[Power] Haiti power questions

[scott at solarnetone.org]

Hi Richard,

On Sat, 4 Dec 2010, Richard A. Smith wrote:

> 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.

I understand.  I doubt I would go stepping up if I could help it... there 
are just more components to buy and watch fail in that.  The wiring should 
be designed to withstand the maximum load at the lowest functionalable 
voltage across your batteries; that also being the Low Voltage Disconnect 
point of your charge or load controller.  Your load will be larger than 
you think... once you put a power source in the field, I'm sure you will 
agree the locals will tend to want to hook more things to it, often in an 
inefficient manner.

>
> Assuming you are only going to 50% DoD I'm guessing your minimum battery
> voltage is about 11.5 to 11.6V.

12.2 is the lowest I have seen my battery array drop in 5 years of 
operation.  That was during a weak but especially large and slow 
moving tropical storm a couple of years ago.

> 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.

If you have half a volt of loss in your line, your line is too small, 
imho. If, under normal seasonal and weather pattern related fluctuations 
in available photons, your battery voltage reaches 11V, either a)your 
solar array is too small b) your battery bank is too small, or c) someone 
has added a significant load to the system outside of the design 
consideration to sizing your power system.


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

I tend to use gold plated bus bars with clear lexan covers and power 
cables with individually tinned strands to prevent as much corrosion as 
possible.  Its wise to vaseline the battery connections as well.


> 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.

OK.  With one day worth of battery, able to be recharged in another single 
day, you will need a little larger than twice your load in watts from your 
panels.  On days with good sun and full batteries, you are going to waste 
half the powoer you make, unless you dump it off to something else.  In 
most cases, you are going to have your highest output from your solar 
array at the time period when the load is largest also. In that case, the 
oversized array is not going to help you much.  A slightly oversized array 
is a good idea, but most of the time your battery bank should be able to 
safely absorb most anything your load does not use from the panels. 
Also, a larger battery bank will maintain a higher voltage longer through 
the discharge process, of course.  I would go for bare minimums of 1.5 
times your projected load in panels, and 3 days of backup battery.  That 
should give you a very stable system that has some 9's in its uptime as 
defined by V >= 12.4 or so.

I would imagine that you would have some equipment that would need 24/7 
operation, such as a wireless access point/mesh node and the link to the 
public internet, but that load is likely really minimal when considering 
the maximum load of all the XO's charging at once.


>
> 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.

XO charge time is 2 hours from a dead battery?  I ask because that will 
effect the maximum load calculations when considering wire size.  1.35kWh 
over a period of 1 hour is much different than over a period of 3h.

You are only expecting 3 hours of use per day?  That changes things a bit 
too.  I was considering 6 hours when doing my mental calculations as to 
array and battery sizes.

>
>>> 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.

I usually ask for building blueprints or at least scale sketches when 
planning out SN-1 installations, and ship with all that predefined and as 
prea-assembled as possible.

>
> 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.

I'm sure that happens more often that anyone is aware of.

>
>> 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.

Thank you.  It is honestly the main factor in making the SN-1 an 
economically feasible solution.  Had we used inverters and standard 
network configurations running on AC, the expense would have been far too 
cumbersome.

>
>>> 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?

Well, understand that the power system is designed for a specific sized 
load.  As it scales either way, the parts list changes.  That being said, 
the load it is made to run is very similar to the 50 XO load we are 
discussing here, with the exception of the discharge cycle.  2.4kWh/day 
either way... I spread it out over 24h, while you need it in a 3h span.
That dictates a few changes to accomodate a slightly different load 
profile.  That being said, I will put together a parts list for an exact 
replication of the power system for you, and a list of parts slightly 
better tuned to the 50 XO load.

>
>>> 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.

Consider that you are doing more than installing power systems for XO 
deployments, as a good power system will likely well outlast the 
particular XO's they are installed to support.  You are installing the 
necessary infrastructure for education;  a clean, stable, renewable energy 
source and wireless connectivity gear to provide internet services.  If 
that is done wisely, it need only be done once every 20 years or so, and 
assuming that the X0 4 has enjoyed further power usage optimizations in 
its ongoing development, then perhaps 75 of them will be running from the 
same system we are speccing out for 50 units.

Cheers,
Scott

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