Battery 4 hour storage limit [Utility scale storage]

[wwhitney]

Question: If a plant is designed with a 100MW power capacity, and 400MWh of storage capacity, does it have to ALWAYS provide 100MW of power when discharging?

No. It should be able to discharge at any rate between 0 and 100MW.

The ratio of a battery plant's capacity (max energy storage in kWh) and its power (max discharge rate in kW) is a choice. Apparently the applications for which utility-scale batteries are most useful or most cost effective at the moment are those for which that ratio is in the range of 2-4 hours. But if it becomes cost-effective to deploy battery plants with a larger "duration" (that ratio), then there's no technical reason it can't be done.

Cheers, Wayne

 

[Tam]

...4 hours of power capacity...

There's no technical reason that the battery system capacity can only last 4 hours. There's such a limit because of the lack of the will to design a system to last longer.


The battery system for Ta’u in American Samoa “allows the island to stay fully powered for three days without sunlight and can recharge to full capacity in only seven hours.” It no longer uses fossil fuel for electricity, and it has solely depended on solar panel/battery system for the past 6 years:

The new Tesla is powering an entire island with solar energy

The island of Ta’u in American Samoa will be nearly 100% powered by solar power that is enabled by a microgrid composed of 60 Tesla Powerpacks.

www.businessinsider.com

Its population is only 600, but this system can scale up for an entire country, so the battery can last for days and not just hours.

 

[ZenRockGarden]

I would expect that cost/benefit drives the sizing of utility scale battery systems.

Of course they could build them to cover any duration desired. Additional time will cost cubic dollars.

Since the most common durations where the battery is needed to augment the grid tend to be in the 1 to 6 hour region, it probably makes economic sense to target that "80%" case vs spending vastly more money to try and cover the rare longer outage.

None of that is very controversial - not sure why this all started with seeming outrage at a non-existent 4 hour max battery duration.

 

[RKCRLR]

There's no technical reason that the battery system capacity can only last 4 hours. There's such a limit because of the lack of the will to design a system to last longer.


The battery system for Ta’u in American Samoa “allows the island to stay fully powered for three days without sunlight and can recharge to full capacity in only seven hours.” It no longer uses fossil fuel for electricity, and it has solely depended on solar panel/battery system for the past 6 years:

The new Tesla is powering an entire island with solar energy

The island of Ta’u in American Samoa will be nearly 100% powered by solar power that is enabled by a microgrid composed of 60 Tesla Powerpacks.

www.businessinsider.com

Its population is only 600, but this system can scale up for an entire country, so the battery can last for days and not just hours.

It isn't the technical feasibility, it is the cost. I'm a net producer on an annual basis but for me to not rely on the grid in the winter I would need to triple my solar production and probably double my powerwalls (currently 3). And that's assuming I still use my dual fuel (propane) system and woodstove the way I do now. And there would still be some times that I would need a generator.

American Samoa probably doesn't need nearly as much energy for heating and cooling and they probably get a lot more sun than many places.

Now ,the way electricity prices are going in California it may pencil out in the future for me to go all solar and be a grid defector. But that will cause problems for the rest of the grid if many people start doing that. And that's assuming they allow grid defecting.

 

[Tam]

...But that will cause problems for the rest of the grid if many people start doing that. And that's assuming they allow grid defecting...

There has been an argument to punish "freeloaders" who use the "grid" for free regarding private residential solar panels.

I think that is a false argument.

It's like when there was an oil embargo in 1970's, when there was not enough oil in the US, we need all the oil we could get and not worry about "freeloaders" who don't pay for the oil pipeline.

We just had several Cell Phone alerts to conserve electricity to prevent a blackout in California. It's about not enough electricity so we need all those private residential solar panels.

Right now there's no problem with too much electricity for the grid. It's the opposite: not enough!

 

[ZenRockGarden]

There has been an argument to punish "freeloaders" who use the "grid" for free regarding private residential solar panels.

I think that is a false argument.

It's like when there was an oil embargo in 1970's, when there was not enough oil in the US, we need all the oil we could get and not worry about "freeloaders" who don't pay for the oil pipeline.

We just had several Cell Phone alerts to conserve electricity to prevent a blackout in California. It's about not enough electricity so we need all those private residential solar panels.

Right now there's no problem with too much electricity for the grid. It's the opposite: not enough!

I hear ya, but it's a bit like how we maintain the highways with a gas tax. Everyone is using the highways - not just for their own car, but for goods and services which come over the highway system.

The grid not only connects to our houses, it makes the streetlights go. It needs to be maintained, hopefully in a fair manner.

Gas taxes worked when all vehicles used gas. We probably have to rethink it (I favor a registration fee based on mileage which would be uniform and fair, and would replace the gas tax instead of trying to tax electricity).

Grid maintenance used to be covered using portions of everyone's electric bill. With some folks going off-grid, there's less money to maintain the grid (which as I noted everyone is kinda using even if they completely disconnect their home). We need to figure this out - I would again favor some kind of uniform fee that everyone who lives in the region contributes to. I would also be fine if we made a uniform system where homes generally connect to the grid, but the utlities get to pay half-price for excess power going back into the grid and use that profit margin to help maintain the wires....

 

[jjrandorin]

I can feel this starting to slip into discussion about policy discussions and not "Tesla megapack" discussions, which would be somewhat related to this particular subforum. If thats the case, I will likely move it to the energy policy subforum, which is where those discussions go.

 

[ZenRockGarden]

OK. So Powerwalls and MegaPacks can be configured to last as long as you want against a given load. Fiscal concerns cause most use cases to land in the 4 to 6 hour region. Maybe we can be done.

 

[1100MCM]

4 hours is just the standard for utility scale battery storage. There is nothing to prevent them from supplying less power at longer duration but grid load peaks are mostly planned for 4 hours. When utility scale storage is procured the 4 hours is the “nominal” duration the batteries will supply power since 4 hours is a typical high load time in summer evenings (5-9 pm) as well as being exactly when solar production falls off rapidly.

 

[petit_bateau]

The link and comment that @Tam made is what I am talking about. How much power can a utility scale battery station supply and for how long. This link covers this exact topic. So "We calculate a battery’s duration by using the ratio of energy capacity (measured in megawatt-hours [MWh]) to power capacity (in MW). Energy capacity refers to the total amount of energy these batteries can store."

A station with a 100MW power capacity is able to generate 100MW of power. If the energy capacity is 400MWh, it can generate that 100MW of power for 4 hours. And the linked article also shows the capacity and duration for energy shifting applications:

View attachment 857381

So this is likely where the "4 hours" number that is typically referenced comes from, though note that there are many sites that provide more than 4 hours of power generation.

The point that I was originally trying to make, is that many articles I read like this one often mention 4 hours of power capacity. While there are another whole universe of articles that complain about needing longer term storage, because 4 hours isn't sufficient, like this one also mention 4 hours as a kind of "upper limit":

View attachment 857382

My point being that utility scale battery storage sites can provide power for any duration required. A 10MW power capacity system with 1000MWh of energy capacity can provide 100 hours of 10MW of power. Now, whether that is practical or not, I have no idea.

Question: If a plant is designed with a 100MW power capacity, and 400MWh of storage capacity, does it have to ALWAYS provide 100MW of power when discharging, or could it be operated to provide 50MW of power, and therefore be able to run for 8 hours?

I totally get that batteries aren't likely to be able to get us from say 90% renewable to 100% renewable due to having to be sized very large for the winter scenario where there are 10 cloudy days in a row. For that you likely need seasonal storage (electrolyzed hydrogen stored and burned in gas turbines).

Apologies for the confusion and not being clearer in my original post. Hope this clears things up.

RT

To answer your direct question - Utilty scale battery storage plants can be operated to discharge variable amounts of power and so can discharge for variable lengths of time depending on how the operator chooses to use them on any given day.

To answer the underlying question - The 4-hour thing is sort of circular. Because it turns out that the general sweet spot in design terms (at the moment) is to design a plant with a 4-hour storage duration (at max discharge rate), then it has also become habitual for the trade press and even general media to start discussing all of them in terms of their nominal 4-hour capacity. This is now quite powerful descriptive norm and therefore gets quoted even if the specific plant is designed/used for another use-case entirely which may have a shorter (more common) or longer (less common) design duration.

 

[mongo]

Question: If a plant is designed with a 100MW power capacity, and 400MWh of storage capacity, does it have to ALWAYS provide 100MW of power when discharging, or could it be operated to provide 50MW of power, and therefore be able to run for 8 hours?

For a Megapack, the inverter capacity in MW is typically one quarter the battery capacity in MWh. So *at max power* it lasts 4 hours. It doesn't need to run at full power and so can last longer (assuming the grid load is less).

 

[Tam]

...4 hours?...

ESS flow battery "will provide enough energy to power 60,000 homes for 10 hours" for Sacramento County, CA.

Accelerating decarbonization ESS Inc and SMUD announce agreement for long-duration energy storage

www.smud.org

Again, longer than 4 hours is doable. It's just a matter of whether you want to do it or not. Not because of unavailable technology.

 

[Tam]

...4 hours...

New tweet about 24/7 and not just few hours:

https://twitter.com/x/status/1575268810109505536

 

[dareed1]

I'm all for battery storage and am glad to see that Oahu is investing in storage. But the coal plant going off-line (as required by law) is 180MW, and this MegaPack installation is about 450MWh, or less than 3 hours. It won't come close to making up the shuttered coal plant. Instead Oahu will import more petroleum and the utility has warned consumers that their electricity bills will go up by about 7%.

Storage is essential to the success of intermittent energy sources like solar, but it also takes a significant investment in the source of energy. Hawaii is investing heavily in solar, but it takes time to ramp up.

See for example

 

[shs1]

I have often heard the 2-4 hour battery storage argument used by those promoting hydrogen or some other energy storage medium. Basically FUD about LI Ion batteries and batteries in general. I recently got into a debate on this issue on another forum stating that the self-discharge rate of Li Ion batteries is about 1/2% a month and so this 2-4 hour thing is not based on storage ability or technology, but rather economics and actual practical storage needs. One hydrogen advocate said that they have electric buses and if they don't keep them plugged in, they are dead in a matter of a week or so, so obviously batteries can't store energy of any significant length of time, e.g to get from summer to winter which he claimed was the real energy storage need. I tried to explain the energy use by cars and buses in standby (vampire drain), but he still would not admit that batteries could store energy for any length of time. Sometimes it is just not worth arguing. When I got my first Powerwall 2, it sat for a good 9 month while I was arguing with Tesla to allow an installation with a generator present. I was glad to see when it was finally activated (with the generator present) that it had plenty of charge and was ready to go.