emekadavid said:
I live in a rural area, in Nigeria, where the power comes rarely and
when it does it lasts for 3 hours of a 9 hour working day. I work with
generators most of the time, with two laptops, one desktop, a tv set
and some handsets which batteries have to be recharged. Can someone
recommend me a cost-effective solution to this energy problem?
Something or an equipment which can store power for close to 9 – 11
hours everyday for the five technological equipment above that I work
with? Someone suggested an inverter here in Nigeria, but the ones in
the market are way too expensive for the needs of these equipments
that do not consume so much wattage, IMHO, though, because I do not
know about engineering. I think setting up a car battery with some
engineers might do the trick, but I do not know much about computer
equipment and do not want to run the risk of damaging my systems.
Can someone suggest a solution? Please take into account that it has
to be used in a rural area and in Africa.
The first part of the process, starts with "load characterization".
One tool for such evaluation, is the Kill-a-Watt meter. These are cheap,
and help you plan for the real loading your appliances are making.
http://www.p3international.com/products/special/P4400/P4400-CE.html
Batteries have an ampere-hour rating. The battery could be 12V
between the terminals or 24V and so on. If you had a 12V 10 amp-hour
battery, it can provide 120 watts for one hour, or 60 watts for two
hours. The product of voltage and amp-hours, gives potential watt-hours.
The watt-hour rating drops, if you make heavy demands of the battery,
so the rating makes some assumptions of how you're discharging the
battery. You may get more power total, from the battery, by drawing
down on it over a longer period of time.
The usage of an "inverter", is half a solution. It allows taking
energy from the battery at 12VDC, and making an appropriate voltage
and frequency for some AC loads. But an "inverter" does not know how
to charge the batteries. A device that handles both battery charging
and inversion from DC to AC, is the "UPS" or uninterruptible power supply.
An inverter is a useful invention when used in your car, because the
car alternator and voltage regulator can recharge the battery later.
Inverters are useful devices, if you've already solved the battery
charging problem. For example, you might already have a solar array
charging the batteries, and then an inverter (with low voltage cutout),
can be used to extract energy.
http://en.wikipedia.org/wiki/Uninterruptible_power_supply
In areas of the world where power is managed via rotating blackouts,
there is usually a cottage industry (local factory) making UPS solutions.
This can result in a lower purchase price to the consumer, as there
will be a local market of product, and competition with overpriced
manufacturers from other parts of the world. So you'd start by
looking in your own neighborhood. I've read of areas in places
like India or Pakistan, where they make their own brands of UPS for
local consumption, and that is to deal with situations of rolling
blackouts.
Now, in terms of battery types, batteries vary in terms of what they
will tolerate. A regular car battery (sulphuric acid electrolyte, lead
plates) cannot withstand deep discharge. You can safely use perhaps one
quarter of the capacity, before it is wise to recharge again. Shallow
discharge cycles, gives more overall charging cycles before the
battery is ruined.
There are marine batteries or batteries with "deep discharge" rating,
where the purchase cost is much higher, but the usable energy and
recharge cycle count may be better than an automotive battery. The
automotive battery is built on the assumption it won't be regularly
run down, and the alternator in the car quickly restores the power
used to turn over that car at startup. Other kinds of batteries
are rated more appropriately for your application, which is
continuous, abusive charge and discharge cycles. (Such batteries
may be used in traction devices, such as electric golf carts,
electric floor cleaners, electric fork lift, and so on.) So while
automotive batteries may be relatively cheap and available, they may
fail after less than a year of usage. This may be acceptable if you're
getting the batteries for free.
Another matter, is explosive gas emission. Automotive batteries, at least
some of them, evolve significant amounts of hydrogen gas at the terminals
when charging. If that gas comes in contact with an ignition source
(spark from a light switch), that can be enough to explode the gas.
My neighbor back home, had such an explosion in his garage, and
battery acid was spewed around the garage. He was not present at
the time, and very lucky. If you are going to use such batteries,
they *must* be located in a well ventilated area. You cannot hoard
your battery collection in your bedroom. Even with an open window,
there is a danger you could be in the middle of an explosion. I have
some personal experience with hydrogen, and would want no part of that.
Another battery type is SLA or sealed lead acid. The electrolyte in that
case, may be a gell, rather than a liquid. Depending on electrode material,
the level of gas emission is lower. That is why they're used in commercial
UPS devices. Such a design can be used in your room.
*******
The following is a ridiculous example, but it's to show what a large unit can do.
The run time is a function of the size of the battery pack, and large battery
packs tend to go with units with large power output capabilities.
This UPS is $9000 and a large add-on external battery (to increase runtime)
costs $17000.
http://www.tripplite.com/en/products/model.cfm?txtModelID=3652
This is a run time chart, with units of minutes.
http://www.tripplite.com/shared/techdoc/Runtime-Chart/SU16000RT4U.pdf
A 9 hour work day would be 540 minutes. The left most column is for
a 1000W load. By purchasing just the $9000 unit, you can draw 1000W
for 95 minutes. You would estimate (but not be accurate), to say you
could get 200W for 5*95=475 minutes. That may be enough to run a couple
of uncharged laptops for that period of time. With regard to your TV,
it depends on what kind of load it represents.
OK, using the same chart, I look at the $17000 additional battery pack.
The $9000 UPS already has batteries inside it, but this external
battery pack adds additional battery capacity.
BP192V1037C-16K
http://www.tripplite.com/en/products/model.cfm?txtSeriesID=752&txtModelID=4085
Using the run time chart, that allows 1000W to be drawn for 958 minutes
or about 15 hours. It would help if you didn't discharge the device
completely, so a bit of overage on time is a good thing. 1000W is
likely more than you need.
For a TV, you should be using an LCD TV, and preferably with a small
screen real estate. That will help keep the TV in the 30-40W range.
A large screen TV would be a mistake, in such a situation where
power is unavailable.
The laptops each have adapter power ratings. Say you had 65W laptop
adapters for example. That sets an upper limit on power usage.
Best case, perhaps your loading is 65+65+40+5+5 or 180W. So now
the $9000 basic UPS package powers that for 475 minutes or about
8 hours.
Now you see why owning the Kill-a-Watt meter is important. It is
part of your run time calculations, and sizing the unit. If your
electrical loads are smaller than my estimate, the UPS ends up smaller
and cheaper.
*******
Now, the above is a dumb way to do it. But it's to illustrate how hard
it is to do this by yourself.
If you do find a local solution for a lower price, as the seller for
references to customers who are already using it. If the thing has
short battery life, and high maintenance cost, an existing customer
is who you want to talk to.
*******
Another note, on "AC waveforms".
http://i285.photobucket.com/albums/ll50/Nanu_28181/SteppedPure-1.jpg
A cheap inverter, might make a crude square wave. That would work OK
with a light bulb. It would work with an older desktop computer which
does not make any mention of "power factor correction".
Some newer ATX power supplies, have demonstrated they're not really happy
with square wave power. And much of the world is switching to PFC type
ATX supplies. (Supplies come in no PFC, passive PFC, and active PFC.) At
least some of those will prefer the closer to sinusoidal waveform in
the diagram.
Luckily for me, none of my ATX power supplies have PFC, so I can continue
to use my square wave output UPS. But I haven't checked my new laptop,
and wouldn't run the laptop off UPS anyway (because thankfully, rolling
blackouts aren't the issue here - we have lots of 1 second switching
transients). So that is another tiny factor to consider, is whether
any of the loads have a preference for power quality. The above diagram
shows the various waveforms a UPS could provide.
If you were to attempt to run AC motors from your UPS, they like a
pure sine wave. And motors also have "stall current ratings". For
example, a fairly small motor, say 2 amps, if I check the nameplate,
may have a 20 amp stall rating. It draws 20 amps for the first second
or two. The above 11 kilowatt UPS can probably supply 120V * 20A without
a problem, and actually manage to start the motor. Some motors don't have
that high a stall ratio, but again, it's one of the reasons they don't
recommend connecting any motorized devices at all, to cheap UPS devices.
Normally, they just wouldn't have the power rating, for the first few
seconds of loading.
*******
With my absurd example, you can see the relative cost of this solution
versus using a generator. UPS with add-on battery pack is a bit
quieter, a bit cleaner (no exhaust), but the initial cost can be
high. If local manufacturers make UPS devices for the local market,
you may do much better on prices. But the quality of the batteries
could be different, and the economics might not work well if the
batteries in fact last a short time.
If you have any question about the quality or type of batteries used
in locally made UPS solutions, put the unit where any hydrogen cannot
collect. Hydrogen floats to the ceiling, and forms a nice ignitable
cloud.
Paul
