Compressed Air

Terraformer · 2023-02-20 08:33:21

A few posts recently have touched on compressed air storage.

In Britain, the relative humidity is very high -- often something like 80+%. What happens to the water when the air is compressed? Compressed air systems will need a way to remove the excess water, and the water condensing out will release a lot of heat.

How much use can we make of this heat? If we had a heat pump that used ambient air as its working fluid, how would the heat of vapourisation of the water in the air affect its performance?

tahanson43206 · 2023-02-20 09:13:20

This post is repurposed to hold an index to notable posts by members...

https://newmars.com/forums/viewtopic.ph … 64#p225464
The post above by kbd512 contains details about how much energy can be stored in a compressed air system.
The post includes explanations for losses, and gives a reasonable/practical energy recovery value

For Terraformer re new topic, and Post #1

Please consider asking this question of ChatGPT and then posting the reply.

The reply you receive will be interesting even if it is wrong, and you will have gained access to a powerful brain amplifier.

The reply might even be right, at which point you could add ** that ** to this interesting new topic.

(th)

SpaceNut · 2023-02-20 11:35:33

Yes, vapor is condensed in the tank to which you keep them from rusting from the inside requires the tank to be drained of the collected water at the bottom with a venting valve. Its, also why most commercial units have a drying stage to allow for the moisture to be removed as much as possible to keep it from happening.

tahanson43206 · 2023-02-20 12:30:17

I'd like to point out that water collected in this way is free of ground contaminants. It still contains debris floating in the air, so it would still require treatment before it is considered potable, but that treatment would be mild in comparison to ground or surface water.

(th)

kbd512 · 2023-02-20 18:26:31

If the inside of the tank is lined with PTFE or Nickel or Chromium or Titanium Nitride, then it takes a lot longer for it to rust. However, in general filtering out atmospheric contaminants and removing water vapor is necessary, because the rest of the components (valves, high pressure lines, pressure gauges, etc) aren't so easy to preserve. I suppose we could use fluoropolymer lines, but at some point that gets rather expensive and you need to do a better job of filtering.

To power a motor vehicle, the cheapest solution is probably Nickel or Chrome with stainless steel lines and fittings. To power a city, you need powder coatings or paints on the exterior, with a good filtration system to keep water and dust out of the interior. You will always have some water present. That's unavoidable in a practical system.

As far as extracting the heat generated during compression, the heat could be fed into a hot water distribution network so the energy is not wasted. There's no practical or technological reason why this cannot be done. We haven't done it because so many people fetishize electricity and electronics, even though they will promptly turn the majority of the generated electricity back into thermal energy (hot water or hot / cold air) for most of their energy usage.

As always, electricity superficially appears to be a more efficient solution because nobody is counting energy inputs vs outputs between apples-to-apples solutions. They don't consider what will be required when the electronics wear out, which is complete replacement with little to no recycling. They don't scale-up their ideas to the level of cities or nations. They obviously don't compare with systems supplying power, 24/7/365. Those "evil" fossil fuels backstop their ideas whenever their favored electronics-based solutions come up short, which is every single day in the case of photovoltaics and any place where there is not constant wind movement for wind turbines. They routinely falsely claim that a system which doesn't supply most of the power or do it most of the time is somehow "cheaper and more efficient".

Compressed air storage systems, especially ones using cryogenics or hot natural materials (rocks / salts / water), are buy-once / cry-once per human lifetime. They are cheap if we're talking about 24/7/365 operation, and they can be built by pretty much anyone, pretty much anywhere on the planet. They don't require special environments or special natural features. Where special environmental features are present, like high insolation deserts or windy coastlines, I take no issue with taking what nature is offering. They do require steel and concrete, but those are the most mass-manufactured materials on the planet.

I think combined heat and power with solar thermal and geothermal, with natural materials sensible heat storage, or compressed air storage, combined with hydrocarbon fuels manufacturing, are the most practical solutions over many thousands of years. Even with fusion, we'll simply consume all the Deuterium. The process for removing it from seawater is horridly inefficient, and will only become worse as we deplete it, no different than any other form of ore extraction. Maybe there's a way to use fission and fusion together as Calliban suggested, but that's about the only other very long term solution I can see with nuclear power. I'm thinking on scales of thousands of years and assuming our population remains between 5 and 10 billion people. That's also why we need access to all the metals and water in the main belt if we're going to eventually have a shot at making it to the nearest star.

AI and other new computing technologies are / will be great until AI attacks us. If we survive it, computerization will be regarded as "black magic", similar to how nuclear energy is regarded by most people today. I'll probably be treated as if I were a witch, for the crime of being a software developer. Nobody can say that they had a "failure of imagination", though. Everyone and their dog who knows anything about AI or where this ends has issued the same warning, which has fallen entirely on deaf ears.

SpaceNut · 2023-02-20 18:56:09

There is also the oil that lubricates the head so are reduce friction.

tahanson43206 · 2023-02-20 19:25:20

For SpaceNut re #6

Good reminder of the need for lubricating oil for any mechanical system.

However, your post reminded ** me ** of the spiral compressor design that I'm "pretty sure" was used in the MOXIE test system on Mars.

It might be worth while for anyone interested in doing long term air compression to see if that system might be capable of the job.

It would still need oil but perhaps less than would be required by a piston pump.

(th)

kbd512 · 2023-02-21 03:16:17

Air or hydrodynamic bearings can reduce the need for continual addition of oil. You still need lubricants for startup and shutdown. There are certain classes of materials like Teflon, or very expensive ceramic coatings, or surface treatments like WPC that can mostly remove the requirement for petroleum-based lubricants. However, whether or not those other solutions work depends greatly on the bearing loads involved, duration / duty cycle, and expected service life of the bearing surfaces. In broad general terms, air compressors are going to use lubricants or they're not going to have high duty cycles or long service lives. Turbines or functional equivalents like scroll compressors (the part doing the compressing only moves in one direction) will require less oil than reciprocating pistons, and are more efficient at compressing air.

Calliban · 2023-02-21 05:29:36

Terraformer, well done for raising an excellant new topic!

For large scale CAES, we would probably use multi-stage axial compressors with intercooling between stages. Axial compressors have lower frictional energy losses and generally better power density than alternatives, but are only practical at large (multi MW - GW) scales. It would make sense chilling the air prior to compression. When air is compressed, temperature rises and a lot of energy is converted to internal heat, which is usually wasted. Pre-chilling would also remove any trapped moisture, which would condense on cold surfaces within a heat exchanger. You could use a heat pump to do the chilling, in which case the pumped heat is a byproduct that you can find other uses for, like putting into a district heating network or injecting into the ground for later use. Interstage cooling would also produce warm water that can be used in the same way.

So far, CAES has mainly been used for grid energy storage and tends to utiluse abandoned mines for air storage. Under sea CAES is another potential option which has broader applicability, given that most of the world population lives in cities close to the coast. CAES using above ground pressure vessels is possible, but pressure vessels have high capital cost. One option here would be the use of pre-stressed concrete pressure vessels. A PCRV is a concrete tank with a steel liner, with tensile strength provided by steel tendons running through the concrete. The advantage here is three fold. Tendons can the added in any number neccesary, making it possible to build enormous pressure tanks without the difficulty of welding steel vessels with enormous wall thickness. Tendons are also replacable. This makes catastrophic failure an incredible event and also obviates problems with fatigue life. A well maintained PCRV could last for centuries. It would be expensive to build, but would have low marginal cost per MWh of energy stored over its lifetime.

Kris DeDecker wrote an article on the future potential of small scale CAES a few years back.
https://www.lowtechmagazine.com/2018/05 … onomy.html

Before the development of AC power networks in the early years of the 20th century, compressed air networks were developed in a number of cities for power transmission. Compressed air is still widely used in industry as air tools are cheaper and have better power to weight ratio. But compressed air does tend to be a less energy efficient means of powering equipment. If large scale CAES is adopted, we could develop air networks and cluster industry around air stores. We could also use air to power short range vehicles. A hybrid vehicle equipped with an air tank would eliminate compression power losses within the engine, increasing mpg.

Last edited by Calliban (2023-02-21 05:43:01)

Terraformer · 2023-02-21 07:38:16

AFAICT, the amount of moisture air can hold depends on its temperature, but not on its pressure? So if we have air that can contains 10g of water per cubic metre at saturation, and has 80% relative humidity, and we compress it to twice the pressure and let it cool to ambient, 6g of water should condense out? Which would release ~12kJ of heat in the process. Am I right in my thinking here?

Calliban · 2023-02-21 09:28:31

Terraformer wrote:

AFAICT, the amount of moisture air can hold depends on its temperature, but not on its pressure? So if we have air that can contains 10g of water per cubic metre at saturation, and has 80% relative humidity, and we compress it to twice the pressure and let it cool to ambient, 6g of water should condense out? Which would release ~12kJ of heat in the process. Am I right in my thinking here?

Correct. The vapour pressure of water is a function of temperature. The humidity is the concentration of water in air that allows the same number of molecules to enter and leave the liquid state simultaneously. If you isothermally compress one unit of air with 100% humidity to 2 bar, say, then half of the water vapour would condense. The problem is that compressing the air also heats it. If the air to be compressed is not dry, you are wasting work trying to compress water vapour. Any water that does condense needs to be drained out of the compressor. Another problem is that air cools as it expands. Any water vapour within the compressed air could turn to ice particles during the expansion. To negate these problems, it makes sense drying the air as much as possible before compression. The easiest wayis to cool it, reducing the vapour pressure of any water it contains. Cooler air also requires less compressor work.

One interesting side note. The atmosphere contains 0.04% CO2 by volume. At temperatures <31°C, CO2 can be liquefied by compression. The CO2 is then something that we can seperate. It can either be sold, used to producesynthetic fuel or sequested from the atmosphere. CAES could provide a means of removing CO2 from the atmosphere.

kbd512 · 2023-02-21 09:55:11

Terraformer,

I think the term you're asking about is pressure dew point. For air compressors that need to supply dry compressed air, this is a figure of merit. The dew point of uncompressed air could be much lower at atmospheric pressure if the air is compressed significantly.

Specific humidity is the mass ratio of water vapor to a given mass of dry air. Compressing the air doesn't change its mass, nor the mass of the water vapor it contained prior to compression, only the volume that the compressed air and water vapor occupies. That's why it's not dependent upon pressure. One cubic meter of air with 80% relative humidity, still contains however many grams of water it started with after it's been compressed.

Calliban · 2023-02-28 13:02:39

This device is so beautifully simple. I wonder if more use could be made of it.
https://en.m.wikipedia.org/wiki/Trompe

If compressed air is used as a source of power, then a trompe can produce dry compressed air without any moving parts. All that is needed is flowing water and a sufficient head height. Small trompes would be a good way of providing air conditioning for a building. In this case, we want lots of cool air at low pressure, so the trompe could be low head.

But the water supplying a trompe could be pumped using wind or solar power. If wind power is used, then a very simple wind pump could deliver water to an elevated reservoir, where it would flow down the trompe inlet. A head height of 140m would produce air at 10 bar pressure, which is suitable for machine tools. The wind machine doing this would not need any rare materials. It could be made from wood, or steel. The wind turbine shaft would drive a centrifugal water pump, either directly coupled or through a gear box.

tahanson43206 · 2023-02-28 14:54:45

For Calliban re #13

Thanks for reminding us of this air compression technique, and casting your previous reports in a new light.

My recollection (subject to correction) is that in previous posts, you might have referred to air compression done in association with mining operations in parts of the UK. The details of the trompe system you showed us look familiar (to me at least).

The big attraction (at least to me) is the aspect you emphasized of "no moving parts".... Of course air and water ** are ** moving "parts", but the system is free of humanly fashioned "moving" parts to do the compression.

I'm hoping you might follow up with a vision of how such a system might work in the context of a community like the one where Terraformer is considering a network of pipes to carry thermal energy. If the community is willing to invest in a system to store and deliver thermal energy, they might be willing to go the next step and consider stored air under pressure.

In flat terrain, I'm presuming (wild guessing?) that it would be necessary to excavate a column for the water and air to descend.

The water would have to be brought back to the surface by the means you suggested, which ** would ** involve moving parts.

However, if the water at the bottom of the hole is heated to steam, it could travel back up to the surface without mechanical assistance.

That would be a system without humanly fashioned moving parts.

(th)

tahanson43206 · 2023-02-28 15:03:13

As a follow up to the post just entered in considering Calliban's post #13....

This is really stretching, but Calliban's post reminded me of the challenge of harvesting low grade energy from geothermal wells.

Certainly a well deep enough to reach temperatures able to evaporate liquid water to steam would be deep enough to compress air as Calliban described.

If the water were converted to steam and returned to the surface on it's own, then the air would be present in the compressed state, able to do useful work, such as driving a piston to drive a generator, to send electric current back up the pipe.

The system described ** would ** require humanly fashioned components, but if well made they ** should ** last for a number of years.

I'd be ** really ** interested in seeing an analysis of performance of such a system, and of it's efficiency compared to one that delivers steam all the way to the surface to be harness there. It seems to me (again just guessing) that the modified Trompe system would achieve a higher efficiency overall, due to harnessing the heat for the secondary water lift operation, instead of for the primary drive-a-generator function.

(th)

Calliban · 2023-02-28 16:26:48

Both the inlet and outlet of the trompe are above ground. Water flows up the return shaft by gravity. Water enters the downflow shaft and entrains air. So the downflow shaft needs to be at least 40% longer than the upflow shaft, because it carries bubbly flow which is less dense. The energy trapped in the compressed air is proportional to the difference in head between the two water columns, taking into account the lower density of the bubbly flow in the downcomer. But the upcomer pressure head is identical to the pressure of the compressed air trapped in the tank at the bottom. Part or all of the trompe water columns could be within the tower of a wind turbine, with the air tank at the bottom. Some turbines of 10+ MW power output have nacelle heights of over 100m. Static pressure of a 100m water column is 10 bar. The turbine pump needs to take water from the top of the upcomer and pump it to the top of the downcomer.

I wonder if the same thing can be achieved using a jet pump without having to build an enormous trompe. We can couple the turbine shaft to a centrifugal water pump. This would generate a stream of water with a dynamic pressure of 10+ bar. The water stream would enter the jet pump, which would entrain air bubbles before entering a seperation tank. The water would drain out of the bottom of the tank and return to the centrifugal pump. Compressed air would exit the top of the tank. By doing this, we are replacing the static pressure of the trompe water column, with the dynamic pressure of the centrifugal pump. But the jet pump is far more compact than the trompe. And this is something that would work even with a very small turbine.
https://www.gea.com/en/products/pumps-v … e-fgv1.jsp

Last edited by Calliban (2023-02-28 16:41:32)

tahanson43206 · 2023-02-28 19:30:46

For Calliban re #16

Thanks for this interesting development of your idea, and for considering the above-ground case.

One question I have is about your wording in this section:

Water flows up the return shaft by gravity.

This wording may have been an abbreviation of a longer sentence.

Water (of course) NEVER flows UP under the influence of gravity.

I apologize if this seems like a nitpick, but your writing may be viewed by others for years to come, and this topic in particular may attract future searches.

You may have meant something along the lines of ... flows up under pressure from the down spout.

Water levels would tend to equalize if left alone, so this is what you may have been thinking.

But if there is no difference in the water levels of the columns, then there is no incentive for water to flow down the down pipe, so the system will remain stagnant.

Obviously under those conditions, no air entrapment will occur.

(th)

Calliban · 2023-03-01 08:36:33

TH, quite right. It is the difference in pressure at the base of the two water colums that allows water to flow through the trompe. The trompe itself is around 80% efficient. The high efficiency is achieved because the water very rapidly absorbs heat produced within the air during compression. Compression is effectively an isothermal process. The 20% energy loss occurs because some air will dissolve into the water as it descends the downcomer.

The main problem with a hydrostatic compressor is its large size and the need to either build towers or drill boreholes in flat areas. The advantage is that once built, it can remain operational for centuries with little maintenance. There is nothing to wear out.

One intriguing possibility is to build a trompe in the sea and store compressed air in flexible bags or concrete containers on the sea bed. This would work well in the North Sea for example, where depths range from 10-150m. Offshore wind turbines could then be fitted with sea water pumps instead of electric generators. Dozens of them could pump water into a single trompe. Compressed air from the trompe would flow through a plastic pipe, to a single large electric generator onshore. The compressed air tanks could be sized to store hours of even days of compressed air supply for the generator.

The advantages of this arrangement are (1) Very simple mechanical systems can be used in the turbines. There would be no need for electronics in the turbines themselves. The generator onshore is the only electrical system. This means much reduced expenditure of materials like copper, aluminium and rare earths. (2) Air stored undersea would not need to be kept in pressure vessels, just ballasted containers. Thin walled concrete or thermoplastics will do the job. So undersea air containers can be cheap to build.

Last edited by Calliban (2023-03-01 08:50:32)

tahanson43206 · 2023-03-01 09:21:00

For Calliban re #18

This idea (or rather set of interrelated ideas) seems (to me for sure) suitable for development.

Please pursue the idea set a bit further, to see if it might tempt investors seeking to drop big amounts into the Green Energy wave.

The only adjustment I would like to offer (for consideration only) is that it may turn out to be more efficient to move electrons to shore, instead of huge masses of baryons. You will recognize this is a constant refrain, so please feel free to discount it.

If you pursue the Trompe idea it might allow you to offer offshore energy storage that would be more reliable than alternatives, and likely to have lower maintenance costs than alternatives.

(th)

Calliban · 2023-03-01 19:22:13

TH, converting the air into electric power at sea, would allow power transmission to shore by cable. A cable would certainly be more compact than an air duct. But we then need to build a power station on a platform in the sea. That is more challeging and the people that work there need to accomodated, much as they would on an oil rig. I don't know how the economic balance would work out. But both are options to be considered in a cost-benefit analysis.

My reason for proposing the use of turbines that generate pumped water instead of electricity, was an attempt at reducing the need for copper, aluminium and rare earths embodied within a wind farm. Whilst less extreme than solar PV, the metal resource requirements of wind power are still problematic if this energy source is the be scaled up to generate thousands of TWh per year. I also wanted an energy storage solution that was relatively affordable. Using hydraulic wind turbines to generate compressed air, would allow a hundred wind turbines to power a single electric generator. This generator can be onshoreeven if the turbines are offshore. This makes transmission and maintenance much easier. And we can use a standard 500MW generator set, coupled to an air turbine. The wind farm itself will be dominated by just three materials: concrete, glass fibre reinforced polymer (for blades) and carbon-manganese steel. The concrete plinth can be reused when the turbine reaches end of life. The steel can be recycled. The plastic converted into synfuel.

tahanson43206 · 2023-03-01 20:56:59

For Calliban re #20

Thanks for thinking more about the compressed air energy storage system you've been describing.

I did find one aspect of your design a bit surprising ... it seems you were planning to put people in a power plant at sea to make electricity.

The existing wind turbines installed around the world (on land and at sea) are able to generate electricity without having people on the scene, except for maintenance.

is there something about your vision that would require a person on site? What would the person do?

I do like your theme of reducing dependence on rare atom types. My observation is that iron can be used for electric power transmission, if is is absolutely necessary to save money, but aluminum is a reasonable substitute, if it is not constrained in some way.

Silver, copper, gold: Aluminum, Zinc, Platinum .... a pattern memorized long ago for a test.

(th)

Calliban · 2023-09-28 01:28:28

Driving on compressed air.
https://m.youtube.com/watch?v=fFoYPj3Ntzc

Two university proffesors produce a prototype. They claim 60-70% energy efficiency and a 140km range.

tahanson43206 · 2023-09-28 06:42:12

For Calliban re #22

Thanks for the link and comment about compressed air ...

The history of machinery using the process was particularly interesting!

The careful review of the principles and technical challenges was helpful.

The end product would be suitable for a go-to-work car, or a shopping car for many folks.

A detail that caught my eye was the tank wrapping machine.

By coincidence, GW Johnson ** just ** advocated use of the Bigelow Aerospace design for a space habitat in another topic.

It occurred to me that a space habitat might be made using a similar machine on a larger scale. The pressure to be contained is (of course) far less, but what I like about the concept is freedom from use of metal atoms. I assume carbon and hydrogen are the major atom types in the wrapping tape.

The wrapping could be done in orbit, so the tape would be transported to LEO in a tightly wrapped coil, which could be as large as the diameter of a Starship. Major components of Large Ship could be fabricated in this way. For example, the spokes might be made of tension cables wrapped with tape to make a component able to withstand the stress of holding the habitat ring and the lesser stress of holding pressure.

In the case of Large Ship designs that rely on the torus instead of flat walls, the entire torus could be wrapped in sections and bonded together.

Returning to the video in closing ...

I would ** hope ** this vehicle might be produced at a modest price. If anyone sees pricing information or even speculation, please post it here.

(th)

Void · 2023-09-28 08:24:07

While it would also be nice to have some sort of auxiliary travel system for a family or individual, I think the incorporation of some of the elements of the systems into home interior climate control might be good.

In the event of a home heat pump system, in the case of cooling a house, the output heat could warm a tank such as Parafin, and also might help to warm the stored air in a pressurized air tank to drive an air turbine. In the case of warming a house, the output cold could cool the air compressed into a pressurized air tank.

Both the compressed air tank and the thermal storage could also act as "Off Peak Power Sinks". Generally, the home system would be an electrical energy sink, but could also send power to the grid at times. So, this might work well with grids that also work with intermittent energy sources. Houses also can have solar panel installations included which would make them one of the intermittent energy sources.

In addition to compressed air and paraffin thermal energy storage such a house might have a power pack battery of a relatively small size.

The house being connected to the grid could also run entirely off of imported electricity in the event that the house local system otherwise had a breakdown. That presumes some functioning climate control systems for the internals of the house.

I suppose when the entire set of machinery was functional you would have the means to charge an air and thermal driven car of some small size.

Done.

Last edited by Void (2023-09-28 08:29:24)

tahanson43206 · 2023-09-28 09:24:15

For Void Re #24

Thank you for extending the idea of compressed air energy storage to the challenge of home energy storage.

As you noted, air becomes hotter as it is compressed, and that thermal energy could be banked for use in winter. Air released from the compressed store sucks thermal energy out of the environment, so it could be used for cooling in summer. It would not be helpful for cooling in winter, so it would need to be vented outside during that time of year.

Please continue developing this interesting line of thought. Transportation is a good starting point, but the potential for home energy storage may be greater.

(th)

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