[Power] Haiti power questions

[scott at solarnetone.org]

Hi Richard,

On Thu, 2 Dec 2010, Richard A. Smith wrote:

> 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.

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

>
> 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.


>
> 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.

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?


>
> 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.
>

Agreed, but... see below.

> 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.

>
> 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.

>
> 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.