information for our energy future
information for our energy future
The energy content of fuels is variously described as HHV and/or LHV. These abbreviations stand for higher heating value and lower heating value. The HHV is the energy released during combustion of the fuel with the combustion products cooled to 25 °C. The LHV is the energy released during combustion but assuming the gaseous combustion products have not cooled to room temperature. The HHV is always greater than the LHV, and the difference is largest for hydrogen, less, but still significant for natural gas, less still for liquid fuels, and not much for solid fuels.
The difference in HHV and LHV depends on the hydrogen content of the fuel, since it is the water vapour in the combustion products that can still retain or provide extra energy.
For example, when 1 m3 of natural gas is burned in air and the combustion products are returned to room temperature, approximately 38.7 MJ of energy are produced (slight variations are due to the slightly different mixtures of gases in natural gas). If the same gas is burned in an engine, and the exhaust is released to the atmosphere above 100 °C (the usual case), the useful energy recovered is the LHV - about 34.6 MJ. This difference is about 10% for natural gas and about 5% for coal, which has a lower hydrogen/carbon ratio. The largest difference is for hydrogen gas - about 15%.
Confusion often arises when engine and turbine manufactures quote their efficiency, since it is common practice to quote the efficiency using the LHV of the fuel, but fuels are purchased on the HHV. So if you are trying to determine the cost of running a gas-fuelled engine for example, you should make sure you are using the same energy basis for the fuel cost and engine efficiency.
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