Fuel for Thought - Choices of Energy Storage Media

Fuel for Thought - Choices of Energy Storage Media

Rob Palin
March 20, 2023


Our transition to a sustainable energy future is dependent on shifting from fossil sources of energy to renewable ones. It is essential that we carefully consider the next steps we take, as there exists the real possibility of making things worse. The sustainability choices being considered by the maritime industry will be a significant factor in our ability to tackle climate change, with maritime currently contributing 3% of global CO2 emissions, and having more than double that impact when we add into consideration the impact of other emissions such as Black Carbon, Methane and Nitrogen Oxides.

The simplest and clearly most appealing option is to replace what we currently burn with something else that we can use in the same way.  The issue here is that the proposed “sustainable” fuel alternatives are less energy dense, often significantly more difficult to generate, distribute, store, and in themselves pose real risks to the environment, human health, and counter-intuitively – climate change. 

Many of these suggestions are finding backers within industry, as they would allow existing fossil fuel suppliers and infrastructure to continue operations largely as-is. One example of this is natural gas being suggested as a ‘bridge’ option, to burn less oil while we develop a real alternative. The front-runner for that alternative is then hydrogen, but more than 98% of the hydrogen in use worldwide is produced from natural gas.  So, the proposal is for a first step to move from fossil oil to fossil gas in LNG, then to use hydrogen produced  from natural gas (eventually with CO2 capture), and lastly produce the hydrogen cleanly, via the electrolysis of water using electricity from renewable sources.  While this may sound like a reasonable and pragmatic approach to transitioning away from fossil sources, we would be substantially increasing the harm we do to the climate in the short term, and leave ourselves open to extended transition periods while the final clean step remains off toward the horizon that bit longer than hoped for.

It is Spaera’s belief that the known and potential problems with these drop-in solutions rule them out as a strategy for easing the transition to a sustainable future, and that a more impactful first step will be required.


“Fossil fuels” is the term we use for substances we extract from the ground that effectively store energy once harvested from the sun by plants.  They are, in essence, condensed and fossilised sunlight.  While it is common to refer to them as energy “sources”, they are really just the middle-men; the holders and carriers of the sun’s energy.  As we know, these stores of energy will eventually run out, and that even independent of climate change, we would need to find ‘fresh’ sources of energy.  Fortunately, we already have a wide variety of ways to harvest  fresh energy from our environment, most of which is ultimately supplied afresh daily by the Sun (the exceptions being geothermal, tidal, and nuclear).

While there is unquestionably sufficient renewable energy available to power our civilisation, the three big challenges here are the intermittency, locality, and low energy density of some renewable energy sources.  Energy farms of solar panels or wind turbines need to be located where the ambient conditions are suitable, require quite a lot of space to get large enough power outputs, and don’t generate consistent amounts of power across days, weeks, or months.  The primary solution to these limitations is to be able to effectively condense, store, and relocate this renewably harvested energy, and this is the focus of a great deal of research and development across the world.  

Energy storage can be divided into two main categories: local buffers, and media for distribution.  Buffers have been an integral part of utility scale power generation for a long time, using facilities like pumped water storage at dams to effectively set aside surplus energy from the optimum generation periods for use in periods of high demand.  There are new developments in this category in the form of batteries, compressed gases stored in underground caverns, and the electrochemical synthesis of substances which can be either used directly via combustion, or indirectly through the effective reversal of the fabrication process.  This latter approach is also the basis for the idea of creating energy storage media for wider distribution before consumption.  Generally referred to as “e-fuels”, substances such as elemental hydrogen, ammonia, methanol, ethanol, or more complex hydrocarbons can all be synthesised, relocated, and used in various ways.

Synthesised energy storage media are a heated topic of conversation in the shipping world.  So far, there hasn’t been a decisive selection by either the existing energy companies, or the regulatory bodies, leaving shipping high & dry as it tries to make decisions that it will have to live with for the next two to three decades.  This quandary is further exacerbated in that ships already in service may have 20+ years of life remaining, and will also require a solution. 


The Good: Cheap, widely available, established, extremely high energy density, relatively easy to contain & store
The Bad: Very high output of CO2 and other harmful emissions

Of the current energy media available to shipping, the derivatives of fossil oils such as Heavy Fuel Oil (HFO), Very Low Sulfur Fuel Oil (VLSFO), Marine Gas Oil (MGO), and Marine Diesel Oil (MDO) clearly dominate on price, availability, and usable energy at the point of use.  These fuels kind of arrive ‘pre-prepared’, and so require relatively little processing before they are usable by a customer, and their availability benefits from more than a century of infrastructure growth and development.  They are mostly relatively safe, too, for the people immediately engaged with them, with only fairly simple training, and protective equipment.  The real twist is that their environmental impact can be catastrophic.  We estimate that fuel oil use in the maritime industry is accountable for over $400 billion in socio-environmental costs and direct taxpayer subsidy, every year. The typical mixes of marine fuel oils emit significant enough volume of particulates, such as Black Carbon, to be directly responsible for between 60 000 and 400 000 premature deaths per year. When we look at the CO2e of all the additional emissions from fuel oil use in maritime, it would equate to over 6% of global emissions. It’s beyond doubt that shipping must move away from fuel oils rapidly.

While some actors point to scrubbers and carbon capture as the answer, in practice these will only have a mitigating effect (and then only if employed rigorously with strict enforcement and verification – typically not possible within the lawless oceans), rather than the elimination of these emissions. Use of fossil fuels with scrubbers and carbon capture cannot ever be described as a sustainable solution.


The good: Cheap, widely available
The bad: Emissions produced are significantly worse for climate change than CO2, requires cryogenic storage, prone to leaks and slip

Liquid Natural Gas, or liquid methane, has benefited greatly from a deliberate rebranding exercise, giving it a veneer of benevolence from an association with nature, but really it is a toxic substance with up to 86 times the negative impact on climate of carbon dioxide – in fact, methane has accounted for roughly 30 per cent of global warming since pre-industrial times. Methane is considered by some scientists to be the biggest threat to keeping below 1.5c of global warming, giving rise to methane sniffing satellites being launched into space, so we can better understand and control this risk.

Fugitive emissions and methane “slip”, where methane escapes storage reservoirs unintentionally or passes through the engine un-combusted, are sadly common phenomenon, leading to a significant amount of critics of the fuel source.

Along with methane’s impact on climate change, it is also the primary contributor to the formation of ground-level (tropospheric) ozone (O3) or smog, by mixing with NOx and Volatile Organic Compounds (VOCs) – both of which are also heavily emitted by shipping. Ground level O3 is a hazardous air pollutant and greenhouse gas in its own right, exposure to which causes 1 million premature deaths every year.

Some LNG characteristics make it fairly convenient for certain purposes, and undeniably effective, but with growing scientific consensus that the environmental benefits of LNG are limited, if not negative, compared to fuel oils, it should not be considered as a transitional step for maritime, let alone a long term replacement.


The good: Widely available, easy to store and transport, high energy density vs. other e & bio-fuels
The bad: Not a significant improvement for CO2 emissions when burned in internal combustion and derived from a non-green source, such as coal or natural gas

Also known as ‘wood alcohol’, methanol is the simplest form of alcohol, and is widely synthesised from fossil sources for use as a solvent for inks & dyes, as a basic building-block feedstock for the synthesis of more complex hydrocarbons, or directly as a combustible fuel for racing cars and ships.  Methanol has around half the energy density of the heavy oils, but when burned releases only around 40% of the carbon dioxide, and none of the sulfur oxides, aromatic hydrocarbons, or particulates.  It is also relatively cheap, and easy to store and distribute, with no recognised impact on climate or the environment from leakages, partly due to its rapid biodegradation over the course of about a week in the ocean.  

While some question the safety of methanol due to its clear burning nature, in a comprehensive risk assessment conducted by a collaboration of industry giants (including Maersk, Shell and Chevron) it was deemed as the lowest risk when compared to LNG, Hydrogen and Ammonia.

The biggest factor in the effectiveness of methanol as a sustainable alternative to replace fuel oils will be how it’s made, and how it’s consumed. Brown or grey methanol produced from coal or natural gas, while still an improvement over fuel oils, has a significant carbon footprint when we consider the entire life cycle. E-methanol and Bio-methanol, derived from either green hydrogen or waste and agricultural by products, are both capable of being either carbon neutral or even carbon negative if emissions that would have been released into the atmosphere are captured and processed. If green methanol is burned via internal combustion with no capture of emissions, while still offering a significant reduction in emissions vs. fossil fuels, it will likely not be enough to shift the industry to where it needs to be. If we extract the energy from methanol with a fuel cell, we reduce CO2 output even further, and make capture of the remaining emissions significantly easier.


The good: Efficient energy carrier of hydrogen, contains no Carbon, so does not produce CO2
The bad: Highly, HIGHLY toxic, and releases by products in its use that can be 300 times worse than CO2 for climate change

Ammonia (NH3) may not be a hydrocarbon, but it does work quite well as an energy carrier.  There are many advocates for ammonia in the shipping industry, emphasising its familiarity as an industrial chemical product, including widespread use as a basis for agricultural fertilisers for many decades.  Ammonia is certainly very much a known quantity, and it can be synthesised from non-fossil sources with current technology, but it is also a horrendously toxic substance to handle and store.  At just 30ppm it can affect health when permanently exposed, at 300ppm significant damage can happen in less than an hour. When ammonia dissolves in water it forms ammonium hydroxide, which is highly toxic to marine life – one study by the EDF stated an ammonia spill would be far more toxic to marine life than a comparable oil spill. In the risk assessment of future fuels conducted by Maersk, Shell and Chevron, it was the only fuel source to have intolerable risks identified, across 6 areas of its distribution and use.

Another key consideration is that most advocates are proposing to use ammonia in combustion engines, but this may have very concerning emissions consequences.  Ammonia does not contain any carbon, so no carbon dioxide is produced during its combustion, but engines currently require that a small amount of diesel be introduced in order to get the combustion process initiated optimally.  If the combustion is not optimal, then the oxygen in the air combines not just with the hydrogen in the ammonia, but with the nitrogen in both the ammonia and the air, too, leading to greater production of nitrous oxides, which, as we’ve mentioned, can be nearly three hundred times more damaging for the climate than carbon dioxide.

Finally, Ammonia must be stored under pressure at atmospheric temperature or fully refrigerated at –33°C, posing significant challenges for its use as a marine fuel.


The good: Completely clean energy medium if generated via green methods (not derived from fossil fuels!)
The bad: Extremely difficult and dangerous to transport and store – no feasible solution has so far been established for a global hydrogen supply network 

Underpinning almost all of these options, to one degree or another, is hydrogen.  By far the most abundant element in the known Universe, hydrogen is amazing, with incredible abilities for storing energy – up to 3x diesel!  As you might expect, though, there’s a catch!  Hydrogen really, really doesn’t like to be confined.  It takes a huge amount of effort to collect enough of it in any one place for it to be useful as an energy storage medium, and extreme measures are required to keep it in that state.  Hydrogen in industrial applications is typically stored at either extremely high pressures (~10,000 PSI / 700 bar), or extremely low temperatures (below -253 °C / -423 °F).  Both require very specialist materials and technologies to be made safe.  Thus far we have yet to see truly compelling answers to the challenges of storing and distributing elemental hydrogen as an energy medium.  These aren’t just a matter of figuring out some new technology; we are right up against the limits of the laws of Physics, trying to confine the smallest element in the Universe, and near to the coldest temperature that can exist.  The answers need to be good.  If any of these systems fail and allow hydrogen to escape, at best we release a substance into the atmosphere estimated at between 10 and 100 times worse than CO2 for climate change, depending on the length of time considered. In the worst case scenario, even small amounts of liquid hydrogen combined with air can be highly explosive, as demonstrated in the NASA Saturn program


The following tables compare the main options currently available for stored energy media for shipping, across five high-level categories.  It is always excruciatingly difficult to digest such rich and complicated subjects down into simple representations like this, but it is also the only viable way to make reasoned decisions.

Fuel Choice Comparison in today's market
  • Cost is perhaps uppermost in the minds of the decision-makers.  Expenses accrued anywhere in the chain from production to distribution and delivery, all get passed through to the end purchaser.  For shipping there is some nuance here, in that normally the people building, and/or operating the vessels pass the fuel costs directly on to those utilizing the transport service.  There is certainly still some competitive advantage to having more efficient vessels, but market pressure is not felt as directly as where the fuel cost is borne by the service provider, subsumed within other considerations as part of setting the price.
  • Availability is a combination of the ubiquity of the energy media, or its constituents, and the widespread distribution of the result at ports across the world.  Fossil fuels have a very well-established infrastructure for mining, refining, and distribution.  Some other energy carriers have at least some infrastructure, but usually their primary purpose is not energy, but some other application, such as feedstock for the synthesis of other chemicals, or other industrial processes.
  • Energy density is possibly the most contentious topic, because there is so much complexity & nuance, from the sourcing of the raw materials, the synthesis of the medium itself, the distribution, the transfer to the ship, the storage onboard, the specific means of extracting the stored energy, and then the treatment of whatever mixture of waste products result.
  • Safety and environmental impact are listed separately here, but are somewhat intertwined, in that we cannot limit our concern about the hazards of an energy medium just to its point of use.  All the way through the production and distribution chain there are risks to personnel engaged in these activities, as well as to the surrounding communities and environmental ecospheres.  
  • Each choice was made after extensive research and debate within the Spaera team, and could in themselves each warrant multiple blog posts, but as the saying goes “ain’t no-one got time for that!”

If we imagine the fuel oils are burdened with the additional socio-economic costs appropropriate to the damage they inflict, and that the manufacturing infrastructure of renewable fuels has developed, we could anticipate an updated cost scenario, and a new order for the energy media.

Fuel choice comparison anticipating cost adjustments for fossil fuel damage and renewable manufacturing scale

Both tables also highlight the benefits of pairing wind power with a renewable fuel source. Wind power can significantly compensate for the lower energy density of the fuel sources, ensuring vessels need to pack with them a fraction of the total voyage energy required, all with zero negative impact on the environment and minimal cost.


There is no clear and definitive winner that matches all of the desired criteria without at least some trail of caveats.  This is the fundamental core of the quandary facing the shipping industry, and resolving it requires a more flexible approach to the factors constraining the solution.  Spaera believes that in the short term, at least, we need all ships to switch to using energy media which emit less harmful substances when combusted.  Looking to the future, combustion itself needs to be deprecated entirely, and more controllable approaches taken to ensuring that only the chemistry we want occurs as energy is released from storage.  Any waste products then have to either be harmless in themselves, or captured & recycled.

Big enablers of future energy media options, with their lower energy densities, are gains in energy efficiency of the ships themselves, of the overall process of moving cargo from one place to another within desired schedules, and even a return to harvesting ambient energy from the marine environment as a ship passes through it.  The idea of bringing sail power back to shipping often meets with mockery, as if it is a backward step, but the reality is that we now have vastly improved abilities to harness the wind.  It makes sound economic sense to pick up energy that’s freely available along the route, rather than to compromise our vessel design to somehow package the storage of all the energy we could possibly need on our journey. Having hybrid supplies of energy can also provide significant risk reduction, and provides more flexibility when it comes to route and schedule optimization for cost.

Through a process of investigation and analysis briefly summerised above, Spaera has determined that methanol is best suited as the energy storage medium for the future of shipping, but again with some caveats.  As discussed, on the good side, it is arguably the simplest hydrocarbon, easily synthesised from carbon dioxide and water.  It is relatively benign and easy to store without specialist equipment.  Methanol is already widely produced and used around the world.  Methanol does contain hydrogen, but having the carbon and oxygen in there as well helps to suppress its natural resistance to being crammed together, and so a litre of methanol can actually contain more hydrogen than a litre of hydrogen! The caveat – we shouldn’t burn it.

While some backers claim that the emissions from using methanol via internal combustion are counteracted by carbon negative production methods, this strategy allows for a lot of mathematical gymnastics that could undo any real benefits, and relies heavily on scaling an industry still in its infancy. By using methanol with fuel cells that control the chemistry so precisely that the waste products are already pure enough to be returned to the start of the cycle, and made back into new methanol by being ‘recharged’ with more hydrogen, we can ensure that the lifecycle is truly net zero.

Novel technologies such as fuel cells can seem unnerving when considered against the context of an industry known to rely heavily on established methods to ensure reliability and robustness, but on a first principle physics level they are actually much more simplistic than internal combustion – fewer moving parts, fewer points of failure, and their material and construction costs are dropping every day as more innovators enter the field. The automotive industry first resisted the electric motor, but it’s now hard to dispute that battery electric will be the format of choice for personal automotive transport. At Spaera we’re confident a similar step change is about to hit the maritime industry, and our money is on methanol fuel cells supported by significant wind assistance.

Part of the Spaera mission, in partnership with our network of experts in industry, is to demonstrate these conclusions in an irrefutable manner, utilising the latest innovations in computational fluid dynamics and computer modelling.