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The Basics of Solar Power
Systems
Learn the essential basics of all solar
power systems so you can understand your solar power project. Planning
your solar power system begins with understanding the basics found in
this section
or contact
us for expert technical assistance.
Producing common electricity with solar
power
Solar power to produce electricity is not
the same as using solar to produce heat. Solar thermal principles
are applied to produce hot fluids or air. Photovoltaic
principles are used to produce electricity. A solar panel
(PV panel) is made of the natural element, silicon, which becomes
charged electrically when subjected
to sun light.
Solar panels are directed at solar south
in the northern hemisphere and solar north in the southern
hemisphere (these are slightly different
than magnetic compass north-south directions) at an angle
dictated by the geographic location and
lattitude of where they are to be installed. Typically, the angle of
the solar array is set within a range of
between site-lattitude-plus 15 degrees and site-lattitude-minus 15
degrees, depending on whether a slight
winter or summer bias is desirable in the solar power system.
Many solar arrays are placed at an angle
equal to the site lattitude with no bias for seasonal periods.
This electrical charge is consolidated in
the PV panel and directed to the output terminals to produce low
voltage (Direct Current) - usually 6 to 24
volts. The most common output is intended for nominal 12 volts,
with an effective output usually up to 17 volts. A 12 volt nominal
output is the reference voltage, but
the operating voltage can be 17 volts or higher much like your car
alternator charges your 12 volt battery
at well over 12 volts. So there's a difference between the reference
voltage and the actual operating
voltage.
The intensity of the Sun's radiation
changes with the hour of the day, time of the year and weather
conditions. To be able to make
calculations in planning a solar power system, the total amount of
solar radiation energy is expressed
in hours of full sunlight per m², or Peak Sun Hours. This term, Peak
Sun Hours, represents the average
amount of sun available per day throughout the year.
It is presumed that at "peak
sun", 1000 W/m² of energy reaches the surface of the
earth. One hour of full sun
provides 1000 Wh per m² = 1 kWh/m² - representing the solar
power received on a cloudless summer
day on a surface directed towards the sun.
The daily average of Peak Sun Hours, based
on either full year statistics, or average worst month of the year
statistics, for example, is used for calculation purposes in the
design of the solar power system.
So it can be concluded that the power of a
solar system varies, depending on the intended geographical location.
Components in a typical
solar power system
The four primary components of a typical
solar power electrical system which produces common 230
volt AC power for daily use are: Solar
panels, charge controller, battery and inverter. Solar panels charge
the battery, and the charge regulator
insures proper charging of the battery. The battery provides DC
voltage to the inverter, and the inverter
converts the DC voltage to normal AC voltage.
Solar Power Panels
The output of a solar panel is usually
stated in watts, and the wattage is determined by multiplying the
rated voltage by the rated amperage. The
formula for wattage is VOLTS times AMPS equals WATTS.
So for example, a 12 volt 60 watt solar
panel measuring about 20 X 44 inches has a rated voltage of
17.1v and a rated 3.5 amperage.
V x A = W
17.2 volts times 3.5 amps equals 60 watts
If an average of 6 hours of peak sun per
day is available in an area, then the above solar panel can
produce an average 360 watt hours of power
per day; 60w times 6 hrs. = 360 watt-hours. Since the
intensity of sunlight contacting the solar
panel varies throughout the day, we use the term "peak sun
hours" as a method to smooth out the
variations into a daily average. Early morning and late-in-the-day
sunlight produces less power than the
mid-day sun. Naturally, cloudy days will produce less power than
bright sunny days as well. When planning a
solar power system your geographical area is rated in average
peak sun hours per day based on yearly sun data. Average peak sun
hours for various geographical
areas is listed in the section "Determining your solar power
requirements".
Solar panels can be wired in series or in
parallel to increase voltage or amperage respectively, and they
can be wired both in series and in
parallel to increase both volts and amps. Series wiring refers
to connecting the positive terminal
of one panel to the negative terminal of another. The resulting outer
positive and negative terminals will
produce voltage the sum of the two panels, but the amperage stays
the same as one panel. So two 12 volt/3.5
amp panels wired in series produces 24 volts at 3.5 amps. Four of
these wired in series would produce 48 volts at 3.5 amps. Parallel wiring
refers to connecting positive terminals
to positive terminals and negative to negative. The result is that
voltage stays the same, but amperage
becomes the sum of the number of panels. So two 12 volt/3.5 amp panels
wired in parallel would produce 12
volts at 7 amps. Four panels would produce 12 volts at 14 amps. Series/parallel
wiring refers to doing both of the above -
increasing volts and amps to achieve the desired system voltage
as in 24 or 48 volt systems.
Solar Power Charge
Controller
A charge controller monitors the battery's
state-of-charge to insure that when the battery needs
charge-current it gets it, and also
insures the battery isn't over-charged. Connecting a solar panel to a
battery without a regulator seriously
risks damaging the battery and potentially causing a safety concern.
Charge controllers (or often called charge
regulator) are rated based on the amount of amperage they can process
from a solar array. If a controller is rated at 20 amps it means that
you can connect up to 20 amps of
solar panel output current to this one controller. The most advanced
charge controllers utilize a charging
principal referred to as Pulse-Width-Modulation (PWM) - which insures
the most efficient battery charging
and extends the life of the battery. Even more advanced controllers
also include Maximum Power Point
Tracking (MPPT) which maximizes the amount of current going into the
battery from the solar array by
lowering the panel's output voltage, which increases the charging amps
to the battery - because if a panel
can produce 60 watts with 17.2 volts and 3.5 amps, then if the
voltage is lowered to say 14 volts
then the amperage increases to 4.28 (14v X 4.28 amps = 60
watts) resulting in a 19% increase
in charging amps for this example.
Many charge controllers also offer Low
Voltage Disconnect (LVD) and Battery Temperature
Compensation (BTC) as an optional feature.
The LVD feature permits connects loads to the LVD
terminals which are voltage sensitive. If
the battery voltage drops too far the loads are disconnected -
preventing potential damage to both the
battery and the loads. BTC adjusts the charge rate based on the
temperature of the battery since batteries
are sensitive to temperature variations above and below about 75
F degrees.
Solar Power Battery
Deep cycle batteries used in solar power
systems are designed to be discharged and then re-charged
hundreds or thousands of times. These
batteries are rated in Amp Hours (ah) - usually at 20 hours and
100 hours. Simply stated, amp hours refers
to the amount of current - in amps - which can be supplied by the
battery over the period of hours. For example, a 350ah battery could
supply 17.5 continuous amps over 20
hours or 35 continuous amps for 10 hours. To quickly express the total
watts potentially available in a 6
volt 360ah battery; 360ah times the nominal 6 volts equals 2160 watts
or 2.16kWh (kilowatt-hours). Like
solar panels, batteries are wired in series and/or parallel to
increase voltage to the desired
level and increase amp hours.
A battery in a solar power system should
have sufficient amp hour capacity to supply needed power
during the longest expected period
"no sun" or extremely cloudy conditions. A lead-acid battery
should be sized at least 20% larger
than this amount. If there is a source of back-up power, such as a
standby generator along with a
battery charger, the battery bank does not have to be sized for worst
case weather conditions.
The size of the battery bank required will
depend on the storage capacity required, the maximum
discharge rate, the maximum charge rate,
and the minimum temperature at which the batteries will be used.
When planning a power system, all of these factors are looked at, and
the one requiring the largest capacity
will dictate battery size.
One of the biggest mistakes made by those
just starting out is not understanding the relationship between
amps and amp-hour requirements of 230 volt
AC items versus the effects on their DC low voltage
batteries. For example, say you have a 24
volt nominal system and an inverter powering a load of 3
amps, 230VAC, which has a duty cycle of 4
hours per day. You would have a 12 amp hour load (3A X 4 hrs=12
ah). However, in order to determine the true drain on your batteries
you have to divide your
nominal battery voltage (24v) into the
voltage of the load (230v), which is 9.5833, and then multiply
this times your 230vac amp hours (9.5833
x 12 ah). So in this case the calculation would be 115
amp hours drained from your
batteries - not the 12 ah. Another simple way is to take the total watt-hours
of your 230VAC device and
divide by nominal system voltage. Using the above example; 3 amps x
230 volts x 4 hours = 2760
watt-hours divided by 24 DC volts = 115 amp hours.
Lead-acid batteries are the most common in
PV systems because their initial cost is lower and because they
are readily available nearly everywhere in the world. There are many
different sizes and designs of lead-acid
batteries, but the most important designation is that they are deep
cycle batteries. Lead-acid batteries
are available in both wet-cell (requires maintenance) and sealed
no-maintenance versions. AGM and
Gel-cell deep-cycle batteries are also popular because they are
maintenance free and they last a lot longer.
Using an Inverter in a
solar power system
An inverter is a device which changes DC
power stored in a battery to standard 240 VAC electricity
Most solar power systems generate DC
current which is stored in batteries. Nearly all lighting,
appliances, motors, etc., are designed to
use ac power, so it takes an inverter to make the switch from battery-stored
DC to standard power (230V,50 Hz).
In an inverter, direct current (DC) is
switched back and forth to produce alternating current (AC). Then it
is transformed, filtered, stepped, etc. to
get it to an acceptable output waveform. The more processing, the
cleaner and quieter the output, but the lower the efficiency of the
conversion. The goal becomes to produce
a waveform that is acceptable to all loads without sacrificing too
much power into the conversion
process.
Inverters come in two basic output designs
- sine wave and modified sine wave. Most 230VAC devices can
use the modified sine wave, but there are some notable exceptions.
Devices such as laser printers which
use triacs and/or silicon controlled rectifiers are damaged when
provided mod-sine wave power.
Motors and power supplies usually run
warmer and less efficiently on mod-sine wave power. Some
things, like fans, amplifiers, and cheap
fluorescent lights, give off an audible buzz on modified sine wave
power. However, modified sine wave
inverters make the conversion from DC to AC very efficiently. They
are relatively inexpensive, and many of the electrical devices we use
every day work fine on them. Sine
wave inverters can virtually operate anything. Your utility company
provides sine wave power, so a sine
wave inverter is equal to or even better than utility supplied power.
A sine wave inverter can "clean up"
utility or generator supplied power because of its internal
processing.
Inverters are made with various internal
features and many permit external equipment interface.
Common internal features are internal
battery chargers which can rapidly charge batteries when an AC source
such as a generator or utility power is connected to the inverter's
INPUT terminals. Auto-transfer switching
is also a common internal feature which enables switching from either
one AC source to another and/or
from utility power to inverter power for designated loads. Battery
temperature
compensation, internal relays to control
loads, automatic remote generator starting/stopping and many other
programmable features are available.
System Efficiency Losses
In all systems there are losses due to
such things as voltage losses as the electricity is carried across the
wires, batteries and inverters not being
100 percent efficient, and other factors. These efficiency losses
vary from component to component, and from
system to system and can be as high as 25 percent. That's why
it's a good idea to speak to someone who has extensive system design
experience - like us! - to properly
configure the right equipment for your system.
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