What is Energy Density?

Energy Density can be measured in two ways:

• Volume: How much energy exists within a specific space

• Weight: How much energy exists within a specific weight

Many times we confuse ourselves by speaking about one of them while forgetting about the other, and more often we forget about how one category may impact the other ie: large volume energies require large storage containers... which are often heavy. When we speak of energy density in maritime applications, both weight and volume can be challenges. At the end of the day, many types of shipping are measured on how many units of cargo they can carry and this can be impacted by the weight or volume of the energy system on board.

Why is this a challenge for the maritime industry?

As you can see in the figure above, the traditional fuels used by the maritime industry are very volume dense, meaning you can get a lot of energy into a small space. The fuels of the future; primarily Ammonia, Liquid Hydrogen (LH2) and Compressed Hydrogen (CH2 350/700 Bar) are quite the opposite, these are gravimetrically dense energies. Batteries are also a useful energy carrier - and often the cheapest overall solution when they are feasible. From the figure you can also see that batteries have low volumetrically & gravimetrically energy density, which means that you need large and heavy batteries to have sufficient energy. This limits the application of batteries to short sea applications (and of course as an energy buffer in all others). 

How does energy efficiency play a roll in this?

Energy efficiency is important because it is the multiplier different energies go through when they are actually used on board. What this means is energy densities are measured in theoretical energy contained per unit (volume or weight), however when the energy is consumed on board it is not the theoretical energy density that matters but the overall system efficiency. If energy has a high density but low efficiency it’s ‘Actual' or 'In Use' energy density is much lower. Batteries shine strongly here because they have high efficiency and therefore are not penalized as severely as other energy carriers.

We've put together an interactive figure that displays the energy density for several conventional fuels such as MGO, and Diesel, and some of the new fuels proposed for use in maritime such as Liquid Hydrogen (LH2) or Ammonia. As a twist, we have added the option to see what these energy densities look like on board when the energy is actually utilized (in a combustion engine or fuel cell, etc). Just click the box under Energy Density: Theory or Reality and select "Reality".

Beyond the density of the energy itself, system (Balance of Plant) weights and volumes are important but require design and operational specifications to compare these appropriately. Beyond energy and system densities are a number of key issues to be considered such as safety, cost, and fuel availability.

If you have a maritime project you'd like to explore zero emissions on, we're always looking to take on projects and offer advisory where we can add value

Do you want to learn more about electrolyser technologies? Check out our technology series: part 1, part 2 and part 3.