[Power] Power Digest, Vol 13, Issue 3

[Nicholas]

George wrote:

The reason for this request:

The XS, running on the XO, really only needs to be running during the 8-10
hours of the school day. A 40 watt-hour daily load seems more solar power
friendly.

George

My reply:

Sorry, I don't know anything about RTC. Like wake-on-lan?

I am a bit of a bogan when it comes to solar and power electronics so
here's my take. After writing this post I realise I've probably
overdone it (having just looked at the XS XO project, looks very
concise and well thought out). However, it might be useful to
someone..

40 watt hours? For a day sounds pretty light. There are always
inefficiencies and it never is as simple as a straight 40w/h
My analog is running a fridge. They are always running but on a duty
cycle. So, when you are doing your calculation you factor amount of
power it takes, there is the duty cycle which varies, and the amount
of times you open the fridge and let out the cold air. Then you have
to be able to recharge the batteries plus the amount of electricity
you used to get to a net zero situation or full battery status. With a
fridge there is also the ambient temperature to consider.

So, the variable bit (duty cycle) for a server might be, how many
computers are connected. How much data throughput? It's not going to
be a static amount. The times you need to keep the computer running
the most are also going to be the times you need to provide the most
electricity. A fridge is always running and in summer it needs the
most electricity (duty cycle ramps up) but fortunately it also
coincides with the sun for solar. The last part--recharging the
batteries and being able to run the computer might not be a huge
factor like a Sawafuji swing motor fridge (25-35w) but it might be
enough that you can't fully charge your battery after a lot of use
gradually running the battery down. Everyone forgets that last bit,
recharging capacity needs to be the static + variable load +
recharging + worst case scenario.

This is just an example but for instance worst case scenario is 2
hours of sunlight per day in Winter. You need to be able to recharge,
run the computer, and grab your complete allotted amount of sun in 2
hours. Lets say 60w/h is what you need to run your computer on a busy
day, and factor another 20w/h for inefficiencies in the system. You
will need to get put 40w into your battery in an hour. That means you
need at least a 60w panel, possibly an 80w panel (because the wattage
is calculated at the peak power point and you aren't going to be able
to harness _most_ of that into 12v without a Maximum Power Point
Tracking converter/regulator. The next thing that becomes obvious is
you aren't going to be able to use the built in battery for this and
then the expense and effort ramps up significantly..

Also like a fridge your really don't want your server going kaput. So,
you will want some extra capacity for the redundancy factor too
(external hard drive?)? It all adds up and the only way to try this
out is in a real world situation with perhaps a data logger such as
the watt's up series of power meters made for RC electronics for ease
of use or use the program Richard wrote to do it properly :D

The best way to do this stuff is to do it! You are going to need an
external battery of some sort. My guess, 10-20aH if it is a 12v
battery and perhaps 60-80w of solar. I like the Tenergy Premium D NiMH
cells on ebay. 20x for US134.99 + shipping or 50x for US319.99 +
shipping. Waay better than any lead acid for cycle-life, handling,
toxicity.. no need for fancy cell balancing with LiFePo4. Solar is
being dumped on the market right now and what seems to be below
manufacturing costs, 20w can be a matter of 20-30$ these days..

Hth, Nicholas