Hydrogen as a fuel: the pros and cons

Although it could be the solution to the greengouse-gas emission problems caused by means of transport, for various reasons this technology is still having difficulty taking off

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For years now, we've been hearing about hydrogen as a non-polluting fuel for cars, but what are the pros and cons of its production? And why is it being used so little? “It could constitute a breakthrough for the planet as the waste generated by its use to produce energy is water vapour, but today more than 96% of hydrogen production still derives from non-renewable sources, effectively cancelling out its green properties”, explains Professor Alessandro Abbotto, a lecturer of Organic Chemistry at the Milan-Bicocca University Department of Material Science and the author of the book entitled “Idrogeno: tutti i colori dell'energia”, (literally, Hydrogen: all the colours of energy) published in 2021 by Edizioni Dedalo that covers all its applications and analyses the related political, economic and social debate.

The three advantages of using hydrogen as a fuel 

1. It is a clean fuel

Hydrogen is a perfectly clean fuel, because the only waste it produces is water vapour. In its free state it consists of two atoms (H2) which, when combined with oxygen (O) during its use (combustion or, more commonly, in a fuel cell), generate water (H2O).“These conditions – explains Professor Abbotto – are sufficient to take on the environmental emergency, something we can no longer postpone.” Hydrocarbons, on the other hand, are made up of carbon and hydrogen and, during combustion, when combined with oxygen, produce carbon dioxide (CO2) and other waste that is harmful to the environment and to human health (nitrogen and sulphur oxides).

Hydrogen is obtained through electrolysis of water, a simple method in which a low voltage current flows through water to form oxygen and hydrogen gas. Hydrogen defined as green is the only sustainable hydrogen because it is obtained through electrolysis of water powered by electricity produced from renewable sources. Grey hydrogen, on the other hand, uses fossil fuel sources, mainly natural gas, which produce greenhouse-gas emissions, thereby undermining its zero impact on the environment.

2. It uses more efficient technology

The combustion engine, launched in the middle of the 19th Century and never abandoned since, allows the car to move thanks to the combustion between fuel and air which is converted into thermal energy and in turn into mechanical energy. In almost 200 years this engine has reached its maximum performance and optimisation and is today no longer sustainable due to the strong environmental impact of the waste produced.
On the contrary, the hydrogen combustion engine uses technology which stands out for the absence of any harmful emissions. Its main use, however, is not in the combustion engine but in a fuel cell, developed for space exploration since the 1960s, whereby an electrochemical process combines hydrogen and oxygen to generate electrical energy, which in turn powers an efficient electric engine.

The petrol engine uses only 20/25 % of the energy introduced and consequently 75/80% of the fuel is dispersed, producing heat. This is why, for example, you cannot touch an engine without getting burnt. These figures are not very well known, but they indicate how inefficient the combustion engine powered by petrol or diesel is. “We had so much energy that we could afford the luxury of wasting it and extracting crude oil was so cheap, but now that it is running out and that all the related environmental issues can no longer be ignored, something will have to change,” explains Professor Abbotto.

In the electric engine, the percentages are entirely reversed. 80% corresponds to the energy used and only 20% corresponds to the energy dispersed. However, hydrogen is not immediately and directly exploited within a car engine because it must first be converted into electrical energy to power the engine. “This passage consumes 50% of the energy and so this 80% is halved, reducing the amount of energy used to 40% which is, however, twice as much as that of a petrol engine. With the studies and experiments already under way, it is assumed that this percentage can be significantly increased, while that of the petrol/diesel engine can no longer be further optimised”, adds Abbotto.

3. It is convenient for heavy transport and trains

Hydrogen propulsion is not yet widespread today and one of the sectors for which it could be convenient from the outset is heavy-duty transport or trains. These are means of transport which, if they were to be supplied with electric batteries as an alternative to the combustion engine, would require enormous, heavy batteries with extremely long charging times. On the contrary, hydrogen offers the advantages of a more compact propulsion system, with rapid refuelling times and a long travel range, which can be powered at charging docks located along the motorways most travelled by HGV fleets, without the creation of a capillary distribution network, or along the railway lines at all the main stations.
In Asia (South Korea) some industrial vehicle manufacturers offer a “turnkey service” by providing HGVs for goods haulage and guaranteeing the distribution network of green hydrogen. “It is easier for heavy goods vehicle manufacturers to do this than for car manufacturers because the HGVs travel along standardised routes and the rail haulage is organised in a very similar way,” Abbotto explains.

Almost 50% of railway lines in Europe are not electrified to date. There are many routes in Europe where an aerial power line is just impossible, so the trains are powered by diesel, a highly polluting fuel. Hydrogen would solve both the problem of lack of electricity and the emission of pollutants. In Valcamonica, for instance, the 104 km-long Brescia-Iseo-Edolo railway line will be served by hydrogen trains from the beginning of 2024, while Germany has had a zero-impact hydrogen line since 2018.

The disadvantages of using hydrogen as fuel 

1. If it is “grey”, it pollutes

If it is not produced using renewable sources, hydrogen pollutes. To date, more than 96% of the hydrogen used is grey. It costs less, but its impact on the environment is so great that 10 kilos of carbon dioxide are produced for every kilo of hydrogen obtained. World hydrogen production is about 70/75 million tonnes, with a waste of almost 1 billion tonnes of carbon dioxide. “The biggest challenge is to produce clean hydrogen at an affordable cost,” Abbotto says.
The spread of green hydrogen can go hand in hand with the increase in electricity production from clean sources. “And the good news is that by 2030, 70% of electricity production will have to be generated from renewable sources,” Abbotto explains.

2. It is a gas that is difficult to handle

Pouring petrol into a tank is quick and easy, just as it is hooking up the cable to recharge the battery of an electric car. Hydrogen, on the other hand, is a difficult gas to handle because, having a low volumetric energy density, it has to be highly compressed at high pressures (from 350 to 700 bar) to be packed into a tank in sufficient quantities to power a car. 5/6 kg of hydrogen are required to cover about 600 km. In a car's tank, if it were not compressed, there would be enough hydrogen to cover just 5 km.

Another difficulty lies in transport, in other words, how to carry hydrogen to the refuelling stations to supply travelling cars. Here, the first problem consists in the fact that in order to be distributed, special pipelines are necessary because those intended for methane and natural gas are not fully compatible, unless natural gas-hydrogen mixtures with a low hydrogen content are used. The alternative would be to carry it in liquid state, like oil derivatives, but the liquid state of hydrogen is reached at a temperature of -253 degrees Centigrade, with heavy energy expenditure to transform it and then keep it in the liquid form. “At present, and in most cases of industrial use, hydrogen is produced in the same place as it is used,” Abbotto explains.
There are a few hydrogen pipelines but they extend for just a few thousand kilometres all over the world. Building an entire network of hydrogen distributors for automotive transport involves considerable costs and delays. Germany holds the leading position for distribution, with almost 100 stations, a number that is constantly rising; they are located along the motorway arteries, making it possible to travel within the country. In Paris, hydrogen is produced locally, allowing a taxi network to run efficiently with the aim of having half the fleet powered by hydrogen by 2024. In Italy there is just one station for cars on the Brenner motorway in Bolzano Sud. Here, hydrogen is locally sourced from the hydroelectric power produced by the mountains. 

3. It is less advantageous than electric power for cars

The battery-powered electric motor is now the most efficient system because it converts 80% of the electricity in the battery into energy. What's more, it is currently cheaper to charge an electric car than to refill on hydrogen. “Grey hydrogen costs $1-2 per kg, while green hydrogen costs $5-7 per kg” Abbotto explains. If the “green” advantage of using hydrogen is preserved also during the production stage, refilling a car must only involve green hydrogen. And suppliers sell green hydrogen at a price of around €14 per kg, which can fall to €9 per kg where infrastructure is more developed such as in Germany.
But the hydrogen engine still remains much more efficient than a conventional petrol/diesel engine. Diesel offers a small financial advantage over hydrogen, but the enormous environmental benefits would offset the higher cost. One thing is certain: hydrogen is less beneficial for light road transport compared to electric power from renewable sources. The environmental benefits are the same, but the cost of refuelling and the availability of a network for charging are better.