Cambodian charity - Solar requirements for a Raspberry Pii and router

TukTuk4Children

I run a charity in Cambodia and we are doing an interesting project. The idea is to have internet in a box installed in our schools. This basically means running a local server and a router that people can connect to like a wifi hotspot. On it will be a bunch of educational material for students and teachers to help them either teach or pass their schooling. As the schools don't have power, we would like to install a solar system for each unit. The power needs are 10w per hour and it must run 24hrs a day. There is a lot of sun in Cambodia so I used California as a comparison for sun hours. We will use a 40ah lead acid battery and have calculated a 30w solar panel should be OK. Do I have my calculations right or is there something that I have missed?

Also there is a lot of cheap solar bits and bobs around the local stores. How important is quality and what should I look for. I imagine that a poor quality invertor for instance could use more power than the rest of the setup it is designed to power. What specs should I look out for when buying the equipment.

mcgivor

What voltage is required, using 12VDC with USB adapters will be an option that would eliminate an inverter.

Dave Angelini

Yes as mcgivor said, do you need an inverter? Maybe for powering cell chargers etc? Probably the router needs AC to run?

The other thing is to get the Raspberry off the AC wall wart and use the battery to power the Raspberry.
In other words, if your battery is 12V, buy a 12V to 5V converter for the raspberry. That way only the router power needs to be considered.

I would not use a 30 watt panel for this.  Use  (2) 30 watt panels), 50 or 100W  panels if the budget can be stretched.  Go higher to enable charging in tropical weather.

Most of California is not like Cambodia in the afternoon
Good Luck

Estragon

10w/hr is 240 watt-hour/day. A 40ah (assuming 12v) battery would be 40x12=480wh, so would be at ~50% state of charge after 1 rainy day. 50%SOC is about as low as you would ever want to go for decent life on a deep-cycle lead acid battery. A small powersports type battery may not handle even 50% well. On the second rainy day, the battery would likely be dead and may suffer some permanent loss of capacity. It may be worth considering lithium for this application, as they can be drawn more deeply than lead-acid, and handle partial state of charge better.

Assuming the next day was sunny, and you got 4 hours of full-sun equivalent, a 30w panel might produce ~30w x 75% (operating temp output) x 4hrs = 90wh. This isn't enough to handle a 240wh daily load. To recharge the battery from 50%SOC, plus handle a 10w load for say 8hrs, 240wh ÷85% battery charging efficiency = 282wh + 8hrs x 10w = roughly 360wh. A 125w panel x 75% x 4hrs would be 375wh.

Looking at insolation data (pvwatts.nrel.gov) for Phnom Pen, full-sun equivalent seems to range from ~3.5 in rainy season to ~6.5 in dry season. Your climate may be different, so you might want to check the data for somewhere close to your actual location. If your rainy season tends to have long runs of gloomy days, either a much larger battery/solar system would be needed, or a secondary charging means (eg small generator).

I think some models of rPi can be set up to be both router and wi-fi hotspot. If so, you may be able to reduce the 10w load. As Mcgivor notes, running USB directly from 12v source is more efficient than AC inverter.

jonr

A key feature to battery life is a low voltage cutoff.   Better for the system to be off than ruining the battery.   

For power draw, trust measurements - not published efficiency or draw specs.

I'm a bit curious about the use of Internet in a school at 2AM.

TukTuk4Children

Thanks all for your comments.
@mcgivor Yes good point, I will source this for both the Raspberry Pii and Router.

@jonr not many people out and about at 2am for sure, what we are worried about is constant shutdown and power up procedures that could cause loading errors. As the units will be remote, we want to ensure that they continue to operate. The Raspberry Pii is only 1.5w/hrs and the router (using 240v and not including the invertor) is 7.5 w/hrs. Therefore it probably makes sense to have this turn off at nights from say 9pm to 6am.

@Dave Angelini The router is 240 to 9v DC adapter so you are correct, maybe it is best to get the 12v to 5v convertor for the raspberry Pii and 12 to 9v for the router, That should save a bit power usage.

@Estragon Thanks for all the detailed information. Our budget can go to 100w panel and 60a/hr battery. If we look at turning the router off between 9pm and 6am plus we remove the invertor then we are looking at approximately 150w/hrs a day. It seems that this system should protect the battery and allow for a rainy day or 2. Your right that the Raspberry Pii does have an internal wi-fi hotspot however it is not powerful enough for the amount of clients we have in the school.

westbranch

or possibly the BBB but it may be too much processor for your needs

https://www.google.com/search?q=beagle+bone+black+features&ie=utf-8&oe=utf-8&client=firefox-b

jonr

On ebay you will find lots of 12V->9V and 12V->5V DC-DC converters.