Properties and energy — light, but hard to store

There is only one contrast to remember first

The starting point for understanding hydrogen as a fuel is the contrast: energy per unit mass is large, but energy per unit volume at ambient conditions is small. That is why summarizing it as "a light, strong fuel" alone is misleading — "how to contain it" is always the next question.

Do not mix up LHV and HHV

When discussing the energy content of hydrogen, it is important not to mix LHV (lower heating value) and HHV (higher heating value). The difference is whether you recover the heat of condensation of the water formed after the reaction. Discussions of vehicles and many energy systems typically use LHV, and the simple calculations in this course also proceed on an LHV basis.

The course uses the constant 1 kg of H₂ ≈ 33.3 kWh (LHV). This is a rounded form of the standard lower heating value of hydrogen (about 120 MJ/kg ≒ 33.33 kWh/kg), and similar values appear in DOE and AFDC reference materials. It is not a precise design value, but it is enough to grasp the relationship between mass and the energy that can be extracted.

How to compensate for the low density

At ambient temperature and pressure, hydrogen has very low density, so you need a large tank to hold it as-is. That is why high-pressure compression appears in vehicles, and liquefaction and carrier conversion appear in logistics. High pressure eases the volume problem, but increases compression energy and container strength requirements. Liquefaction is favorable by volume but brings in cryogenic temperatures and the issue of boil-off.

Boil-off refers to the gradual evaporation of cryogenic liquid (the boiling point of liquid hydrogen is about −253 °C) due to heat ingress from outside. Tank insulation is never perfect, so during long parking, mooring, or after a full fill, the internal pressure rises and the design vents a small amount through a relief valve. In other words, "the energy needed to keep it cold" and "the loss from evaporated hydrogen" come together as a single design issue.

Practice 1–3 — Mass and volume, and reading things on an LHV basis

Check the mass-vs-volume contrast and how to read things on an LHV basis.

Q1. Which contrast about hydrogen's properties does this course emphasize most?

Do not stop at 'light and strong' — pair it with 'hard to store'.

Energy per mass is small, but energy per volume is large Energy per mass is large, but energy per volume is small Both per-mass and per-volume are always ten times that of gasoline Both per-mass and per-volume are always at their minimum

Q2. Which is closest to the main reason for wanting to compress or liquefy hydrogen?

Recall what you do to mitigate the low density.

To change its color Because it easily takes up a large volume at ambient temperature and pressure Because hydrogen's chemical symbol changes unless it is compressed Because liquefaction always makes production-stage CO₂ zero

Q3. Using the course constants, give the LHV-based energy content of 5 kg of H₂ in kWh.

Use 1 kg of H₂ ≈ 33.3 kWh (LHV).

kWh

Practice 4–6 — LHV / HHV and storage conditions

Check the difference between LHV / HHV and how to read storage conditions.

Chapter 2 summary

Hydrogen is strong per mass but weak per volume at ambient conditions. That is the starting point.
The course uses 1 kg of H₂ ≈ 33.3 kWh (LHV) to roughly estimate the mass-to-energy relationship.
LHV and HHV differ in whether the heat of condensation of water formed after the reaction is counted. Do not mix them.
For mobile platforms, volume constraints are tight; liquid hydrogen brings in the new issues of cryogenic temperatures and boil-off.