Compressed Air

tahanson43206 · 2024-07-22 06:36:57

Finally, here is a formatted version of the table:

Energy Stored in a 1000 Liter Tank at Various Pressures
Hello everyone,
Here's a table showing the energy stored in a 1000-liter tank at different pressures. The calculations assume isothermal expansion:
Pressure (bar) - Energy Stored (kWh)
10 - 0.64
20 - 1.66
30 - 2.83
40 - 4.10
50 - 5.43
60 - 6.82
70 - 8.26
80 - 9.74
90 - 11.25
100 - 12.79
150 - 20.88
200 - 29.44
250 - 38.34
300 - 47.53
This table illustrates the energy storage potential at pressures ranging from 10 to 300 bar.
Looking forward to your thoughts and further discussions!
Best regards,
ChatGPT4o supervised by tahanson43206

(th)

tahanson43206 · 2024-07-22 06:44:29

I was less than secure in my understanding of the two kinds of expansion, so I asked ChatGPT4o to explain the difference...

Understanding Isothermal vs Adiabatic Expansion
Hello everyone,
Here’s a summary to help understand the difference between isothermal and adiabatic expansion, particularly in the context of our compressed air discussions:
Isothermal Expansion
Temperature: Remains constant during expansion.
Heat Exchange: The gas exchanges heat with the environment to maintain a constant temperature.
Practical Example: An ideal isothermal process can be approximated if the gas expands slowly enough that heat can transfer in and out of the system.
Energy Calculation: The work done by the gas is calculated using:
\[W = nRT \ln \left( \frac{V_f}{V_i} \right)\]
Adiabatic Expansion
Temperature: Changes during expansion.
Heat Exchange: No heat is exchanged with the environment. The system is insulated.
Practical Example: Adiabatic processes occur when expansion or compression happens rapidly, like in a piston engine.
Energy Calculation: The work done by the gas is calculated using the adiabatic equation:
\[
P V^\gamma = \text{constant}
\]
Practical Application
Pneumatic Tools: Generally experience adiabatic expansion because the air expands rapidly to do work, with little time for heat exchange.
Stored Gas at Ambient Temperature: If gas releases thermal energy to the environment until it reaches ambient temperature before use, the initial expansion can be considered isothermal. However, if the gas is used rapidly, the process is more adiabatic.
Looking forward to more discussions!
Best regards,
ChatGPT4o supervised by tahanson43206

(th)

Calliban · 2024-07-22 06:54:34

High pressures are necessary if a vehicle is to achieve a usable amount of range between refills. But to use HP air in a vehicle, it really is necessary to include a heat storage material that can absorb compression heat during charging and then provide reheat during expansion. Paraffin wax has been used. Water could also be used. The compressor is onboard the vehicle, allowing plug in charging in much the same way as a BEV. Prototype air vehicles have been made that achieve 130km range. I think that would work for most people and it would be cheaper to buy and service than a BEV.

But air cars would require the same infrastructure as EVs and would place similar demands on the grid. There isn't a way around that, because the energy has to come from somewhere. One advantage that air has over a BEV is that mechanical energy could be used for charging. We could store energy in a raised weight hydrauluc accumulator and then use hydraulic power to run the compressor. Alternatively, air could be provided from an external tank, in much the same way as ICEs are refuelled. In that case, the car must also be provided with heat that can be used for interstage reheat.

An HP air system could work for home energy storage as well. In fact, if we are making them for automotive use, economy of scale will bring down cost.

Last edited by Calliban (2024-07-22 07:01:08)

Terraformer · 2024-07-22 08:10:57

Calliban,

Washers and (tumble) dryers primarily use heat also. There's no good reason to use electricity to heat the water over simply plumbing it in to the hot water tank. I think its 400W for just the spinning? So a load might take 200Whr. Likewise, kitchens should not require much storage microwave and oven aside, since near boiling water can be stored and only requires a small amount of power to keep it hot (or phase change materials?). The baseload electricity demand for a house that's optimised heat storage would be significantly lower than it is now. Maybe we could get away with about a kWh a day...

Compressed air takes time to cool. How does this affect its use for transmission? Could a series of windmills directly pump air into a chamber that is then tapped for steady power? Thinking about underwater air pipelines here.

Calliban · 2024-07-22 09:06:49

Terraformer wrote:

Washers and (tumble) dryers primarily use heat also. There's no good reason to use electricity to heat the water over simply plumbing it in to the hot water tank. I think its 400W for just the spinning? So a load might take 200Whr. Likewise, kitchens should not require much storage microwave and oven aside, since near boiling water can be stored and only requires a small amount of power to keep it hot (or phase change materials?). The baseload electricity demand for a house that's optimised heat storage would be significantly lower than it is now. Maybe we could get away with about a kWh a day...

Yes, indeed. If the compressed air store can be brought down to 1kWh, then a 3m3 compressed air tank is enough to cover a whole day of baseload power, assuming no power generation is coming in from home wind and solar. A single 1000 gallon propane tank would do it. That is COTS equipment. A 1000 gallon tank would cost $1500 - 3000 in the US. Not sure about the UK. The vapour pressure of propane is about 10bar on a warm day. So a propane tank should work OK as an air storage tank.
https://www.affordabletanks.com/post/ev … pane-tanks

A 1000 gallon tank is 1m wide and 5m long. This is sufficiently compact to provide an offgrid solution for suburban areas in the UK as well. Small scale wind many be practical if you live in the countryside without neighbours. But it isn't practical in towns. So going offgrid isn't going to work for most people. But if the grid became unreliable in the UK with frequent power outages, a 1000 gallon tank would allow people to store 1 day of electric power. Maybe more if they stretch it out and limit power usage to lighting and personal electronics whilst the grid is down. Thermal energy storage is compact enough to actually integrate into the house. So I think this is something that people could use.

Terraformer wrote:

Compressed air takes time to cool. How does this affect its use for transmission? Could a series of windmills directly pump air into a chamber that is then tapped for steady power? Thinking about underwater air pipelines here.

Pipelines and containers can be insulated to reduce heat loss. That is possible, but tougher to do under the sea. It is certainly possible on land. Low alloy steels retain 90% of their strength up to temperatures of 400°C. After that, strength drops rapidly to 50% at 500°C. So it should be safe to use steel air tanks with air temperatures up to a few hundred degrees C. Any moisture in the air will be gas at those temperatures. Concrete is less forgiving of temperature extremes. But is unlikely to take damage beneath the boiling point of water. You could extend operating temperature using an insulating material.

Last edited by Calliban (2024-07-22 09:43:18)

Calliban · 2024-07-22 10:06:28

Building a 1kW wind turbine for less than £100.
https://m.youtube.com/playlist?list=PLb … yXNB64Zurf

This is interesting. If we couple one of these with a 3kWp solar system and a 1kWh compressed air storage system, we have the makings of an offgrid power system that can be sold to suburban customers in the UK.

tahanson43206 · 2024-07-22 12:41:40

This topic seems to me to be exploring the low end of the range of possibilities.

Here is a snippet found by Google when I asked about high pressure storage...

High-Pressure Air Compressors: Your Complete Guide
Arctic Compressor
https://www.arcticcompressor.com › resources › high-pre...
Search for: What is the highest pressure of compressed air?
What is the maximum PSI for compressed air?
You can use compressed air in pressure ranges from 14 PSI to 6004 PSI (1 to 414 bar) at flow rates from as little as 3.5 CFM (0.1 m³) and up. Fortunately, most people have no reason to memorize or use this formula.

A few posts ago I offered an estimate that 336 kwh could be stored in a set of 7 1000 liter tanks at 300 bar.

The performance of the system would be even better if the pressure could be raised all the way to 400 bar.

I'm hoping one or more of our members will work up a scenario at 300 bar or perhaps even 400 bar.

Because of the high pressure, the storage facility needs to be located in an underground location where an unwanted release of gas would be contained by layers of regolith overhead.

Pumps to fill the tanks could be located some distance from the tanks, and perhaps those could be located in a facility accessible to personnel for service. The same seems reasonable to consider for the output side, where the air will pass through an air motor to drive a generator.

I still have not seen a solid estimate of how much of the energy invested in the store would be recovered as useful electrical power.

If the system efficiency reaches 50% I'd be surprised, but it might.

If 50% is the overall performance, then the 336 kwh is insufficient, and double that would be needed to reach the (arbitrary) performance goal of 2 kw for a week.

The complete system must include tanks, compressor(s), air motor(s) and generator(s), as well as control electronics.

(th)

tahanson43206 · 2024-07-23 06:22:48

This topic has the potential to discover a practical energy storage solution using compressed gas for a home emergency backup system.

We have a range of possible solutions, from 10 bar to 400 bar. Each solution has advantages and disadvantages. It should be possible to find Real Universe products that can be combined to meet the t a r g e t performance of 2 kw for a week, delivered as electric current.

Calliban, since you have chosen to advocate the 10 bar option, please work up a detailed proposal for how that system would be constructed, and how it would operate. I ** think ** you were predicting the system would need 500 1000 liter tanks, but it would certainly be safer than the higher pressure version.

kbd512, since you have expressed interest in the automobile application of this technology, please see if you can work up a proposal for a system able to operate at 400 bar, or higher if you can find tanks on the open market. That system might need as few as 7 - 10 tanks, to meet the t a r g e t performance.

Besides the tanks, both solutions need to include compressors, regulators for discharge, air motors and generators, as well as control electronics. It seems to me reasonable to suppose there is a market, because there is already a market for petroleum burning generators of various kinds.

There is an opportunity to study a hybrid system as suggested by Calliban. That system would combine gas for energy storage with fluid for exchange from potential to kinetic energy. Such a system would (presumably?) have advantages since the temperature of the working fluid would remain near ambient, while the gas cools as it expands. They hybrid system may be limited in pressure it can handle.

SpaceNut has provided a Category for Projects, visible to NewMars members, so the alternative designs can be set up there.

(th)

Calliban · 2024-07-23 07:37:11

TH, why do you think 2kW x 1 week (336kWh) is the right amount of energy to store? Building a system that can do that will be expensive. Those LPG propane tanks I mentioned earlier cost about $3000 each. You would need 250 of them to store that much energy, so $750,000. That isn't going to be affordable for most people. I suggest we first determine minimal storage requirements to satisfy a customer need and then look at what can be done to meet it.

The cheaper the system is, the larger the customer base. As Terraformer pointed out earlier, combining a compressed air system with thermal energy storage greatly reduces the amount of electrical energy needed to be stored. If people are willing to adjust behaviour and work with the weather, that reduces it even more. If we can get away with storing just 1kWh in compressed air, then we have a system that most people could afford.

Last edited by Calliban (2024-07-23 07:43:37)

tahanson43206 · 2024-07-23 07:59:49

For Calliban!

Thank you for a terrific question!

2 kw for a week is precisely the amount of power that a family in Texas could have used to keep the refrigerator running during a week long power outage.

The purpose of the refrigerator was to keep medicine safe.

I'm sure you can relate to the motivation of a family to invest whatever it takes to protect family members.

Your observation about lower cost systems certainly makes sense. However, the high end is the goal, and the lower cost systems are a welcome byproduct. Folks who think they only need 1 kw for a week can choose that option.

The challenge here is to put ** something ** out into the marketplace of ideas for study.

At the moment we have nothing but hand waving in our archive.

I'd like to see this group put some points on the board. We've been running back and forth and building up skills to take a shot at the basket, but so far only GW Johnson has been making some serious attempts to put points on the board.

Speaking of GW Johnson... he has asked to clarify that the work I have published does not qualify as a "design". It is a preliminary study to find the boundaries of the problem.

(th)

Terraformer · 2024-07-23 09:38:13

Are you sure it's 2kW, or 2kWh? 2kW is a ridiculous amount of power consumpton for a fridge. Literally would put the national grid at capacity if that's how much power it required. 2kWh *per day* is pretty typical on the lower end.

But in the case of refridgeration, you could just use ice instead to store cold. Far cheaper than any kind of power storage, be it compressed air, batteries, or pumped hydro.

kbd512 · 2024-07-23 10:07:34

I think the real issue here is the power consumption of a refrigerator, which was stated as between 300W and 800W for a typical home refrigerator unit. Our refrigerator is on the larger side, so I would guess it's closer to 800W than 300W. That is quite a lot of power, and likely the reason why the batteries couldn't make it all the way to 9AM when the solar panels took over and started producing power again. Running a single fan at night for 8 hours to keep cool using air movement and cooler expanding air, a few flashlights, and a laptop computer to work from home is a far lower energy requirement, which would remain within the realm of practicality to meet, using a combination of air expansion from a reasonably-sized high pressure storage tank, when combined with a small air turbine powered electric generator to power the home electronics that allow for work and basic life to continue.

We never lost water pressure or our natural gas, so our ability to cook, wash, and flush toilets was never affected, thankfully.

Modern homes equipped with centralized heating and cooling are clearly not designed to permit any air movement, and that was the most overwhelming part of the entire experience, especially at night while trying to sleep because it was hard to simply breathe air that hot and humid, when the house was the hottest from the previous day's heating from the Sun. I wore a set of normal "home clothes" (a T-shirt or thicker polo shirt, a pair of pants, and sneakers) the entire time, yet wasn't "dying of heat" while working all day long, without a fan or even air movement, during the days that we did have power. I've been to a literal handful of places with Houston's combination of heat and humidity, which overwhelmed our solar panel reinstallers, who hailed from the deserts of Utah, and had no problems installing solar panels there, but couldn't tolerate the heat here.

I would have gone outside and done yard work, despite the heat, but had no good way to "come inside and cool down", due to the lack of air movement. It was definitely more comfortable to be outside than in our home, once the Sun started heating up the house during the day. I would say 3PM in the afternoon to about midnight was the most unbearable for all of us, and then the house became livable again, but the total lack of air movement was clearly the worst aspect of it.

I noticed that my wife and kids had the most problems with the heat, but our youngest son seemed fine, whereas our older son, my wife, and our daughter all had more problems with the heat. This tracks with what I recall as a kid growing up in Austin, where I often went without AC during the summer months while off at camp or visiting relatives. Us boys did fine outside, where we spent most of our time, my younger sister didn't tolerate the heat quite as well, and my mother also had some problems. My father never had AC as a kid, and was also fine outside. Our dogs didn't much appreciate being baked in the house, and were constantly panting as well as guzzling down water. They also wanted to go outside constantly, probably because there was more air movement.

If we were the same age as many of our neighbors, it would have been more than "merely very uncomfortable". Some of my gray and white haired neighbors are more used to the heat from growing up in Texas without AC, but a few of them left to go to hotels. A few of the women with small children also left their homes for hotels outside Houston. This is all perfectly understandable. My wife had to stay at home with our daughter right after she was born, when we went without power for 2 weeks, following Hurricane Ike, back in 2008. That was our first hurricane. It was pretty miserable then as well, but we were also 15 years younger, and being young really helps with resiliency.

Anyway, compressed air expanded through a small fan would easily eliminate the worst aspect of the entire experience by providing air movement and localized cooling. If you can get a good night's sleep because it's comfortable enough to sleep, then facing the rest of the day is a lot easier to do.

Calliban · 2024-07-23 11:41:31

Power consumption of typical household items. A fridge is shown as 100-200W. With most fridges, the compressor is on a thermostat. So the fridge won't be drawing power all of the time. I don't know if 100-200W is the power of the compressor motor or the average power consumption.
https://powerstuffs.com/power-consumpti … ppliances/

It should be possible to build fridges that store energy in ice. The heat of fusion of water is 0.093kWh per kg. So 100 litres of water could store 9.3kWh of thermal energy. The fridge will remain at a temperature of 0°C until the ice has melted. That should take around 20 hours.

Then again, a fridge like that is bespoke equipment that isn't COTS. If a house is equipped with a cooker, fridge, freezer and hot water tank, each with phase change thermal storage, then 1kWh of compressed air energy storage may be sufficient for the other power uses in the house. Maybe one day those sorts of systems will be standard. If we are to live well on intermittent energy we need to fully exploit opportunities for thermal energy storage because it is one of the few ways of storing large amounts of energy cheaply. But as things stand, these systems aren't commonly available.

Last edited by Calliban (2024-07-23 11:56:14)

Terraformer · 2024-07-23 12:04:28

Hot water tanks are standard in a lot of houses. It's an oversized one that's the non standard part here. Phase change thermal storage is available for use with heat pumps.

For fridges and freezers, adding in water should not be a particularly difficult thing to do. And hot water dispensors are off the shelf too, though probably need some better insulation.

More excited about this kind of energy storage than others because of this. It's not a massive leap from what is already there, and multi-tariff deals from energy suppliers that provide cheap off peak energy, especially if flexible (so you can buy wind energy when it's basically being dumped onto the grid) should provide a strong incentive to install it.

Calliban · 2024-07-23 12:53:39

A typical washer uses 0.79kWh per cycle and is used 200x per year.
https://checkappliance.co.uk/how-much-e … chine-use/

This UK company manufactures a hot fill washing machine that they claim reduces electric energy consumption by 64%. This reduces the electrical energy consumption to 288Wh per cycle or 158Wh per day (average).
https://ebac.com/washing-machines/e-care-8kg-hot-fill

A dryer uses somewhat more power. But it isn't difficult to dry clothes on a line. My wife and I use a combination of outside drying on a line and an indoor airer.

A typical fridge uses 166kWh/year in the UK. That is 454Wh/day.
https://www.homebuilding.co.uk/advice/h … fridge-use

A 40W-equivalent LED bulb uses about 5W of power. Some 5 of these bulbs, running 8h aday, will use 200Wh of energy per day.

Total = 812Wh. If we can take the fridge off of baseload by including thermal storage, the total is reduced to 358Wh.

How about watching TV? A 22" LED TV consumes 17W. So 5 hours of watching amounts to 85Wh.
https://powerstuffs.com/power-consumpti … ppliances/

Running total = 443 - 897Wh.

Last edited by Calliban (2024-07-23 13:18:35)

Terraformer · 2024-07-23 13:22:17

I have made a new topic in home improvements to talk about home thermal storage -- https://newmars.com/forums/viewtopic.ph … 38#p225338

kbd512 · 2024-07-30 01:04:36

tahanson43206,

This is an answer to your question about how much energy can be stored in compressed air:

Compressed Air Energy Storage

Ideal air compression and expansion is described by the isothermal process. Compressing air however heats it up and expanding it cools it down. Therefore practical air engines require heat exchangers in order to avoid excessively high or low temperatures and even so don't reach ideal constant temperature conditions. Nevertheless it is useful to describe the maximum energy storable using the isothermal case, which works out to about 110 ln(Pa/Pb)/Nm3 at 24°Celsius. A Nm3 is a cubic meter of gas volume at normal, i.e. atmospheric pressure, conditions. Thus if 1.0 m3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bar, the potential energy stored is 583 kJ (or 0.16 kWh). A highly efficient air motor could transfer this into kinetic energy if it runs very slowly and manages to expand the air from its initial 200 bar pressure completely down to 1 bar (bottle completely "empty" at ambient pressure). This is practically impossible and if the bottle is emptied down to 10 bar, the energy extractable is about 330 kJ. The efficiency of isothermal compressed gas storage is theoretically 100% but in practice the process is not isothermal and the two engines (compressor and motor) have various losses.
A standard 200 bar 5 liter steel bottle has a mass of 7.5 kg, a superior one 5 kg. Bottles reinforced with or built from high-tensile fibers can be below 2 kg in this size, always regarding legal safety codes. Thus we get energy densities from roughly 75 up to 300 kJ/kg. Ordinary steel bottles thus have about the same energy density as lead-acid batteries and advanced fiber-reinforced bottles that of superior electrochemical storage batteries. However, modern batteries provide almost their full energy at a nearly constant voltage, whereas the pressure of compressed air storage varies greatly. It is technologically difficult for air engines to maintain high efficiency and sufficient power values over such pressure swings.

300kJ/kg = 83Wh/kg (of air compressed to 200bar)
1,500kJ/kg = 417Wh/kg (of air compressed to 1,000bar)

I don't expect to get all of that energy back out, but if I can get 65% back out, which appears to be achievable using existing refrigerant loop technology found in air conditioners / motor vehicle AC compressors, then it should be about as good as Nickel Metal Hydride at the battery pack level (90Wh/kg), if we figure on 2kg of CFRP tank material for every 1kg of 1,000bar compressed air. Practical range falls somewhere between 100 and 200 miles. That's good enough for a short-range vehicle that doesn't require sophisticated control electronics, nor any electricity for that matter.

Since all EVs are recharged by grids that burn something, whereas these vehicles are designed for natural energy grids that don't require backups and integrate the storage into the grid design, then it is in point of fact a major leap forward.

You cannot "brick" a compressed air car by zapping its nonexistent control electronics, its battery doesn't lose any capacity as you put more cycles on it, and it can be recharged in a matter of a few minutes, just like refilling a gasoline tank. There are no horrendously toxic and environmentally destructive mining and refining processes to obtain Lithium and Copper. The waste product that comes out of the tailpipe is the same thing all humans and most other animals must breathe in, in order to be counted amongst the living. It's good enough to get the job done, and won't become another toxic waste problem we have no scalable or energy-favorable solutions for, unlike electronics.

Given the absurd amount of money we've squandered on battery solutions that have yet to solve any transportation or other energy storage applications at a meaningful scale, I figure experimenting with this solution is worth a shot. It's something we haven't tried in a serious way, it doesn't have the severe and intractable materials limitations issues that continue to inhibit deployment of electro-chemical batteries at scale, and the existing CFRP storage tank technology already lasts for 15,000 (875bar- this is the "test to failure" point conducted by US DoE on 700bar Type IV CFRP H2 storage tanks) to 25,000 (700bar) cycles. Lithium-ion hasn't progressed much beyond 3,000 cycles, except by giving up a significant portion of the battery pack's capacity by slow-charging to only 80% and then discharging to 30% or 50% prior to recharging. If you have to do that to get your 10,000 cycles out of the battery, then you may as well use easier-to-replace lower embodied energy materials.

All the fundamental technology limitations associated with compressed air energy storage tech have already been "solved" to the extent that they need to be. I think we should give it a serious "tire kicking" to see how well the entire operational concept holds up in the real world. If it turns out that we can solve a lot of our energy problems in much cheaper and more practical ways, then reasonable people will eventually discard unreasonable or otherwise impractical solutions.

I don't see us doing much more with electronics, batteries, and photovoltaics than merely making all our existing energy and environmental problems worse than they already are, because the physics behind using the entire solution is horrendously energy intensive, short-lived, and functionally non-repairable and non-recyclable. If someone concocts a much-improved battery than can go from 100% capacity to 5% capacity in mere minutes, lasts for 10,000+ cycles, and achieves greater energy density using cheaper materials like Sodium, then let's pursue that solution instead. We'd need to do the same thing with photovoltaics and the electric grids. There's no there there. Until then, this is a more viable alternative.

I think the battery and photovoltaics tech we have right now is quite good, relative to what we had when I was a kid, but still nowhere good enough. I believe its proponents never considered what the rate of progress would be when they first started developing it, or that all the major gains would fall far short of expectations. They counted on the market solving the tech and materials limitations problems, but markets overcome intractable problems with alternatives, rather than "more of the same". The real question is why no serious funding or research effort was allocated to the alternatives. Fighting against the physics and mechanics of actually implementing an electronics-based solution at scale was probably always going to be a losing battle. Rather than fixate on what's not happening and not likely to happen with batteries and photovoltaics, I'm choosing to focus my attention on what I view as a viable alternative.

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#76 2024-07-22 06:36:57

Re: Compressed Air

Energy Stored in a 1000 Liter Tank at Various Pressures

#77 2024-07-22 06:44:29

Re: Compressed Air

Understanding Isothermal vs Adiabatic Expansion

#78 2024-07-22 06:54:34

Re: Compressed Air

#79 2024-07-22 08:10:57

Re: Compressed Air

#80 2024-07-22 09:06:49

Re: Compressed Air

#81 2024-07-22 10:06:28

Re: Compressed Air

#82 2024-07-22 12:41:40

Re: Compressed Air

#83 2024-07-23 06:22:48

Re: Compressed Air

#84 2024-07-23 07:37:11

Re: Compressed Air

#85 2024-07-23 07:59:49

Re: Compressed Air

#86 2024-07-23 09:38:13

Re: Compressed Air

#87 2024-07-23 10:07:34

Re: Compressed Air

#88 2024-07-23 11:41:31

Re: Compressed Air

#89 2024-07-23 12:04:28

Re: Compressed Air

#90 2024-07-23 12:53:39

Re: Compressed Air

#91 2024-07-23 13:22:17

Re: Compressed Air

#92 2024-07-30 01:04:36

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