Well let's see:
Energy @ 40,000uF 20v= 0.5*C*V^2=8J
Energy @ 7.5V=1.125J (this is where a stepdown switcher would fail to 
produce 7.5V anymore).

delta-energy: 6.875J

Energy needed= W*sec= 2A * 7.5v * 1 sec= 15J

Not nearly enough.  We have not considered switching losses or the 
capacitor's ESR either.  2A is not a problem for many 40,000uF caps but 
do not simply assume the ESR is low enough.

When reviewing the design idea, note that a cap of twice the voltage is 
4x the energy.

If this is a DC motor load, it should be possible to PWM a higher 
voltage cap to produce the desired speed without an additional inductor 
or cap or anything.

Battery can be more complicated than you think.  A NiMH does not like to 
be "floated" with a constant voltage OR constant current and 
standard/simple state-of-charge detection schemes don't work for 
floating NiMHs.  You could do a lead-acid batt but it will be larger and 
have a somewhat limited life.  Actually a lithium CAN be floated at 
constant voltage, and have long life, if you simply select the float 
voltage that leaves it at only 70% charge or so.  Full charge and it may 
overcharge if your float voltage is not very accurate, and the lifespan 
is far lower when sitting on the shelf at 100% SOC than 70% (total 
opposite of lead acid).

Danny

Enki wrote:
> 	Your design is interesting. What is the generator used?
>
> 	I have to power a load that draws 2A @ 7,5V during one second each 
> 15 seconds. So I think a 40,000uF capacitor charged to 20V powering a 
> switcher would do the job.
> 	The charger circuit (air turbine generator) would need to charge the 
> capacitor to 20V in 14 seconds. The air pressure available is ~10BAR.
> 	I could use a battery instead of capacitors.
>
> 	Mark Jordan
>
>
> On 4 Nov 2008 at 5:59, plpeter wrote:
>
>   
>> I wrote:
>>     
>>>> Would my design help ? :
>>>> http://www.geocities.com/plpeter2006/piclist-airgen.html
>>>>         
>> make that 8W @ 9V
>>
>>     
>>> The claimed 20% efficiency sounds a bit optimistic.
>>> E&OE / what have I missed?
>>>       
>> The "measurements" were done very approximately (l/sec estimated
>> from the
>> pressure drop/time on a 25l air compressor tank before pressure
>> regulator),
>> and generously rounded up by me (I don't like to lie to myself, it
>> tends to
>> come back and haunt one).
>>
>>     
>>> I like the self dismantlement avoiding shunt regulator.
>>>       
>> Sufficiently optimally "designed" turbines that do not have one
>> "live"
>> through some of exactly one load dump.
>>
>> I think that the design is good and that it should be redone with a
>> carbon
>> wheel and a different generator to take the 60,000 rpm where it
>> would be
>> optimal (maybe 4 times better than now). I guess reaching 50%
>> over-all
>> efficiency can be tried (0.7 for the turbine and 0.7 for the
>> generator). The
>> shown wheel is mild steel sheet ca. 0.4mm thick and too hard to bend
>> by
>> hand, and will start to visibly bend outwards after 30,000 rpm.
>>
>> Peter
>>
>> -- 
>> View this message in context:
>> http://www.nabble.com/Compressed-air-electric-generator--tp20293599p
>> 20322786.html
>> Sent from the PIC - [EE] mailing list archive at Nabble.com.
>>
>> -- 
>> http://www.piclist.com PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>     
>
>
>   

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist