Jonas

Off-grid without aeration nightmares... Compressed air?!

21 posts in this topic

Hi all,

 

I'm currently preparing setting up a fairly serious sized system in Myanmar, obviously a place struggling for decent power which needs to be keptin mind. Naturally, I'm worried about aeration, especially during nights (during day time fortunately solar should provide good energy even though obviously sun intensity varies throughout the days).

 

Whilst toying around with the classic solar panel/battery solution for aeration, I was, mainly from an intellectual stimulating point of view looking at various other energy storage options (flywheels, alternate batteries, pumped storage hydroelectricity etc.) to come up with something with high levels of resilience at low cost. I do not know too much about this area though so currently this is pretty much just an abstract, theoretical thought experiment with solar/battery being fallback option.

 

However, one thing I was wondering was if it might be possible to directly use an air compressor which very slowly releases air directly into the fish tanks at night if power goes down ("leaking" air over a 12h period or so)? Again, I'm not sure if such equipment exists, if efficiencies even remotely make sense etc. but would it be possible to compress air into an extra air cylinder during daytime and then when power goes down during night slowly have that air released from the unit into fish tanks through simple airstones? Thoughts?

 

Thanks,

Jonas

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That's possible.  I would thnk you would need a sizable tank and compressor.  I have no direct experience with this setup, but an oil less compressor is a recommendation I have read about.  Some other options are to oversize your tank,  store water in an elevated tank and let it run through the system, or use a fish that is an air breather too like Clarius sp.

 

One thing that helps is making sure the system if filtered of solids as solids breakdown in the system consumes a great amount of water.

 

 

Due to compressing of air power loss increasing with the volume and pressure increasing, you may be money ahead to oversize your battery and use a low power air compressor for aeration at night.

Edited by Ravnis (see edit history)

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I guess if you are comparing something to solar it would help to have a ballpark figure on cost. Your timing is perfect! I made myself a little spreadsheet last weekend to help me with just this sort of rough calculations.

 

equipment

 

4 x 200watt 24 volt cheap solar panels

60 amp MPPT solar controller (one that converts the 24 volt panel voltage to 12 volt)

panel racks, hookup wire, switches

350 Amp hour of good quality gel deep cycle batteries (wired to give 12 volt) calculation gives 115 amps, 33% of battery capacity for overnight use for maximum battery life.

25 amp 12 volt mains battery charger

a small sine wave inverter

 

calculations based on 100 watts per hour of aeration pumping.

 

This is what it would cost me to install with good quality bits in Australia. There is also a margin factored in that ensures I will make it through my winter months without damaging the batteries from over discharging them.

 

38 cents per Kilowatt-hour seems pretty good value to me, that is only about 25% more than I pay now for a connection to the grid. Of course the sun and season will be different in the tropics but it is a good starting point.

 

what does everyone think? Am I being realistic?

 

post-3002-0-46748300-1412813652_thumb.gi

 

EDIT I am probably a bit low on the solar controller price more like $850 rather than $370, I was thinking 24 volt originally.

Edited by yahoo2 (see edit history)

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Hey Yahoo2, excellent, looks like the kind of simple sheet required - unfortunately I can't really comment on whether it's realistic or not as I'm at a similar place to you but hopefully somebody else can! :-p

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That's possible.  I would thnk you would need a sizable tank and compressor.  I have no direct experience with this setup, but an oil less compressor is a recommendation I have read about.  Some other options are to oversize your tank,  store water in an elevated tank and let it run through the system, or use a fish that is an air breather too like Clarius sp.

 

One thing that helps is making sure the system if filtered of solids as solids breakdown in the system consumes a great amount of water.

 

 

Due to compressing of air power loss increasing with the volume and pressure increasing, you may be money ahead to oversize your battery and use a low power air compressor for aeration at night.

Great points (especially the "obvious" solution of using an air breather - commercially though unfortunately not quite as attractive here...)

 

Not sure I understand your last sentence though - do you mean that it would probably be cheaper to oversize the battery and just run a low power air compressor off that battery?

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I was referring to the greater power loss of high pressure compressors vs the power loss of the low pressure compressors used in to power air stones.  To store sufficient air to power for an extended period of time you will need to compress air to a much higher psi than the 3-7 psi aerator pumps typically put out.

 

I was following research from some MIT students on this vary subject, but their goal was to use the  compressed air to power generators and use compressed air as batteries.   They have now switched to using "super high capacity capacitors" but that's still currently in development.

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In theory hydro works, the energy storage is the flowing water, it is not hard to make a simple diaphram pump or three.

 

I have built a couple and driven them with a tiny windmill and tapped the pressurized air into my workshop system. Works fine for that, not sure I would trust it for life support. These sort of things tend to be very difficult to fix quickly when they break down.

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I was referring to the greater power loss of high pressure compressors vs the power loss of the low pressure compressors used in to power air stones.  To store sufficient air to power for an extended period of time you will need to compress air to a much higher psi than the 3-7 psi aerator pumps typically put out.

 

I was following research from some MIT students on this vary subject, but their goal was to use the  compressed air to power generators and use compressed air as batteries.   They have now switched to using "super high capacity capacitors" but that's still currently in development.

 

Are you close to that group? I am quite interested in the energy storage topic (including flywheels, hydro, compressed air etc.) - if you have any papers/research which looks close to market-ready I'd be very curious to see them. Interestingly, I just saw something that looks like this outside a university here in Yangon, a big compressed air silo hooked up to a generator for what seemed to be emergency power (as obviously outages are prevalent here) - the claim on the machine was 5.5 HP!

 

My rationale was actually simpler - I'm more interested in oxygen (or rather aeration to be precise, obviously won't pump pure oxygen) than electricity anyways so there could be one (inefficient) step removed in this process. A quick look at diving equipment being sold on Alibaba for example shows that it should be possible to get a large bottle for <$100. These have about 15-18l @ 210bar so we're looking at roughly between 3,000 and 3,800l of air. So if I want to release this air over night (12h) where there is 0 light for solar generation, I'd be looking at a flow of 250 - 300+ L/h. Simple air pumps I have seen use roughly 1W/l airflow per hour, assuming some efficiency at scale, lets just say we need a 150W air pump to get 250-300l/h. If I do that for 12 hours, I need about a total of 1,8kWh. Now, assuming that air pump would run on 12V (not sure), that would mean I need a battery of 150Ah which would be fully discharged just to get the same total output (ignoring that full discharge obviously is very detrimental to life expectancy of battery). Such batteries go for more than $100 on Alibaba.

 

Obviously, I would need to have a fairly serious air compressor to be able to fill the diving bottles in the first place but that's a different question (and looks like I probably have such a solution on hand anyways) - but could it be true that this kind of aeration backup is cheaper than the normal battery option (again as a disclaimer, I fear I'm at the dangerous stage of knowing a tiny bit about the topic but having massive blind spots, hence, might be missing something obvious here?)! One thing I still have to find/build/solve is the valve which in this case would slowly release the air in a controlled manner to ensure airflow for the actual 12 hours and not just release everything in one big swoosh within a few minutes.

 

In theory hydro works, the energy storage is the flowing water, it is not hard to make a simple diaphram pump or three.

 

I have built a couple and driven them with a tiny windmill and tapped the pressurized air into my workshop system. Works fine for that, not sure I would trust it for life support. These sort of things tend to be very difficult to fix quickly when they break down.

 

Hydro was interesting as well, If scaling up though the structural requirements get quite significant to hold multiple tons of water high up in the air by consequence again increasing costs quite significantly... Very interesting comment on the difficulty to fix such things - makes perfect sense that this is a risk and something to keep in mind, don't want Murphy to succeed too often ;-)

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If this is a thought experiment, I dont need to build any structure I can run the pretend river past the front door giving hydro a cost effective advantage. But I still have to clear the imaginary duckweed when it blocks the impeller.

 

No such luck with high pressure air storage, friction, thermodynamics and humidity means the efficiency can be very low. 12 bar is about the limit that off the shelf equipment can handle safely and the tanks are still going to have to be tested frequently.

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I have put a lot of thought into this over the last week or so and will have a prototype built, hopefully, in the not too distant future. I am planning on using wind power to turn a compressor to fill an air tank. Then properly regulated I should have a decent amount of compressed air. On the not so windy days I will use solar power. I am a big dreamer and tinkerer....

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I have put a lot of thought into this over the last week or so and will have a prototype built, hopefully, in the not too distant future. I am planning on using wind power to turn a compressor to fill an air tank. Then properly regulated I should have a decent amount of compressed air. On the not so windy days I will use solar power. I am a big dreamer and tinkerer....

 

I'd like to see the wind powered compressor idea....

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My approach to aeration is to (as far as practicable) minimise the consumption of oxygen by limiting the amount of particulate matter that helps to consume it......while, at the same time, taking every opportunity to encourage effective aeration in the system.

 

Arguably the most effective way to offset the risk of catastrophic failure arising from power or equipment failure, is through the use of bottled oxygen.

 

Obviously, this is difficult to justify (in terms of capital cost) in the case of very small systems but, for those situations where it is appropriate, it is the best way to keep fish alive when the poop hits the fan.  The incorporation of a battery backup systems - and an extra pump - are other useful risk mitigation strategies.

 

I think that the most useful (and practical role) for wind-powered devices is for supplementary aeration......through beating/stirring the water.....or with a simply axial flow pump arrangement.

 

Gary

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I'd like to see a side by side comparison of cascade aeration(as expounded by Dr. McMurtry), air stone aeration, and axial flow aeration to see which is the most energy and cost effective. 

 

 

Regardless, the problem seems to require either store potential energy by hydraulic means, potential energy by electrical means, or potential energy by high pressure.  Just a guess, but seems to me like the latter two would require the least space vs the hydraulic method, unless you employ some really deadly level pressure.

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I think that the most useful (and practical role) for wind-powered devices is for supplementary aeration......through beating/stirring the water.....or with a simply axial flow pump arrangement.

 

Gary

 

Perhaps, so.  It does make some sense to maximize efficiency by hooking the shaft from the windmill blades, directly to device in the water to minimize energy loss from friction.  Much like electrical heaters are as efficient as they could be because there is no energy transfer to another medium before heating the tank, like an external water heater, for example.

 

I wonder if using compressed air as an energy storage mechanism is feasable, much like how batteries store any excess energy from solar panels.  Though that might be completely moot, considering it's very easy to exceed your energy generation power with solar, or wind anyway.

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I'd like to see a side by side comparison of cascade aeration(as expounded by Dr. McMurtry), air stone aeration, and axial flow aeration to see which is the most energy and cost effective. 

 

 

Regardless, the problem seems to require either store potential energy by hydraulic means, potential energy by electrical means, or potential energy by high pressure.  Just a guess, but seems to me like the latter two would require the least space vs the hydraulic method, unless you employ some really deadly level pressure.

Axial flow aeration is most energy efficient, followed by air stones then cascade aeration. Lots of work has been done to work this out for commercial aquaculture. Obvious large peddlewheels don't fit a small aquaculture tank while airstones do.

 

Instead of looking at aeration during power black-outs you also could keep densities of fish so low that they can survive a night without forced aeration. You can improve this by feeding only in the first half of the day so that oxygen demand is low over night.

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The 'lo tech' iterations of iAVs featured cascade aeration because that's what best suited the circumstances of the African villagers for whom the system was designed.

 

I like axial flow pumps for their efficiency.......and a simple axial flow pump (effectively a fan blade in a shroud) could be direct-coupled to the shaft of a vertical axis wind machine (like a Savonius rotor).

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I know I am going a bit of topic here, cascade aeration is 2 steps in one; oxygenation and degassing. While airstones haver limited value as degassers. This is related to the air/water ratios passing the units. Thus it also depends what you want to achieve.

In some of my commercial aquaculture designs I use cascade aeration (water cascades over several rows of perforate plates) and axial flow pumps for low head-high volume water displacement.

 

The litte R&D unit I have at home has a moving bed filter running of a small airpump (100 Watt pond membrane compressor); this is all the aeration I use and this is enough to handle a feed load of 600-700 gram/day with murray cod. btw I do have a small drum installed to keep solid load low.

 

These membrane compressors don't have a high start current thus can start from a small $120 ebay sine wave inverter and will run for a long time from a big battery. This is in contrast to a blower which require serious power to start (powersource with a low internal resistance). I tried to run a little 400W sidechannel blower from a cheap 1200W sine wave inverter but could not get it going especially on load (1 meter water collum)

 

Back to your idea: My little 100W airpump gives around 7000 l/h of air at 1 meter. If you would have a compressor which go's to 7 bar (still doable for a small unit) you need a tank volume of 1 m3 compressed air/per hour backup. Looks not feasable to me.

Forget the compressors used to fill dive cylinders, I worked with them and just the filters required to clean the air of dust and oil fumes makes running cost very high, and if you run them without filters you risk internal damage.

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I have uploaded a couple of images of what I was thinking about building. The wind would turn the shaft that powers the compressor which in turn would build pressure in the tank. The air pressure in the tank would be released, as slowly as possible but with enough volume to supply the fish with adequate oxygen, via a pressure relief valve. Not sure how much wind be required to keep the tank filled with enough positive pressure to keep the system working but when I build it, I will make sure to post updates and pictures.

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