[Power] Haiti power questions

[Richard A. Smith]

On 12/02/2010 02:51 PM, scott at solarnetone.org wrote:

>> 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.
>
> Under most conditions where everything you need to power is under
> one roof, DC wins hands down.

Indeed since you are removing 2 conversions.  That's a big win.

>> The technical and logistical challenges that one faces with a DC
>> only system however put it out of the reach to most deployments.
>
> Well, I guess its a good thing we have solved all of that in an
> off-grid open source system that would require only minor
> modifications to be appliend in this situation.

Sounds like Tim Falconer should talk to you.  He's searching for some
sort of template setup for his Haiti trips.  Perhaps you have something 
he can use.

>> use the same plug so hopefully this will change in the future but
>> for now that plug is really hard to get.
>
> This should not be a rate limiter, I think, since the distribution
> chain must pass through the XO community anyway.  Is it not possible
> to source these on a bulk scale from the vendor who supplies them for
> the XO?

Until recently it was.  We now have a spare parts item for a bare cable 
but that's a very new item and for it to be available to people like Tim 
in quantity 50 will require someone like ilovemyxo or someone else to 
bulk order a bunch and piece them out.

I have a drawing for a compatible cable and the DC jack specification is 
here: http://wiki.laptop.org/go/File:DCJack.pdf

My cable drawing needs a slight tweak to extend the barrel length. 
Sourcing from the vendor that provides to quanta is not necessary.  Any 
cable maker can make this its just a question of ordering the minimum 
qty.  If you want he drawing for the cable let me know.

>> 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.
>
> It works a little different than that.  The role of the charge
> controller is to protect the batteries.  You will find most 12V
> panels will actually give you 17V or so.  My charge controller, as I
> look at its display now, is pushing 14.6V into my batteries and load,
> to prevent the electrolyte in the battery from boiling away.  During
> the day, you will commonly see these higher voltages.

What about when you have to run off of the batteries? LA batteries don't 
have an output voltage that high.  Also the XO input is only good down 
to 11.0V (10.8V officially but I say 11V due to component tolerance 
variations).  So IR drop across the length of the network needs be kept 
to a minimum.

>> I know it really sucks to have to invert up to 120 or 240 Vac just
>> to convert it back down to 12Vdc.
>
> You waste almost half the power produced this way.
>
>> 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.
>
> It actually works pretty well.  I am able to run a dual core server,
> 5 geode based thin clients w/ 15" LCD displays (tho I would love the
> PixelQi displays), a hub, and a SIP phone on about 100W in the
> field.

Sorry. I meant with the specified load of 50 XO's.  Scaling by almost a 
factor of 10 has a way of revealing weak areas.  But sounds like you 
have a great starting point to to create a testbed.

>> 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.
>
> I use twin #4AWG runs (two #4's for + and two #4's for ground) from
> the combiner box for my array (where all the panels get paralleled)
> to the main breaker box, where the batteries, charge controller, and
> loads also connect.  The dual #4 runs only have to travel about 10
> feet or so. From there, #8 or #10 out to busses should work... figure
> 10 XO's per bus. The same AWG you use between the wall wart
> transformer and the XO in an AC setup should suffice between the bus
> and the XO... 14-16 at most.

I thought your system was only 100W? Twin #4s suggests much more juice 
or are you just allowing for scaling?

> I use #14 to power the thin clients and displays on runs of greater
> than 50 feet without issue.  At one point, I was using the unused
> pairs in CAT6 for this purpose... 1,2,3 and 6 were my network signal,
> as normal, and 4,5,7 and 8 carried the current.  I never had a
> problem with this arrangement.

The wiring charts are pretty conservative.  As the official power 
spokesperson for OLPC I have to also be conservative and go with the 
accepted values.  Especially with children involved.  Some areas have 
electrical codes even though they may not have an extensive grid.

> Correct, and once again, already done on the design;)

Is the design and BOM available somewhere to look at?

>> 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]
>
> In this case, you have to go a very long way on a very small cable to
> lose half your energy.
>

I don't agree its that simple. Its load dependent and goes up with the 
square of the current.  The same system that makes sense at 300W may not 
make sense at 3kW.

>>> 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.
>
> There are circuits run by a microcontroller that monitor power
> factor on load and parallel different cap values in or out as
> necessary... real time PF correction.

Sounds nifty, cool and expensive.  Any idea on how that compares to to 
the cost of over-sizing the generator?  Over-sizing generators can be 
tricky if they are diesel because of the need to run them periodically 
at full load to prevent carbon buildup.

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