Contents
Introduction
Heating value is a fundamental property of fuels, representing the amount of energy released during combustion. For hydrogen, two values are commonly used: Lower Heating Value (LHV) and Higher Heating Value (HHV). Understanding the difference between these values is essential for accurate energy calculations, project feasibility studies, and comparing hydrogen to other fuels.
What is Heating Value?
The heating value of a fuel is the amount of heat released when a specified amount of the fuel is combusted completely with oxygen. It is typically expressed in units of energy per mass (e.g., kWh/kg, MJ/kg) or per volume (e.g., kWh/Nm³).
Heating values are crucial for:
- Calculating fuel consumption
- Designing energy systems
- Comparing different fuels
Hydrogen LHV and HHV: Definitions
Lower Heating Value (LHV)
The LHV of hydrogen is the amount of heat released when hydrogen is combusted and the water produced remains in vapor form. The latent heat of vaporization of water is not recovered. For hydrogen, the typical LHV is 33.33 kWh/kg (or 120 MJ/kg).
Higher Heating Value (HHV)
The HHV of hydrogen is the amount of heat released when hydrogen is combusted and the water produced is condensed back to liquid, recovering the latent heat of vaporization. For hydrogen, the typical HHV is 39.44 kWh/kg (or 142 MJ/kg).
Key Differences Between LHV and HHV
- Water State: LHV assumes water remains as vapor; HHV assumes water is condensed to liquid.
- Energy Value: HHV is always higher than LHV for the same fuel.
- Application: LHV is commonly used in Europe and for real-world efficiency in most combustion systems; HHV is often used in the US and for fuel cell/electrolyser efficiency.
- Reporting: Always check whether efficiency or energy content is reported as LHV or HHV, as this can significantly affect calculations and comparisons.
Why Does It Matter?
The choice between LHV and HHV affects:
- Efficiency calculations for electrolysers, fuel cells, and combustion systems
- Project feasibility and cost estimates
- Comparisons between hydrogen and other fuels
Example: Electrolyser Efficiency
If an electrolyser uses 50 kWh of electricity to produce 1 kg of hydrogen:
- Efficiency (LHV basis) = 33.33 / 50 = 66.7%
- Efficiency (HHV basis) = 39.44 / 50 = 78.9%
Always check which basis is used when comparing technologies or reporting results.
When to Use LHV vs HHV
1. Use LHV (Lower Heating Value) When:
- Calculating Real-World Efficiency: In most combustion systems (gas turbines, engines, boilers), exhaust gases exit above 100°C, so water vapor does not condense. The latent heat in vapor is not recovered → LHV reflects usable energy.
- Comparing Fuel Efficiency: Automotive/aviation industries use LHV for fuel economy metrics (e.g., km/kg H₂), as condensation is impractical.
- High-Temperature Processes: Steel/glass manufacturing, where waste heat isn't recovered for condensation.
- European Standards: EU energy policies often mandate LHV for efficiency reporting.
2. Use HHV (Higher Heating Value) When:
- Assessing Total Chemical Energy: HHV includes condensation heat → it's the maximum theoretical energy from complete combustion.
- Fuel Production/Electrolysis: Electrolyzer efficiency is calculated using HHV (e.g., "50 kWh/kg" based on HHV ≈ 39.4 kWh/kg). Why? Electrolysis produces H₂ from liquid water, requiring energy equivalent to HHV.
- Condensing Boilers/Heat Recovery: If exhaust is cooled below 100°C to condense water vapor (recovering latent heat), HHV applies.
- US/Japanese Standards: US energy regulations (e.g., DOE) often use HHV for fuel comparisons.
- Carbon Intensity Calculations: GHG protocols (e.g., GREET model) may use HHV for consistent well-to-wheel analysis.
- Safety/Codes: NFPA/ISO standards reference HHV for hydrogen storage/transport classification.
Conversion Example
To convert between LHV and HHV for hydrogen:
HHV = LHV × 1.183
LHV = HHV ÷ 1.183
For example, if you have an LHV-based value of 1000 kWh, the equivalent HHV-based value is 1183 kWh.
Related Calculators
Need Help with Hydrogen Calculations?
Understanding LHV and HHV is essential for accurate hydrogen project design and reporting. If you have questions or need support, our team is here to help.