As a subject matter expert in the field of energy and combustion, I can provide a detailed explanation of the difference between Lower Heating Value (LHV) and Higher Heating Value (HHV) of a fuel.

Lower Heating Value (LHV), also known as net calorific value, is the amount of heat released when a specified amount of fuel is completely combusted under constant pressure conditions. During this process, the water produced is in the form of vapor. The LHV does not account for the energy required to condense the water vapor back into liquid form. It is a measure of the energy content of the fuel that is actually available for use in practical applications, such as in power generation or heating systems.

Higher Heating Value (HHV), on the other hand, also referred to as gross calorific value, is the total amount of heat released when a specified amount of fuel is completely combusted under constant pressure, including the heat that would be released if the water vapor produced were condensed back into liquid water. This value includes the latent heat of vaporization of the water, which is the energy required to change the state of water from vapor to liquid. The HHV is a theoretical measure of the energy content of the fuel, as it assumes that all the water vapor produced during combustion can be condensed and the energy from this phase change is recovered.

The numerical difference between the LHV and HHV of a fuel is indeed roughly equivalent to the latent heat of vaporization that can be practically recovered in a secondary condenser per unit of fuel burned. However, this recovery is not always feasible in real-world applications due to various practical constraints such as the efficiency of the condenser and the operating conditions of the system.

In most internal-combustion engines, such as diesel and gasoline engines, the LHV is more relevant because these engines typically do not have the capability to condense the water vapor produced during combustion. As a result, the energy from the condensation of water vapor is not utilized, and the LHV is the value that more accurately reflects the energy available for work.

In summary, the key difference between LHV and HHV lies in whether the energy from the condensation of water vapor is considered. LHV does not include this energy, making it a more practical measure for most applications, while HHV includes it, providing a theoretical maximum of the energy content of the fuel.

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That is the case, for example, with most internal-combustion engines, such as diesel and gasoline engines. ... The numerical difference between the LHV and HHV of a fuel is roughly equivalent to the amount of latent heat of vaporization that can be practically recovered in a secondary condenser per unit of fuel burned.Feb 4, 2008read more >>