One problem facing fuel cell cars is storing enough hydrogen...

One problem facing fuel cell cars is storing enough hydrogen for a reasonable driving distance. Hydrogen's energy density is $142 \mathrm{MJ} / \mathrm{kg},$ higher than gasoline's $44 \mathrm{MJ} / \mathrm{kg} .$ But gasoline is a liquid (density approximately $720 \mathrm{~kg} / \mathrm{m}^{3}$ ), and hydrogen is a gas. Suppose you want to store the energy equivalent of a full tank of gasoline in a tank with the same volume, but containing hydrogen gas $\left(\mathrm{H}_{2}\right)$. What pressure would you need, assuming a temperature of $20^{\circ} \mathrm{C}$ ? Is this practical?

Key Concepts

Energy Density

Energy density refers to the amount of energy stored per unit mass or volume. It is crucial in comparing different fuels because it helps in evaluating how much energy can be stored in a given volume, which is especially important for transportation applications where space is limited.

Phase and Density of Substances

The phase of a substance (gas or liquid) significantly affects its density. Liquids typically have much higher density than gases, so even if a gas has a high energy content per unit mass, its energy content per unit volume can be much lower compared to a liquid. This concept is central to understanding the challenges in storing energy-dense fuels like hydrogen in compressed gas form.

Ideal Gas Law

The ideal gas law (PV=nRT) is the fundamental equation used to relate the pressure, volume, and temperature of a gas. It allows for the calculation of the pressure needed to store a specific amount of gas in a given volume, making it an essential tool for analyzing practical storage conditions for fuels like hydrogen.

High-Pressure Gas Storage

High-pressure gas storage involves compressing a gas to store a greater amount of energy in a limited volume. However, as pressures increase, practical issues such as material strength, safety, and the efficiency of compression and decompression processes become significant, which are critical considerations when evaluating the feasibility of using compressed hydrogen in fuel cell vehicles.

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