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Hydrogen for renewable energy storage- total system efficiency?

This is a question that has been bugging me for some time, especially since the questions about methanol synthesis revived my interest in energy storage.

Say I have a wind turbine or solar panel or whatever, that produces 1000 Wh per day.  If I use that electricity to electrolyse water, store the generated hydrogen at roughly atmospheric pressure in an upside-down water butt or a big gas-tight bag in my shed (don't worry, I'm not going to actually do this) and then feed it into a generator converted to run on H2, what percentage of that initial energy input would I get back out?  20%?  5%? 1%?

The follow-up questions to this are
a) How does that compare against a battery bank? What about a similar DIY-style pumped water storage system?
b) What one component of the system should be improved to raise the overall system efficiency? Electrolyser, storage, generator?
c) Are there any other DIY-friendly methods for storing intermittently generated electricity that I'm not thinking of?

And, I suppose,
d) Does doing this and providing 5-10x your overall power requirements in wind turbines work out cheaper than spending thousands on batteries?

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Kiteman5 years ago
As a rough rule-of-thumb, energy conversions tend to be roughly 30-35% efficient (except electromagnetic transformers).

So, you're looking at putting 1000Wh in this idea and getting 100-150Wh out.

PKM (author)  Kiteman5 years ago
To partly answer my own question, this presentation seems to imply that a small utility-scale (output power of up to 1MW) system could have a total round-trip efficiency of 25%. They quote 80% for electrolysis and compression, which I suspect is ambitious, though, and that's using commercial kit- other sources tend to peg it closer to 50%.

Still, 10-15% efficiency is actually better than I'd thought.  
I suppose replacing the hydrogen powered generator with a fuel cell might push efficiency up a bit, but is probably impractical for home manufacture at that scale.

Also, I suspect that supercapacitors may be more efficient than batteries, although they would cost even more.
Oh, fuel cell!

@PKM - did you see the latest "James May's Toy Stories"? The one where they make another attempt at the ten-mile train set?

They had an extra challenge, with "alternative power" trains as well, and one was fuel cells fuelled by hydrogen generated the day before with a solar cell.

I didn't know he was still making those...I've only seen about 3-4 episodes....
That's about all there were in the old series, and this may have been a one-off (it's not been advertised very well).
Hmm, I I suppose even less so over here :-)
THERMAL energy conversions are Carnot-limited, other energy conversions are not. Thermal efficiency improve as the ratio of the inlet and outlet temperatures increases.
[boilerplate comment about how the impossibility of total efficiency]
PKM (author)  Lithium Rain5 years ago
Well, that's partly my point. I know the boilerplate response is "it's not worth doing because the efficiency is rubbish", but I'm trying to quantify "rubbish". I'd guess that overall we're looking at under 10% but I really don't know.

My other question still stands- is there any other way of storing and retrieving electricity that's open to hacking? Deep down I know batteries are probably the most practical right now, but they're just... boring.  Generating a shedful of explosive gas is so much more exciting!

Unless, of course, you meant that you can't get more than 100% efficiency because that's a ridiculous idea that violates fundamental laws of physics, in which case the appropriate response is probably "duh" :)
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