# Developments in Electric and Green Marine Ships
## 1 Introduction  
In the present decade, one of the most discussed issues is that of tackling and mitigating society's negative impact both on human health and the environment [1]. Although there is still great debate on the actual size of the human contribution to the problem, cutting down emissions is surely a positive step [2; 3; 4; 5; 6]. Society pollutes and harms the environment through various ways, such as unhealthy emissions such as sulfur, mainly through SO\({}_{2}\), NOx and greenhouse gases (GHGs) [7; 8; 9; 10; 11]. The maritime industry is part of the transportation sector, which is responsible for 14% of the global GHGs, making it the fourth biggest sector in emissions globally [12]. Although ships are thought to be some of the biggest polluters, the maritime transport of goods is a relatively clean form of transportation, per kilogram of material, and is therefore given an increasing weight over air and road trade and transport [13; 14; 15]. In European coastal areas, shipping emissions contribute 1-7% of ambient air PM\({}_{10}\) levels, 1-14% of PM\({}_{2.5}\), and at least 11% of PM\({}_{1}\). Contributions from shipping to ambient NO\({}_{2}\) levels range between 7-24%, with the highest values being recorded in the Netherlands and Denmark [16]. Maritime produced SOx and NOx create significant problems that will be further explained below. Taking into account the huge and constantly growing sector of worldwide trade and transportation, it becomes clear that even the cleanest means of transport per kg of material/good has devastating effects on our health and the environment [17]. The many sources of air pollution caused from the maritime industry can be seen in Figure 1.  
Reducing transportation and trade cannot be considered a realistic solution, as it is critical for international growth. Therefore, making the maritime industry more efficient and environmentally friendly is unavoidable. The idea of reducing the negative environmental impact is present in every economic sector, making it the ideal time to implement that in maritime as well [18; 19; 20; 21]. Previously taken steps toward this direction were mostly indirect, through technologies primarily targeting the improvement of the vessel's engine efficiency. However, green technologies and renewable energy sources (RES) have strongly infiltrated sectors such as energy production and storage and cars [22; 23; 24]. Following the same logic, the shipping industry is already starting to change and implement technologies that reduce the environmental impact. There are numerous research studies, experiments, and prototypes trying to implement almost every alternative technology and fuel in vessels. Although it is not yet known which of these technologies will become dominant in the future, some of them  
Figure 1: Sources of maritime air pollution. have a strong statement of qualifications, and seem to be commonly accepted at present. As far as the alternative fuels are concerned, biofuels, hydrogen, liquefied natural gas (LNG), liquefied petroleum gas (LPG), and methanol are among the most representative. Ship engine manufacturers have started producing dual-fuel gas engines to allow the use of both natural gas and conventional fuels. In terms of new technologies, batteries, fuel cells, and wind-assisted propulsion are being strongly considered [25]. The above-mentioned alternatives have different advantages and disadvantages, regardless of their short or long-run implementation, or just being part of the transitional stage of the evolving maritime industry. A solution that suits all different types of vessels is probably impossible, as there are different capabilities and needs depending on teach type. Nowadays, digitalization is rapidly growing and affects almost every industry. In the naval sector, this impact is mostly through big data gathering and analysis, as well as real-time monitoring (temperatures, emissions, etc. ), and it brings along with automation, another growing field, fundamental changes such as autonomous shipping [26].