"Storing" energy when it is not being used?

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Summary:

Understand the idea of storing energy when energy is not being instantly used and "sending it back" to the power plant.
Hello,

When discussing alternative sources of energy, like solar, wind, etc., we often hear that if the energy produced by these sources which is not immediately used, can be "sent" to the power plant and used later... I don't think the idea that is energy being transferred "somewhere" and stored "somewhere" is accurate at all. What is happening exactly? For example, let's consider wind energy: kinetic energy is converted into "electrical energy which could be either stored in a battery or "sent back" to the power plant. Or solar energy produced by solar panels and not being used by our home turning the meter backward..

What happens exactly when energy is not being used and said to be "stored"? At the power plant, I see how nuclear, coal, etc. are eventually producing hot and high pressure water vapor that spins turbine which in turn produce electricity that is used by people through the grid (which is just a big network of wires). By connecting devices to the grid in parallel, we "use" electricity and pay for it. Sometimes, the energy request is larger than other times. What happens at the power plant in that case?

Thanks for any clarifications.
 

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  • #2
russ_watters
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When discussing alternative sources of energy, like solar, wind, etc., we often hear that if the energy produced by these sources which is not immediately used, can be "sent" to the power plant and used later... I don't think the idea that is energy being transferred "somewhere" and stored "somewhere" is accurate at all. What is happening exactly? ...

What happens exactly when energy is not being used and said to be "stored"?
....
What happens at the power plant in that case?
The power plant throttles down to match the load. The "storage" is virtual.
 
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  • #3
Stephen Tashi
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Methods of (explicitly) storing energy are used. e.g. https://www.epa.gov/energy/electricity-storage

As to whether you can say the excess energy produced by a given household is stored in such a manner, perhaps not. We could regard conserved resources for generating power as a type of energy storage. For example, if a powerplant burns coal and the demands for power are less because of households feeding excess power into the system, then less coal is required. So the unused coal can be regarded as storing energy.
 
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  • #4
DaveE
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I don't think there is a general answer. You need to look into the details of each power plant & associated storage system to figure out what "sent back" means.
 
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  • #5
anorlunda
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Pumped storage hydro has been used for decades to store electric energy in the form of potential energy. Recently, batteries have been used.

But in terms of adjusting power plants to compensate for changes in load, we have an insights article about that. The article tells you all the things that happen in the grid and the plants after you flip a light switch.


https://www.physicsforums.com/insights/what-happens-when-you-flip-the-light-switch/
 
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  • #6
Baluncore
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This thread appears to arise from a conflation of two concepts.

Firstly; In the real world, the grid distributes a regulated voltage, V, over a fixed impedance, Zo, transmission line, but allows the many customers to adjust their currents, I, to control the flow of energy. When the load on a transmission line is mismatched, Zo ≠ V/I, the unused energy is reflected back to the generator.
That assumes the unwanted energy was generated in the first place. Networks of short transmission lines can be treated as lumped constants. When the load is largely resistive, distributed over the whole area of the grid, and the mismatch is made up by generator regulation, then the impedance of the transmission line does not dominate the equations.

Secondly; Some sources of energy, such as solar power and wind energy must be used immediately, or be stored using some other technology, or be foregone.
Stored energy not used now, has been conserved for later. That is true for energy sources derived from a stored material, such as a fuel, a charged battery, or the water in a hydroelectric reservoir. Delaying the release of stored energy is essential to regulation of the grid voltage, while the herds of ignorant customers vary their loads.
 
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  • #7
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Thank you. So when we don't use the electrical energy produced by our solar panel and sell it back to the energy company, the unused energy is instantly being used somewhere else by somebody else.
 
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  • #8
Baluncore
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Yes. And the fossil fuels they would otherwise have burnt can remain in the ground.
 
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Thank you. So when we don't use the electrical energy produced by our solar panel and sell it back to the energy company, the unused energy is instantly being used somewhere else by somebody else.
That's right. It might be better to call that "displacing" energy rather than "storing" it. But the long term effect is the same as storage: Because you had power from solar cells, you DIDN'T need to burn fossil fuels, so now you have that extra fuel to burn later when the sun isn't out.
 
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I have heard that during some time of the day, like at night, wind turbines are not operated because there is low need of energy at that time of the day...Is that possibly true? Can the electrical energy produced by those wind turbines be rerouted elsewhere?
 
  • #11
anorlunda
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E.g. SaP 😒
I have heard that during some time of the day, like at night, wind turbines are not operated because there is low need of energy at that time of the day...Is that possibly true? Can the electrical energy produced by those wind turbines be rerouted elsewhere?
No. When they say not operated, it means that they stop generating any power. That's not the same thing as generating anyhow and rerouting.
 
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  • #12
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E.g. SaP 😒

No. When they say not operated, it means that they stop generating any power. That's not the same thing as generating anyhow and rerouting.
Yes, sorry, I didn't communicated well.

Why wouldn't they be operated all the time to produce energy all the time? I get that there are moments of the day when there is less energy demand. But why stop generating power?
 
  • #13
anorlunda
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Why wouldn't they be operated all the time to produce energy all the time? I get that there are moments of the day when there is less energy demand. But why stop generating power?
The reason is money. They get paid to generate power. No pay, no generation. However, sometimes power producers would rather generate no matter what. They can put in a negative bid price to sell their power at. That means that they have to pay the power company to generate during those hours when the prices go negative. In most locations, that happens seldom or never, but in Southern California, prices frequently go negative in summer afternoon hours.

It is the same with solar PV power. If the sun is shining why not make all the power possible? But still the power has to be transported and distributed to somewhere else, and some other power plant must reduce their power generated to compensate. Depending on where you live, the cost of generating the power may be only 1/3 to 1/2 of your power bill. So, although the primary motivation is to generate as much as possible all the time, secondary motivations can override.
 
  • #14
berkeman
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I have heard that during some time of the day, like at night, wind turbines are not operated because there is low need of energy at that time of the day...Is that possibly true?
One of the reasons is that in locations like the wind farms near me (east of Silicon Valley, California), the wind typically dies down at night. Below is a plot of the wind velocity and direction for the next 10 days:

https://www.wunderground.com/foreca...7114,-121.80632767983757?cm_ven=localwx_10day

1604250124123.png


BTW, here in Northern California we have "rolling blackouts" that can happen on very hot days when lots of air conditioners are running. They typically take place between 5-`10PM, because the large solar farms start losing sunlight in the early evening, which reduces the available power for supplying all of that air conditioning while the air is still hot outside.
 
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  • #15
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I see, thanks berkeman!

So, in essence:
  • there is not such idea or technology to "store" electrical energy (unless we talk about recharging batteries or pumped hydro storage, as suggested by anorlunda).
  • To save energy, we can either burn less coal or reduce energy production by turning off or throttling solar, wind, hydro, nuclear, etc. when the electricity demand is not high. So energy production and delivery are addressed (almost) in real time.
  • In some scenarios, producing extra energy all the time via alternative energy sources (solar, wind, etc.) is actually not ideal and can cost more at the end...
Thanks!
 
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  • #16
berkeman
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there is not such idea or technology to "store" electrical energy (unless we talk about recharging batteries) on a large scale.
Did you see the reply a while back by @anorlunda about pumped hydro storage? It is a practical solution in some situations...
 
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Yes! I did. I forgot to include it. I am now.
 
  • #18
berkeman
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Yes! I did. I forgot to include it. I am now.
The GIF attachment comes up blank for me...
 
  • #19
anorlunda
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BTW, here in Northern California we have "rolling blackouts" that can happen on very hot days when lots of air conditioners are running.
That's enough to bring tears to the eyes of any power engineer. The credo is "keep the light on no matter what." To be forced to turn them off manually seems like an unforgivable crime. No doubt, the blacked out customers have even stronger words to say about that.

FERC requires all utilities in all states (except Texas) to provide enough reserve capacity so that it almost never runs out. But year after year, California seems to not comply with that. I don't know how they get away with it.
 
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  • #20
Tom.G
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FERC requires all utilities in all states (except Texas) to provide enough reserve capacity so that it almost never runs out. But year after year, California seems to not comply with that. I don't know how they get away with it.
Because the Peaking plants burn fossil fuel, which California is trying avoid. The 'avoidance' is to decommission the oldest/least efficient plants.

Unfortunately much of the data for peaking requirements is from when it was a bit cooler. Lately, every time one faction wants to decommission an old plant another faction points out that there is not yet enough storage to handle the peaks. (They seem to be working on the storage problem, Lithium batteries keep getting cheaper.)

So some plants get torn down, resulting in occassional outages, or replaced with more efficient ones.

The other reason for outages in California is that electric utilities have "strict liability" if their equipment causes a wildfire. Consequently, in high wind and temperature conditions with low humidity, the utilities shut off power in their long distance, high voltage, transmission lines. Even if the wind blows a tree branch into a substation, they are liable; they have the responsibility of trimming all vegetation that could cause a problem for their equipment. With occassional 100MPH gusts that is almost impossible. (IIRC, there are about 60,000 miles of transmission lines, seems high though.)

"Strict liability", in its strictest interpretation, means that the power utility is responsible for ALL costs of a fire caused by their equipment. That includes firefighting costs, replacement of all property damage (houses, cars, people), plus penalties.

In reality, they don't get hit with all of the costs because their net worth and/or capitalization would not be enough to cover it all, even if liquidated. (assuming a buyer could even be found.)

Pacific Gas and Electric (PG&E), the states largest privately owned utility, got hit with this; they emerged from chapter 11 bankruptcy on July 1, 2020.
(https://www.google.com/search?&q=pg&e+bankruptcy+plan)

Cheers,
Tom

(hmm.. guess I got carried away and drifted off topic, sorry)
 
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  • #21
anorlunda
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Everything you said is true Tom, but they are not valid excuses.

California's wildfire problem is severe. But the utility's response to red flag conditions is to shut things off in the red flag region until the weather changes. Rolling blackouts or brownouts that last a few hours are not because of fires, they are an attempt to remedy shortage of supply.

The difficulties you cite in California are analogous to the difficulties in every other state and other countries in the world. Difficulty is no excuse for breaking the law. "But officer, I was speeding because I'm late for work."

I am saying that California can meet and should meet FERC's and NERC's and CAISO requirements, even if they find the necessary steps distasteful. Other states with their own challenges do comply.

For those who don't know, here are the acronyms:
  • FERC = Federal Energy Regulatory Commission
  • NERC = North American Electric Reliability Corporation
  • CAISO = California Independent System Operator
Edit: and here is the curve that describes what I'm talking about.
https://www.xcelenergy.com/staticfi...SCo-ERP-2011/Attachment-2.10-1-LOLP-Study.pdf

1604323317201.png
 
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In post #19 you mentioned an exception for the state of Texas. Could you elaborate on that?
 
  • #23
anorlunda
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In post #19 you mentioned an exception for the state of Texas. Could you elaborate on that?
Yeah, it's an odd case.

Texas had (and has) an aversion to federal regulation. So Texas historically made sure that no power lines cross their borders with other states. Therefore, the federal government has no authority to regulate the Texas power grid.

Rest assured though that they have their own state regulators.

1604328166454.png

The Texas grid is called ERCOT, note that it no longer follows the borders exactly. Quebec is another maverick, but not as drastic as Texas.
 
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  • #24
berkeman
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Texas had (and has) an aversion to federal regulation. So Texas historically made sure that no power lines cross their borders with other states.
Huh, I did not know that. So most/all of their power generation is oil and coal powered? Do they have any renewable power sources in Texas?
 
  • #25
anorlunda
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Huh, I did not know that. So most/all of their power generation is oil and coal powered? Do they have any renewable power sources in Texas?
Sure. There is so much wind power in West Texas that they don't know what to do with it. One record day in 2016, 45% of their power came from wind.

Much of the wind was financed privately by Boon Pickens. But post-fracking, Pickens changed his mind, deprecated wind, and started investing in gas.
 
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