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What is Energy Density?

Energy Density is the amount of energy stored in a given system or region of space per unit volume or per unit mass. The use of volume or mass to describe energy density is dependant upon the context in which the energy density of an element or compound is described. For example, in most cases, the energy density of a liquid is best described in terms of mass. In the case of a gas, using a per unit volume description may be more appropriate because a gas, even when compressed, occupies an overwhelming volume as compared to a liquid. Therefore the energy density of a gas by mass appears quite substantial, but when one looks to the volume of gas required to
achieve the desired energy density, it can become impractical.

In terms of fuel cells, considering the energy density of each fuel by volume is extremely significant in determining what may be practical in commercial off-the-shelf applications. Many hydrogen fuel cells may require numerous hydrogen tanks to support electrical loads over what is deemed a practical duration. These tanks involve increased weight and space that must be accounted for in the system design. 

By referring to Table 1 below, we can see that Hydrogen has the highest energy to mass ratio of all the fuels at more than two to one. This is why hydrogen is so attractive to the fuel cell community as the basis for fuel cell power. When comparing the energy density of hydrogen by mass, 1 [kg] of hydrogen contains the same amount of energy as 2.4 [kg] of natural gas or 2.6 [kg] of gasoline. However, although energy density by mass yields a favorable comparison to fossil fuels, energy density by volume reveals how inefficient hydrogen gas is when one considers storage and delivery of hydrogen to a given fuel cell system.

                                                              Energy Density               Energy Density
                                                                      by Mass                         by Volume

                           Hydrogen              33,300 [Wh/kg]                530 [Wh/l]
                           Natural Gas           13,900 [Wh/kg]              5,800 [Wh/l]
                           Gasoline                 12,700 [Wh/kg]              8,760 [Wh/l]
                           Propane                 12,900 [Wh/kg]              7,500 [Wh/l]
                           Methanol                5,600  [Wh/kg]               4,420 [Wh/l] 
                                 Table 1 - Energy Density by Mass vs. by Volume

By referring to Table 1 above, to equal the energy density in 20 gallons of gasoline, 1,195 liters of hydrogen would be required or almost 16 times the amount of pure hydrogen by volume will be required to sustain the same energy density.  Comparatively, if a fuel such as propane is utilized instead of hydrogen, the equivalent amount of propane by volume is close to 22 gallons.

As you are looking to size a particular fuel cell to a given application it is important not only to match the voltage and power rating of the fuel cell to your application but also to understand the consequences of the fuel that is utilized and the volume of fuel required to sustain your power application over a reasonable period of time. In some cases this volume requirement might force you to reconsider either the requirements of your application or the power source you intend to use.