Why hydrogen is no route to renewables
All the attention in the nation appears to be on hydrogen as the ideal medium for energy in a renewable economy. It has a lot going for it, in particular the fact that it can be produced from nothing more than water and energy. But this comes at a high (and hidden) price, especially for production from renewable energy; it is far from obvious that the use of hydrogen is worth the additional costs. The consequence is that we should downgrade hydrogen research, and cease deployment efforts immediately.
Hydrogen is certainly a wonderful molecule. It's the lightest element and has a very high energy/mass ratio. It's also the foundation of many chemical synthesis processes, both artificial and natural; when plants make sugar, they begin by splitting a water molecule to make hydrogen. There are even some ways to persuade plants to yield hydrogen directly. And when hydrogen is required, nothing else will do. You need hydrogen to make ammonia (for nitrogen fertilizer) or synthesize hydrocarbons.
We can learn a lot from plants (biomimicry has yielded a lot of good concepts), but there are limits to how far this can go and still be useful. It's one thing to borrow inventions and techniques from nature when they are well-suited to the task at hand, and quite another thing to cut the engineering problem to fit the Procrustean bed of a biological prototype. I intend to show that the "hydrogen economy", and particularly the hydrogen fuel-cell car, is a poor way to accomplish this.
The efficiency of any system has to be considered in context. Grass is perhaps 1% efficient converting sunlight to biomass? That's bad, isn't it? Not compared to the efficiency of forests. I understand that an acre of woodlot captures energy at an average rate of something like 500 watts. By way of contrast, you can average 500 watts by taking the rather small fraction of a 1/5 acre lot covered by the roof of the house standing on it and covering it with PV.
Relative efficiency is important for many things: what's better at getting energy from where it is to where you want it to go. This is where hydrogen falls down. Worse, it hurts exactly where most people would expect that a "clean" energy source would do the best: carrying renewable energy from source to end use. Hydrogen is far more efficient (and thus cheaper) at carrying energy from fossil fuels than renewable sources like wind and solar.
When comparing different approaches, there's nothing like an example. Suppose that we've got two sources of energy, coal and wind. The goal is to power a car. We have two options for using coal: gasify and convert to hydrogen, or burn and generate electricity to charge batteries. We have similar options for wind: generate electricity and either electrolyze water to hydrogen or charge batteries.
The theoretical efficiency of proton-exchange membrane (PEM) electrolyzers is quite high, about 85%. Unfortunately this efficiency is only achieved at low power levels and PEM systems cost too much to use them to make trivial amounts of product. Quoted efficiencies for real systems are in the range of 65% to 75%. Let's use 75% for now.
Fuel cells are the essential part of the hydrogen economy. According to the Rocky Mountain Institute
, PEM fuel cells have efficiencies ranging from 35% to 60%. Let's use 60%.
Coal can be used to make either electricity or hydrogen; IGCC powerplants make large amounts of hydrogen as part of their syngas. The quoted efficiency of the gasifier at the Wabash River powerplant is 76%. The composition of the combustible (non-nitrogen, non-CO2) syngas is described as being 37% hydrogen, with most of the rest being carbon monoxide; unfortunately, I cannot unequivocally determine whether this is by volume (mol %) or by mass. The conversion of carbon monoxide to hydrogen via the water-gas shift loses energy, but without a clear distinction between the two cases it is impossible to calculate how much. If the product syngas is 37% hydrogen by mass, the losses in conversion to pure hydrogen will be relatively small; for the sake of argument, let us assume that they are zero. Let's also assume that the efficiency of electric generation from coal is 40%.
Last, let's assume that the efficiency of batteries is 90%.
There are four possible routes for energy depending on the source and the storage option:
coal -> electricity -> batteries -> motor
coal -> hydrogen -> fuel cell -> motor
wind -> electricity -> electrolyzer -> hydrogen -> fuel cell -> motor
wind -> electricity -> batteries -> motor
Each path has a different efficiency (ignoring losses in transmission, compression, etc):
| coal -> electricity -> batteries -> motor || 40%*90% = 36% |
| coal -> hydrogen -> fuel cell -> motor || 76%*60% = 45.6% |
| wind -> electricity -> electrolyzer -> hydrogen -> fuel cell -> motor || 75%*60% = 45% |
| wind -> electricity -> batteries -> motor || 90% |
Interesting things become apparent in this comparison:
- When hydrogen is used as the medium, coal and wind have roughly the same end-to-end efficiency.
- If electricity is used as the medium, coal loses about 20% compared to hydrogen.
- If electricity is used as the medium, wind GAINS 100% compared to hydrogen!
In short, hydrogen fuel for vehicles is moderately advantageous for fossil energy sources, but it applies a 50% penalty to renewable sources such as wind (also solar and hydro). This handicap is completely artificial, and comes on top of hydrogen's very poor storage density and high cost of hydrogen fuel cells.
There you have it: hydrogen isn't the way to a renewable future, it's just a boondoggle.
Labels: hydrogen, renewable energy