Fuel Comparisons Must Consider Energy Density

Steve Esau, general manager, SEA-LNG explains why fuel assessments can be distorted if energy density is overlooked

LNG tanker
The LNG bunkering vessel Kairos (file image courtesy Babcock Schulte Energy)

Published Sep 17, 2021 4:46 PM by Steve Esau

Comparing fuel price and performance on a per-tonne basis has sufficed for liquid fossil fuels, but as shipowners evaluate new fuels as part of their decarbonization plans, this method is no longer accurate.

SEA-LNG has identified a number of situations where misleading information has been disseminated because energy density has been overlooked when comparing the economic viability of liquid fossil fuels, LNG, and new fuels such as ammonia, hydrogen and methanol. These inaccurate comparisons are a legacy of marine fuel oil being historically sold in metric tonnes.

Energy density is the amount of energy that can be stored in a given mass of fuel. It can be specified as energy content per unit of mass (gravimetric energy density) or energy content per unit of volume (volumetric energy density). Gravimetric energy density is critical when comparing the operating costs of different fuels - one tonne of LNG bunker fuel releases more energy during combustion than one tonne of very low sulfur fuel oil (VLSFO), for example. Volumetric energy density is relevant when making newbuild investment decisions, as less space needed for high energy density fuel storage means more space available for cargo.

Gravimetric energy density

LNG’s energy cost per tonne is about 16 percent lower than VLSFO because it contains more energy for a given mass. LNG provides approximately 46.7 MMBtu (or 13.7 MWh) of energy per metric tonne, whereas VLSFO provides about 40.2 MMBtu (or 11.8 MWh) per metric tonne. Hence LNG at $100 per tonne is price neutral against LSFO at $84 per tonne.

It is important to reference credible sources of pricing information that are adjusted for energy content and regularly updated. For example, the Platts monthly average bunker price assessments for LNG (available on the SEA-LNG website) provide a true cost comparison with marine gasoil, LSFO and heavy fuel oil.

Looking forward, it is important to recognize that when we see fuel price estimates quoted for ammonia, hydrogen and methanol on a per tonne basis, then these prices will need to be adjusted for energy density. For example, one tonne of ammonia contains only 33 percent of the energy of one tonne of LNG and its zero-emissions cousins, bioLNG and synthetic LNG. For methanol, the comparable figure is 36 percent, whereas for hydrogen the number is 216 percent.

Volumetric energy density

LNG has a volumetric energy density advantage compared to new fuels. Liquid hydrogen, ammonia and methanol have 34 percent, 51 percent and 63 percent of the volumetric energy density of LNG (respectively). In other words, it takes about two cubic meters of ammonia to match the energy output of one cubic meter of LNG. To achieve the same sailing distance, fuel tanks for liquid hydrogen would need to be at least three times the volume of those for LNG as a consequence of the large amounts of insulation required. For ammonia, the tank size ratio is approximately two to one compared with LNG and in the case of methanol, tank sizes are equivalent. The potential difference in sailing distance would not be clear if the fuels were simply compared on a per-tonne basis.

From a ship design perspective, bunker storage tank size will clearly be an important consideration, as it can impact on cargo carrying capacity. This can be illustrated by examining the latest CMA CGM 22,000 TEU ultra-large container vessels. Designed with a 18,600 cubic meter capacity LNG storage tank, the ships would need a substantially larger, 35,340 cubic meter capacity fuel tanks if they were to run on ammonia. This equates to approximately 1,000 TEUs of space for ammonia storage compared to roughly 500 TEUs required for LNG.

Looking ahead to 2050 and beyond, when hydrogen-based fuels such as synthetic LNG and potentially green ammonia become available from renewable energy sources, volumetric energy density considerations will remain critical to emissions calculations, vessel design, deadweight tonnage, cargo volume and passenger space availability.  This is in addition to the safety and operational challenges faced by many new marine fuels. 

Ship operators, financiers, class societies, terminal operators and other stakeholders therefore need to change their mindset now and think about fuel in terms of its energy per unit volume, rather than just in terms of its weight or volume. The industry needs to understand and standardize methodologies so that fuel comparisons are made from a level baseline. Misunderstandings will come at a cost – not only for individual businesses but for the maritime industry’s sustainability efforts as a whole.

Steve Esau is the General Manager of the LNG bunkering industry association SEA\LNG. He previously served as Head of Energy at the consultancy Xyntéo. He began his career as a geophysicist at BP and worked in a variety of business development, strategy, and analysis roles in the company’s gas, power, & renewables and energy trading businesses.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.