[Stoves] Powering a TLUD Fan

[ajheggie at gmail.com]

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), rongretlarson at comcast.net
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.