How to store energy -3

2022/12/12 2023/01/03

lenemas

Let's first look back what we have discussed in the previous posts regarding how too store energy.

Hydrogen has been considered as an energy carrier, because it can contain large amout of energy on a graviometric density basis
However, volmetric energy density of hydrogen is so small that its transportation requires some measures
(see Hydrogen as an energy carrier?)
Ways to increase the volmetric energy of hydrogen include;
- liquefaction or compression to make hydrogen small (see How to store energy -1)
- physically or chemically attaching hydrogen to other substance (see How to store energy -2)

This post now compares methods of energy/hydrogen storage.

Comparison of energy density

The next figure summarizes and compares the methodes of storing hydrogen in terms of volmetric and graviometric energy densities. The data plotted is adopted from DOE (U.S. Department of Energy).^[1,2]

volmetric-and-weight-density-of-energy-carrier

Sorry that the figure is in Japanese, but will be translated into English.
体積密度 in the y-axis title is the volmetric density, and 重量密度 in the x-axis title measn the graviometric density.
From the top to the bottom, 金属水素化物 is metal hydride, ケミカルハイドライド is chemical hydride, 水素吸着材料 is hydrogen adsorbent, and 圧縮 is compression.
各種電池 at the bottom means battery.

Firstly, let's compare these approches in terms of graviometric energy density (horizontal direction).
The compressed hydrogen, adsorbent, chmeical hydride is characterized by graviometric energy density as high as > 1 [kWh kg^-1]. The energy density of metal hydride is smaller than these, appearing at the left-side of figure below 0.5 [kWh kg^-1].

A more detailed data regarding the graviometric energy density is summarized by DOE,^[2] adopted in the figure below.

The y-axis of this figure is graviometric energy density in the unit of weight percentage. The x-axis dictates the temperature, in which the temperature range lower and higher than room temperature corresponds to sorption and release of hydrogen, respectively. Metal or chemical hydrites require substantially high temperature for the release, while adosrbents require quite low temperature to adsorbe hydrogen.
It is ideal to operate the sorption and release at temperatures closer to ambient condition, meaning that these is a large room for improvement in the properties.

Let's now look again the first figure.
Also in terms of volmetric energy density (y-axis in the figure), the metal hydride is as low as 0.5 [kWh L^-1], while chemical hydride is the largest, approaching a value of 1.5 [kWh L^-1].

It is noteworthy that all approaches discussed exceed the batteries in terms of both grviometric and volmetric energy density. One may consider battery when hearing "to store energy", however, chemical substances can actually store more energy than battery does.

However, it is also critical to consider the weight of container of enregy carrier when discussing the energy density. According to the data by New Energy and Industrial Technology Development Organization (NEDO) in Japan, the weight of storage tank on car for 1 [kg], 35 [MPa] of hydrogen with a volume of 39 [L] is 23 [kg].^[3] The weight of 23 [kg] for 1 [kg] of hydrogen...

Next posts will dive into the energy density in more detail.