Hydrogen as an energy carrier?

Hurdles to clear in implementing renewable energy

Globally, the implementation of renewable energy is an agreed way to go. However, just increasing the number of its installation would not be sufficient to achive this goal!

Critically, there is a gap between locations where we can generate electric power using renwable energy and where we need energy, as discussed in another post (see Issues associated with renewable energy utilization). Without technologies to transport or store "energy", if the electric power generated is larger than the demand of electric power, the excess energy need to be thrown away. Such scenario came true in the Kyushu region, where supply of electricity by electric power generation exceeded the demand.^[1]

To effectively use the renewable energy, the concept of "energy carrier" has been intensively considered (see Issues associated with renewable energy utilization). Electric power generated from renewable energy is converted into energy carrier for storate or transportation. Therefore, the energy carrier preferably contains "large amount of energy" in "small volume, small weight".

How much of energy does hydrogen contain?

What is the most suitable material for the energy carrier? A close look at the periodic table may point to hydrogen, located at that top-left corner. In fact, the weight energy density of hydrogen molecule is one of the highest.

Let's calculate its energy density!

When extracting energy, we often make use of combustion reaction, which is exothermic reaction and therefore emits heat (How much of energy can be extracted from fossil fuels?). The following describes the combustion reaction of hydrogen.
H₂ + 1/2 O₂ --> H₂O (l) ...(1)
* Note that character in the parenthesis dictates the phase of the chemical. H₂O (l) means that the phase of H₂O is liquid, i.e., not ice or vapor, but water.

One can thermodynamically calculate the change of heat by this reaction, and obtaine an value of +285.8 [kJ mol^-1]. Because the molar mass of hydrogen molecule is 2 [g mol^-1], the combustion energy of hydrgen molecule was found to be 142.9 [kJ g^-1] (= 285.8 [kJ mol^-1] / 2 [g mol^-1]). 1.0 [cal] equals 4.2 [J], such that the energy content of water molecule in 1.0 [g] corresponds to 34.0 [kcal].

34 [kcal] .... How big/small is this?

For the comparison purpose, let me now consider about gasoline. The gasoline is the fuel to the vehicle, and possesses heating value of 7970 [kcal L^-1] with its density of ca. 0.75 [g mL^-1].^[2] On the weight basis, this value corresponds to 6.0 [kcal g^-1] (=7970 [kcal L^-1] * 0.75 [g mL^-1] / 1000). Therefore, the gravimetric energy density of hydrogen molecule is 5.7 times higher than that of gasoline. As a side note, a dish filled with rice (140 [g]) contains 235 [kcal].^[3] These quantities make it clear that the gravimetric energy density of molecular hydrogen is substantially high.

However, we must remember that the hydrogen under standard condition is in the gasous form. The gas is quite high in volume, therefore difficult to transport. Hence, how to make its volumetric energy density small is the key challenge.

There are several approaches proposed. One is to store the hydrogen in another energy carrier such as ammonia, methyl cyclohexane, or hydride. Another post shall summarize it... Stay tuned!

References

1. 九州電力送配電株式会社, “再エネ出力制御について知りたい”, https://www.kyuden.co.jp/td_renewable-energy_purchase_control.html (accessed on 2021/11/01)
2. 石油連盟, “換算係数一覧”, https://www.paj.gr.jp/statis/kansan/ (accessed on 2021/10/07)
3. 株式会社 タニタ , “接種カロリー早見表”, https://www.tanita.co.jp/content/calorism/table/index2.html (accessed on 2022/02/03)