First to report that Frans Peeters has tried a small 1 W laptop fan and it failed to achieve any pressure which is a disappointment, he points out that vacuum cleaners give better pressure performance but they are thousands of times too big. To achieve pressure with a centrifugal (snail) fan needs high rpm and diameter, volume comes with the depth of the rotor. Centrifugal fans for moving grain are large diameter and narrow so perhaps we need to stick with the small PC axial fan motor and shaft and put a purpose built vane on it.
The main point of my previous post was to point out the apparent disparity between the power consumed by the fan compared with the power theoretically necessary to move the required amount of air and look for suggestions for improvement. On Sat, 1 Sep 2012 16:55:43 +0000 (UTC), [email protected] wrote: >Andrew and list: > >This is to ask for your thoughts on a different approach to air control than a >fan (with or without a battery). That is to use a (pre-pumped) air storage >"bladder" . >My rationale for this query includes: >1. energy storage via air (CAES) has quite a few proponents. Large vehicles >have operated on air storage (where electricity was a hazard) and there are >companies building cars based on air storage.. Yes and in rigid tanks air storage is maintained for many years without loss. There is a caveat though and that is that when you expand the air as it does work it has to leave the "engine" it's doing work on at the same temperature as before it was compressed, this means adding heat, otherwise you lose all the energy that went into heating up the gas as you stored it. Overall I'm still of the opinion that a charger using PV or TEG technology is the most hope because of the benefits of mass production, no moving parts and simplicity offered. >2. compressed air supply is common in many places - as air-motors are lighter >weight and cheaper than electric. Yes but does anyone know of their efficiency, from Frans' comments it seems that small fans are not efficient. I'm wondering about the motors used in dentists' drills, has anyone ever seen one apart? >3. What is needed in TLUDs is controllable primary air - not electricity. In >Josh Kearn's bicycle-flywheel-fan proposal, one can compress air perhaps as >easily as putting energy into a flywheel (using inner tubes or similar) and >then conversion to a fan/blower. That is - one is storing exactly what you >want to supply - no energy conversions needed. You compress air adiabatically, so the pump gets hot, this heat is lost as the air enters the tank, in fact it is best to keep the tank as cool as possible because as it gets warm the pressure needed to pump into the tank increases. Over time as the tank cools pressure is lost, this is why it is necessary to add heat before the stored air is used to do work. Similarly in the case of a rubber balloon the pressure-volume characteristic is different from a rigid tank and energy is expended in stretching the fabric, not all of this energy is given up as the balloon deflates. I do not think bicycle tyre pumps are very efficient compared with a metal piston with compression rings and as you go for higher pressures with reciprocation pumps it is necessary to have stages with cooling in between. A wind turbine powered air pump?? >4. I think there are controllable constant volume-flow valves - that would >also allow controlling power levels (turn down ratios). Yes but what throttling losses do they incur? It's probably much like using resistors to control current as opposed to quickly switching the full voltage to control current with less I2R losses. The thing about storing compressed gas is a bit like storing electricity in super capacitors, which Crispin suggests, as the reserve empties the potential of the remaining store reduces, this makes control difficult, but doable with modern electronics but with compressed air engines??. With an electric cell the energy stored for a large part of the discharge is at a fairly constant voltage, I see Andrew Parker points out a way around this by pumping a separate bellows which raises a weight to maintain constant pressure. This is much how town gas was stored before we went to high pressure mains and natural gas. I think the natural gas supply to my house is about 21 millibar which is around 7 inches of water gauge so Paul's requirement does seem modest. >5. Your use below of the term 1 cubic meter per 1 kg biomass sounds >potentially doable with moderate size "bladders". I have no idea how many >atmospheres of pressure is reasonable, nor what material (if any) might be >appropriate with simple TLUDs. The issue only is whether air storage might be >cheaper than other approaches. As Crispin pointed out, the low pressure above >the fuel supply reduces the needed pressure (but not the amount of needed >air). >6. The fact that the air will be heated going into the "bladder" and cooled >coming out might not be a problem is the air is preheated by flowing downward >through an outer air-tight "jacket" surrounding the fuel chamber. Okay I see you have the point of adiabatic compression and isothermal storage on board, yes you would need to preheat the gas before it is needed to do work, some sort of coiled skirt around the pot may do. >7. My gut feeling is that this will only work economically if a second use can >be found for the "bladder". Solar plus rechargeable batteries can be pretty >cheap when you are also getting lighting, radio, cell-phone-recharging, etc. Which is why I prefer this route, small scale battery power has much more utility for lighting and IT. With many devices now being charged and powered by USB I would plump for this as being the standard to aim for. _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://www.bioenergylists.org/
