[crmd]

> Developers and power plant owners plan to significantly increase utility-scale battery storage capacity in the United States over the next three years, reaching 30.0 gigawatts (GW) by the end of 2025

Can someone explain how/why they are measuring storage capacity in watts?

[kevinventullo]

This doc has more information: https://www.nrel.gov/docs/fy19osti/74426.pdf

They’re talking about the power capacity, which is the maximum instantaneous rate of discharge as measured in kW, as opposed to the energy capacity which is measured in kWh. From what I can glean, it appears that for many applications, the power capacity is actually the more important number.

[lazide]

It is a bit weird, and seems to come from a history of fossil fuel, nuclear, and/or hydropower where the total capacity is always some (very large) multiple of the instantaneous power level.

With storage currently just supplementing other power sources as renewables aren’t the majority (or all) of available capacity to provide grid stability, it makes sense I guess. None of these battery storage systems need to provide meaningful load beyond a few hours now.

Hard to imagine that won’t change at some point though, probably soonish?

[epistasis]

"Long duration" storage doesn't have many economic use cases right now, unless the cost is a tiny tiny fraction of current storage costs. There are many startups developing tech for this, and one with the most hype (and the one I'm most excited about) is Form Energy, which is using iron-air chemistry.

Nonetheless, I do not expect to see a significant market demand for such batteries until into the 2030s; it takes a really high level of renewables penetration before it makes sense to start having long duration storage at the moment.

If there is a large need for it, I would expect that a lot of existing installations will be upgraded with longer duration, so that the same inverters and grid connection can be reused. New connections to the grid are becoming quite difficult to build on a reasonable time scale, and there's starting to be significant economic rent from just the permitting and connection stages.

[lazide]

I agree, thanks for writing that up.

Do you think it will require the phase out of things like Natural Gas peaker plants to make longer term storage more worthwhile?

Off grid, the most economic setup right now in California is essentially solar + batteries with battery storage sized up to about a day/24 hrs of load, with a propane fueled generator + propane backup heaters for ‘emergencies’ like long winter storms.

Generators get exercised regularly to ensure functionality, but otherwise don’t get run unless needed, which is rare. Maybe 16-24 hrs a season. They also charge the batteries when their full load isn’t needed elsewhere, which cuts run time.

Natural Gas would of course be the replacement for propane in a grid situation, where it was available.

The paperwork and permitting in general is a sign of the future for sure. sigh

[epistasis]

Last I heard, lithium ion is cheaper than the OpEx for the average existing natural gas peaker, but I haven't checked in a year or two. With the fluctuating cost of gas, and the recent small rise in battery cost from supply constraints, I could see that flipping back and forth. And there will be a long delay between having a new cost king and a switchover due to utilities being able to charge rate payers for stranded capital, and also the generally slow utility process for developing new resources, which is often on a five year time scale, using decisions made on five year old data.

That's pretty exciting with regards to off-grid use, I don't know much about it. 24 hours of fossil gas use per season is just absolutely tiny, one could definitely even imagine synthetic fuel production from electricity and air being enough to generate that. Thanks for the info!

[jupp0r]

If this was true, we'd be building capacitors instead of batteries...

[cptskippy]

Or perhaps your understanding of the problem is incomplete?

The point of the batteries are to store power when it's cheap and abundant, then deliver it during peak demand for sustained periods. The time between those periods is measured in tens of hours.

[jupp0r]

So your point is that we care about energy storage capacity more than about maximum discharge power after all?

[nl]

Of course we care about the energy amount stored - some. But total power capacity is more important for a large number of applications.

The usage the OP listed (arbitrage) is only one of the 9 function listed by the National Renewable Energy Lab[1]. Many of the functions need immediate power capacity in the seconds to an hour (so MW seconds or MW minutes), and more important is the speed of response and the amount of power deliverable.

[1] https://www.nrel.gov/docs/fy19osti/74426.pdf see table 1

[jupp0r]

You are describing perfect use cases for large capacitors, which was my point earlier.

[bee_rider]

I believe they mean:

(Battery storage) capacity

not

Battery (storage capacity).

That is, they are talking about power capacity (power is probably implied by default), which is provided by a type of device. The type of device is “battery storage.”

[digitalscribe]

Because this is for utility companies where the value of the rate of discharge might be even more important than the energy capacity?

A quick look at their site shows "battery power capacity" is measured in watts, whereas "battery energy capacity" is measured in watt-hours.

https://www.eia.gov/todayinenergy/detail.php?id=51798

[jupp0r]

I came here to complain about the same thing. Don't they have anybody who can proofread these diagrams and texts? For some reason, electric power related articles are super prone to people getting units wrong.

[bamboozled]

What should it be measured in?

[jdsully]

watt/hours is the typical measurement. Raw wattage is insufficient without a time component.

[rightbyte]

Wh, not W/h. W/h is a change in power.

"Scotty, increase engine power by 1 gigawatt per hour!"

[epistasis]

And for a lot the initial lithium ion storage on the grid, which was used for frequency regulation, the W/h or ability to respond to powe change, was a far far more important characteristic of the batteries than the Wh.