I decided
to use
a photovoltaic
(PV) module
to make
electricity from
the sun,
and a
battery to
store it
in, but what kind
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 windup
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 Watthours per day.
In
the future,
I may
need over
212 Watthours.
2.
How much Storage?
The
capacity of a battery
— how
much it
can store
— is
rated in Amperehours.
To figure
out how
big of
a battery
bank I need,
I converted Watthours
to Amperehours.
Since power
(Watts)
equals voltage
(Volts) times
current (Amperes),
I divided
the number of
Watthours by
the Volts. I
will use
a 12 Volt
battery, so I
divide 100
Watthours by
12 Volts to
get 8.3
Amphours.
Another
concern is the
usable capacity
of
the
battery.
Leadacid batteries should not be fully discharged — you
can’t regularly use
the
full
capacity.
A 40 Amphour
leadacid battery cannot deliver
40 Amphours.
If the
battery is a deepcycle
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
Amphours by
30% to
get 27.7 Amphours.
I need
a battery
rated at
least 27.7 Amphours; the car battery is rated 40
Amphours.
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
Amphours per
day x
3 days =
83.3 Amphours.
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
Amperehours
at
certain
charge/
discharge rates.
For
example,
a
battery
may
be
rated
40
Amphours 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
Amperehours) divided
by the cycle
time (time
to totally
charge or
discharge the
battery in
hours). In
this case,
C/10 equals
40 Amphours
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
NickelCadmium or
NickelIron 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 selfregulating
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 BobO tested
different photovoltaic modules. Let’s
see, I_{sc}
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 V_{oc},
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
P_{max}. The
voltage (V_{pmax})
and current (I_{pmax}) at maximum power
are also
listed. The part of
the “soup”
important to
me was P_{max} (maximum power), V_{pmax},
and I_{pmax}.
The final decision
was fairly
arbitrary. I
called up my local
dealer, BobO,
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
:
