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Electric drive

Now, that we have all that renewable energy produced - most efficient way to use it, would be using batteries and drive the electric propulsion engines directly by wind and solar generated power. It works i.e. for small scale such as water taxi / bus or on other fix routes such as ferries. 

The electric drive by example with Azimuth pod thrusters is rather a great solution by handling different workloads (peaks) economically, high grade of efficiency, low vibration - so less noise emission, but: how to get the power onboard? Huge accumulators / batteries would weight and cost to much to install on a vessel as main power supply as big vessels have big power needs. A different story is using them for short-term energy storage like a buffer for peak demand in combination with main engines supplying the power i.e. diesel-electric drive on AIDA-ships. These internal combustion (diesel) engines in MW size could be modified to run also on methane (LNG), but unfortunately not on hydrogen due to the higher temperature and pressure. It wouldn't be 'zero-emission' anyway considering around hundred litres lub oil being burned every day together with the fuel. 

Revival of Steamships - Turbo-Electric Drive

When I say 'hydrogen', most of us immediately have 'fuel cell' technology in mind. There are existing passenger vessels running on hydrogen (also small scale) in the sea-port cities Hamburg and Amsterdam on fuel cells. But large vessels need more power. The biggest fuel cell I could 'search and find' is a 1 MW fuel cell build in a submarine which costs according to my information approximately 5 million Euro and needs the 'stacks' replaced after approx. 5 years.

Thus, regarding 'feasibility', internal combustion engines and fuel cells are not the means of choice as they need further research and development. What we do have 'ready to buy' are 'external combustion engines' or more precise: Steam turbines.

Many people may think now about the old days steamships that were fired with wood, coal and oil. That steam engines ran commonly with pistons and they were by far not that efficient and could not cope with the „new“ smaller internal combustion engine. So they slid into obscurity.

Nowadays there are high efficient steam turbines on the market (i.e. in most power plants as well as on ships) - ready to buy - with up to 40% efficiency, even capable handling a range of different workloads efficiently. The lubricated, rotating parts are in a separate, external chamber - thus it’s called an external combustion engine. In the combustion chamber itself the hydrogen can burn slowly and completely. In this manner no NOx or other emissions from burnt lub oil occur. These can be used as generators. Until the day that fuel cells become more powerful and the prices more reasonable - the steamship is the means of choice when realizing a seagoing zero-emission fleet. In recent years hybrid propulsion systems proved increased efficiency due to better handling of different workloads. Combining the advantages of these two technologies into a turboelectric drive is the idea especially suited for feeders, short sea shipping and inland navigation were different workloads are more often the case.