Background on hydrogen

This page provides some background information on the use of hydrogen as an energy carrier. Please note that the sole purpose is to give a concise overview of aspects related to hydrogen, the interested visitor is referred to the links page for sources containing more detailed information.

Internal combustion engines versus fuel cells

The incentives for a hydrogen economy are the absence of harmful emissions, the sustainability (potentially CO2-free) and the energy security. When considering hydrogen as a fuel for transport applications, most people make the link to fuel cells (FC). However, our laboratory focuses on hydrogen-fuelled internal combustion engines (ICEs) for the following reasons.

The internal combustion engine has benefited from a continuous development during more than a century and is still showing potential for further optimisation. Fuel cell technology on the other hand is still in its infancy. This also reflects in the price, with a prohibitive cost for fuel cells. Naturally, the conversion of an ICE to hydrogen increases its cost but this cost is very limited (not counting the cost of the hydrogen storage and safety devices, as these are needed regardless of the propulsion unit, the only additional cost of hydrogen fuelled ICEs is the cost of additional H2 injectors, a modified engine control unit and possibly some changes to the ignition and crankcase ventilation system).

Using ICEs allows bi-fuel operation (e.g. the engine can run on gasoline as well as on hydrogen), alleviating fuel station density and autonomy requirements. This could facilitate the start-up of a hydrogen economy, where the experience gained with transport, fuelling and storage directly translates to fuel cell vehicles. Fuel cells are currently still handicapped by cold-start problems (freezing of the fuel cell stack) and the necessity of very pure hydrogen to avoid poisoning of the FC. The hydrogen fuelled ICE does not suffer from these problems.

The most frequently hailed advantage of fuel cells is their high theoretical efficiency. However, not only do practical fuel cells not (yet) reach these high efficiencies, the fuel cell stack (of which the efficiency is mostly cited) is also part of a fuel cell system and the overall efficiency is thus lower. Furthermore, the efficiency decreases as the load increases. This is not an important disadvantage for light-duty applications as these are in part load most of the time, but could become important for heavy duty. The large difference between the theoretical efficiency of the fuel cell stack and the effective efficiency of an ICE thus mostly exists on paper and is much smaller in practice. Furthermore, hydrogen fuelled ICEs also have the potential for an increased engine efficiency, with a demonstrated indicated efficiency of 52% for a hydrogen fuelled spark-ignition engine [1] and a power generation efficiency of 49% for a hydrogen fuelled compression-ignition engine [2].

Finally, the current number of motor vehicles worldwide is estimated at about 800 million. To replace these in a relatively short time by fuel cell vehicles is impossible.

In summary: the hydrogen fuelled ICE and FC both have their own advantages and both merit research to show their full potential. The hydrogen fuelled ICE can function as a transition technology to fuel cells or might take up its own share of the market next to fuel cells (and other technologies).

[1] Tang, X. et al. Ford P2000 hydrogen engine dynamometer development. SAE, paper nr 2002-01-0242, 2002.
[2] Akagawa, H. et al. Development of hydrogen injection clean engine. 15th World Hydrogen Energy Conference, paper nr 28J-05, Yokohama, Japan, July 2004.

Storage

Vehicle manufacturers and research institutes now seem to agree that direct fuelling of hydrogen is the best option. In the past, on-board reforming was frequently suggested, fuelling e.g. methanol, gasoline, sodium borohydride (from borax), ... and producing hydrogen on board the vehicle. This idea seems to have been abandoned, as this reduces the overal efficiency. The US Department of Energy has stopped financing research into reforming technology.

There are several methods of storing hydrogen on board a vehicle:

  • In liquid form (LH2)
  • In gaseous form (GH2)
  • In 'solid' form, adsorbed on metal hydrides (MH)
  • Others that are still at a laboratory stage, e.g. in carbon nanotubes (CH2)

The single greatest challenge for hydrogen storage is its ultra low density. At atmospheric pressure and temperature, hydrogen is gaseous with a density of about 0.08 kg/m³. This is 15 times lighter than air, and for an equivalent energy content, it takes about 3500 times as much volume as gasoline. Reducing this volume implies pressurizing up to very high pressures (700 bar) or liquefying (at -253 °C) or adsorbing hydrogen on a large internal surface solid material.

The following figures give an idea of the challenge of replacing the current liquid fuels with gaseous fuels such as hydrogen.

  • In energy equivalent terms, 1 kg of diesel fuel is equivalent to 0.35 kg of hydrogen, 1 liter of diesel fuel being equivalent to 0.293 kg H2, which is 3.50 Nm³ H2
  • 1 kg of gasoline is equivalent to 0.36 kg of hydrogen, 1 liter of gasoline being equivalent to 0.276 kg H2, which is 3.29 Nm³ H2
  • Stored in compressed gaseous form at 700 bar, it takes about 10 liter of compressed hydrogen for the energy equivalent of 1 liter of diesel (9.2 liter GH2 for 1 liter of gasoline)
  • Stored in liquid form at -253 °C, it takes about 4 liter of liquid hydrogen for the energy equivalent of 1 liter of diesel

Production

In contrast to fossil fuels, hydrogen cannot be found in free form and must be produced. It can be viewed as a high quality energy carrier, similar to electricity. However, hydrogen is more easily stored than electricity which makes it suitable for e.g. transport applications. Several methods exist for producing hydrogen:

  • Steam reforming of natural gas
  • Partial oxidation of heavy fuel oil or coal
  • Electrolysis using e.g. renewable electricity
  • Hydrogen from biomass
  • Direct thermal solar
  • Other methods that are still at a laboratory stage, e.g. photochemical, photobiological etc.

Steam reforming of natural gas is currently the cheapest method of producing hydrogen. However, this method as well as the partial oxidation method, produces CO2. Thus, using hydrogen produced by these methods will in practice increase total CO2 emissions compared to a direct use of natural gas, oil or coal. Consequently, it is not a long term option.

A sustainable method of producing hydrogen must use renewables. Possibilities are using renewable electricity (from wind, hydro, photovoltaics, geothermal, tidal etc.) for water electrolysis, hydrogen from biomass and others. Peak shaving of wind turbine power through hydrogen production is an especially interesting option, using electricity that would otherwise be considered 'junk' electricity, to produce hydrogen.

Transportation

Did you know that the world's longest hydrogen pipeline runs through Belgium? It is operated by Air Liquide and runs from the north of France to Antwerp in Belgium, for a total of 400km.

Hydrogen can be distributed in gaseous or liquid form through pipelines. A number of the existing natural gas pipelines can be used without change for hydrogen, others can be retrofitted with a special liner to be compatible with hydrogen. This is necessary to avoid hydrogen embrittlement. Studies have shown that up to 20 vol% of hydrogen could be added to the existing natural gas grid without changes to the appliances.

However, it is still unclear what would be preferable: central hydrogen production and distribution to the users, or on-site production without need for transportation.