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Old 28-02-2010, 01:26   #1
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Stored Energy-Pneumatic Systems on Board

As part of my "Dream Boat" project (more like exercise) I've been looking at the possibility of using Pneumatics for some systems.

Applications may include;

pressurizing the fresh water and raw water systems,

evacuating the grey/black water holding tank/s,

operating actuators or bladders to raise/lower electric propulsion pods, lifting of the rudder, dingy garage door, boom vang?

possibly refrigeration and air conditioning??

At this point everything is on the table, but my gut feeling is that pneumatics may not be well suited for rotary applications that need a lot of power, like the windless or other winches.....My very limited experience with rotary pneumatic motors is that they need a lot of volume ...(I have got some thoughts around a linier actuated windless)

The boat would be total electric via regenerative while sailing, large amounts of Photovoltaic and some wind generation.
The boat is technically a tri, but is set up more like long skinny shallow draft full keel mono with outrigger hulls (no deck between hulls)

The recharging of the air bank would probably be electric driven compressors, but there may also me some regenerative possibilities here as well.

Air could be stored in high pressure vessels like a bank of aqua lungs or other size/shaped tanks.
Given the size of the vessel there is lots space for this.

The biggest advantage I see to this system is the possibility of lots of stored clean energy….and getting rid of a lot of electric motors and pumps on board.

One down side is the heat created in compressing all this air.
One possibility is to store this heat at the tanks or in another location….I recently started a thread A Discussion On How To Store Heat or a way to get rid of the heat before the air gets to the tanks.

Another concern is the longevity of the components…the tanks should be good if we keep our air dry (water will be a by product, but the amount may be negligible....or not?) I would also think the supply lines and fittings could be very long term…but I don’t know about seals and valving?

I know that volume for volume pneumatics are less capable than hyraulics. That may require larger volumes and pressures, but given the possible advantages it seem worth looking into.
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Old 28-02-2010, 02:39   #2
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I don't think think compressed air is a good idea. When you compress air you generate heat (you loose this by cooling and I wonder if you can put all that lost heat to good use); getting compression efficiency at a reasonable level in small scale applications seems far away. When you have a supply of compressed air it isn't a problem putting it to use; unfortunately at the expansion process you loose more energy. Pneumatic motors are powerful, small, reliable and simple of construction. Total efficiency is probably below 25%. The advantages (simple, relatively save) of air would loose against storing energy in batteries; maybe that's why see so many batteries to store energy?
It is not possible to store energy in hydraulics (oil is not compressible); of course you can use accumulators (based on gas pressure) to store some energy but you would still rely on (electric?) driven pumps to re-energize the system. In fact most if not all hydraulic power packs work on gas (air) pressure. Usable systems for both air and hydraulics will be bulky and heavy (dealing with high pressures requires a lot of steel).
For the time being I think batteries are the best way to store energy.

For some applications air is good, you probably use air pressure to maintain drinking water pressurized already. It is the same as with hydraulics; you will rely on other motors to recharge that system as well.
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Old 28-02-2010, 02:54   #3
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Good notes on the motors...I need to do my home work and see whats out there.
The problem I see with the batteries is how long they last...dont last.
They are also nasty.
I like the idea of another system thats completely unrelated to the electrical system.
Even at 25% efficiency's it may be worth while...if the initial "charge" energy is PV.

Also, if I go all electric, it means several individual electrohydaulic units (or a big hydraulic unit) shattered around and their associated heavy circuits, spares, maintenance issues....... .

I've still got losses...but I think its not just how the energy is stored...its how that particular energy is used.
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Old 28-02-2010, 04:11   #4
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Even plain old lead acid batteries, when treated well, will give you about 5 years of trouble free service, AGM and Lithium batteries are even more reliable. I doubt that any technical installation on a yacht will give you more than 5 years of maintenance and trouble free service.
If a battery fails you can simply replace it with a new one. This change will alway be easier (and probably cheaper) than maintenance and repairs on any other technical installations. Modern, well designed electrical components have no moving parts (solid state) and do seem to work trouble free for extended periods. Bonus: they are more efficient as well.

With efficienies of PV alone not even reaching 25% nothing much of efficiency will be left if you store energy in air (0,25*0.25 = 6%).

As for the motors: I think you will have a hard time adapting available motors to the marine applications. I have never seen anything like that. OK if you really like a challenge. Are you familiar with the noise air motors make (think if the whine a Formula 1 team makes when changing tires); that's something else you would need to solve. In most work shops workers are required to use hearing protection against noice; air tools result in a fair share of noise produced.
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Old 28-02-2010, 20:57   #5
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We had air starts on Tug Engines.....They required a lot of tanks, big compressor and big genset.....

The idea of a pressurized holding tank is too scary to even think about.
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Old 28-02-2010, 22:08   #6
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I like your thinking.
However the inefficiency with compressing air plus the inefficiency of using the air plus the marine environment, and high pressure tanks, all seems too hard to me.
I find that the whole battery charging power system, bad enough, very inefficent and expensive even when used with solar, wind or generator.
Using air storage would be worse.
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Old 01-03-2010, 00:08   #7
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OK...so let me take some smaller steps.
Lets say you're a diver and keep tanks on board.
Are those tanks safe?
Lets say you need the occasional use of a linear lifting device.
Would a pneumatic ram operated off one of your existing tanks make sense or would it be better to install an electric-hydraulic pack.... considering the maintenance and electric infrastructure required?
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Old 01-03-2010, 01:15   #8
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I think the pnumatic systems used in manufacturing are sucessful and have evolved as being the best tool for that particular job. The infrastructure is versatile, adaptable to a mirriad of applications and its cost effective to have a large compressor and central storeage.
I can see the potential uses for something similar aboard. No heavy cables and subsequent losses, small compact motors or rams, small bore supplies which are easilly installed etc.

Ineficiences of conversions of one energy to another and subsequent losses back again to mechanical power are the biggest drawback when your initial power source to compress air is costly to opperate.
Add to that the marine environment, internal corrosion of motors, rams and tanks etc would make it a high maintenence application.
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Old 01-03-2010, 01:21   #9
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A central hydraulic system is the prefered method of powering winches, steering etc on trawlers and warships?, but those vessels have large engines which are always running, and of course hydraulic power cannot be stored.

Electro hyd still requires an electrical supply to the hyd motor at point of use so you have a two technology cost, plus a conversion inefficiency, so I would rule that out
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Old 01-03-2010, 02:34   #10
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Quote:
Originally Posted by Chief Engineer View Post
We had air starts on Tug Engines.....They required a lot of tanks, big compressor and big genset.....

The idea of a pressurized holding tank is too scary to even think about.
I remember those from my merchant navy days, big tanks 30 feet long, 7 feet tall (2 off) pressurized at 30 bars; scary. Compressors 150 kW each; you needed two of four auxiliary engines online just to start the air compressor and were constantly lapping valves. I can also vividly remember when we had a fire when our topping-up compressor decided it was time to become a diesel engine and blew itself up.

Even with todays 'plastic' tanks (at 300 bars!!) not something I would like on my boat; OK for some applications (SCUBA) but to be avoided if possible. I will take batteries any time to store energy the way we need it on yachts.
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Old 01-03-2010, 19:49   #11
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I was engineer on an 85 footer (sail) that had a central hydraulic unit for everything from inmast furling, winches and bow thruster....I was a Ron Holland Design Maxi-Hull

I found the winches to be curiously dangerous...the switches would stick....or they would energize when the sun hit them....it was some kind of air pressure switch that would operate the solenoid valve....
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Old 01-03-2010, 19:58   #12
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There's a french guy that may have already solved the air powered propulsion problem.......maybe the windlass too.
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Old 02-03-2010, 01:23   #13
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All this French guy has done is put a 300 bar tank (that's a mere 3400 PSI) in a car and develop an air engine (steam engines exist a couple of 100 years and are nothing new). It doesn't tell how he will get his tank energized when empty; to do this in a reasonable time you need a lot of power (probably either nuclear or made by burning something) and a big compressor. We have to remember that when converting energy from one form to another you always loose some. If someone would invent a solar panel giving instant 300 bars then that would be a nice development. The total efficiency of compressing air and expanding it in engines to produce mechanical energy is probably less than 25%; better to use electric motors which can have efficiencies in the 90 percent range.
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