Power and Propulsion

Doing away with the Diesel engine

Sarah Carter
Sarah Carter

05 December 2018

Doing away with the Diesel engine

Another new technology that is forecast to revolutionise shipping is the use of fuel cells. It has to be said that the experience to date has been below expectations and for a while most companies that were pursuing the idea seemed to have put it on the back burner. Only a tiny number of fuel cells have been installed and all have been of quite low output considering the typical demands of the ships they were installed on.

Fuel cells are generally seen as being clean with zero emissions other than water and heat. While that may be true of the stack that is the main component of a fuel cell, there are more aspects to the technology that need to be taken into account.

The usual description of a fuel cell is a device for combining hydrogen with oxygen in an electrochemical reaction with electrical energy being produced for power utilisation and water and heat as by-products. What is often omitted is that there are many more components needed to reform the fuel if it is not pure hydrogen: heaters, pumps and more that make up the fuel conditioning and delivery systems along with starting air compressors and exhausts of a conventional Diesel engine. These ancillary systems require their own power source which, depending upon the type of fuel cell, may produce undesirable by products and pollutants.

In its simplest form as a proton exchange membrane (PEM) fuel cell, the only requirements are a constant feed of hydrogen to the anode and oxygen via air to the cathode. The hydrogen is fed to the anode of the fuel cell where the electrons in the hydrogen are separated to pass as an electric current to the motor or other system. Splitting of the hydrogen molecule is relatively easy by using a platinum catalyst. The electrons continue on to the cathode to meet the hydrogen ions (protons) which have passed across the membrane and are combined with oxygen from the air to form water.

To function, the membrane must conduct hydrogen ions but not electrons as this would in effect “short circuit” the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as to the harsh oxidative environment at the anode.

Using hydrogen as fuel presents problems for ships that may be unsurmountable for large vessels making long voyages. Hydrogen in gaseous form is by a large measure the least energy-dense fuel possible so must be refrigerated and kept under pressure as a liquid for a PEM fuel cell to be viable for marine use. Although it is the hydrogen that is important for a fuel cell to function, it is not necessary for it to be kept as pure hydrogen. Any fuel that contains hydrogen – including diesel, LNG, methanol and many more can be used instead. However, except for a very few fuel types, the fuel has to be reformed to extract the hydrogen before it can be used in the fuel.

As an example, methane as LNG (CH4) can be reformed by using steam (H20) to produce hydrogen and carbon monoxide. The hydrogen is then used to power the fuel cell while the carbon monoxide reacts further with some of the oxygen making the waste products of the fuel cell water and carbon dioxide. In another type of fuel cell, methanol (CH3OH) can be used directly as a liquid alone or mixed with water with all of the hydrogen atom electrons producing the current. Water and CO2 are again the waste products.

Although it is possible for oil fuels to be used, any sulphur and metals present will contaminate the fuel cell and rapidly degrade it unless great care is taken to remove the contaminants before the hydrogen is fed to the fuel cell. With oils,

as with any fuels containing carbon, CO2 will be one of the waste products and this characteristic must be considered as not meeting the decarbonisation targets.