[Power] Haiti power questions

[Richard A. Smith]

On 12/03/2010 02:12 AM, scott at solarnetone.org wrote:

>> 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.
>
> How many hours of continuous battery usage do you project being necessary?
> If your battery and solar arrays are sized properly, you should have no
> low voltage issues.  We designed the SN-1 power system so that we could
> provide a week of network uptime in the event the panels went missing,
> blew off the roof, hit by a meteor, that type of thing... 0 input in other
> words.  If the thin client terminals are not up, there is close to a month
> of backup battery for the server only... long enough to get help almost
> anywhere.

My question wasn't clear.  I was discussing the high currents involved 
if the bus voltage is 12V and you mentioned that because you drive your 
bus at the solar output voltage that often your bus voltage is higher 
than 12V.  My statement about battery voltage was that if are going to 
run the distribution bus at a higher voltage to reduce the necessary 
current then you have to have some sort of DC/DC converter between the 
bus and the batteries because the operational voltage of a 12V LA system 
is going to be in the 12.8 to 10.5 V range.

Assuming you are only going to 50% DoD I'm guessing your minimum battery 
voltage is about 11.5 to 11.6V.  The XO input starts to roll off at 
approx 11V.  So while on battery only the end to end voltage drop can 
only be .5 or .6 V.  That or the system has to be designed to operate at 
a much higher %DoD.

Thats not hard to accomplish but it does involve additional care at the 
high current junctions.

For the amount of continuous battery use necessary I usually ask the 
deployment how many days they think they would need to go if there was 
no sun.  They usually don't know and so I suggest 1 day.  Meaning that 
if there was zero solar input from the array that data that there would 
be enough battery capacity to recharge all of the XO's and that it would 
recover on the next day that good sun returned.

I don't actually have a good measurement of the Wh it takes to recharge 
the battery from a DC source but based on the power delivered to the 
battery and the 90% efficiency of the XO's DC/DC converter I'm going to 
estimate it at 27Wh/XO.  So 50 XO's needs 1.35kWh.  Most of the time the 
charging happens with the XOs powerd on so in Tim's case thats another 
1kWh (6W/.9 * 3h * 50 XO).  So 2.35kWh minimum.

>> 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.
>>
>
> Its simply a matter of observing your safety requirements in each segment
> of the power distribution system.

Yes.  However, it does require a level of planning, resources, and 
execution that can be beyond a lot of the people I'm contacted by.  Many 
of them don't have the electrical background to do the necessary site 
calculations and installation rigor that a DC only system would require.

Even the local "experts" sometimes give me a quote for a AC system that 
doesn't have chance of providing the necessary power for 50 or 100 XO's.

> As to cost... to power the same load
> via AC, you have to buy almost twice the size battery and solar arrays AND
> an inverter vs regulated DC off the panels.  So double your initial
> investment, since you are going to have to run some power wiring anyway,
> be it AC or DC that you use to distribute it.  You may pay marginally more
> for the DC gear, as the wires will be a little bigger, but that will be
> far outweighed by savings from the smaller panel and battery arrays.

That's a great observation.  One that I had not fully considered. 
Batteries and panels are expensive. A 30% to 40% reduction there would 
buy a lot of big cable.

>> Is the design and BOM available somewhere to look at?
>>
> take a look at http://www.gnuveau.net/cgi-bin/wiki.cgi
>
> I will be happy to answer any questions as regards specifics or details
> not therein covered.

It looks like most of the BOM info is up on the site but its all in 
different locations.  Do you have some sort of table with all the parts, 
quantity and cost info?

ie if someone wanted to duplicate your setup what would the shopping 
list be?

>> 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.
>
> I agree, its not that simple, but if you have clusters of say 5 or 10
> students around a power distribution point, you are moving 200W max to
> that particular point, and 20W max to each XO, correct?

XO-1:  17W
XO-1.5 25W

> And the latter
> will be a run of what, 5-10 feet max?  Sure, make the trunk of the tree
> coming from the battery bank stout, but neither each branch of the tree,
> nor the twigs which split from those need be of such magnitude for the
> leaves to grow;)

Indeed.  You have convinced me its certainly an option I should be 
considering when thinking about power options for deployments.

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