My friend Krzysztof has sent me details of the compressed air driven generator he has been experimenting with. It's currently in Devon so he can't get pics of his actual kit at present, and has in the meantime sent me the following eBay links of the equipment he purchased.
That is quite big stuff. The turbine is fit for over 5 hp at 2500rpm. I read that it runs on lubricated air and I wonder what is involved in that. What sort of air supply it uses and and how big it is and from what source? I would love to hear it do its thing. Where will the set up be? c
the compressed air will be provided by a small compressor powered from a PV panel.
The idea here (I think) is to come up with a means of power storage which could be used as a dump load for domestic PV.
I did some simple sums - A standard 3kg cooking gas cylinder could hold 200wh of energy if used to store compressed air. Whilst this doesn't sound like much, it could run a 600w electric chainsaw for 20 mins, or a 200w motor for an hour, or a 25w bank of LED lights for 8 hours (enough for the main rooms in a house), and so on. With a storage efficiency of about 40% it's not great, but if the heat given off when compressing and the chilling effect (or expanding gas) when using the airpower can be employed somehow, then perhaps the efficiency rate can be improved.
Sorry to be tedious and dull but would batteries not be a better bet? With the heat losses and the inherently poor harvest from PV this looks like it would be doing well to usefully store 7% of the suns energy. On the other hand I may just have made the mistake I am most critical of when others make it. I (we) usually judge these ideas in the wrong context. The 200Whs in Cye's example is nearly nothing if I compare it with the power output of our car (about 60kW) or the amount of energy I would consume if I used an immersion heater to heat water for a bath. However if I adjust my frame of reference to reflect a more sensible attitude towards consumption, 200Whrs of power is quite a lot. It is 2 hours hard labour on my bicycle or all our lighting, laptop and water pumping needs for a day. Really, it is canned luxury! I heard a story from Co-op member that, at the end of a conversation about these matters with a 'Newbie' the individual said "That sort of power wouldn't iron my shirt". The answer (I propose) to this sort of comment is "Yes, that is right, it won't iron your shirt." So, on a nippy day like today, do I light the stove after breakfast or do I pull on the long-johns and light the stove later? It will be the long johns and by the way, they haven't been ironed. c
Thanks for that Cye, It makes sense now. Mind you if I were to be setting up a system like this I would not be running compressed air into a cooking gas cylinder, for a couple of reasons, they are way too small to get any decent volume of air into and a half decent compressor would fill that size in no time.
Any idea what pressure a heating gas cylinder can take my compressor runs at about 140psi.
I'd be really interested to see this setup in operation, just to see how well it works, because it seems to me that you couldn't store enough air to get this setup to produce electric for very long. I'd guess that your probably looking at it consuming air at a rate of at least 10l/s to generate the minimum voltage. At that rate an air receiver of 500l will provide enough air for about 30s before the pressure drops too low to be of use.
Of course I may be way of track with this in both my understanding and calculations. It would be interesting if something like this worked, if even to eliminate all of those chemicals in a battery pack.
Post by Colin Lloyd on Nov 10, 2013 5:55:04 GMT -5
Hi all, Cye has asked me to comment on the air motor/generator and first of all I will refer to the motor itself. This is made by the Gast manufacturing Company in USA and is a well respected and hard wearing unit made from cast iron. It will have carbon fibre blades( maybe cast iron) inside and is a lubricated unit ie a small amount of vapourised oil should be constantly introduced to prevent excessive wear, especially in the blades and rotor end faces. The oil also acts as a sealant between blades and stator. A lubricator can be purchased to do this but in the meantime I suggest that a fine oil ( Shell Tellus 27 or equivalent) is inserted into the air input port on the motor. ( a teaspoon every two hours of running time will do). Wrong type or insufficient oil will cause " gumming" resulting in the unit slowing down or stopping altogether as the result of blades glueing into their slot. In my experience I recommend that a suitable solvent be poured into the air input and then compressed air applied, this will clean out all old carbon dust and dried oil and the motor will be heard to increase in speed, do this several times until the exhaust runs clear and then lubricate. Now the physics of the unit, which will be quite scary. This motor requires 100psi (7 bar )to run at its optimum requirement and will tolerate pressures to either side but there is a minimum pressure where the resulting speed is too low to allow centrifugal force to "throw" the blades out of their slot and the unit will stall and air escape through the exhaust. As pressure and volume are relative to each other you need enough of both ie you can have a matchbox and a normal room filled to 100 psi but the room will run the motor for longer. The volume of air required at 100 psi is where it gets scary. This unit can run at up to 2500rpm and using air at 100 psi it will require approx. 170 scfm (standard cubic feet per minute) @ 500 rpm it will use 60 scfm. If you take the output of a compressor in fad (free air delivered) the maximum of of a rotary screw or vane compressor is 4 scfm and a normal reciprocating garage compressor is 3 scfm, or less per horsepower, then you would need a compressor with a motor of about 55Hp 42KW approx. to power this air motorti its maximum. A normal home compressor with a 50 litre tank is about 2 or 3 HP and this is why the motor emptied the receivor in a few seconds. The continuous output from a garage compressor will be insufficient to run the unit continuously as once the receivor is exhausted the pressure will fall to a level where insufficinet is required to turn the generator or throw the blades and the unit will stall. You could of course put a normally open solenoid valve on the outlet of the compressor (wired into the pressure swith on the compressor) that opens when the pressure switch registers maximum pressure and switches the compressor motor off and the solenoid will close when the the pressure switch registers minimum pressure and brings the compressor back on. The upper pressure on the compressor should be set to maximum recommened ( usually 140 psi) to have as great a reserve as possible and the outlet on the compressor pressure regulator set to minimum the air motor will tolerate (approx 30 psi). This will give maximum running time for the air motor as long as the demands of the generator are met ie HP and rpm. I will leave it to the forum to make conclusions.
Hi Colin, Thanks for that, although its a tough subject to get your head around you have helped with some of the info.
As I understand what your saying, you will be able to run the electric generator at an acceptable rate as long as the air turbine is going at least 500rpm which will require 60cfm, which I think is about 30 l/s give or take. A compressor to make this sort of air on an ongoing basis would need to be pretty beefy.
The thing is, as I understand it, the air is to be produced ahead of time and run through a compressor to create a reservoir of air that can be used at a later date, which is why I thought that a large air receiver would be appropriate.
If you used a bigish compressor with a 50l tank on it, then you will only have a certain amount of air available to run the kit, so using the numbers above I calculate
(P1)(V1) = (P2)(V2) (140)(50) = (80)(v2) assuming 80 psi is its minimum operating pressure 7000/80 = v2 v2 = 87.5l Available air would be 37.5 l
This means that it will run at this speed for a second or 2 before the air supply is exhausted. This seems very low to me but from from personal experience our 50l compressor runs every couple of mins and our cfm is less than the 60cfm of this thing.
I guess what I'm saying is that either this isn't working the way I think it is or my maths is messed up. :-)
I passed on Colin's advice and received the following response from Krzysztof............
Yes he [Colin] is right this motor is beast and need lubricated air supply I have lubricator included on generator. I am planning to design 1kW array just to power compressor and filling up 500L tank during a day and use generator to boost batteries at night when voltage drop, all the control i have done like he said by air pressure.
Big tanks and air flow required for motor are scary hehe. I even think about bigger tank add dig it underground in garden. It can be decomissioned calor gas tank. But I have no idea where to get one in belfast.
Unfortunately look like my generator is in Poland now i can get it back in january i did not realise i left it in poland until I look in my garage in Devon.