The key difference between HHV and LHV is that HHV can be determined by bringing all the products of combustion back to the original pre-combustion temperature while allowing any produced vapour to condense. Meanwhile, LHV can be determined by subtracting the heat of vaporization of water from the higher heating value.
Heating value is a property of a substance that describes the amount of energy (heat energy) released during the combustion of a specified amount of the particular substance. Usually, the substance we consider here is either fuel or food. There are two types of heating values as higher heating value (HHV) and lower heating value (LHV).
CONTENTS
1. Overview and Key Difference
2. What is HHV
3. What is LHV
4. Side by Side Comparison – HHV vs LHV in Tabular Form
5. Summary
What is HHV?
The term HHV stands for higher heating value. We can determine the higher heating value by bringing all the products of combustion back to the original pre-combustion temperature. In this process, we need to allow any produced vapour to condense. The standard temperature we use for this determination is 25°C. The higher heating value is equal to the thermodynamic heat of combustion because the enthalpy change for the combustion reaction also assumes a common temperature for compounds before and after the combustion. In addition, if water vapour is produced during the combustion reaction, it undergoes condensation to form liquid water.
Figure 01: Phase Transitions of Water
Furthermore, we have to take the latent heat of vaporization of water into account when measuring the higher heating value. This value is important in calculating heating values for fuels where condensation occurs. This means, when doing measurements, we assume that the water component occurs in the liquid state throughout the reaction.
What is LHV?
The term LHV stands for lower heating value. We can determine the lower heating value via subtracting the heat of vaporization of water from the higher heating value. This includes any water molecules that are formed as vapour. Therefore, the energy required to vaporize water is not considered as considerable heat energy.
According to the calculations of lower heating value, we need to assume that the water component of the combustion reaction is in the vapour state after the completion of the combustion. Moreover, this is an opposite assumption to that of the higher heating value calculations; in higher heating value calculations, we assume that the water occurs only in the liquid form due to condensation.
What is the Difference Between HHV and LHV?
HHV stands for higher heating value while LHV stands for lower heating value. The key difference between HHV and LHV is that HHV can be determined by bringing all the products of combustion back to the original pre-combustion temperature while allowing any produced vapour to condense. Meanwhile, LHV can be determined by subtracting the heat of vaporization of water from the higher heating value.
The following table summarizes the difference between HHV and LHV.
Summary – HHV vs LHV
The term HHV stands for higher heating value while the term LHV stands for lower heating value. HHV can be determined by bringing all the products of combustion back to the original pre-combustion temperature while allowing any produced vapour to condense. Meanwhile, LHV can be determined by subtracting the heat of vaporization of water from the higher heating value. So, this is the key difference between HHV and LHV.
Reference:
1. “Heat of Combustion.” Wikipedia, Wikimedia Foundation, 20 Feb. 2020, Available here.
Image Courtesy:
1. “Heating of water with phase transitions – no energy values” By MikeRun – Own work (CC BY-SA 4.0) via Commons Wikimedia