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Me and "My panel " ...
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I did it. I decided to make my own power. I live in a trailer about a 100 foot extension cord length from Home Power Office & Power. The batteries are filled with electricity from photovoltaic panels and a wind generator — I can’t complain about the source. But, well, sometimes the extension cord gets “borrowed”, and there are “black outs” when Richard changes inverters (he counters that I haven’t been paying my bill). I decided to create my own system to learn and to do a system for my folks.
1. The Plan PV-Power-Pack (5W, 10W, 25W, 50W, 75W, 100W )
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I decided
to use
a photovoltaic
(PV) module
to make
electricity from
the sun,
and a
battery to
store it
in, but whatkind
of PV module and what size battery? First I made
a list
of the
appliances
I use
and whether they
use Direct
Current (DC)
or Alternating
Current (AC).
I also
have a
clock, but
it is
a wind-up
model. Since my
only ac
loads right
now are
lights, I may
buy a
DC light
instead of
buying an
inverter to
change DC
to ac current.
I’ve seen
DC compact
fluorescents and halogen lights ranging from 11 to 50 Watts. Then
I looked
at how much power
or watts
each appliance draws. The
figure is
usually stamped
on the back
or bottom
of the
appliance and
often is
not exact, but gives a
fair estimate.
Next, I
listed how
long I use each
during the
week. I
also thought
about expanding in the
future. Our
area is
really dusty,
so a small
car vacuum would
be nice.
I’ve been
thinking about getting a
computer someday,
too. I multiplied the wattage drawn by each appliance by the hours used per day to get an idea of how much power I need. Now I have an idea of how much electricity I need — about 100 Watt-hours per day. In the future, I may need over 212 Watt-hours.
2.
How much Storage?
The
capacity of a battery
— how
much it
can store
— is
rated in Ampere-hours.
To figure
out how
big of
a battery
bank I need,
I converted Watt-hours
to Ampere-hours.
Since power Another
concern is the
usable capacity
of
the
battery.
Lead-acid batteries should not be fully discharged — you
can’t regularly use
the
full
capacity.
A 40 Amp-hour
lead-acid battery cannot deliver
40 Amp-hours.
If the
battery is a deep-cycle
battery (designed
for deeper
or fuller
discharges), one should never
use more
than 80%
of the
capacity. For
car batteries, only
20% of
the capacity
should be
used —
any more will
decrease the
life of
the battery.
I’ll use
a car
battery a friend gave me
for now,
and maybe
get a
deep cycle
or alkaline battery in
the future.
So I
divided 8.3
Amp-hours by
30% to
get 27.7 Amp-hours.
I need
a battery
rated at
least 27.7 Amp-hours; the car battery is rated 40
Amp-hours. But what if the sun doesn’t shine? We have stretches of cloudy days, about three in a row on average. Since I want to be able to turn on my lights during this period, I want to have a battery capacity of at least three days: 27.7 Amp-hours per day x 3 days = 83.3 Amp-hours. I don’t have this capacity right now, but will make do with what I have. I will just watch my use on cloudy days until I get a different battery.
Batteries
are
rated
in
Ampere-hours
at
certain
charge/
discharge rates.
For
example,
a
battery
may
be
rated
40
Amp-hours at
a C/10
rate. A
C/10 rate
is the
rate of
charge or
discharge. The
rate of
charge (in
Amperes) is
equal to
the rated capacity
of the
battery (in
Ampere-hours) divided
by the cycle
time (time
to totally
charge or
discharge the
battery in
hours). In
this case,
C/10 equals
40 Amp-hours
divided by
10 hours, or
4
Amps.
If
you
discharge
a
battery
at
a
higher
amperage than
its rating,
you won’t
get the
full capacity
of the
battery. If
I plug
in a
load that
draws more
than 4
Amps, I
would deplete
the battery
faster. If
the load
is only
on for
a few
minutes, no
big deal. I looked at my consumption chart to see how much current each appliance draws for how long. (Watts divided by Volts equals Amperes.) Currently, the maximum amps drawn are three Amps. The vacuum uses 8 Amps — a C/5 rate — but only for a few minutes. A C/10 rate would work for my current and future loads. The car battery can take a high discharge rate for a short time, so no problem there!
3. Choosing a panel
Next I wanted to buy a photovoltaic module. But which one? There are so many brands and sizes! I decided to buy a new panel; I want this to be a portable system, so greater power per size is a factor. Another factor is voltage. I may use Nickel-Cadmium or Nickel-Iron batteries someday; these alkaline batteries may be fully discharged. Generally, these batteries get up past 16 Volts under charge; lead acid batteries generally do not get past 15 Volts. Some voltage will be lost through wiring and a regulator (to prevent the PV from overcharging the battery), and also due to heat. We can get five months of 90° F weather here, and heat degrades the voltage output of most PVs (about 15–25% for every 25°C above 25°C (77°F)). Modules heat up to 50°C on a sunny day. I need a panel that has a high enough rated voltage to deliver a respectable current to fill nicad batteries. With a panel rated at 17 Volts, 15 Volts may reach the battery. Crystalline PV modules are made up of many cells wired in series; each cell produces about 0.5 Volts. So I need a module with at least 36 cells (36 x 0.5 V = 18 V). Modules with 33 cells are called self-regulating for a reason — the 13 Volts or so that reaches the battery cannot overcharge lead acid batteries and is not enough to fully charge NiCads. Heat does not seem to affect amorphous silicone PVs, but currently these are more expensive per watt. Yes, another factor is price. I was willing to pay for a new module, but wanted the most watts per dollar! I
looked at
the specifications of a
few modules.
Time for a
lesson in
alphabet soup!
This is
another area
that has
always confused
me. I
flipped through
Home Power
#24 to
the article
where Richard
and Bob-O tested
different photovoltaic modules. Let’s
see, Isc
is the short circuit current. If I directly connect the
positive terminal
of the module to
the negative
terminal, I create
a
short
circuit
pathway
for
the
electrons set into
motion when
sun hits
the panel.
There is no
load and
very little
voltage. Next
is Voc,
the open circuit
voltage. With
full sun
on my
panel, this is the voltage
difference from
the positive to
the negative terminal. No
current is
flowing. The
panel’s maximum power is labeled
Pmax. The
voltage (Vpmax)
and current (Ipmax) at maximum power
are also
listed. The part of
the “soup”
important to
me was Pmax (maximum power), Vpmax,
and Ipmax.
The final decision
was fairly
arbitrary. I
called up my local
dealer, Bob-O,
and was
told he
dealt primarily
in Solarex modules.
Since the
Solarex modules
have 36 cells,
produce 17.1
Volts at
peak power,
and were a
fair price per
watt, I
opted for
the Solarex
MSX60 photovoltaic module. I
decided to
buy the 60
Watt panel instead of
the 50
Watt, because
I wanted
plenty of power for
future expansion.
Maybe I’ll
run my toaster oven in my trailer.... The
specifications for
my particular
module are
:
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Solarex
provides
real
figures
at
49°C
and
800
Watts-meter2
to account
for the
loss in
power due
to heat. For
my panel,
at 49°C,
maximum power
(Pmax)
drops to
44.4 Watts
and the
current at
max power (Ipmax) is 2.91 Amps — voltage drops to 15.3 Volts!
4.
The Frame
I had my panel! The next step was to find a place for it and mount it. I found a fairly clear place about 20 feet from the trailer. Using the Solar Pathfinder, a device that shows the sun’s path across a particular spot over the course of the year, I found that my site will get 4.5 hours of sun in the winter and about 8 hours in the summer. The hours in a day are not equal in the eyes of PV’s; photovoltaic produce the most power when perpendicular to the sun. My panel faces south on a stationary mount and will not track the east to west path of the sun. When looking for a site for the panel, the two hours just before and after solar noon are the most important. The 60-Watt panel will deliver the energy I need. In the winter, my panel will produce about 3.5 Amps times 4.5 hours of sun per day, which equals 16 Amp-hours, or 189 Watt-hours. In the summertime, my panel will only produce 2.9 Amps due to heat but for 8 hours — 23.2 Amp-hours or 278 Watts. Great! I’ll have plenty of power.
At
first I looked at
some perforated
angle iron
rack to mount
the panel, but
found out
that we
had some
Echo Lite PV
racks. I
understand this company
is out
of
business, but
the racks
work great!
I had
to modify
the rack to
fit the Solarex
module (the
module is
about 20
inches by 44
inches long);
I drilled
two extra
holes in
the two-module rack holder.
I screwed
the bottom
of the
rack into two 3 foot long 2 by 4s, and that was it.
5.
Shall I compare Thee to a Nose...
Photovoltaic
modules perform best when
perpendicular to the sun,
just like
your nose
gets burnt
from the
sun before
your arms or legs.
Your nose
is at
an angle
to catch
more of the
sun’s rays.
Unlike your
nose, you
can adjust
your panels throughout
the year
to follow
the sun’s
elevation in
the sky:
low in
the winter
and high
in the
summer. I
have the panel
set at 45° from
horizontal for fall
sun, but
the rack is adjustable to three more angles: 60° for
winter sun, 30° for summer sun, and 0° folds up for easier
carrying.
6.
Next Time
Whew! In
planning a system, I can see why using efficient appliances
and shutting that light off when not in use is so important. I feel
I’ve done
and learned
a lot,
but I’m
not finished yet! The
next step
is wiring,
and building
a homebrew regulator, which I’ll write about next
time. |
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