As a semi tradition, one of our summer intern assignments is to do a little project that summarizes issues that run through Rik’s head.
In this case Abigail Crow, UVA Systems Engineering Sophomore, was asked to get organized on ship alternative fuel efficiencies.
This is her summary:
Determining the most suitable fuel for a ship is a complex task influenced by numerous factors.
Some may even consider it nearly impossible, as there is rarely a single definitive answer, and
preferences often vary from one vessel to another. Nonetheless, it is essential to evaluate all trade offs. For example, a fuel that may be cost effective could also pose significant safety risks to the crew.
The decision should not rest solely with shipowners as port authorities and other stakeholders
must also be involved in providing the fuel to the ships. In addition to addressing the ship’s operational needs, fuel selection must consider the shore based requirements, such as manufacturing, storage, and distribution infrastructure. Therefore, the chosen fuel must support both onboard operations and land based logistics.
This diagram provides a summary for providing propulsion fuel to ships.
The diagram illustrates the energy efficiencies of various fuels, both derived from the
sun and otherwise.
At the top of the diagram, direct solar electricity demonstrates approximately 30%
efficiency, meaning 30% of solar energy is converted into usable power using PV.
Fossil fuels, though originally solar-derived, are considered a “dead end” in terms of renewability, as none of their energy is reusable. Since fossil fuels are the product of a transformed organic matter, they can never be reused. Other energy sources show varying efficiencies for producing electricity:
● Wave energy: 40%
● Tidal energy: 35%
● Hydropower: 90% (the highest)
● Wind energy: 60%
Biofuels bypass the electric phase, but have inherently very low efficiencies and many negative side effects.
Several E-fuels are currently under consideration for marine use. Hydrogen generation has an efficiency slightly under 60%, making it a relatively efficient option. Ammonia generation efficiencies range between 24% and 31%, methanol around 29%, methane at 35%, and ethanol has a similar range at 20–30%. These fuels can then serve as onboard inputs to generate power.
Nuclear energy, while not solar-derived, remains a highly efficient alternative. Whether directly utilized or transformed into another form, it offers consistent and potent energy output, although it introduces concerns around safety, cost, and regulatory complexity. On large ships nuclear power could be the prime propulsion option. While batteries as propulsion energy have limited usefulness aboard ships, they are a very efficient approach due to low conversion losses.
Though the diagram effectively highlights efficiency differences, it does not account for the broader context of each fuel type. For example, hydropower may offer 90% efficiency, but it often entails high capital costs and potential ecological damage. Tidal energy offers low operating costs and long-term potential but is limited by location and intermittency. These trade-offs, absent from the diagrams, are essential to making a well-informed decision.
Selecting the best marine fuel requires a balance of multiple considerations such as efficiency, safety, environmental impact, infrastructure compatibility, and stakeholder input. While the diagram provides helpful comparisons of energy efficiency, it overlooks real world trade-offs and context-specific challenges. Furthermore, recent history has shown that these conversion efficiencies can change rapidly when new technologies come online. Ultimately, the ideal fuel choice depends on a comprehensive evaluation of both technical data and practical constraints.
Rik comments:
I have often wished for a diagram like this for inclusion in further technical discussions on fuel selection.
I particularly like that the diagram shows that, with the exception of nuclear, all energy is solar.
Once the fuel is in the ship there are further conversion losses such as occur in fuel cells and IC motors.
Too often individual component promoters ignore the whole efficiency chain. Having said that, if there is ample electrical power, downstream efficiencies become less significant.
While wind is only occasionally a viable option, its direct simplicity should not be ignored.
