[Power] Haiti power questions

[Richard A. Smith]

On 12/02/2010 05:25 AM, scott at solarnetone.org wrote:

>> Updated Chart.
>>
>> XO	 AC
>> --	 --
>> 4  W	  6   W
>> 5  W	  7.5 W
>> 6  W      9   W
>> 7  W     10   W
>> 8  W	 11   W
>> 9  W	 12.4 W
>> 17 W     23.5 W
>
> Here we area showing a waste to heat of 30-35% of our power in a
> conversion step not always necessary.

>> Thats the eff% of the AC inverter.
>> Typically those are 90% or more.  That means that you should take all
>> the numbers above and divide them by .9 if you are on solar.  Thats the
>> juice that your batteries will have to provide.
>
> I disagree with this part.  There is no need to run the XO load through an
> AC inverter, and incur the additional waste therein, on top of the wall
> wart conversion waste.  We have no real utility for switching to
> alternating current in this application, then back down to DC.  The XO's
> have regulated power supplies.  Voltage swings will not effect them so
> much, so we only need a charge controller and battery between the XO and
> the solar panels, assuming the voltage range is correct... nominal 12V
> panels.

I agree that its totally possible for you to setup a DC only system and 
that under various conditions that will yeild a more efficient setup.

The technical and logistical challenges that one faces with a DC only 
system however put it out of the reach to most deployments.

At the top of the list is the availability of a cable that can plug 
directly up to the XO.  The DC jack on the XO is difficult to find in 
most non-Asian markets and digikey or mouser don't have a good match. 
Several flavours of the Acer Aspire and Classmate also use the same plug 
so hopefully this will change in the future but for now that plug is 
really hard to get.

The cabling requirements are also very significant.  Consider Tim's 50 
XO's in a class setup.  Thats 850W peak (17W * 50) so for a 12V system 
you are going to be dealing with currents up in the 70A range.  To 
distribute that effectively you need pretty beefy cable and very good 
connections at your junctions. Copper is quite expensive these days and 
if you have any sort of high resistance hot spot in the wiring your are 
certainly going to melt some wires.

The XO-1 allows for you to increase the voltage up to near 18V. That 
would help decrease the amount of current on the wires but now you have 
to have an extra DC/DC converter with an adjustable output to get the 
voltage up that high.

I know it really sucks to have to invert up to 120 or 240 Vac just to 
convert it back down to 12Vdc.

I'd love to see some computations and tables that try to show where the 
breaking point for AC vs DC distribution are based on efficiency and 
cost.  With some follow up real world experiments on how well it 
actually works.

The AWG claims that to deal with a 70A load you need 4 guage unless you 
have high temp insulation. A quick google search shows a low price (US 
price, usually much higher in the rest of the world) of $.60/foot.  8 
guage is only $.20/ft.  Since the 70A load will only show up near the 
top of the distribution chain can you get away with 8 or even 10 guage 
for most of the system and only have 4 guage where its really needed? 
Its non-trivial for the non-technical to design that system though.

To wire up a area of 50 kids how much wire do you need?  Whats the I2R 
losses associated with distribution?  At some cable length even with the 
conversion losses AC becomes more efficient than DC unless you have 
_really_ thick cable. [1]

> One could always correct the power factor to the load with a
> tuned capacitor bank.

With an SMPS the input draw is non-linear so you can't correct it with a 
simple capacitor bank.

http://en.wikipedia.org/wiki/Switched-mode_power_supply#Power_factor

[1] Interestingly enough at super high voltage DC distribution now seems 
to be winning due to our ability to deal with it via high voltage 
semi-conductors.

http://en.wikipedia.org/wiki/High-voltage_direct_current

-- 
Richard A. Smith  <richard at laptop.org>
One Laptop per Child