Could someone explain Lower Heating Value and Higher Heating Value in relation to hydrogen?
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When you burn hydrogen (or any fuels containing hydrogen, such as oil/gas hydrocarbons), one of the combustion products is water – H2O.
As the combustion typically occurs at high temperature, this is usually in the gaseous form of steam.
It turns out that a decent chunk of energy is locked up in keeping the steam a gas, which can be released by condensing it back into liquid water. Therefore, there are two quoted figures for calorific (heating) value: the Lower figure that assumes the steam is not condensed, and the Higher figure that assumes it is.
For maximum efficiency, the flue gases should be condensed to achieve the higher heating value, but there are a number of circumstances when that is not desirable or possible. Perhaps you want to heat something above the boiling point of water – in which case it’s difficult to cool the gases enough to condense.
Another reason is that for many hydrocarbon fuels, impurities in the fuels mean that the condensed liquid is quite acidic which can cause issues if the exhaust materials are not designed to cope with it.
When judging the usefulness of an energy source, e.g. H2 “fuel”, we often consider energy density, which we might measure by mass or volume. We also consider how easy it is to turn the energy into the form we want, e.g. kinetic for a car, or heat for a house.
H2 has a good energy density by mass (120 MJ/kg, almost three times more than diesel or gasoline). So for stationary storage or a large vehicle it’s quite good. But it has a poor density by volume (roughly a quarter that of diesel), so is a relatively poor choice for a smaller vehicle like a car.
H2 can easily be turned to heat, by burning, or electricity e.g. in a fuel cell, so again it’s useful for stationary use like a building.
The two big problems with H2 are making it in the first place, which isn’t that efficient, and storage, which requires cryogenic refrigeration at -240C or at high pressure (over 5000psi), this is in expensive bulky and heavy tanks.
The fossil fuel industry is pushing “blue” hydrogen, derived from fossil fuels, which is inefficient and wasteful and worse than simply using the fossil fuel source.
As more wind and solar plants are built, I think we’ll see more green H2 being made as a way of using unwanted energy during peak production. When the energy is almost free, the inefficiency of electrolysing water to get H2 doesn’t matter too much.
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