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.

The Trouble with Energy Storage

Energy Storage - The Holy Grail for intermittent electrical generators.

Reading the tweets and articles from the wind and solar industries (and their avid followers) you could be forgiven for thinking that large scale energy storage was a done deal. Something that just needed the bureaucrats in Westminster or Berlin or Washington to rubber stamp.

Sadly though, large scale wind/solar energy storage is not only not a done deal it is not even on the horizon. Even if it was it would still be very far from being a “solution” to intermittent and dilute electrical generation. But more on why that is in a later post.

First of all lets be clear about what I mean by energy storage.

In a way, all current thermal generation and hydro depend on “energy storage” The difference between energy storage at (say) a coal plant and a wind farm is that the coal plant stores its energy pre-generation (i.e. as raw fuel) whereas a wind turbine has to convert its energy into a non-electrical form after excessive generation in order to store it. So the wind turbine has to convert its excess energy into some form of fuel to be stored for later use. The coal plant simply does not use the fuel until it is needed.

(by the way I am using coal plant here because it is a good comparator – not because I am a fan of coal generation – I prefer nuclear)

Typically the front runners for renewable energy post generation fuel storage revolve around two technologies:

1. Pumped hydro (pumping water up hill into a reservoir)
2. Or as is the fashion - in some form of Battery.

Pumped Hydro.

Pumped hydro is an old and proven technology. It existed a long time before the current wind/solar obsessions. Originally pumped storage facilities ( like Dinorwig in Wales) were built to store energy when the price was low (typically at night) and then sell that stored energy at peak demand (when prices were high). Using this model, pumped hydro works very well. It is a profitable and very worthwhile addition to the Grid.

But things change when you try and use it to store excess solar and wind energy. You essentially break the pumped hydro economic model, especially with solar PV. (See Speigel Online article here )

You have to buy in energy when prices and demand is high while sacrificing your profitable market as well. Then you then have to sell on when prices and demand is low.

It does not work. Even if you created some subsidy regime to support this broken model, the number of potential pumped hydro sites are very limited anyway.

But at least, individual pumped hydro sites can store relatively large quantities of electricity.

Although UK pumped hydro could not deliver the energy quickly enough to actually take over the whole UK grid, they do hold enough energy to power the entire UK grid for about 1 hour. 

While that may not sound much, it is overwhelmingly better than any form of battery storage.

When we get to battery style storage the practicality and price viability of large scale energy storage falls off a very high cliff.

Batteries

The biggest battery in Europe is in Leighton Buzzard in the UK. It can store 10MWh of electricity. It could (say) store half the output from a single small 10MW wind farm running at maximum output for two hours. It cost £20 Million. An average UK demand is around 30GWh. So this single battery would power the entire UK grid for about 1.2 seconds.

OK, you may say – let us distribute/duplicate it and use a cheaper technology After all why not have a cheaper 10MWh battery for every (say) 5 wind turbines? You know - Spread it out a bit.

Lets look at the (arguably) most viable and cost effective large scale battery technology available today – Vanadium Redox flow batteries.

(Incidentally - this is VERY clever technology and has many potential applications – I am not knocking the technology – only the application) .

Flow batteries store the energy in the electrolyte. The consequence of this is that theoretically the only limitation to the their storage capacity is the amount of electrolyte you can to store. 

Currently Vanadium Redox batteries store about 20 Wh per litre of electrolyte. So for 10MWh you need to store around 500,000 litres of highly corrosive Sulphuric Acid based electrolyte.

Lets say technical innovation decreases that by a factor of 10. You would still need to pump/store/process 50,000 litres or nearly 100 tonnes of electrolyte. 

That is for 10 MWh. Or 1.2 seconds of nationwide supply.

So, why not just store more electrolyte? Simple eh?

But remember, this stuff is lethal. It is massively corrosive and is a liquid. Then remember this is one SMALL wind farm.

Multiply that by thousands of wind farms. Then avoid killing anyone or regularly risking massive environmental pollution. That really is a challenge!

Of course there are other technologies (Lithium-ion being the other main and more expensive player) but whatever you look at, the problems of large scale energy storage are immense. They are effectively intractable.

Remember, what I have discussed above is the LATEST and most promising technologies. On Twitter people often eulogize about lead-acid batteries or compressed air, but really their capabilities are far below pumped hydro or flow batteries.

But Tom Murphy on his appropriately named blog Do The Math has done a very good analysis of a theoretical (USA) National Lead Acid Battery and its practicalities On this Link

A very interesting post on the EROEI (Energy Returned On Energy Invested) on  storage with RE has been written by John Morgan  On This Link

Large scale post generation energy storage is not viable.

Bit even worse – not only is it not viable, it is also potentially very, very (and appallingly) dangerous.

But more on that in another post.

(Here I am not considering issues with charge/discharge rates, resource availability or lifetime cycle expectancy – they all just make things worse)