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I just read an article about some entrepreneurs investing money into ways for producing power that do not rely on fossil fuels.  A worthy cause in my opinion.  The article mentions one aspect of the big picture, calling energy storage the "holy grail that will speed the green power revolution."

Specifically the article mentions one approach:

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The process involves towing nine-metre-tall balloons off shore and weighing them down. Compressed air is pumped in and held there by water pressure. When power is needed, the air is released into a turbine that sends the power back into the electrical grid.

So it got me thinking that what we really need to do is come up with ways to store potential energy.  We can collect solar and wind power during the day, but need ways to store it for use during the night, and days that are calm and cloudy.  

My first thought is to use gravity: pumping water into an elevated tank, then when needed we can release the water into a lower one, passing through turbines along the way.  Could this be done on a scale small enough where each dwelling would have it's own mini hydro dam room?

I would like to hear your own ideas, or perhaps you've heard of other existing concepts out there.  How would you go about storing potential energy, to be converted into electricity on demand?  Or am I way off on this potential energy thing?

Article is found here: entrepreneurs-seek-path-to-fossil-fuel-free-future

Edited by justidutch
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Wouldn't storing energy chemically a more efficient way to do it? I mean, better than pumping water up high, wasting energy transporting it, energy expenditure to create necessary structures, and energy loss to the process of converting it back to usable energy.

If we have to store it mechanically, why not use fly wheels ?

Edited by RainDreamer
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The suggested idea of pumping water up to an elevated location isn't exactly new.  It's called pump storage and it is in actual use on utility scales in nations like Germany.  While not the most energy efficient, it is one of the cheaper storage systems to build, (most of it is little more than a big hole on top of a hill).

It, like all storage systems, has problems with efficiency but once you have a large fraction of your power coming from renewables you often have electricity that is so cheap that running the pumps only really costs you in wear.

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1 hour ago, RainDreamer said:

Wouldn't storing energy chemically a more efficient way to do it? I mean, better than pumping water up high, wasting energy transporting it, energy expenditure to create necessary structures, and energy loss to the process of converting it back to usable energy.

If we have to store it mechanically, why not use fly wheels ?

Because all motion and no stop makes the flywheel an unreliable flywheel over time.

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34 minutes ago, Chakat Firepaw said:

The suggested idea of pumping water up to an elevated location isn't exactly new.  It's called pump storage and it is in actual use on utility scales in nations like Germany.  While not the most energy efficient, it is one of the cheaper storage systems to build, (most of it is little more than a big hole on top of a hill).

It, like all storage systems, has problems with efficiency but once you have a large fraction of your power coming from renewables you often have electricity that is so cheap that running the pumps only really costs you in wear.

Cool, I like reinventing the wheel.  I guess I was thinking more small-scale: each house has it's own storage capacity, two tanks, one elevated and one below it.  I have not done any looking into numbers, how much energy could be stored in what sized tanks, it's just a thought from a guy with fairly weak math skills!

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oh.. a big change in the forum looks, I thought that I miss the page.

Here you go:

https://www.easycalculation.com/physics/classical-physics/potential-energy.php

You need to calculate the potential energy, lets said that you have a building 100m tall. In the top you have a 30m3 tank.

The same pump that is used to raise the water can work as generator.. Electric motors has close to 95% efficiency.

30m3 = 30000kg if we rise this 100 m then we have 29400000 Joules --> 8,1kwh  -10%= 7,3kwh  Which is not much for a 100m building :(

Also not sure how to do to not waste a part of that water without big changes.

But it was not a bad idea...  People should think in ways to improve the efficiency of all our systems. This is not so hard as many may think, because we never needed before (when oil was cheap and co2 dint matter)

 

Other company plans to do the air compress bag idea but at 1000m deep, capturing the heat of the compression before send it to the bag. 

 

 

Edited by AngelLestat
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As Chakat mentioned, it's already done in places with elevated terrain; I suspect that you'll need a large scale project for this to marginalize the inevitable losses due to friction, etc.

Let's do some math. According to the US Energy Information Administration, the average US home uses about 10,932 kWh per year. Divide that by 365×24 and you get an average power useage of about 1.25 kW which doesn't sound outlandish.  For an 8 hour night (you can expand this number according to your wishes) you'd need a storage capacity then of around 10 MJ. Now that's an easy number to work with!

Storing one cubic meter of water (1000 kg) at a height of 1 m will take about 9.8 kJ. Let's say for argument's sake that you store your upper tank about 10 m above the lower tank. That's between 3 and 4 floors (depending on how tall your house is), and especially if the lower tank is buried under ground not unreasonable. So your one cubic meter will now store (theoretically!) about 98 kJ; you'll need about 10 of those. If you take a spheroid tank (we love those after all!) it would take a tank with a diameter of a little bit less than 2.8m.

Of course, that assumes that there are no losses in pumping the water up, and no losses in turning the water flowing back in the lower tank into energy. Let's be optimistic (because I think your losses will be far higher than that) and assume 50% loss in both directions; so in total 50%×50% is 75% loss. So your tank will need to be 40 m3 or about 2.24 m in diameter (or about 7'4" if you're into those units).

While this doesn't sound too outrageous, the main questions are:

  • Do you need more or less than 8 hours of reserve (adjust accordingly)
  • How much energy storage does 40 m3 in batteries (even fairly simple lead-acid ones) get you?

I think it works for a utility company that creates an artificial lake because of the sheer size they can operate on. At home, a battery oriented solution seems the better way to go.

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http://www.cnet.com/news/teslas-elon-musk-introduces-beautiful-money-saving-home-battery-back-up-system/

http://www.forbes.com/sites/uciliawang/2015/05/01/from-elon-musk-the-battery-packs-he-hopes-will-rule-them-all/

Elon thinks hydrogen is a loser fuel.

http://www.wired.com/2013/10/elon-musk-hydrogen/

Toyota and Honda would disagree, even though they don't have a decent proposal to make hydrogen.

http://world.honda.com/FuelCell/

https://ssl.toyota.com/mirai/?dfaid=DFA:1698801:120387914:295362048:62973859

Interesting both musk and toyota used the same 8-letter word, Musk as a mockery and toyota as a source of fuel.

Hydrogen is the problem, http://esc.fsu.edu/electrolysis.html

Hydrogen can be made efficiently on very small scales, but not cheaply or efficiently on large scales. The modern motor fleet uses 100,000s of tons of hydrogen bond energy equivilents a day.
We have to consider the two equivilancies. Lithium is abundant if you don't mind separating sodium and potassium from lithium in sea water. The ratio of sodium:potassium:lithium in the earths crust is 8000:266:1. There are only so many uses for sodium and potassium hydroxide that are economically useful. Of course one of them is to absorb C02, but then you have residual HCl you have to store. So lithium is expensive. Then there is platinum. Electrolysis is not efficient, needs alot of platinum or platinum treated palladium and generates alot of waste heat, mostly of little use because the heat is at sub boiling temperatures (unless you do pressurized electrolysis). Electrolysis does not work well heated either. At best hydrogen would cost as low as $5.30 the equivilent of one gallon of gasoline. Since gas is now running $1.90 a gallon hydrogen fuel cells are not feasible unless cities subsidize them. And there are good reasons cities would not do this. 1. It would be a tax on the grid, whereas electric cars and gas/electric hybrids would be less so. In fact in japan you see now electric power companies tapping into the hybrids ability to generate electricity to feed the grid during peak demand.  2. It requires a whole new infrastructure. 3. Consider the optimal compliment of solar and wind power distributed globally to support the worlds needs, we would need 2 to 3 times that capacity to convert it all to hydrogen and then back to electricity or to power vehicles, and BTW that still does no give us the chemicals we need to build plastics or electronics.

There are some reasons why hydrogen is attractive. Wind power is often generated far from where it is used, and at peak supply the price of electricity often drops to zero (as in parts of North Central Texas). Another example is that European plan on building solar farms in Africa, but transporting electricity from Africa to Europe by any known means is inefficient and subject to disruptive acts over long distances. Hydrogen could be made in africa and piped to Europe almost invisible to disruptions and burnt cleanly where it is needed, including neighborhood fuel cells. Electric battery is no substitute for bond energy for long distance transportation such as on Jets. In some places solar power will not be possible for long stretches. Nuclear power plants can be used but often represent a third or less capacity because they tend to produce at a constant rate, if we increased nuclear energy we could feed all electric during peak demand and part electric and part hydrogenesis and lowest demand.

Lithium ion storage like solar panels are the most useful at the load, not the source, and are best coupled to each other, and while mobile are less mobile than hydrogen.

The choice of storage has some dependency on function. You would not want a lithium ion battery running a transpacific wide-bodied aircraft, it would be kind of silly to use solar panels on a home to create electrolytic hydrogen and then convert that back to electricity. You might have a hydrogen/lithium-ion hybrid car that one might mandate in a highly polluted city, like Delhi or Beijing, but the politics there is decades to centuries away.

 


 

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How can I do to not let spaces in each line that I press Enter?  Also.. there is a way to use the last quote tag system?

1 hour ago, PB666 said:

Elon thinks hydrogen is a loser fuel.

Elon just said that (in my opinion) to increase the confidence of his clients in his car. But that could help in the short term, but those words would come back as a boomerang to hit his ass.

We already had this discussion in the topic lithium vs hydrogen, in that topic I said that for small vehicles (less than 1500 kg or for high acceleration cars) batteries has sense but for anything big than that fuel cell has more sense.

Maybe not just now.. but for sure in the next 3 years (depending location) will be.  

Quote

Interesting both musk and toyota used the same 8-letter word, Musk as a mockery and toyota as a source of fuel.

Now all big car companies are making their hydrogen car, not just toyota and honda. So you think they are all wrong and Elon is right? I am a Elon fan.. but I know to disguise between strategic propaganda vs reality.

Quote

Hydrogen can be made efficiently on very small scales, but not cheaply or efficiently on large scales.

???  ehh?   Hydrogen is made it at big scales using steam reforming, but is not used in cars because that market is not really here yet. Is used to make fertilizers. About electrolysis is the same at low scale than big scale, the only thing that matter here is the catalyst, right now the best is platinum which is very expensive, but a lot of new technologies are rising to remplace platinum with other materials without losing efficiency. But even with today hydrogen production disadvantages, Germany already had a net of stations that produce hydrogen at 700bar or in the new standard 350 cryo-compressed (not liquid) when they need to storage energy.

 

The math is easy, batteries become very expensive to move heavy things or for long range. But in the hydrogen case, you just need a bigger tank which is not expensive, also you can recharge the tank in no time.

Right now you will find a lot of articles talking about hydrogen new techs, cars and long scale country plans. In the Paris convection all countries are reaching an agreement that they need to move one step further into the hydrogen economy.

Because when you talk about energy storage, there is not battery, flywheell or any other technology that can be close to match the storage requirements needed at country scales as hydrogen or methane (capturing co2) can provide.

Countries can be 100% renewable, and many are taking that step right now. It was after a study ordered by europe community which was made in finland. Where they find that they can change to fully solar and wind (including transport) without increase more than a 10% their current energy budget.

Is easy, when you have extra energy (that it will be most of the time) you make hydrogen and methane.

Compress air comes second as big scale energy storage alternative.

Of course, as always, I can backup everything that I said with evidence and logic in case someone needs. 

Edited by AngelLestat
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1 hour ago, AngelLestat said:

How can I do to not let spaces in each line that I press Enter?  Also.. there is a way to use the last quote tag system?

Elon just said that (in my opinion) to increase the confidence of his clients in his car. But that could help in the short term, but those words would come back as a boomerang to hit his ass.

We already had this discussion in the topic lithium vs hydrogen, in that topic I said that for small vehicles (less than 1500 kg or for high acceleration cars) batteries has sense but for anything big than that fuel cell has more sense.

Maybe not just now.. but for sure in the next 3 years (depending location) will be.  

Now all big car companies are making their hydrogen car, not just toyota and honda. So you think they are all wrong and Elon is right? I am a Elon fan.. but I know to disguise between strategic propaganda vs reality.

???  ehh?   Hydrogen is made it at big scales using steam reforming, but is not used in cars because that market is not really here yet. Is used to make fertilizers. About electrolysis is the same at low scale than big scale, the only thing that matter here is the catalyst, right now the best is platinum which is very expensive, but a lot of new technologies are rising to remplace platinum with other materials without losing efficiency. But even with today hydrogen production disadvantages, Germany already had a net of stations that produce hydrogen at 700bar or in the new standard 350 cryo-compressed (not liquid) when they need to storage energy.

 

The math is easy, batteries become very expensive to move heavy things or for long range. But in the hydrogen case, you just need a bigger tank which is not expensive, also you can recharge the tank in no time.

Right now you will find a lot of articles talking about hydrogen new techs, cars and long scale country plans. In the Paris convection all countries are reaching an agreement that they need to move one step further into the hydrogen economy.

Because when you talk about energy storage, there is not battery, flywheell or any other technology that can be close to match the storage requirements needed at country scales as hydrogen or methane (capturing co2) can provide.

Countries can be 100% renewable, and many are taking that step right now. It was after a study ordered by europe community which was made in finland. Where they find that they can change to fully solar and wind (including transport) without increase more than a 10% their current energy budget.

Is easy, when you have extra energy (that it will be most of the time) you make hydrogen and methane.

Compress air comes second as big scale energy storage alternative.

Of course, as always, I can backup everything that I said with evidence and logic in case someone needs. 

Hydrogen is currently made from fossil fuels, making the entire endevour pointless. You would need to make hydrogen production switch to electrolysis, which is energy-intensitive itself.

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10 hours ago, Chakat Firepaw said:

The suggested idea of pumping water up to an elevated location isn't exactly new.  It's called pump storage and it is in actual use on utility scales in nations like Germany.  While not the most energy efficient, it is one of the cheaper storage systems to build, (most of it is little more than a big hole on top of a hill).

It, like all storage systems, has problems with efficiency but once you have a large fraction of your power coming from renewables you often have electricity that is so cheap that running the pumps only really costs you in wear.

One benefit of pump storage is that its an adition to existing hydro plants. This make it cheap and very high capacity.
A huge problem with power generation is that coal and nuclear is slow to throttle, renewable is free but unpredictable. 
Hydro is nice as its fast to throttle, gas is also fast to throttle.

 

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4 hours ago, AngelLestat said:

How can I do to not let spaces in each line that I press Enter?  Also.. there is a way to use the last quote tag system?

Try Shift + Enter. It makes it look like 
this.

What do you mean last quote tag system? There is a "+" and "Quote" at the bottom of each post. The "Quote" quotes only that one post, "+" selects and remembers the post for multiple quotes. After pressing it you can click the "+" on additional posts to include them too in you reply.

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Pumped storage is efficient and relatively cheap if you have the natural topography to do it (ideally you want two preexisting lakes, although you can also create one or both the reservoirs naturally). Efficiencies are about 70-80%.

Flywheels are better than you might expect, if you aren't storing energy for a long period of time. For a few hours, or even days. They can get about 90% efficiency in this way. Capacitor banks are similar, but have even shorter storage times.

Batteries are very expensive up front, but are also efficient, you can get back 90+% of the energy you put in.

Compressed air systems are similar to pumped storage, but don't rely on topography. The turbomachinery is also generally slightly less efficient, you're looking at around a 20-30% loss each way, so 50-60% efficient, at the trade-off of a generally reduced capital cost.

Chemical energy is probably the worst way of doing things. I have no idea how efficient the production side would be, but the energy generation side would be limited by the Carnot efficiency, and would probably be 40% efficient, tops. (Unless you use a fuel cell, which is expensive and unproven, but not limited by the Carnot efficiency)

Other ways that don't get talked about so much are things such as storing the energy as low-grade heat. A huge proportion of our energy needs are just heating our buildings or making hot water. Generating heat is easy, and can be close to 100% efficient if you're using electricity to do it. If you store the energy in a eutectic salt, or even just a big cavern of water, at maybe 80-90 degrees, it will stay very warm for months, and can be used to heat homes.

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Using pumping storage has its issues. It damages the enviroment (if combined with a dam) and also its energy density is not very high, enough for peak loads, but not enough for a cloudy day without wind.

To compare: The pump storage systems of germany can store 0,04TWh, the gas grid can store about 200TWh. Germany needs about 30TWh storage to compensate shifting production and demand over the year (calculated for 80% renewables in year 2050), there you can see quite clear that pump storage alone wont be enough. It still has its applications, the efficency is higher than using electrolysed hydrogen.

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29 minutes ago, Elthy said:

Using pumping storage has its issues. It damages the enviroment (if combined with a dam) and also its energy density is not very high, enough for peak loads, but not enough for a cloudy day without wind.

To compare: The pump storage systems of germany can store 0,04TWh, the gas grid can store about 200TWh. Germany needs about 30TWh storage to compensate shifting production and demand over the year (calculated for 80% renewables in year 2050), there you can see quite clear that pump storage alone wont be enough. It still has its applications, the efficency is higher than using electrolysed hydrogen.

You could however use most existing hydro plants for pumped storage as long as you have water on the bottom then the turbines are not running. No this will not work if the dam is full but then you would run it on full anyway. 

Gas is nice in that its also fast to throttle up and down, but yes both require that hydro or gas is an decent part of your base load. 

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Is it unreasonable of me to still think of producing hydrocarbons from industrial exhausts and seawater?

I mean, if the energy comes basically free and in abundance from sun, wind, water, tidal, geothermal ... ?

Well, I have to confess I have no real concept of the energies needed to reconfigure molecules. :wink:

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15 hours ago, Kerbart said:

Of course, that assumes that there are no losses in pumping the water up, and no losses in turning the water flowing back in the lower tank into energy. Let's be optimistic (because I think your losses will be far higher than that) and assume 50% loss in both directions; so in total 50%×50% is 75% loss. So your tank will need to be 40 m3 or about 2.24 m in diameter (or about 7'4" if you're into those units).

50% loses on friction in the pipe??  In each trip? 

Those loses are close to 5% for the pipe that goes to the tank.  To reach 50% loses in friction you will need a labyrinth of pipes of small section.

 
9 hours ago, fredinno said:

Hydrogen is currently made from fossil fuels, making the entire endevour pointless. You would need to make hydrogen production switch to electrolysis, which is energy-intensitive itself.

Why you quote me if you dont read my quote?  Another thing that I dint mention is that a new tech left its prototype stage and now is entering for commercial use with plants already in construction, you can convert natural gas or methane into hydrogen capturing all the co2 in form of soot that you can also sale. The system has a efficiency of 75% and a production cost of 2eu by m3.

 

2 hours ago, peadar1987 said:

Pumped storage is efficient and relatively cheap if you have the natural topography to do it (ideally you want two preexisting lakes, although you can also create one or both the reservoirs naturally). Efficiencies are about 70-80%.

Flywheels are better than you might expect, if you aren't storing energy for a long period of time. For a few hours, or even days. They can get about 90% efficiency in this way. Capacitor banks are similar, but have even shorter storage times.

Batteries are very expensive up front, but are also efficient, you can get back 90+% of the energy you put in.

Compressed air systems are similar to pumped storage, but don't rely on topography. The turbomachinery is also generally slightly less efficient, you're looking at around a 20-30% loss each way, so 50-60% efficient, at the trade-off of a generally reduced capital cost.

Chemical energy is probably the worst way of doing things. I have no idea how efficient the production side would be, but the energy generation side would be limited by the Carnot efficiency, and would probably be 40% efficient, tops. (Unless you use a fuel cell, which is expensive and unproven, but not limited by the Carnot efficiency)

Other ways that don't get talked about so much are things such as storing the energy as low-grade heat. A huge proportion of our energy needs are just heating our buildings or making hot water. Generating heat is easy, and can be close to 100% efficient if you're using electricity to do it. If you store the energy in a eutectic salt, or even just a big cavern of water, at maybe 80-90 degrees, it will stay very warm for months, and can be used to heat homes.

Ok, but is not all about efficiencies, you need to include the cost of each method. Also let me correct you with air storage, today the efficiency reach 75% because you capture and reuse the heat that before was waste it.

Also.. Fuel cells are unproven?  The chemical efficiency can reach 60% in a triple cycle, fuel cells can reach 80% if you use the heat as cogeneration.

But yes, we need to learn to use the energy in a more efficient way, today most factories waste a lot of energy that can be used to increase the efficiency of other factories or homes.

Solve the intermittence problem of renewable energy sources is super easy and cheap when you include smart grid, cogeneration and hydrogen and methane production.

 

1 hour ago, Elthy said:

Using pumping storage has its issues. It damages the enviroment (if combined with a dam) and also its energy density is not very high, enough for peak loads, but not enough for a cloudy day without wind.

Is not for county scale, but when the terrain is appropriate is a very good option.  The part that is energy density is not high, that is not quite accurate.

If you have a cube tank of 2 meters, you get 8m3...  but if you increase the side just for 1 meter, then you tank now holds 27m3.

That is how a small and cheap lake like this can storage 30mwh.

okinawa_ps.jpg 

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1 hour ago, AngelLestat said:

50% loses on friction in the pipe??  In each trip? 

Those loses are close to 5% for the pipe that goes to the tank.  To reach 50% loses in friction you will need a labyrinth of pipes of small section.

Pray tell, how are you getting the water from the bottom to the top. Through the pipes yes. But you're going to need a pump. That pump needs to be powered. And then when it gets back, you'll have to convert the energy of that flowing water somehow in electrical energy. By all means tell me where you find the turbine/generator combo (working with the volumes we're talking about, not some hydro-electric powerplant monstrosity) that has an overall efficiency of over 95%; at that scale I think you can be very happy with 50% in both directions.

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5 minutes ago, Kerbart said:

Pray tell, how are you getting the water from the bottom to the top. Through the pipes yes. But you're going to need a pump. That pump needs to be powered. And then when it gets back, you'll have to convert the energy of that flowing water somehow in electrical energy. By all means tell me where you find the turbine/generator combo (working with the volumes we're talking about, not some hydro-electric powerplant monstrosity) that has an overall efficiency of over 95%; at that scale I think you can be very happy with 50% in both directions.

Actually, hydro pumps and turbines are pretty efficient. You can expect an isentropic efficiency of upwards of 90%. Pipe losses are pretty small in the grand scheme of things, unless you catastrophically undersize them. 70% is reasonable, 80% is achievable if you're willing to throw money at the problem and never operate at off-design conditions. 50% is a pretty poor design. Dinorwig in Wales operates at about 75% efficiency, and is a 1970s design. Turlough Hill near my home in Ireland operates at about 64%, and it was built in the '30s.

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20 hours ago, AngelLestat said:

The same pump that is used to raise the water can work as generator.. Electric motors has close to 95% efficiency.

30m3 = 30000kg if we rise this 100 m then we have 29400000 Joules --> 8,1kwh  -10%= 7,3kwh  Which is not much for a 100m building :(

Also not sure how to do to not waste a part of that water without big changes.

 

95% efficiency for an electric motor that also does alternator, maybe if your pump is from the 178th century where magic is a thing and breaking laws the laws of physics is possible... ^^

I'd rather say 20 percent efficiency... Where did you even get that number ? 

 

The whole point of water energy storage like this is not having 30m³ tanks, more like thousands and thousands transferred between giant pools, one being on top of a hill.

 

Actually it' not this good of a way to store energy : it's more like a way to make money : in the middle of the way when power is cheap, you pump the water up. At night when the electricity is way more expensive, you let the water down and sell the power. That's how they do it in Italy at least, probably the same in Germany. Not sure about the other countries

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2 hours ago, peadar1987 said:

Actually, hydro pumps and turbines are pretty efficient. You can expect an isentropic efficiency of upwards of 90%. Pipe losses are pretty small in the grand scheme of things, unless you catastrophically undersize them. 70% is reasonable, 80% is achievable if you're willing to throw money at the problem and never operate at off-design conditions. 50% is a pretty poor design. Dinorwig in Wales operates at about 75% efficiency, and is a 1970s design. Turlough Hill near my home in Ireland operates at about 64%, and it was built in the '30s.

I assume you missed the part where I said "not some hydro-electric powerplant monstrosity" — the discussion was about an energy storage solution on a house-by-house basis, Not at a powerplant level. I've visited the Dinorwig site. I doubt anyone will have that in their back yard.

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1 hour ago, Hcube said:

95% efficiency for an electric motor that also does alternator, maybe if your pump is from the 178th century where magic is a thing and breaking laws the laws of physics is possible... ^^

I'd rather say 20 percent efficiency... Where did you even get that number ? 

 

The whole point of water energy storage like this is not having 30m³ tanks, more like thousands and thousands transferred between giant pools, one being on top of a hill.

 

Actually it' not this good of a way to store energy : it's more like a way to make money : in the middle of the way when power is cheap, you pump the water up. At night when the electricity is way more expensive, you let the water down and sell the power. That's how they do it in Italy at least, probably the same in Germany. Not sure about the other countries

Guess what?  It seems that we are even further than the 178th century according to your scale.

In my example of 100m building, I imagine the motor has close to 20hp, at that power any motor has 93% of efficiency. Also any motor is a generator.. Just turn a DC motor and touch the cables and see what happen :), or connect a generator to power. The only thing that can reduce efficiency (which I am not sure) is the pump pressure disk that push the water if is designed in a way that in reverse would not catch much water. I will later search that in google if I have time.

But today we have even better electrical engines that can reach 98% of efficiency even at low power scale, and I am not talking about superconductor motors. 

http://www.enstroj.si/Electric-products/emrax-motorsgenerators.html 

" EMRAX motor is a completely new type of pancake axial flux brushless synchronous three phase AC (Alternating Current) electric motor. It can also work as a generator - technical data are the same - either EMRAX is used as a motor or as a generator.   We achieved our objective and built a high-powered, high torque, extremely light, direct drive, low noise electric motor, which efficiency is up to 98%. Because of the high torque EMRAX engine can achieve high power also at relatively low rotation speeds. "

 

1 hour ago, Kerbart said:

I assume you missed the part where I said "not some hydro-electric powerplant monstrosity" — the discussion was about an energy storage solution on a house-by-house basis, Not at a powerplant level. I've visited the Dinorwig site. I doubt anyone will have that in their back yard.

I did many calculations on pipe pressure loses before. So I have a very good idea on the effect of long pipes, curves, "T" and different type of faucet. Independent of how the house-building design is made, you will find a section between the pump and the main tank without many obstructions and a good diameter, so the flow almost not change.

One of my friend´s house has a terrible pipe design, and even there the lost of load from a boost pump (not higher tank) to the worst place of the house was not more than 30%.

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