There have been predictions of a hydrogen economy for decades. At first glance it seems like the ideal fuel. Its specific energy - how much energy you get from a kilogram - is three times higher than gasoline. And the byproducts of combustion with oxygen are heat and water. So why hasn't it happened? It turns out you should think of hydrogen as more of a battery than a fuel.
Hydrogen is the most common element in the Universe and common on the surface of the Earth. Unfortunately the kind we need, molecular hydrogen H2 , is very rare. The only practical way to get it is to use a lot of energy to break down more complex molecules. Methane is the most common, but other hydrocarbons like coal are used. The process uses more energy than burning hydrogen releases and usually produces a lot of carbon dioxide and other waste products.
There are many industrial uses of hydrogen, but the fossil fuel industry has been interested in expanding to new markets like transportation and heating. They have the knowhow - all they need to do is scale their operations. In fact they've been lobbying governments since the 80s and billions of government money have been spent investigating a fossil fuel driven hydrogen economy. A good fraction of the funding went into the development of practical automotive fuel cells - more on that later.
In grade school you probably used a battery and two wires to make a simple electrolysis machine that broke water into hydrogen and oxygen gas. These days the best electrolyzers are something over 70% efficient. Sounds great but there's another problem. The energy per unit volume (energy density) of hydrogen gas at atmospheric pressure is about 2,800 times lower than gasoline. A gas tank for a car would be bus sized. The trick is to compress or liquify it. Compressed hydrogen has about one seventh the amount of energy as the same volume of gasoline. Liquid hydrogen is closer to a third. Larger, heavier and more expensive than gas tanks, but more practical than attaching a blimp to your car.
A hydrogen fuel cell uses hydrogen and oxygen to produce electricity and water. The efficiency of a fuel cell running an electric motor is higher than that of an internal combustion engine so the hydrogen tank is small enough to fit in a car. Before you can fuel the car the hydrogen needs to be compressed or liquified, both of which take a lot of energy. About a third of the energy produced as electricity where electrolysis takes place makes it to the tank. From there you about half of that getting it though the fuel cells and motor to the wheels. But with the right power source it can be carbon-free. Unfortunately most of the hydrogen cars to date use a dirtier hydrogen. And that brings up a marketing term: the color of hydrogen.
Green hydrogen is produced by electrolysis using electricity from a zero carbon source. Brown hydrogen is produced from methane and black hydrogen from coal (much of the industrial hydrogen in Germany and China is black) Both forms have serious production carbon emissions that, when used in a hydrogen car, emit more than a conventional car. Blue hydrogen is an attempt to deal with brown and black carbon emission problem. It't still produced from methane or coal, but the carbon is captured and stored. I consider that something of a pipe dream, but Japan is interested in blue hydrogen produced in Australia.1
Hydrogen is frequently mentioned as the future for low carbon airplanes. Liquid hydrogen would be necessary and huge tanks would be needed. There are several designs on the drawing board that couild happen in fifteen years using turbofans designed to burn hydrogen. They'd still produce some pollution (NOx) from the heat of combustion and the extra water vapor would leave thicker contrails.2
Recent geopolitical issues have raised the issue of using hydrogen to heat homes and businesses rather than natural gas.. after all .. there's a large and very expensive infrastructure in place already. Countries like England are experimenting with hydrogen/natural gas mixtures. You can get away with as much as 20% hydrogen and use the same furnace burners and conventional pipe infrastructure. In the UK new furnaces will have to have a dual fuel mode - natural gas or hydrogen. An infrastructure problem is it takes about three times as much energy to move equivalent amount of hydrogen as methane. Also current pipelines may be too brittle over time. Hydrogen leaks much more easily than methane (which is already a problem) and hydrogen leakage is an indirect greenhouse gas. There are a lot of potentially expensive questions to answer, so I wouldn't pencil this in as a slam dunk. On top of it you need to produce about three times as much energy to heat a house with hydrogen as you would with just electricity. Use a heat pump and electric heating looks even better.
Hydrogen could be used store power for the grid, but that gets into comparing many forms of energy storage, so not now.
My gut tells me hydrogen could see niche applications.. Long distance trucking if battery development plateaus soon (I wouldn't bet on that, but I wouldn't want to be a truck manufacturer) and aviation in the long term (15 years at the earliest).
But a final comment - if you have enough it will gravitationally collapse and fusion ignites - you have a star. We happen to have one. The trick is just one of collection. So in that sense life on Earth and almost everything we do is part of an hydrogen economy, but the convertor is about 150 million kilometers away.
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1 Japan is big on hydrogen and had originally envisioned scaling up nuclear power, but that pathdied with a tidal wave.) At this point I think blue hydrogen has underpants gnome issues.
2 Contrails appear to be serious global warming issues. It's possible that leaving them at night doesn't make sense and flights may have to occur at lower altitudes - say 20,000 feet rather than over 30,000. That would mean slower speeds which means propellers may be practical so liquid hydrogen powering fuel cells to run electric motors may make sense.
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