Energy Storage: The Trouble with Power to Gas

There is a fundamental law of Physics called the Law of Conservation of Energy.

It's a real bitch.

Basically it demands that whatever the system you use, you can never get more energy out of it than you put in. So many beautiful dreams have come to naught - just because of this one damn law.

Even so there should be an adjunct to the law of conservation of energy. Something more like a serious health warning really.

Something like:

"Whenever you change from one form of energy to another - you will get screwed."

And I mean really screwed.

If on your last foreign holiday you thought changing currency was robbery then believe me, that was as nothing compared to the losses when changing energy form.

Truly, energy can neither be created or destroyed. But Oh Boy can it get "mislaid" dispersed or changed into unwanted useless forms whenever you try to convert one form into another.

You always (and I mean ALWAYS) end up with less than you started with. Mostly you end up with significantly less.

How good a system is at converting energy is its energy efficiency. It can never-ever be more than 100%.

90% is mind bogglingly good.

An old 19th century steam engine by comparison is about 10% efficient - on a good day.

By burning fossil fuels and liberating their stored chemical energy as heat and then changing that heat (from burning coal or gas) into electricity - you will lose about 50% of the energy as waste heat up the chimney. Still, we do it because electricity is far more useful to us than a lump of coal or a pocket of trapped gas.

Now, what would be the best way to store the energy in (say) gas for a rainy day?

Would it be by just not burning it until needed? Or would be by converting it to electriciy (50% loss) and then post generation converting it back to something else? (say another 50% loss)

I hope it is obvious that by doing a "gas->electricity->something else" you will get royally screwed. In this example you would end up with 25% of what you started with.

You are far, far better off not burning the gas until needed.

Wind turbines and solar PV do not have the luxury of having their energy pre-stored like coal, gas or nuclear. If there is excess generation by wind or solar they either waste the available energy by not converting it or they have to convert it to electricity and then convert it again to some other storeable energy form.

So, with wind (or solar) you have no option but to bear the pain and go with:

"wind->electricity->something else".

One of the much hyped "something elses" for wind/solar is called Power to Gas (Wikipedia article Here).

Twitter is alive with excited windies who see Power to Gas as the the "Great Breakthough" - The mythical silver bullet that will slay the demon problem of energy storage for wind/solar.

To be fair, Power to Gas is technically clever. It takes any excess electricity from wind/solar and via some clever chemistry uses it to generate flammable gas - either hydrogen or methane. This can then be stored and used at some later date either for heat or electricity generation..

The trouble with Power to Gas is is not the cleverness of the technology. The trouble is that damn law of physics about energy conservation. Especially the health warning attached to it.

Turning electricity to potential chemical energy (gas) is at best 75% efficient but more realistically it is around 60%. Then turning that gas back to electricity again reduces the overall efficiency to around 40% at best or more realistically about 30%. In other words we lose about two thirds of the energy - Of course the missing 2/3rds is not destroyed. It is simply just dissipated and lost to the system.

Remember this proposed technique is there to bale-out an already massively subsidized generator. Even if you totally ignore the actual cost of plant and plant operation the price of the re-generated electricity would have to be three times that of the source price just to stay level.

In reality though the price of Power to Gas has been estimated at anything from £500-1000 MWh. Or from 10 to 20 times as expensive as current gas/coal/nuclear generation.

A whole order of magnitude more expensive. Breathtaking!

Power to Gas is a nice idea. It may even have some practical niche applications. But storing excess energy from wind and solar? Dream on.

And all because of that damn law about the Conservation of Energy.