Storing Renewable Energy in Stones

exchemist said:

That's quite wrong. It looks as if you may get only half the energy you put in back again.

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The plan is to make that up in volume

(But seriously, the folks doing the work here MUST believe that that doesn't have to be the case.)

Twilight Hue said:

That's what I meant. Using more energy to get less in return. It's a loss no matter what.

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Hang on, that's a totally different thing.

If you put in 2kWh of energy and only get 1kWh back, you get back half what you put in. But you can still boil a few kettles with 1kWh. So even getting half back is better than just wasting the surplus, if the sun is shining or the wind is blowing and you are producing more than your immediate needs.

So it's not a loss at all. It's a gain. Just not very efficient at storing the extra. Whereas with a battery you might get 80-90% of it back, rather than 50%.

icehorse said:

The plan is to make that up in volume

(But seriously, the folks doing the work here MUST believe that that doesn't have to be the case.)

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I agree. In fact, now re-reading the article, it looks as if they intend to run this thing as a reversible heat pump. I had missed that first time round. That way they can possibly boost the efficiency rather higher than my guesstimate of 50% max. They might get 60-70% overall, which would not be too bad, considering the system may be a lot cheaper (and less resource-intensive) than an expensive battery system.

Perhaps I should dig around for a more detailed article on it later.

exchemist said:

I agree. In fact, now re-reading the article, it looks as if they intend to run this thing as a reversible heat pump. I had missed that first time round. That way they can possibly boost the efficiency rather higher than my guesstimate of 50% max. They might get 60-70% overall, which would not be too bad, considering the system may be a lot cheaper (and less resource-intensive) than an expensive battery system.

Perhaps I should dig around for a more detailed article on it later.

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A battery has a limited life before it needs remanufacturing. A rock is always a rock. The equipment that they need for energy transfer, clearly has a running cost and an overhead, but nothing like that of a battery system.

icehorse said:

Sounds like this solution might be cheap, safe, and durable, and probably inefficient.

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2 +/- 1

exchemist said:

Hang on, that's a totally different thing.

If you put in 2kWh of energy and only get 1kWh back, you get back half what you put in. But you can still boil a few kettles with 1kWh. So even getting half back is better than just wasting the surplus, if the sun is shining or the wind is blowing and you are producing more than your immediate needs.

So it's not a loss at all. It's a gain. Just not very efficient at storing the extra. Whereas with a battery you might get 80-90% of it back, rather than 50%.

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Let's look at the efficiency of using that heat....
- Steam engines can hit 40%. Treat this as an unattainable
maximum, given that the compressed air temp air will be
below the operating temps of the most efficient engines.
- At lower temps, stirling cycle would be best. 40% could
be achievable. But again, this is an unattainable maximum,
because it applies only to the stored energy, which will
endure losses in the storage process.

I imagine that 25% efficiency could be possible.

Terrywoodenpic said:

Storage is never efficient.
but it can be very cost effective

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Bingo. It's all about cost effectiveness and sufficient capacity. If it's cheap enough efficiency does not matter.

Revoltingest said:

Let's look at the efficiency of using that heat....
- Steam engines can hit 40%. Treat this as an unattainable
maximum, given that the compressed air temp air will be
below the operating temps of the most efficient engines.
- At lower temps, stirling cycle would be best. 40% could
be achievable. But again, this is an unattainable maximum,
because it applies only to the stored energy, which will
endure losses in the storage process.

I imagine that 25% efficiency could be possible.

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Those are interesting numbers and I think reasonable.

exchemist said:

I agree. In fact, now re-reading the article, it looks as if they intend to run this thing as a reversible heat pump. I had missed that first time round. That way they can possibly boost the efficiency rather higher than my guesstimate of 50% max. They might get 60-70% overall, which would not be too bad, considering the system may be a lot cheaper (and less resource-intensive) than an expensive battery system.

Perhaps I should dig around for a more detailed article on it later.

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Thanks. I'm learning something in this thread.

sun rise said:

Thanks. I'm learning something in this thread.

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I've found a couple more articles about this project but all of them are very cagey about exactly what the pumping system is.

They talk, vaguely, about "turbines and compressors". None of them mentions a working fluid, of the type you would need to have to operate a heat pump cycle (i.e. like a refrigerator, which is a heat pump). But it is noteworthy that they speak of pumping heat from the cold stone to the hot ones, thereby making the cold stones colder and hot stones hotter. That can only be a heat pump. If I were a betting man I might take a punt on them using ammonia as the working fluid. Ammonia has been widely used in industrial scale refrigeration processes for a century, is easily made and does not damage the environment - though of course it is very poisonous and irritating to the eyes and lungs if it leaks and is inhaled.

It looks as if there is something commercially confidential about the details of the design that they don't want to reveal. I note also that some serious commercial organisations are involved. BWSC (Burmeister & Wain Scandinavian Contractors) is a power plant builder owned by Mitsui, though the name comes from Burmeister Wain, a long-standing Danish maker of marine engines.

So this is a real business venture. Intriguing and one to watch.

Yes. With power storage you store when the energy is free or cheap and e

Revoltingest said:

Let's look at the efficiency of using that heat....
- Steam engines can hit 40%. Treat this as an unattainable
maximum, given that the compressed air temp air will be
below the operating temps of the most efficient engines.
- At lower temps, stirling cycle would be best. 40% could
be achievable. But again, this is an unattainable maximum,
because it applies only to the stored energy, which will
endure losses in the storage process.

I imagine that 25% efficiency could be possible.

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Was this comment intended for the other thread?
Compressed Air Grid 'Battery'

Brickjectivity said:

Yes. With power storage you store when the energy is free or cheap and e

Was this comment intended for the other thread?
Compressed Air Grid 'Battery'

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Uh oh.

Revoltingest said:

Uh oh.

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I was posting at half efficiency.

Brickjectivity said:

I was posting at half efficiency.

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I hit a max of 7% posting efficiency.