Electrolysis efficiency

[Approaching infinity]

I have been thinking of different ways of storing energy in times of electricity overproduction, in particular electrolysis as it can form relatively good quality energy (chemical energy) that isn't too volatile. Electrolyzing H20 and CO2 into methanol is a way of doing this, but if you want to store electrical energy instead of chemical energy it's not very efficient to just burn the methanol. What would be more efficient is if you could reverse the electrolysis process to turn the chemical energy immideately into electrical energy again, (theoretically) a much more efficient transformation.

 

I have been searching for how high efficiency these energy transformations have but I can't seem to find any numbers of this. Without any numbers on how efficient these reactions might be it's impossible to say if it can be viable. So essentially what I'm actually searching for is an reversible electrolysis reaction that uses relatively cheap materials and is efficient in both directions, but failing that a general picture of how efficient electrolysis is right now and how efficient it could theoretically/reasonably become will more than suffice.

[Enthalpy]

The reverse operation is called a fuel cell and it propels several marketed cars.

 

Neither electrolysis nor fuel cells are very efficient. 60% each is a mean value. That would not suffice, by far, as an energy storage for electricity networks. Present applications concentrate on cars or on laptops. I see a use in aeroplanes and helicopters.

 

To store efficiently excess electricity from networks, batteries are not too bad. Almost acceptable efficiency, cost affordable. Lithium is too scarce for that use, so one Japanese university researches batteries based on sodium.

 

Alternatives: pump and turbine water between two lakes at different heights; that has been done for decades. Or have flywheels (very efficient, looks affordable), store air under bags on the sea floor (could be affordable from my estimates), pump water out of a ring-shaped artificial island built on shallow sea floor (prototyped in Belgium, cost?).

 

Very few methods are cheap enough. For instance voiding a stiff container at the seabed is worse than the air bags.

[Approaching infinity]

Very informative post. A bit embarrased that I didn't know that what I was actually was looking for are fuel cells.

 

Also, I weren't very specific in my first post but the case I've been thinking about is wind energy. The thing about wind energy is that, at least according to what I've read (Smart Power Generation), you would have to be able to store the excess electricity of 3 windy days in a row, an absolutely enormous amount of energy. Now that is probably a bit of an extreme case as you would probably cut back on other energy production in areas with lots of wind energy during windy days but that is still a lot of electricity to store. Obviously cost/energy stored is important here, and according to the previous mentioned book using batteries to store this amount of electricity would be way too expensive. Do you think fuel cells made of cheap materials could in this case be viable in the abscence of any hydropower that could be used as a balancing factor?

[Enthalpy]

Last time I checked the price of batteries, they were affordable. It's more a matter of lithium availability than price, presently - hence the attempt with sodium. The battery solution works in Japan, at least as a demonstrator, to smoothen out the demand, because most nukes are stopped and the running power plants can't cover the peak demand.

 

Wind energy is presently the cheapest renewable (cheaper electricity than from the planned EPR nuclear reactor), hence thinking for it is reasonable. A first answer is to transport electricity over big distances, from places where wind blows at the time it's needed; this costs some power lines but is long done (13GW over 800km from Itaipu, 16GW over a similar distance in Québec, and more) with small losses. In Europe, wind would always be available among Scotland, Brittany and Galicia.

 

I don't want to smoothen out the production by using other energy sources. This would mean that (1) we still emit dioxide, while the goal should be zero (2) we would have to operate backup plants, making wind energy too expensive.

 

The amount of electricity to be stored demands good methods. Some of them look possible, more must still be invented. Imagine one giant 5GW wind turbine that produces mean 1.5GW. Providing this power over 1 day (with interconnected regions) needs to store 130GJ.

 

That's for instance <26,000m3 air at 500m water depth: a half-ellipsoid bubble of 10m height and 70m diameter, smaller than the wind turbine - and at the same pressure as water essentially, so it takes just a strong plastic film and good anchors to hold it down.

 

Or it's a ring-shaped island of 290m diameter where water is 40m lower than outside, where 5m water are pumped or turbined. It looks less cheap than the air bubble, but many turbines can share one island, whose wall length increases slower than the storage area.

 

My flywheel design is meant for 12GJ a pair (hence 10 pairs per giant wind turbine) and I estimated the cost at 570k€.

http://www.scienceforums.net/topic/59338-flywheels-store-electricity-cheap-enough/

while not extremely cheap, they would be cheaper than presently building extra power plants to cover the peak demand - and are the most efficient storage method over a day.

 

My general feeling is that hundreds and thousands of brilliant scientists invest their work, and the society many billions, to get nuclear fusion or fission reactors that won't work soon and will be expensive, while some millions would permit to store electricity at the proper scale - and the electricity source is already here. Believe me, this intellectual endeavour is an interesting and challenging one.