a certain period of time, beyond which its entire replacement is necessary. UNR134 specifies that suitable materials for the construction of storage tanks are Glass Fiber Reinforced Polymer (GFRP) for the external cladding and Carbon Fiber Reinforced Polymers (CFRP) for the internal cladding. Thanks to its high density, GFRP is very resistant, while CFRP is characterized by high tensile strength, excellent corrosion resistance and remarkable lightness.
