# Developments in Electric and Green Marine Ships
## 3 Technologies that Did or Will Shape the Future of Naval Travel and Trade
### Shaft Generators and Other Mechanical Developments Improving Efficiency  
A very good example of such a technology is shaft generators (SG), which reduce the fuel consumption, and thus the emissions generated, for electricity production. In order for that technology to be understood by an audience not familiar with maritime technologies, a better understanding of a ship's power system is required [41].  
A vessel generates power for both the ship's operational equipment, as well as crew accommodation, such as the galley and cabin lighting. To achieve this, a standard generator burns a large volume of marine diesel fuel, which increases the operational cost, requires more frequent maintenance of the generator, and contributes to air pollution. Merchant vessels typically spend most of their operational life sailing long distances, and fuel economy is the most important factor after safety and reliability. As an alternative, an SG is driven by the ship's main engines--which, when compared to auxiliary diesel generators, do generally have lower fuel consumption and can run on less expensive heavy fuel oil (HFO) or LNG. However, saving money on fuel is only part of the equation. Tightening regulations on ship emissions means that reduced fuel consumption not only increases efficiency and reduces cost, but also lowers overall emissions and complies with the regulations.  
Similar to any other technology, SGs come with problems and barriers that have to be overcome before the implementation. According to [41], the downside of this arrangement was that the propulsion machinery could only operate at a constant speed in order to maintain the network frequency within limits when operating with a shaft generator arrangement. This occurs because the ship's network frequency and propeller rotational speed are interconnected. As a result, any change in speed directly affects the network frequency, which was traditionally overcome by controlling propulsion thrust and ship speed, by changing the propeller pitch. However, that operation can lead to decreased efficiency and increased CO\({}_{2}\) emissions, contradicting the reason a shaft generator was installed in the first place.  
In consequence, the ideal operational scenario would be to enable efficient and reliable power generation when the ship's propulsion system operates at varying speeds, such as during maneuvering or heavy weather conditions. Adding a frequency converter to the shaft generator makes this possible. Apart from the aforementioned benefits, once the frequency converter is fitted as part of the shaft generator system, it can also be used to adapt various shore supply voltages and frequencies, with no need for additional panels in the main switchboard. Connection to an onshore power supply is possible by utilizing the existing synchronizing equipment, having the advantage of not using the auxiliary diesel engines in the harbor, which is an additional environmental benefit. The arrangement is often used to supply the thruster motors via the SG system with split busbars during maneuvering, further improving the efficiency and system flexibility.  
As mentioned above, SGs are only one among many other technologies already operating in vessels to improve their efficiency and reduce emissions. Examples of these include exhaust scrubber, waste heat recovery systems), exhaust gas recirculation (EGR), air lubrication systems, fuel cells, Propeller Boss Cap Fin (PBCF), de-rating engines, slow steaming, operational data analysis, and optimized [42; 43].