Electric boats might be grabbing the headlines but, with diesel-powered craft still dominant, isn’t it time we looked at greener fuel alternatives?
While there’s understandable excitement surrounding the new generation of electric boats from the likes of X Shore, Candela and Delphia, the truth is that battery power won’t be replacing the internal combustion engine in larger planing craft any time soon.
The physics just don’t add up due to the weight, volume and cost of batteries required. Depending how you calculate it, a kilogram of diesel packs 30-100 times the energy of a kg of lithium ion battery.
There’s still a place for hybrid diesel-electric drivetrains that will allow boats to creep silently into port, but the combustion engine is likely to remain the primary source of power on larger craft.
Instead, the focus is shifting away from the engines to the fuels that they burn. Biodiesel manufactured from plant oils is one possible solution, but scientists are also developing new forms of fuel that emit fewer greenhouse gases and don’t depend on extracting them from the ground in countries with unsavoury regimes.
So what are the most promising diesel alternatives?
5 of the best diesel alternatives
Hydrogen can be used in fuel cells to generate electricity or burnt in a combustion engine. In both cases the primary emission is water rather than carbon dioxide. It can also be produced cleanly by electrolysing water into its constituent elements.
However, it has a number of drawbacks. Electrolysis requires a lot of power so at the moment the majority of commercially produced hydrogen is made by reacting natural gas (a fossil fuel) with steam to create hydrogen, carbon monoxide and carbon dioxide.
It’s also a very voluminous gas with a relatively low calorific value that needs compressing to at least 350 bar and storing in high-pressure tanks or freezing to -253c into a liquid form to carry on board a boat in sufficient quantities, both of which are energy-intensive processes.
Pros: Burns cleanly, non-toxic, efficient source of energy
Cons: Expensive to produce, highly flammable, no smell, leaks easily
Methanol has a number of advantages over hydrogen in that it’s a clear liquid that doesn’t need to be stored under pressure or at freezing temperatures and can be burnt in conventional petrol and diesel combustion engines.
However, its energy density is around half that of petrol so requires twice the volume to store it. It is also toxic to humans and corrosive, requiring a number of adaptations to the fuel system.
On the plus side it burns cleanly, producing water and carbon dioxide but fewer particulates and sulphur/nitrogen oxides than diesel, making it an attractive option for boats.
Currently, the majority of methanol is produced from natural gas by reforming it with steam and distilling the resulting gas into methanol. However, it is also possible to produce it cleanly by fermenting biomass waste from wood.
Pros: Liquid at normal temperatures, simple conversion from diesel engines, reduced emissions, not as explosive as petrol
Cons: Low energy density, toxic to humans, highly corrosive
Like hydrogen, ammonia is a flammable gas that can be burnt in a combustion engine without creating any carbon dioxide but whereas hydrogen can only be stored under extreme pressure or temperatures, ammonia can be stored at room temperature at a relatively modest 10 bar or as a liquid at -33C.
It also has a higher energy density of 12.7MJ/L compared to 8.5MJ/L for liquid hydrogen. And because ammonia is used in the production of fertiliser, the infrastructure for producing, transporting and storing it already exists.
When ammonia burns it produces water, nitrogen and nitrogen oxides but no soot or CO2. The vast majority of ammonia is currently produced by converting natural gas or liquid petroleum gas into hydrogen, which is then reacted with nitrogen to create ammonia.
However, if that hydrogen is created through solar or wind-powered electrolysis then it can be considered a green, renewable fuel.
Pros: No CO2 or soot when burning. Can be produced from renewable energy, and can be stored as a pressurised gas or liquid
Cons: Toxic at low quantities, production still uses a lot of energy
Formic or methanoic acid is still being tested but it holds some promise both as a fuel and a means of carbon capture.
In theory carbon dioxide can be converted into formic acid in the presence of a catalyst under considerable heat and pressure, although it is currently produced commercially for use in cleaning products by a chemical reaction between methanol and carbon monoxide.
As a fuel it can be used to generate electricity directly in a formic acid fuel cell or as a carrier for hydrogen. When converted into hydrogen in the presence of a catalyst, the only emissions produced are water and CO2, which can be captured and used to create more formic acid.
Being a liquid at room temperature it’s easier to store than hydrogen but it is corrosive and prone to biodegrading into carbon monoxide and water, which can cause troublesome build-up in a boat’s fuel tanks.
Pros: Liquid at room temperature and can be converted to hydrogen. There is no emission of nitrogen oxides, sulphur oxides or soot
Cons: Highly corrosive, prone to biodegrading
Liquid natural gas (LNG)
Liquid natural gas is largely composed of methane and ethane (natural gas) that has been cooled down to its liquid form at -162C for easier transportation and storage.
This does require considerable energy to chill it and insulated (but not pressurised) tanks to store it. In this form it has an energy density of around 23 MJ/litre – half that of diesel.
It also burns more cleanly than diesel with emissions of harmful NOX reduced by up to 80% and a reduction in CO2 of around 25%.
It is possible to create LNG cleanly using bio-methane but the majority of current production is sourced from the usual gas and oil drilling operations.
Pros: Burns more cleanly than diesel, and is relatively easy to produce, store and transport
Cons: Lower density than diesel, requires a gas-specific engine, currently still a fossil fuel
Conclusion: Which diesel alternatives are the most promising?
The sheer number and variety of fuels being tested by engine manufacturers, fuel companies and scientists gives hope to boat owners. Which diesel alternatives come to the fore and which fall by the wayside is hard to predict but in the medium term it’s likely to involve several different solutions.
There are already a number of hydrogen-powered craft in development, and LNG and methanol are also being trialled by the shipping industry. Hopefully, at least one will form the basis of a greener future for all of us.
Diesel alternatives energy density table
First published in the July 2022 issue of MBY. Thanks to MarQuip exhausts for their help with this article.