Vacuum Energy Storage

[svetz]

No, not a free energy device. Also, not very practical.... so don't bother reading further unless you just like weird stuff or happen to live underwater.

This is the reverse of compressed air energy storage. I was curious how the math would work out after researching an earlier post. Compressed Air technology has a few things working against it. First is that while compressing the air the work put into it also has to pay for the heat being squished into a smaller volume. Second is that increasing pressure isn’t proportional to the energy gain, it’s limited by

.

With all those things working against it, I wondered would they work for you if you created a vacuum rather than compressed the gas? From these rough calculations, it looks like there are some things that work for you and some things to be careful about; but not really practical overall.

First, let’s propose how it might work. Imagine a tank filled with water, that’s the battery discharged. Pump all the water out leaving a vacuum behind, that the battery fully charged. We’ll add some ethylene glycol to the water to reduce it’s vapor pressure to say .1 psi (that is it’s not really a vacuum, just as well as the tank would collapse). The atmospheric tank is nothing special, but the vacuum tank needs to be strong to keep from getting crushed.

​

The pressure gradient between the two is ~14.6 psi. Since it’s a near vacuum on the other side there isn’t much heat so with a near-isothermal process we could expect an efficiency of 92% and a pump efficiency of 80%, for a combined efficiency of 73%. That's a lot better than CAES too.


From Wikipedia for CAES,

So, how many kWh per cubic meter?
0.1013529 x 1 x ln (14.7/.1) + (0.1013529 - 0.000689476) x 1 = 0.5057948149 + 0.100663424 = .6 MJ = .16 kWh/cubic meter = 4W/cuft

But, that's not what's happening here as the pressure is always atmospheric on one side and .1 on the other. In CAES as the fluid moves the pressure changes.

The vacuum system has constant head as long as you're under the systems C-Rate (in this case that would be the rate of the vapor on the vacuum side returning to a liquid form, e.g., condensation). So, the equation would be:

Ph(kW) = q ρ g h / (3,600,000); or with pressure = q p / (3.6 106)

where:

Ph(kW) = hydraulic power (kW)​

q = flow (m3/h)​

ρ = density of fluid (kg/m3) = 1000 kg/m3 for water​

p = differential pressure (N/m2, Pa)​

Solving for q for 1 kW, q = 3,600,000 / 100,000 = 3.6 m3/kW

The average U.S. home is 2687 sqft and consumes 30kWh/day. So, how many m3 would you need for 60 kWh?

60x3.6=216 cubic meters, 57,000 gallons, or about 3' deep for each vacuum tank excavated under your 2687 sqft home.
If you assume 70% efficiency, it would be more like 4' deep per tank. That's a lot better than low-pressure CAES if the math/assumptions are correct.

The cost of getting a vacuum tank that size that could withstand the pressure would probably be prohibitive, I'm not sure that burying it confers any significant advantage.

So all in all it doesn't look all that practical considering the size/cost of the tank needed. Even though increasing the pressure on CAES is a losing proposition, it does allow you to get a higher energy density.

What? You made it this far and wondering what the "you live underwater" had to do with it? Simple... If you lived just 33' under water than "atmospheric" pressure is two bar, that halves the volume requirement. The deeper you go the less volume you need.

 

[BoloMKXXVIII]

Interesting read. Not sure this is any more impractical than the gravity storage systems that have been proposed (lifting large blocks when energy is available and using gravity to turn a generator as the blocks are lowered back down). When you get into the numbers they just don't make financial sense.

 

[svetz]

Hmmm, this guy claims vacuum energy storage is practical...

...by utilizing thin wall concrete domes produced by overlaying an inflatable bladder with concrete and employing modern low labor concrete construction practice. Concrete is cheap and plentiful, has a compression strength of from 4000 to 10,000 PSI, and adapts to scaling and safe usage in urban and outlying locations.

I know concrete is strong under compression...but I'm dubious - anyone have any first-hand practical experience?

 

[svetz]

Found a U.S. Navy paper where they built 5.5' concrete spheres with a 4" thickness of concrete: NCEL started the program in September 1971, when it deployed 18 concrete spheres in water depths ranging from 1800 to 5000 feet. All 18 spheres were designed for a nominal working depth of about 3000 feet at 1300 psi pressure. Thus, as expected, the greater pressures crushed specimens placed at 3700 and 5000 feet. Each specimen, 66 inches in diameter with 4-inch thick walls, was anchored to the ocean floor by the 2600- pound weight of a 21 ⁄2-inch anchor chain 53 feet long. Some spheres were coated with a waterproof phenolic material; others remained uncoated. Two spheres had half of their surface coated and we re the only specimens to contain steel reinforcement. Engineers wanted to study the reaction of steel to the seawater environment . One of these spheres recovered after 10 years at a depth of 1800 feet had no visible corrosion of the steel, even though in some areas of the model the steel had less than 1 inch of concrete cover. Nor was there visible deterioration of the concrete material itself in any of the five spheres and blocks retrieved to date.

So, 4" of concrete for 1300 psi, and a vacuum chamber would only need 14.7 psi.

 

[svetz]

Let's do some math and see if we can get the $/kWh

Volume of a sphere is V = 4/3 πr^3
so the total volume of the 5.5' sphere is 4/3 π 2.75^3 = 87 cuft, and the hollow interior volume is 4/3 π 2.41^3 = 59.1cuft.
So the concrete volume is 87-59.1=27.9 cuft.

HomeDepot has 80lbs of concrete for $4.60 and 150 lbs of concrete / cuft... so, that's only $241 worth of concrete (plus tylenol costs).

From the OP we have 60 kWh would need 216 cubic meters, and converting that to cuft and dividing by .7 for the efficiency loss, that's 10,900 cuft. Assuming we want a spherical tank, solving for r gives 13' 9". Let's assume 3" thick, to give the outer radius 14'. That's a volume of 11,488; so the delta is 588 cuft of concrete to make the vacuum chamber. That's about $5,000 in concrete. If we stopped there it would be $84/kWh. Everything else is probably fairly cheap, let's say another $5000 for the turbine, excavation (depends on terrain, could be as low as $400 for soft soil), plumbing, etc. So, $10,000 for 60 kWh storage, which brings us to $170 kWh storage. LiFePo4 is $500 to $1000 kWh and lead acid is around $400 per usable kWh (assumes 50% DoD).

The next factor is life cycle, and LiFePO4 typically wins out as it has a higher number of cycles than SLAs. But what about the vacuum cycle? The navy's test spheres did great, but they were subject to near constant pressure. In the vacuum chamber the stresses against the walls change with the SoC; will that prematurely age them reducing the number of cycles? Perhaps this is where burying them might play an important role (weight of the soil on top would keep the stress more constant).

 

[svetz]

If you run a pool installation business, you probably have everything you need to be a VES (Vacuum Energy Storage) installer.

Build a pool Dig a hole fill shape with concrete add piping add pump add wiring add water $37,440 for 12x30 (2880 cuft)

Build a VES Dig a hole fill shape with concrete add piping add turbine add wiring add water & ethylene glycol $1,652/kWh?

2880 cuft / 127.133 cuft/kWh = 22 kWh; so if installation costs were comparable that would be $1,652/kWh. So in this case a lot more than LiFePO4.

 

[Fred S]

Fun to think about. So basically use solar or wind to drive a pump to evacuate a very large dome or a tank. Then bleed off the vacuum through a small turbine generator to recover the energy. The equivalent to pressurizing the same size pressure vessel to 14.7 psi, and then exhausting it through a turbine. If you've got the room, why not? Catastrophic failure is much safer with an implosion rather than explosion. Seems like construction cost per unit of power would be pretty high though.

As far as the water shown in the OP's figure with the two tanks, the water won't "co-exist" with the vacuum, it will boil into vapor. So I think you'd have to stick with just using air to generate the power.

 

[svetz]

...As far as the water shown in the OP's figure with the two tanks, the water won't "co-exist" with the vacuum, it will boil into vapor....

It won't be a pure vacuum, the calculations in the OP work with the assumption the vapor pressure will be chemically reduced to prevent boil off:

...We’ll add some ethylene glycol to the water to reduce it’s vapor pressure to say .1 psi (that is it’s not really a vacuum ....

Unlike the reverse of CAES, it also implies this implementation of VAES has a thermally driven C-Rate for the condensation/evaporation for maximum efficiency (although exceeding the C-Rate won't permanently damage the device).

...stick with just using air ...

You could do that, it won't be as efficient as the pressure change won't be constant; you'd have to switch back to

 

[svetz]

...which brings us to $170 kWh storage....

Just realized that the costs are fairly static, but you can easily increase the power with external pressure (i.e., from the OP, putting them underwater). So at 33' deep it would $85/kWh, and at 120' it would be $45/kWh. Doesn't necessarily have to be coastal, as you could dig a hole and keep a static water column over it (although you'd have to add in the cost to dig the hole). My well in Colorado was about 620' deep, so $10/kWh?

At least this crazy idea is all mechanical and doesn't require new electrodes, chemistries, or manufacturing processes... ?

 

[jjparady]

Just realized that the costs are fairly static, but you can easily increase the power with external pressure (i.e., from the OP, putting them underwater). So at 33' deep it would $85/kWh, and at 120' it would be $45/kWh. Doesn't necessarily have to be coastal, as you could dig a hole and keep a static water column over it (although you'd have to add in the cost to dig the hole). My well in Colorado was about 620' deep, so $10/kWh?

At least this crazy idea is all mechanical and doesn't require new electrodes, chemistries, or manufacturing processes... ?

Wouldn't the energy required to pump out the water increase with the depth? I guess it doesn't matter since we are trying to find somewhere to dump extra energy.

 

[svetz]

Wouldn't the energy required to pump out the water increase with the depth? I guess it doesn't matter since we are trying to find somewhere to dump extra energy.

There probably is some efficiency change with depth (e.g., a greater delta pressure), it probably runs similar to any hydrodynamic pump (but I don't know what that is).

 

[copec]

Also, the higher the head pressure, the more efficient the pumping and turbine can be.

I saw a proposed derivative for this years ago that had wind turbine columns hollow at the bottom at sea - although pressurized to atmospheric - and so they could do pumped-storage with a really high head pressure.

I think the water/ethylene glycol mix would have to be contained in a bladder on the atmospheric side too, otherwise the water with gasses dissolved in it will emit them once under a vacuum.

 

[copec]

This form of energy storage really sucks!

 

[DMBrennan51]

No, not a free energy device. Also, not very practical.... so don't bother reading further unless you just like weird stuff or happen to live underwater.

This is the reverse of compressed air energy storage. I was curious how the math would work out after researching an earlier post. Compressed Air technology has a few things working against it. First is that while compressing the air the work put into it also has to pay for the heat being squished into a smaller volume. Second is that increasing pressure isn’t proportional to the energy gain, it’s limited by

.

With all those things working against it, I wondered would they work for you if you created a vacuum rather than compressed the gas? From these rough calculations, it looks like there are some things that work for you and some things to be careful about; but not really practical overall.

First, let’s propose how it might work. Imagine a tank filled with water, that’s the battery discharged. Pump all the water out leaving a vacuum behind, that the battery fully charged. We’ll add some ethylene glycol to the water to reduce it’s vapor pressure to say .1 psi (that is it’s not really a vacuum, just as well as the tank would collapse). The atmospheric tank is nothing special, but the vacuum tank needs to be strong to keep from getting crushed.

View attachment 12818​

The pressure gradient between the two is ~14.6 psi. Since it’s a near vacuum on the other side there isn’t much heat so with a near-isothermal process we could expect an efficiency of 92% and a pump efficiency of 80%, for a combined efficiency of 73%. That's a lot better than CAES too.


From Wikipedia for CAES, View attachment 12819

So, how many kWh per cubic meter?
0.1013529 x 1 x ln (14.7/.1) + (0.1013529 - 0.000689476) x 1 = 0.5057948149 + 0.100663424 = .6 MJ = .16 kWh/cubic meter = 4W/cuft

But, that's not what's happening here as the pressure is always atmospheric on one side and .1 on the other. In CAES as the fluid moves the pressure changes.

The vacuum system has constant head as long as you're under the systems C-Rate (in this case that would be the rate of the vapor on the vacuum side returning to a liquid form, e.g., condensation). So, the equation would be:

Ph(kW) = q ρ g h / (3,600,000); or with pressure = q p / (3.6 106)

where:

Ph(kW) = hydraulic power (kW)​

q = flow (m3/h)​

ρ = density of fluid (kg/m3) = 1000 kg/m3 for water​

p = differential pressure (N/m2, Pa)​

Solving for q for 1 kW, q = 3,600,000 / 100,000 = 3.6 m3/kW

The average U.S. home is 2687 sqft and consumes 30kWh/day. So, how many m3 would you need for 60 kWh?

60x3.6=216 cubic meters, 57,000 gallons, or about 3' deep for each vacuum tank excavated under your 2687 sqft home.
If you assume 70% efficiency, it would be more like 4' deep per tank. That's a lot better than low-pressure CAES if the math/assumptions are correct.

The cost of getting a vacuum tank that size that could withstand the pressure would probably be prohibitive, I'm not sure that burying it confers any significant advantage.

So all in all it doesn't look all that practical considering the size/cost of the tank needed. Even though increasing the pressure on CAES is a losing proposition, it does allow you to get a higher energy density.

What? You made it this far and wondering what the "you live underwater" had to do with it? Simple... If you lived just 33' under water than "atmospheric" pressure is two bar, that halves the volume requirement. The deeper you go the less volume you need.

Could a multi chamber system that uses pressurized gas to increase the speed be helpful? (Several chambers, say for a solar plant that has been asked to slow down production due to not enough energy being taken from the grid in comparison to what is being added so they can use it on a cloudy day.)
Would simply dumping money into it like we did with all other technically and develop the idea more. I mean in your life alone solar has gone from where to where?
70% efficiency would also be good enough for some areas that often have more energy than they can handle.
Could become more efficient by segmenting the chamber and removing the atoms from one segment at a time considering the larger the space the less effective any pump would be at removing 98-99% of atoms.
Obviously not a free source of energy although an easy to implement way to store potential energy, at least from my understanding.
Sorry if you addressed this. I am good at talking about concepts although reading math, not the best.

 

[svetz]

Could a multi chamber system that uses pressurized gas to increase the speed be helpful? I don't know if there's an advantage to a multi-chambered system or not...but Compressed Air Energy Storage (CAES) is the opposite of vacuum storage and is one of the least expensive forms of energy storage if the storage containment is free (e.g., cave system or deplete reservoir). It's not used everywhere, like hydro the geology has to be right. Not sure if it's true, but heard Russia created some huge underground areas via nuclear testing and are now using them to store Helium. I can sort of see where a multi-chamber system (e.g., cambers nestled inside one another like Russian dolls would reduce the strain on any one shell and therefore decrease the cost per shell, but not sure if there would be overall savings or not. I'd say it's something someone designing such a system might want to consider.