In the face of dwindling fossil fuel reserves and tightening environmental restrictions, the shipping world is turning to low-carbon propulsion technologies. Future ships will rely on a combination of power sources and power-saving technologies.

In terms of noxious emissions per tonne of cargo carried, ocean shipping is still the least polluting mode of transport. Nevertheless, conventional fuel oils are a threat to the environment and to world climate. They are not sustainable and will eventually be simply too expensive. Since no single, universal alternative source of propulsion energy is available, future ships will likely be powered by a variety of sources.

With tighter emission limits coming into force in Emission Control Areas (ECA), the next logical step towards low-emission propulsion systems is liquefied natural gas (LNG), which is much cleaner than heavy fuel oil. Provided that the necessary supply infrastructure is established, LNG could be the ship fuel of choice for decades to come. Using LNG would eliminate SOX emissions and significantly reduce NOX, particle and greenhouse gas emissions.

The worldwide fleet of LNG-powered ships is increasing rapidly. But while LNG is still available in vast quantities, it is a fossil fuel nevertheless and does emit CO2. Achieving full independence from fossil fuels in the long term will be inevitable.

Technical challenge

Efforts continue to explore other alternative energy sources and develop technology for their economical use. In ports, ships can practice “cold ironing”, powering their on-board systems with electricity from the grid. This can even be carbon-neutral if renewable energy is used. Fuel cells and battery systems can power smaller craft and some on-board systems but may not be practical for larger propulsion machines. However, new on-board electricity management concepts, such as DC grids, can help reduce overall energy consumption.

While storing major volumes of electricity directly remains an unsolved technical challenge, many experts recommend using excess offshore wind energy or photovoltaic electricity to produce hydrogen by electrolysis. Hydrogen can power fuel cells or be combined with trapped CO2 to produce methane (synthetic natural gas, SNG) for use as a natural gas substitute. SNG avoids the problems associated with transporting and storing hydrogen, which is extremely volatile. Furthermore, SNG can be distributed using the existing natural-gas supply infrastructure.

Biofuels produced from agricultural waste, highenergy crops or algae could contribute significantly to low-carbon shipping. Biofuels can be mixed with conventional fuels to compensate for dwindling supplies. While expected to play a significant role in the future energy mix, they have drawn criticism for driving up the cost of food and animal feed crops and competing with food crops for the available cultivable acreage.

Carbon capture systems (CCS) allow continued, climate-neutral use of fossil fuels by trapping the carbon emissions and storing them or converting them into an ecologically neutral form. CCS are an established technology and may prove useful on board ships, provided the costs can be brought down and the trapped carbon can be recycled economically. New power sources require new propulsion machinery. Advanced gas-only and dual-fuel engines or hybrid systems have been developed and installed in some commercial vessels. Research continues to develop cost-efficient, eco-friendly on-board energy systems.