Defining the Hydrogen Economy from A to Z: D is for Density
Continuing in our Defining the Hydrogen Economy from A to Z series, we’re focusing on the letter D for Density. Density is perhaps the most important fluid property used in our industry. The density of a fluid tells a lot about it, much more so than its temperature or pressure, as density is reliant on both. By definition, density is the mass per volume of a substance, also known as the volumetric mass density. The volumetric mass density can become vitally important for a real system, considering all the system components as a complete assembly and the space available to house that system (on a truck, for example). There’s also energy density, defined by energy per unit mass or unit volume: Gravimetric and Volumetric energy density. Let’s explore.
As mentioned in previous posts, we strive to take advantage of the density of fluids not only in cryogenics but particularly in the hydrogen industry. We do this in two ways, increasing the pressure or decreasing the temperature. These two properties set the boundaries for a fluid. Water can boil at room temperature, if you decrease the atmospheric pressure. Additionally, almost any fluid can freeze if you decrease the temperature. With Hydrogen, decreasing the temperature creates a phase change from gas to liquid and liquid to solid. At GenH2, we believe that liquid hydrogen is the best way to store and subsequently use hydrogen.
Let’s look at some numbers. At standard atmospheric pressure and temperature, hydrogen gas is about 0.083 g/L. In its liquid phase at atmospheric pressure, hydrogen is 71.1 g/L, making it about 853 times denser than its gaseous form at atmospheric pressure. The change in density through phase change is incredible. Trying to achieve the same density through compressed gas would be a monumental task, as even at 700 bar, hydrogen gas has a density of 39.6 g/L.
Now on to energy density. This is how we tend to gauge hydrogen as an energy carrier among other energy carriers or fuels. As mentioned above, there are two important things to note about liquid hydrogen when speaking about energy: its gravimetric energy density and its volumetric energy density. Although Hydrogen has the highest gravimetric energy density of all known substances (120-142 MJ/kg), it falls short to natural fuel sources when it comes to volumetric energy density (9 MJ/L). What does this mean? Well, if you had a 1-kilogram weight limit, then liquid hydrogen would contain the most energy. If you had a 1-liter tank volume limit, then there are other fuels that would carry more energy. Of course, the conversation is a lot more nuanced than weight or volume. When cost, environmental impact and renewable energy comes into play, it’s a different story.
With hydrogen, we believe that the best way to utilize its volumetric mass density and energy density is through liquid. You get more density for less work, and in liquid form it opens the avenue for cryocompressed, liquid to liquid, or liquid to gas transfer. It’s truly fascinating how fluid phases can result in drastic differences, and it all starts with understanding its density.
For more information on density comparisons of different ways of storing hydrogen see Cold Facts’ latest article, ‘Hydrogen Storage and Density: Gas vs Liquid’ published in the Buyers Guide 2022 edition on page 82.