Monday, July 27, 2009

Power to the masses(of XOs)

Date: 10:15 AM, July 26th 2009

Venue: Our quarters, Sahn Malen

We knew it wouldn't be easy deploying 130 laptops(we integrated the 30 deployed in December into this deployment too), a server and three wireless access points in an off-grid village like Sahn Malen. The only electricity here is supplied by small, privately owned generators(I've seen only two in my time here) and for most people the only source of power is their battery pack in their flashlight. But the team took the challenge head-on and it's been rewarding to get things going the way they are.

Since gasoline is pretty expensive in Sierra Leone, we decided to make our power setup solar-based. OLPC gave the Corps teams the option of buying small 10W/12-14V solar panels for 25$(bulk bought from GP Solar to reduce price I assume). These are convenient because they're mobile, durable and most importantly, can plug in directly to XOs without further engineering. Apparently, with good sunlight, each panel provides enough power to keep an XOs running without charging and to charge an XO in 2-3 hours if it is turned off.

We were only able to buy 90 initially because of the limited number in stock but 30 more became available in early July as the team in Senegal didn't want theirs any more. So in all we have 120, enough to take care of the XOs(15 of the 130 users will have to share them with their siblings/parents).

That only left us with the problem of powering the server and the APs. We didn't want to buy expensive, commercially available panels to run the server and APs and so hacked our very own solar powered system into place like this:

1. With some nifty splicing, took the pins off four solar panel wire ends. A polarity test with an ammeter showed that the inside of the pin is positive and the outside negative.

2. Hooked the four panels to a regular 12V car battery through a solar charge controller. The panels are to be connected in series so that the input voltage remains the same(12) but the current increases. This charge controller is central to the whole system as it prevents the battery from deep-discharging or overcharging(and consequently exploding). They are rated in ammeter-hours but I'm not sure if that is the variable the controller uses to decide when to cut the power off. The user manual implies that it does so by voltage and that if the voltage falls below 11.4, it cuts power off. I got a 10 Ah charge controller and it's working fine for our setup, but if anyone has insight into how these things work, a comment or two would be appreciated.

3. The charge controller takes three pairs of input wires: one from the solar panels, another from the battery and one is the output. To this output, we connected a commercially available charge inverter(usually used for car sound systems). This would convert the 12V DC current to 110V AC. And finally, to this we connect a power strip which can power a server and an access point.

We found out that with the AP connected,the battery was running out too fast if there wasn't enough sunlight. I figured this might be because the DD-WRT firmware that we flashed these routers with automatically ups the throughput to 70 mW. Since the program takes place in a single school building, I decreased this to 35 mW on all the routers. That seems to have solved the problem.

This solar-based system should work fine for most of the dry season when there is ample sunlight but during the rainy season(like now), we needed backup, to make sure that the children are able to charge their XOs at least 2 hours every day. For this we bought a backup 2.7 kVA Honda generator and worked out a scheme with Alfred(described in previous posts) so that he would provide an average 2 gallons/week for the generator to be run. This, plus 30m of good copper wire, lots of PVC tape, pliers, screwdrivers, 9 power strips and an eager team of students and local leads and we have a solid setup going in Sahn Malen.

It really is rewarding to see the burst of activity in the village centered around the XOs. Just now as I write, a group of kids in the verandah nextdoors are standing in front of an XO, recording a song that Clem taught them(boom boom chang chang boom chang :D). Another kid is sitting next to me, looking up the Chelsea FC entry on Wikipedia: Amazing things to happen in an off-grid village on an average Sunday morning, wouldn't you say?

Stay safe,


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