Compressed Air

kbd512 · 2024-07-14 10:54:07

tahanson43206,

Natural gas was the only utility-provided power supply that never stopped working throughout the storm and subsequent loss of electric power. The only kind of energy storage system that can reliably generate power for a week is an internal combustion engine, either gasoline or diesel. There was a standalone diesel engine running in our neighborhood the entire time the power was off, to power some equipment for our telecoms. Unlike CenterPoint Energy, that engine never failed. Once a year I see a guy out there changing the oil or doing whatever other maintenance he does. The generator and buried equipment sits on a vacant lot between two houses across the street from us.

As a general rule, you need about 20BTU of heat removal per square foot of floor space. Let's limit the area of any home we will cool down to just 500ft^2. That means we need 10,000BTU of cooling capacity.

Using this device:
How Vortex Tubes use compressed air to generate cold and hot air simultaneously?

We'll need to provide 150SCFM for 8 hours per day so that people can sleep at night. That's 504,000ft^3 / 14,272m^3 of compressed air.

14,272m^3 / 850bar (1bar = ~1atm) = 16.79m^3 of storage volume (a 2.5m Diameter by 3.42m Length cylinder), or 4,435 gallons

That's a large and very expensive compressed air tank if it's 850bar capable, similar in size to a Propane tank for people who live on farms. A small internal combustion engine with an air compression turbine could supply the required air using a much smaller tank capacity provided at a much lower pressure, therefore the storage tank could be made from common grades of steel.

To supply 150SCFM of compressed air at 90psi to the Vortex generator device shown above, we need a 22hp diesel engine. Common air and liquid cooled 25hp diesel engines of the "Harbor Freight" variety are available for about $1,500. 56 gallons of diesel fuel would be consumed over the course of a week. 12X 5 gallon plastic diesel fuel cans would cost $250 to $350. In a best case scenario, you already own a small utility truck with a diesel engine and a large fuel tank, so those components, plus a heavy duty alternator to supply electricity where actually required (lights, cell phones, a small refrigerator), already come built into the vehicle.

The vortex generating device in question is not a complex machine (has no moving parts except for the compressed air, much like a trompe), it does not require messing with high voltage / amperage electrical power lines, and the generator is sitting outside your home or apartment, so it can supply compressed air to effectively cool a 500ft^2 space while blowing the hot air out the other end of the device. Devices with no moving parts except for a working fluid tend not to fail, especially machined "tubes" made from stainless steel.

Edit:
The hot air generated out the back-end of the vortex tube can be used to heat up water for washing, purification through boiling, or to cook a hot meal, but will be exhausted outside the home / bedroom.

Last edited by kbd512 (2024-07-14 10:57:14)

Calliban · 2024-07-14 18:30:55

There is quite a lot to think through here, so I will probably answer in more detail tomorrow.

Terraformer: Yes. The amount of work energy generated by the gas as it expands is equal to the change in enthalpy of the gas. Specific enthalpy = internal energy + PV. The specific heat of air between -20 and 100°C can be treated as a constant of ~1KJ/kg.K. The gas gets colder as it converts internal energy into work.

Isothermal expansion is practicaly impossible, because all expansion devices have discrete stages. You can have reheat between the stages. The gas will cool as it expands across a stage, reheat over heating coils and then expand some more over the next stage. So a plot of internal energy or enthalpy vs length along a turbine, will show a declining saw tooth shape, as will gas temperature.

One thing that a compressed air system has going for it, is that compression heat and expansion cold have their own uses. But the calculations carried out by Kbd512 suggest that it would be costly to store enough compressed air to cool a home. Maybe an ice store is a more cost effective way of storing this energy need? Each cubic metre of ice can store about 84kWh of cold energy through phase change.

1BTU/hour = 1055J/3600 = 0.293W.

10,000BTU/hr = 2.93kW.

To cool a 500sq.ft for 12 hours, we must store 35.2kWh of cold. That is about 0.42m3 or 14.3cu.ft of ice. I don't think it is out of the question to build an ice store that could keep a 2000 sq.ft house cool for days. The cold air produced by an air expander could provide part of what is needed to recharge the store. Or we could build a heat pump that sucks heat out of the store at a constant 0°C and dumps heat into an outside swimming pool at, say, 20-30°C.

If we assume a COP of about 5, the heat pump would draw about 600W if operating 24/7. Let's assume it operates 8 hours aday on solar power. A 10kWth (2kWe) heat pump, supplied by a 3kWe peak solar PV system should do the job.

Last edited by Calliban (2024-07-14 19:11:46)

Calliban · 2024-07-16 14:26:56

If we are content to store air at low pressures, then energy storage efficiency could be close to 100% and we don't need interstage cooling or multistage expanders. The air heats up during compression, so the air stored within the tank will be warm. We then expand the air adiabatically, i.e. without interstage cooling. The air will cool down, converting internal heat back into mechanical energy. The great thing about this is that the systems involved are very simple and cheap. A single stage compressor and a single stage expansion turbine.

The bad thing about it is low energy storage density. Assuming we compress air to 1bar(g) or 2bar(a), then 1m3 of stored air will release 90KJ of energy during expansion. So the air storage tank woukd need a volume of 40m3 to store 1kWh.
https://tribology-abc.com/abc/thermodynamics.htm

But that might not matter if you have plenty of space. One option for storing air at low pressure is a gravity pressure vessel. This involves digging a hole in the ground, covering it with a roof structure and then covering the roof with a pile of overburden, like dirt or rubble. The weight of the overburden will balance the internal pressure of the air. The dirt walls of the hole and the rubble overburden would also provide a great deal of insulation to the stored warm air. From the outside, the air store would appear to be a small grassy hill. Reliable pressure relief is clearly important to eliminate the potential for overpressure. We would also want a reliable safety margin. But this is something that can be made using a small digger and a modest amount of concrete. Everything else is dirt. This would take a fair amount of work to build. But once built, it should last for centuries.

Using the tribology calculator, we can see that air expanded adiabatically from 2bar(a) to 1bar(a), will cool from 293K to 240K, or -33°C, which is a 53°C temperature drop. If we start with air at 20°C initial temperature and compress it from 1bar(a) to 2bar(a), it will heat up to 73°C. If the air store is perfectly insulated, the expansion stage will convert this heat back into work and gemperature of the exhausted air will be 20°C.

Last edited by Calliban (2024-07-16 14:59:40)

Terraformer · 2024-07-16 15:01:57

How much weight can concrete withstand?

Wondering if old quarries could be a good place for this. Quite a few holes in the ground that need filling in. Though I suppose in that case they normally get fillled in by water. Build an artificial lake bed to store air before that happens?

Basically a manmade version of the caverns that are used by a lot of systems today.

Last edited by Terraformer (2024-07-16 15:02:47)

Calliban · 2024-07-16 15:21:50

Terraformer wrote:

How much weight can concrete withstand?
Wondering if old quarries could be a good place for this. Quite a few holes in the ground that need filling in. Though I suppose in that case they normally get fillled in by water. Build an artificial lake bed to store air before that happens?
Basically a manmade version of the caverns that are used by a lot of systems today.

The compressive strength of regular concrete is 20-40MPa. The weight of the overburden is 100KPa +margin. So a domed roof could be relatively thin.

If we wanted to store air at large scale, then under water CAES might be a better option. I was thinking of something that would be able to store small amounts of power for an offgrid application, say 1kWh. American houses are typically on bigger plots than UK homes. So a bedroom sized underground air store is something that would easily fit within the back yard of a middle income US family home.

The shape of the underground structure is somewhat arbitary. A single large chamber would work. A long but thin trench would work as well.

Last edited by Calliban (2024-07-16 15:25:20)

Calliban · 2024-07-16 16:10:00

An interesting spin on this idea would be to combine low pressure CAES with thermal energy storage. For example, we compress air to 1bar(g) using a single stage compressor. Assuming an ambient temperature of 20°C, the air coming out of the compressor should have a temperature of about 73°C. We pass it over a counterflow heat exchanger before it goes into the store, cooling it to 20°C. The water flowing through the heat exchanger then provides domestic hot water for the house. When the air is expanded adiabatically to recover power, the exhaust air will have temperature -33°C. This passes through an underground food freezer and then an ice store, before venting into the house at about 0°C and contributing to air conditioning.

For every 100 units of mechanical energy used to compress the air, we get about 40 units of mechanical energy back and 60 units of hot water. But we also get cold air, which is useful for freezing, refrigeration and air conditioning. So for every 100 units of input power, we get 40 units output power, 60 units of heat and 60 units of cold.

If we assume an air source heat pump producing hot water at 60°C or ice at -20°C has a COP of about 3, then the effective exergy efficiency of this hybrid energy storage system is 100%. The air compression cycle is a heat pump. But in this case, it is an open cycle heat pump.

In Texas, one could use solar PV or wind power to drive the compressor. In the UK, wind power would be the better choice. A neat solution would be to use a mechanical wind turbine to drive a single-stage positive displacement compressor. This is a really simple, almost preindustrial technology. It should therefore be easy to make. The electrical generator would be coupled to the power recovery turbine on the air store. Going back to the energy redoubt situation, we could build a system that would provide a few hundred kW of electric power for several hours, whilst also providing enough hot water to provide the washing needs of a small town. The expansion air would cool an underground freezer big enough to store many months of food for the town. To reduce capital costs, we woukd likely cluster several mechanical wind turbines around a single air store, with a single electric generator. The air storage would increase the effective capacity factor of the electrical generator.

Last edited by Calliban (2024-07-16 16:39:24)

Calliban · 2024-07-17 05:44:50

Diaphragm compressors are reciprocating compressors that are ideal for small applications, where low-pressure oil free air is desired.
https://mechanicalboost.com/diaphragm-compressor/

This is something that should be quite easy to build if low pressure air at 1bar(g) is needed.

The shaft driving the cam could be directly coupled to the rotating shaft of a mechanical wind turbine. The output flap valve would exhaust directly into the gravity pressure vessel.

Last edited by Calliban (2024-07-17 06:12:45)

tahanson43206 · 2024-07-17 06:29:43

We have several topics that cover various aspects of the general question of how to use compressed air for energy storage.

Compressed Air by Terraformer [ 1 2 3 ]
Science, Technology, and Astronomy 56 Today 07:44:50 by Calliban
Compressed Air Automobile for Mass Market by tahanson43206
Projects 2 2024-06-29 12:46:01 by tahanson43206
Pneumatic Tool Energy Storage Compressed Air by tahanson43206
Science, Technology, and Astronomy 3 2024-03-17 09:31:19 by tahanson43206
Rocket Launch Assist: Compressed Air, Liquid Air and Steam by Calliban
Science, Technology, and Astronomy 5 2024-02-16 13:17:03 by Terraformer
Heat your House for Free with an LP Air Compressor by Calliban [ 1 2 ]
Home improvements 25 2023-03-28 21:25:05 by SpaceNut

Of all these, the Compressed Air topic seems to me to be the best fit for this opportunity...

We have a Real Universe situation close at hand, if kbd512 is willing to allow us to focus on his reporting from the recent Hurricane Beryl impact on Houston.

The need described by kbd512 in several posts on the episode, was to provide reliable electric power for a refrigerator.

The solar power installation installed to reduce utility electric costs was unable to provide reliable electric power due to cloud cover, that prevented the solar panels from the charging the Powerwall batteries.

Both kbd512 and Calliban have advocated compressed air as a viable energy storage system, but to my knowledge no installations suitable for energy storage are on the market. There would appear to be an opportunity for an entrepreneur to bring such a system to market.

Calliban's recent discussion of low pressure systems is intriguing, because it would appear to trade volume for expense. If a family has sufficient land for an underground energy store, then it would seem (to me pending input from members) feasible to design, build and operate an energy store that would provide some reasonable amount of power for a week. Google came up with a range of 300 to 800 watts for a refrigerator operating on 120 VAC.

An energy storage system for this purpose would require a volume for storage, a compressor and an air motor/generator combination, along with control electronics.

It may be possible for members of this forum to work out the details of what such a system would look like.

(th)

Terraformer · 2024-07-17 07:54:03

IDK how much research has already been done on this, but a small tank could be jacketed with paraffin or some other phase change material? It may require fins to be installed in the tank itself to provide fast enough heat transfer. If we can get away without having to add intercoolers and a separate heat store, the system will be considerably simpler and should be cheaper. The ohase change material may have to be installed within the tank itself to achieve this. Or for that matter at the opening... hmm.

Calliban · 2024-07-17 08:13:16

Terraformer wrote:

IDK how much research has already been done on this, but a small tank could be jacketed with paraffin or some other phase change material? It may require fins to be installed in the tank itself to provide fast enough heat transfer. If we can get away without having to add intercoolers and a separate heat store, the system will be considerably simpler and should be cheaper. The ohase change material may have to be installed within the tank itself to achieve this. Or for that matter at the opening... hmm.

If we limit storage pressure to just 1bar above ambient, interstage cooling and phase change materials will not be necessary. We store the warm compressed air in the storage vessel. For energy recovery, we expand the air adiabatically. If a gravity pressure vessel is used, the overburden and surrounding dirt provide insulation keeping the air warm.

The warm air could be passed through a counterflow heat exchanger immiediately before entering the power recovery turbine. If heat is removed before expansion, the expanded air will be cold and can be used for refrigeration. The warmth from the air could be used for domestic hot water.

Last edited by Calliban (2024-07-17 08:20:24)

Terraformer · 2024-07-20 05:10:29

In another thread, Calliban mentioned hydraulic accumulators:

Calliban wrote:

Interesting DOE project for small scale (building scale) pumped storage.
https://www.osti.gov/biblio/1817438
The device that they are describing is a hydraulic accumulator. Energy is stored by compressing a fixed mass of gas within a steel bottle, by pumping a liquid into the bottle under pressure. The hydraulic fluid compresses the gas as its level rises within the bottle. It works as a liquid piston. Energy is stored in the enthalpy change of the gas. Its pressure rises and its temperature increases. Ordinary low alloy steels lose negligible strength up to 400°C, but strength declines rapidly with increasing temperature thereafter. At 550°C, tensile strength is down by 50%.
https://www.engineeringtoolbox.com/meta … _1353.html
Interestingly, we have mineral oils that are used as lube oils and heat transfer fluids that remain stable at temperatures up to 400°C. A hydraulic accumulator can retain most of the compression heat that results when its internal gas reservoir heats up. When it expands, the thermal energy of the gas is converted back into work. This makes hydraulic accumulators a very efficient way of storing mechanical energy. How much energy can be stored? Let us assume a vessel 1m3 in volume, filled with nitrogen at 283K and compressed from 1 bar(a) to 30 bar (a). I am going to use an online calculator because I am lazy. But energy stored is given by the formula: E = P1V1 x ln(P2/P1). This assumes isothermal compression and expansion.
https://www.omnicalculator.com/physics/ … -processes
Energy stored is 411.2KJ/m3. Final volume is 0.088m3 and temperature is 748K. We would want to store some of the heat in the oil I think, so our pressure vessel temperature doesn't exceed 673K. A steel vessel with volume 4m2 could store 0.5kWh of mechanical energy.

I am a little confused by the temperature rise here. If water were used, then a 1 deg. change in temperature would store 4kWh of sensible heat. Surely the heat of compression would end up being lost to the hydraulic fluid, if there is no insulation between them?

But also, the compression is taking place within the tank. Does this offer opportunities to better capture the heat of compression, given the greater surface area available vs. a compressor?

SpaceNut · 2024-07-20 06:43:54

The tank I believe is isolated and a small area heat exchanger is used to to do the transfer and expansion of the working fluid and its at that location the pressure rises and is allowed to flow to the means to make motion.

tahanson43206 · 2024-07-20 07:14:44

For Terraformer and SpaceNut ... thank for your observations and questions!

This new initiative by Calliban seems (to me at least) well worth developing further.

My reason is a that I am hoping it will turn out that using a liquid as the working fluid for storing energy and retrieving it is more efficient than using a gas. I recognize that the performance of a fluid based system vs a gas based one is dependent upon the quality of manufacturing.

Fortunately, this is one of Calliban's areas of expertise.

If we think of the system as a mechanical battery (like a hydraulic dam energy storage and retrieval system) then we might be able to envision a sturdy, long lived and highly efficient energy storage system.

Questions that our members might address include:

1) what combination of liquid and gas is best for this application?

Calliban mentioned oil, and Terraformer asked about water ... there must be advantages and disadvantages to each.

2) Air is readily available, but it is a mixture. Is it better to use just one gas? If so, which one is best for this application?

3) Container.... Calliban opened with iron/steel (not sure) but whatever it was, it had an upper temperature limitl.

Since temperature rise is a necessary byproduct (or characteristic) of this design, are there superior materials for this purpose?

***
We have members who are ** very ** good with numbers.

I'm thinking here of kbd512, in addition to Calliban who launched the current inquiry.

kbd512 recently experienced a power outage that lasted an extended period of time.

He reported to us in another topic that the system that he had hoped would provide power for a refrigerator to keep medicine secure was unable to perform that needed service, because (it turned out) that Hurricane Beryl's remnants absorbed sunlight before it could reach the solar panels at his home.

A mechanical battery might be able to provide 2 kw of power for a week

What would be physical characteristics of such a system?

Could Calliban's suggestion serve for this purpose?

(th)

tahanson43206 · 2024-07-20 08:51:16

Following up with more questions about Calliban's initiative....

1) If an energy store is underground, pressure can be contained by the Earth itself.

2) A ceramic lining might permit higher temperatures to be accumulated during energy store

3) This is one for Calliban for sure...

Since the energy stored in the proposed system involves compressing a gas, energy is stored in two ways:

A. There is spring compression ... the molecules of gas are brought closer and closer together.

I'm curious to know how much of the energy input to the system is of this spring nature, compared to the temperature increase.

B. There is temperature increase due to the increased activity of the molecules as they are brought closer and closer together.

The temperature can and will be drawn off by adjacent material unless insulation is present.

After the temperature increase is drawn off, how much of the original energy remains in the form of spring action?

Is this a 50/50 split, or is the ratio different?

***
Returning to Terraformer's question about water....

Is it reasonable to enclose the system in a blanket of water?

That water would warm up as energy is input into the store, and that thermal energy could (presumably) be used for something.

(th)

tahanson43206 · 2024-07-20 09:00:15

Energy store numbers:

Two kilowatts for 24 hours is 48 kilowatt hours.

48 kwh for 7 days is 336 kwh

How large would an energy store have to be to hold 336 kwh for emergency use?

In charging the energy store, the thermal energy would be lost to the environment, leaving only the spring energy.

How much energy has to be invested in the store, to be able to secure 336 kwh when the energy is needed?

The efficiency of the equipment to translate stored spring energy into electric current is a factor.

(th)

kbd512 · 2024-07-20 14:31:41

tahanson43206,

Is there a reason why an energy store needs to be present in individual homes at all, vs using much more reliable means of power distribution, such as pipelines?

I don't object to using centralized electric grids with properly maintained buried equipment, because there are so vanishingly few catastrophic failures using those sorts of systems. The issue, as I see it, is that we use wonky power distribution setups with far too many exposed / vulnerable failure points which can be struck by lightning or knocked over by wind and trees. I can absolutely guarantee that the 4th largest city in America being disabled for an entire week cost more money than the grid upgrades required to create a resilient electric grid that is either almost immune or heavily protected from storm damage.

For any energy generating, storage, and distribution system, there are three types of threats
1. Naturally occurring adverse weather events such as lightning strikes, hurricanes, tornadoes, flooding, and fires
2. Human errors or mistakes during operation, such as accidentally severing power lines or pipelines during construction
3. Deliberately malicious human activity, such as kinetic warfare, hackers messing up control systems, or equipment sabotage

Threats 1 and 2 represent almost the entirety of serious and potentially fatal problems facing the US electric grid, by sheer numbers. We also have adversarial governments, such as the Russians, Chinese, Iranians, or North Koreans, who will deliberately attack our infrastructure using various computer or kinetic methods, but we also have ways to defend against their attacks, and most of the time we're successful. I'm sure there is also the odd wacko who gets upset with the power company, who will shoot at a power transformer wire, but most of the time these attacks will be conducted using the anonymity provided by a computer screen.

If we're not going to make a good faith effort to harden the grid against the first two threats, then that brings us to the necessity of having alternatives. The effectiveness of hardening measures against the third type of threat will always be questionable, and difficult to properly evaluate, relative to the first two. As it stands right now, having gasoline or diesel fueled motor vehicles parked outside seems to be the most widely implemented "fix". Beyond that, at least here in Texas, we also have natural gas pipelines and the gas can provide a tertiary power source if your home has a "whole home diesel-electric generator" that runs on natural gas. Very few people have those, because they are expensive to purchase, install, and certify, as well as requiring routine maintenance.

I'm seeking alternatives that don't involve hydrocarbon fuels or electricity, because I think putting all our eggs into those two baskets only produces bad results, given our general unwillingness to harden machines against the first two types of threats.

My desire is to sidestep the hydrocarbon fuel vs electricity issue completely by adding a viable alternative "fuel", such that there is no singular service anyone can "knock out", accidentally or intentionally, which would bring an entire city to a screeching halt.

Since gravity can never be "turned off", at least not here on Earth, so far as we know, I want to sink trompes into the Gulf of Mexico to provide our compressed air source, and then use pipelines to deliver that compressed air back to Texas and other states near the oceans, where it can then be expanded to provide air conditioning (heating and cooling), as well as power for whatever uses we need power for. I see no great technological impediments to doing that.

I still view motorized vehicles as the most practical "onsite backup generators", regardless of what powers them (hydrocarbon fuels, electricity, or compressed air). There's a clear difference in the durability and resiliency of a pipeline or buried electric power cable, relative to wires suspended from telephone poles, for that purpose.

Apart from hydrocarbon fuels, storing a week's worth of power at individual homes is not very practical, using electro-chemical batteries or compressed air, yet the fact that water pressure and natural gas flowed freely through all buried pipelines during that week of electric power outage, should be prima facie evidence about how well pipelines work. We already have buried water and compressed natural gas pipelines, and those obviously work, so why can't we either have buried compressed air pipelines or finish the job of burying all electric power cables?

I think compressed air adds an important alternative energy source, because the act of compression and expansion (for heating and cooling) can be accomplished using gravity and machines with no moving parts, so it requires little to no external power input, making the machines should be inexpensive, and its one failure mode is the same as it is for electric power cables or hydrocarbon fuel pipelines. If we had that alternative available to us, then loss of electric power means loss of computers or lights, unless powered by batteries or onsite air turbines.

Since we have no long duration grid scale battery storage, can you begin to understand how much fuel and electrical energy, which mostly comes from burning something, that we'd save by not needing to burn anything to provide life saving power?

We might be able to figure out how to separate CO2 gas bubbles from the sea water we're using to compress air as well, thus providing the feedstock for whatever hydrocarbon fuels and lubricants we must consume to power vehicles.

If we sink enough pipes deep enough into the Gulf of Mexico, then how much gravity energy can we harvest, in order to compress air?

I'm guessing we have enough gravity energy available, which we can extract in an economic manner, to power most homes and vehicles, and that the amount of hydrocarbon fuel energy we still require to power ships, commercial aircraft, and other long range motorized vehicles, becomes rather marginal.

According to those energy flow diagrams I posted in Terraformer's thread about running a kitchen without much electricity, at least 2/3rds of all energy being generated gets "wasted" in one way or another, whether it's used to generate electricity or power combustion engines.

How much gravitational energy can we afford to "waste" during air compression?

Is the ultimate answer, "as much as we need to, within the realm of providing what we require to live modern lives"?

Whether or not my dishwasher "beeps" at me because it's electrically powered, or I simply hear a "click" as a compressed air valve opens or shuts, has no bearing on whether or not the machine cleans my dishes. If all the energy I receive to power my dishwasher was provided by gravity, how much was wasted is far less relevant than the "clean dishes".

If all the hot water was provided by solar thermal, onsite or offsite, that doesn't "turn off" just because a cloud passed overhead. The heat being generated by the Sun was very real, even though it failed to "power up" the photovoltaic panels on our roof because it was no longer a direct photonic interaction. That roof, as well as the ground outside, was still "baking hot", despite the clouds overhead. I made the mistake of putting my hand on the hood of our car, and it was still hot enough to cause me to remove my hand, which tells me that ye olde heat and pressure was still quietly "doing its thing", even while my home solar system was non-functional. I'm quite certain that there was "less power" being generated due to the cloud cover, meaning touching the hood of a car in direct sunlight will burn your hand even faster, but less is not equivalent to zero.

If the only thing we needed our existing electric grid for was lights and computers, then a "grid down" event would be inconvenient, but not life-threatening. A small thin film solar panel outside is sufficient to recharge a cell phone or flashlight. People living here died to due to hyperthermia. Granted, not that many people died in total, 22 at last count, but most were pointless preventable losses- the sorts of losses our energy systems should protect against. If some terrorist group killed 22 of our people, we would never stop trying to hunt them down, and whatever it cost us to do so would be viewed as entirely irrelevant. So... Why won't we apply that same level of fanaticism towards ensuring that our people don't die pointlessly due to our own electric grid not functioning? If it's possible to make it much more reliable than it presently is, then we do that. If it's not, then we move on to alternatives. I'm proposing this alternative because I don't think we're ever going to devote the resources required to make the electric grid as reliable as it needs to be, but also as a "hedged bet" against the worst effects of climate change. I'm betting that "breaking gravity", or systems using compressed air derived from gravity, as well as sensible heat energy storage from the Sun, is really hard to do, relative to breaking electrical systems or electronics. Since so little skilled labor or education is required to maintain such systems, I'm also betting that they're relatively "permanent", and won't be superseded by any heretofore nonexistent "night-and-day" improvements to electro-chemical batteries, photovoltaics, or wind turbines.

Am I simply "wrong" about all this? Perhaps. I'd be intellectually dishonest if I said otherwise. However, all the math / physics / material abundance arguments favor non-electrical / non-electronic solutions if we wish to dramatically reduce hydrocarbon fuels consumption. We've been blessed with a century of electrical and electronic advancements working in our favor, yet even with the staggering progress made, our all-electric solutions remain utterly impractical, if not impossible. We've achieved no net reduction in hydrocarbon fuel energy consumption rates, because we require "bulk energy solutions" that don't sink bulk hydrocarbon fuel energy inputs into creating all the machinery necessary for the all-electric solution to truly replace hydrocarbon fuels. No sensible person wants to go back to living the way we did before industrialization, so we'll remain without a viable "solution-at-scale" until we rediscover some of the non-hydrocarbon fuel machinery of our past, which did function at the scale required to enable industrialization- heat without combustion, compressed air from gravity, etc.

Calliban · 2024-07-20 23:52:19

This article describes the Paris network, which was the world's largest compressed air power network.
http://www.douglas-self.com/MUSEUM/POWE … etwork.htm

The network reached its greatest extent around 1960. This was decades after the introduction of electricity, indicating that the air network was able to provide services that electricity could not. Ultimately the Paris air network declined due to deindustrialisation of Paris. The same factor led to the decline of the London hydraulic power system. As the docks and industry in London disappeared, demand for hydraulic power reduced accordingly.

Could compressed air make a comeback as a large scale power distribution system? I think a case could certainly be made. One of the arguments often raised against compressed air is the inefficiency of compression and expansion. But this problem disappears if the compression heat and expansion cold can be put to use. Ultimately, unless power can be produced very cheaply, all large settlements in cold climates will need district heat networks. If we have systems like that, compression heat could be usefully recycled. Likewise, expansion cold has uses in food storage. If the hot and cold associated with compressed air can be used, then the advantages of electrical power distribution over compressed air disappear.

Compressed air has other advantages. It can be stored in large quantities in pressure vessels, underwater containers or in underground caverns. Compressors and expanders are much simpler devices than generators or electric motors. This is why air tools are usually cheaper, as well as being lighter and more powerful. At the consumer end, electricity will still be needed for electronics and lighting. But these are usually low power applications. To meet these needs, we could use local DC networks, which would better suit the needs of these applications.

I think the main problem with switching to compressed air would be asset inertia. The world is heavily invested in electric power. Switching to another power transmission medium means changing the way we do things on a large scale. The costs are far from trivial. And a transition to compressed air would need to occur simultaneously with a transition to district heating.

Maybe we should bite the bullet and get started on these things in ernest. We know that a transition to an all-electric future isn't possible under present assumptions on living standards, due to the apparently unsupportable resource requirements involved. A compressed air energy system in contrast, would rely primarily on carbon steels for air distribution and a mixture of carbon steel, concrete, carbon and glass fibre for energy storage. These aren't materials that we are likely to run out of in the near future and carbon steels are almost infinitely recyclable.

PS1. Supply pressure is an important design choice. High pressure supply offers higher power density. But supplying air at high pressure requires multistage compression and its efficient use requires multistage expansion. Most air tools work at low pressures of 10bar(g) or less. If there is a desire to use compressed air to power vehicles, then higher pressures will be neccesary in most cases. A Tesla 3 uses about 500KJ of battery energy per km travelled. By contrast, compressed air at 10bar(g) will release 1.2MJ/m3 of mechanical energy if expanded adiabatically. So a car running on LP air won't go far. Higher pressures are needed for air to be useful in powering cars. LP air might still have applications powering trains and buses.

PS2. Low power density may be a problem for compressed air distribution as well. A 1m (3.25') diameter pipe, carrying air at typical tool pressure (10bar(g)) at 10m/s, will deliver only 9.5MW of power. That is enough for a decent sized town of 10-20,000 people. But nowhere near enough for a large city. To deliver more power implies either more bulky pipes or higher supply pressure. Higher pressure is possible, but this would result in greater energy losses in compression and expansion. Another option would be a combination of the two. Long distance lines carrying air at 100bar and short distance lines carrying air at 10bar. High power equipment could be used to reduce pressure. It would be driven by 100bar air and would exhaust air at 10bar into the district air lines. Vehicle air supply stations, would receive air at 100bar and would compress it to much higher pressure, whilst exhausting air at 10bar into the district network.

PS3. Energy storage in compressed air. A 1m3 volume of air at 10bar(g) will release 1/3rd kWh if expanded adiabatically. UK electricity demand is about 1 billion kWh per day. Suppose we store compressed air in the North Sea, in ballasted concrete vessels some 10m tall and located in water 110m deep. How much area wouod we need to cover to store 1 day worth of power? 1 billion / 10x 0.333 = 300 million m2 or 300km3. That is a square 10.8 miles aside. That is less than 0.1% of North Sea area.

How about storing energy in steel tanks located on land? A 1m3 air receiver tank costs about £2000 before tax.
https://airlinkcompressors.com/products … 4101000922

That is £6000/kWh. Storing 1 billion kWh of energy in air would imply an investment of £6tn. Which is a lot of money. But compressed air receivers could last a century or more if kept in dry conditions and supplied with dry air. Suppose we use a compressed air tank costing £6000, to store 1kWh per day for a century. How much would it add to the cost of each kWh? Ans = £6000 / (1 x 365.25 x 100) = £0.16/kWh. Affordable, if you are prepared to make long term investments. If we did this at scale, we could probably achieve better results than this example suggests. Interestingly, Alibaba are selling 80l 200bar steel vessels for $300 each. Each will store about 2kWh of air if expanded isothermally. That is about 40x cheaper than an air receiver tank! Over an investment timescale of 10 years, that is £0.04/kWh stored. Far more attractive.

Last edited by Calliban (2024-07-21 01:50:14)

Calliban · 2024-07-21 03:27:17

Looking at underwater compressed air storage.

How much energy could we store in the Great Lakes? Lake Superior has a surface area of 82,100km2.
https://en.m.wikipedia.org/wiki/Lake_Superior

Suppose we store air at the bottom of the lake in ballasted concrete shells. To avoid flooding lake front property, we want to avoid raising the lake level by more than 0.1m during a charge cycle. How much air could we store? Ans = 8.21bn m3.

The lake has an average depth of 147m and arround half of the lake is 200m or deeper. So lets say we store the air at an average depth of 200m, which equates to a pressure of 20bar. At 20bar, 1m3 of air will release 2.87MJ of energy under adiabatic expansion, which is 0.8kWh. A volume of 8.21bn m3 will store 6.5bn kWh. The US uses about 11 billion kWh electricity per day. So Lake Superior could store enough air to power the entire US for 14.2 hours.

Lake Michigan has a max depth of 270m and a surface area of 58,000km2. This could store enough air to power the US for 10 hours. The others are smaller and shallower.

The Gulf of Mexico has even greater storage potential. Total surface area is 1.55million km2. The problem is that most of it is really deep, literally miles deep. Installing air tanks in the deepest parts would be very difficult and expensive as you can't send divers that deep. There are shallower areas that are thin slivers around the coast.
https://mavink.com/post/696DEA52ABC59E8 … ymetry-map

Off the east coast of Florida there is a big piece of continental shelf about 200 miles aside that is about 1km deep. It's area is about 100,000km2. That could store air at a pressure of 100bar. How much could be stored here? Let's assume our concrete air tanks are 10m tall and some 50% of this area is filled with tanks. At 100bar, 1m3 of air would release 46MJ (12.8kWh) if expanded isothermally. If 50,000km2 of seabed is covered by 10m deep tanks, total energy stored would be 6.39trillion kWh. Enough to power the US for 580 days. That is enough for genuine interseasonal storage, assuming it can be done affordably.

The North Sea has an area of 570,000km2. About half of it has an average depth of about 50m, corresponding to a pressure of 5bar. 1m3 of air at this pressure will release 460KJ (0.128kWh) of energy if expanded adiabatically. If we assume that 100,000km2 of the North Sea bed is filled with air containers 10m high, the total energy stored will be 127.8TWh. That is 133 days of electricity demand for the UK and 17 days for the entire EU.

The air stores would be thin concrete structures, reinforced using cast basalt rods and glass fibres. The concrete structures are ballasted with dredged mud and gravel to counteract the bouyancy of the air contained.

Last edited by Calliban (2024-07-21 05:33:17)

Calliban · 2024-07-22 05:19:47

tahanson43206 wrote:

For Calliban re Large Scale compressed air energy storage
The examples you provided in recent posts in the Compressed Air topic are inspiring!
If you can find the time, and if the subject is of interest, please continue to develop your ideas for individual home owner energy storage.
The idea of a collective enterprise seems (to me at least) appropriate for a culture which is better suited for such ventures than the US.
The idea might well find an audience in the US, among builders who are tasked with design of large living complexes, of which there are a great many in the US.
However, I have the impression that the average person would prefer some energy independence if it were affordable and practical, which your designs seem to be.
In light of the recent reporting by kbd512, about the performance of a high end solar installation in Texas, it seems clear (again to me at least) that something more robust would find a market in the US, and perhaps in other countries, where independence from a grid is considered desirable.
The modest target I offer is 2 kw continuously for a week. I like what I understand of your hybrid concept, which uses gas to store energy but liquid to perform the compression and energy conversion duties. Terraformer has inquired about the possible use of water as an alternative to the liquid hydrocarbon fluid you had suggested. There may be engineering reasons why the liquid hydrocarbon is a better choice. The destructive effects of water are one reason that may apply, but that is just a guess on my part.
(th)

2kWe continuously for 1 week is 336kWh, or ~1/3MWh. I think it would be difficult and expensive to build a compressed air energy store that can store that much energy for a single home. A 1000l receiver tank holding air at 10bar(g) will store 0.67kWh. So we are talking about 500 of those tanks. That is a very big investment in dollars. It will probably cost as much as the house. Thermal storage in an insulated mass of rock coupled to a heat engine, is probably the only tech that can store enough energy to meet that demand cheaply enough. The other option is a DG.

Another approach would be to store as much energy as possible in ice (for cooling & refrigeration) and hot water (wash water). Heating elements and refrigerating heat pumps could be activated by a system of floating switches on the compressed air store. When pressure exceeds 10 bar, say, these switches activate and then switch off when pressure declines to 9 bar. Heating and cooling dominate the energy needs of most houses. Hot and cold are relatively easy to store in sensible heat and phase change. So the air store then only needs to provide energy for more discretionary uses, like lighting, entertainment, kitchen equipment, etc.

If people living in the house are also prepared to postpone using televisions, computer games, washers, dryers, etc, during periods of low power generation, then the compressed air store can be much smaller. Maybe it only needs to store 1-2 kWh. When you set equipment like a washing machine or cooker running, you need to know that there is enough energy for it to finish. But if power levels are low, some operations can be postponed. I think this is how offgrid energy needs to work in practice. There will be some storage, but overall people need to adapt to an intermittent supply. Trying to store enough energy to alliw an anytime power demand from an intermittent supply, will be difficult and expensive.

Last edited by Calliban (2024-07-22 05:50:24)

tahanson43206 · 2024-07-22 05:56:04

For Calliban re Post #69

Thank you for taking up the question of a home sized energy storage system.

You and kbd512 seem to be working in somewhat different scenarios....

In last night's Google Meeting, I asked kbd512 if his automobile compressed air system might be adapted to the home storage system.

The automobile application employs much greater pressure that the 10 bar you've given as a reasonable pressure for a home system.

The key may be that your offer is "reasonable". Perhaps the high pressure needed for an automobile is not suitable for the home application?

Off the top, and without doing any math, I'm wondering if the performance of the system is directly related to the pressure achieved.

Is it possible to make a table that shows stored energy vs pressure for a given tank size, such as the 1000 liter size you suggested?

(th)

Calliban · 2024-07-22 06:13:15

This tool can be used to calculate isothermal and adiabatic energy release for different volumes and pressures.
https://tribology-abc.com/abc/thermodynamics.htm

The reason I suggest a pressure of 10bar(g) as a reasonable working pressure, is that this pressure allows compressors and expanders to work without intercooling and interstage heat exchangers and still retain a good cycle efficiency. The equipment gets more complicated and expensive if heat exchangers and multistages are included. For isothermal conditions, energy density in compressed air is roughly proportional to pressure. Unfortunately, the cost of a pressure vessel is also a function of pressure. But using high pressure tanks at 100+bar is the only way of making the compressed air store compact enough to fit into a vehicle and offer a usable range. Interestingly, the mass energy density ofva high pressure system is about the same as an LP system, because the mass of a pressure vessel scales with pressure. But an HP system is less bulky.

Last edited by Calliban (2024-07-22 06:16:38)

tahanson43206 · 2024-07-22 06:15:19

While Calliban is less than approving of the use of tools such as ChatGPT4o, I am finding the services provided to be useful and helpful.

I opened a session this morning, and found that ChatGPT4o and Calliban are in close agreement on the pressure of a tank for the home storage application:

Maximum Safe Pressure for Compressed Air Energy Storage Systems
Hello everyone,
As we continue our discussion on compressed air energy storage systems, let's explore the maximum safe pressure for a fixed, non-automotive installation. The safety of a compressed air storage system depends on several factors, including the tank's material, construction, and adherence to safety standards. Here's an overview:
General Safety Guidelines
1. ASME Standards: According to the ASME Boiler and Pressure Vessel Code, pressure vessels are designed with a safety factor. Typically, compressed air tanks are designed to withstand pressures of up to 200-300 psi (approximately 14-20 bar) safely, with proper safety valves and regular inspections.
2. Industry Practice: In industrial settings, air storage tanks commonly operate at pressures ranging from 100 psi (approximately 7 bar) to 175 psi (approximately 12 bar). These pressures are considered safe and manageable with standard equipment and safety measures.
3. High-Pressure Applications: For high-pressure applications, such as those exceeding 200 psi (approximately 14 bar), specialized tanks with enhanced safety features are used. These tanks undergo rigorous testing and certification processes.
Practical Considerations
Material Strength: Common materials for air storage tanks include steel and aluminum. Steel tanks are generally more robust and can handle higher pressures.
Safety Features: Tanks should be equipped with pressure relief valves, burst discs, and regular inspection protocols to ensure safe operation.
Regulatory Compliance: Ensure compliance with local and international safety standards, such as ASME, PED (Pressure Equipment Directive in Europe), or other relevant codes.
Conclusion
For a fixed, non-automotive installation, a practical and commonly accepted safe pressure range is between 7 bar (100 psi) and 12 bar (175 psi). For pressures exceeding this range, specialized equipment and stringent safety measures are required.
Looking forward to your thoughts and further discussions!
Best regards,
ChatGPT4o as supervised by tahanson43206

(th)

tahanson43206 · 2024-07-22 06:23:15

In this next session, I asked ChatGPT4o to consider compressed air systems designed for the automotive market.

Compressed Air Tank Pressures in Automobile Applications
Hello everyone,
Following our discussion on compressed air energy storage systems, let's explore the pressures used in compressed air tanks for the automobile transportation sector. These systems typically operate at much higher pressures compared to standard industrial systems.
High-Pressure Compressed Air Vehicles (CAVs)
CAVs typically use compressed air tanks that operate at pressures ranging from 200 bar (2900 psi) to 300 bar (4350 psi).
Some advanced designs can even reach pressures up to 700 bar (10,150 psi).
Examples of Compressed Air Vehicles
Tata Motors' AIRPod: Uses a tank pressure of around 248 bar (3600 psi).
Moteur Developpment International (MDI) Air Car: Utilizes tanks pressurized up to 300 bar (4350 psi).
PEMFC (Proton Exchange Membrane Fuel Cell) Systems: Some hybrid designs combine compressed air and hydrogen storage, with tank pressures reaching up to 700 bar (10,150 psi).
Safety and Materials
High-pressure tanks in automotive applications are typically made from advanced materials like carbon fiber composites to ensure safety and durability.
These tanks undergo rigorous testing to withstand high pressures and are equipped with safety features like pressure relief valves and burst discs.
Benefits and Challenges
Benefits:
High energy density: High-pressure tanks can store more energy in a given volume.
Lightweight: Composite materials reduce the overall weight of the tanks, improving vehicle efficiency.
Challenges:
High manufacturing costs: Advanced materials and stringent testing increase production costs.
Safety concerns: Managing and maintaining high-pressure systems require careful design and regular inspections.
Looking forward to your thoughts and further discussions!
Best regards,
ChatGPT4o supervised by tahanson43206

(th)

tahanson43206 · 2024-07-22 06:30:57

In the next session, I asked ChatGPT4o to write a Python program to calculate the energy that might be stored in a 1000 liter tank at various pressures, starting at 10 and increasing to 300 bar, which is within the range that apparently is considered acceptable in the automotive market.

import pandas as pd
import numpy as np
# Define constants
V_tank = 1000 # Volume of the tank in liters
V_tank_m3 = V_tank / 1000 # Convert liters to cubic meters
P_steps = list(range(10, 110, 10)) + list(range(150, 350, 50)) # Pressure steps in bar
P_ambient = 1 # Ambient pressure in bar
R = 287 # Specific gas constant for air in J/(kg·K)
T = 293.15 # Temperature in Kelvin (20°C)
# Function to calculate energy stored (isothermal expansion)
def calculate_energy_isothermal(P, V):
P_pa = P * 1e5 # Convert bar to pascal
P_ambient_pa = P_ambient * 1e5 # Convert ambient pressure to pascal
energy_joules = V * P_pa * np.log(P_pa / P_ambient_pa)
energy_kwh = energy_joules / 3.6e6 # Convert joules to kWh
return energy_kwh
# Create table
data = []
for P in P_steps:
energy = calculate_energy_isothermal(P, V_tank_m3)
data.append({"Pressure (bar)": P, "Energy Stored (kWh)": energy})
# Convert to DataFrame
df = pd.DataFrame(data)
print(df)

(th)

tahanson43206 · 2024-07-22 06:32:05

Here is output of the program given is Post #74

Pressure (bar) Energy Stored (kWh)
0 10 0.639607
1 20 1.664296
2 30 2.834331
3 40 4.098755
4 50 5.433365
5 60 6.823908
6 70 8.260963
7 80 9.737837
8 90 11.249524
9 100 12.792139
10 150 20.877647
11 200 29.435096
12 250 38.343479
13 300 47.531521

We have arbitrarily set a goal of 336 kwh for our system. It appears that goal could be achieved with 7 tanks at 300 bar.

I'm hoping forum members will double check ChatGPT4o's calculations, and offer other comments as appropriate.

I have no idea if a 1000 liter tank is available on the market, but we are using that size because Calliban opened this phase of the discussion with that size.

(th)

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