Also called a solar panel or photovoltaic (PV)
module, an integrated assembly of interconnected solar
cells designed to deliver a selected level of working voltage and current
at its output terminals, and suited for incorporation in a solar
power system. In addition to the cells, a typical solar module includes
the following components:
Solar modules are normally mounted on top of a roof as part of a roof-mounted
solar power system, or a holding rack of some sort, within a frame structure.
A solar module is the smallest building block of the power generating part
of a solar array.
- A transparent top surface, usually glass
- An encapsulant – usually thin sheets of ethyl vinyl acetate
that hold together the top surface, solar cells, and rear surface
- A rear layer – a thin polymer sheet, typically Tedlar, that
prevents the ingress of water and gases
- A frame around the outer edge, typically aluminum
How a solar module is made
Solar cells are connected together by electrical contacts that are very
thin (at least in the front) so as not to block sunlight to the cell. Metals
such as palladium/silver, nickel, or copper are vacuum-evaporated through
a photoresist, silkscreened, or merely deposited on the exposed portion
of cells that have been partially covered with wax. All three methods involve
a system in which the part of the cell on which a contact is not desired
is protected, while the rest of the cell is exposed to the metal.
Because pure silicon is shiny, it can reflect
up to 35 percent of sunlight that falls on it. To reduce the amount of sunlight
lost, an anti-reflective coating is put on the silicon wafer. The most commonly
used coatings are titanium dioxide and silicon oxide, though others are
used. The material used for coating is either heated until its molecules
boil off and travel to the silicon and condense, or the material undergoes
sputtering. In this process, a high voltage knocks molecules off the material
and deposits them onto the silicon at the opposite electrode. Yet another
method is to allow the silicon itself to react with oxygen- or nitrogen-containing
gases to form silicon dioxide or silicon nitride. Commercial solar cell
manufacturers use silicon nitride.
The finished solar cells are encapsulated; that is, sealed into silicon
rubber or ethylene vinyl acetate.
The encapsulated solar cells are then placed into an aluminum frame that
has a mylar or tedlar backsheet and a glass or plastic cover.
Uses of solar modules
Solar modules can be used singly or interconnected in solar
arrays for a variety of applications, from charging batteries and driving
motors to powering entire communities. Sample configurations and uses (courtesy
of GE Power website) include:
Directly connected sysems
- Solar module, mounting hardware, DC motor or pump, and disconnect
switch or circuit breaker
The solar module produces direct
current that is used immediately by a motor. As sunlight rises and falls,
current and voltage rise and fall, and the motor speeds up and slows
down proportionally. The motor operates slowly during cloudy or stormy
weather and does not operate at night. Applications: Remote water pumping,
a ceiling or attic fan or a solar thermal (hot water) circulation pump.
- Solar module, fuse and/or fused disconnect switch
A small current flows from the solar module through a starting battery
to counteract any inherent self-discharge in the battery. A trickle
charge flows only during daylight hours, but on average offsets any
Trickle charging of vehicle starting batteries (fleet vehicles, seasonal
road equipment like snowplows) and boat batteries.
- Solar modules and mounting hardware, a charge regulator, storage
batteries, and disconnect switches or circuit breakers
A solar array produces DC current that passes through the charge
controller into storage batteries. The charge regulator reduces
or stops charging current to prevent battery overcharge. Small DC loads
may be connected to the charge regulator, which can then prevent battery
over-discharge. The battery operates loads at night and during overcast
or stormy days. Solar modules recharge the batteries when average or
good weather returns. Applications: Remote industrial areas (telecommunications,
navigational aids, cathodic protection and traffic systems) and remote
- Above components with the addition of an inverter and an AC distribution
The inverter draws power from the
battery and changes DC to AC
current and voltage. For safety, power is sent to the distribution center
which houses circuit breakers
for individual AC circuits. The inverter operates from battery energy
day or night. Applications: Remote home systems.
- Above components with the addition of a fuel generator (gasoline,
diesel or propane), a rectifier and a sophisticated hybrid system controller
The system controller monitors the battery voltage. When the voltage
drops to a safe but low level, the generator
is turned on. AC output is converted to DC power and recharges the battery.
AC output can also be used directly to power AC loads. When the battery
reaches an almost full recharge level, the generator is turned off.
The solar array can be sized to supply average GE needs throughout the
year, and the generator is used to fill in during seasonal low output
periods and prolonged bad weather. Applications: Village power systems.
- Solar modules and mounting hardware, disconnect switches or circuit
breakers and a grid interactive inverter
The solar array produces DC current that passes through inverter, which
converts to AC current and voltage. Power is sent to the utility meter
and is either consumed immediately by home or business loads, or is
sent out to the general utility grid network. The utility meter spins
backwards, or two meters are used to record incoming and outgoing power.
At night, loads operate from utility power since the solar power system
does not produce power. The inverter shuts down automatically in case
of utility power failure for safety, and reconnects automatically when
utility power resumes. Applications: Urban residential and commercial
systems and utility-scale power plants.
- Above components with the addition of a battery bank, charge regulator
and bi-directional inverter
The solar array charges the battery bank through a charge regulator.
DC power from the battery passes through the inverter and is converted
to AC current and voltage. Power is sent to the utility meter and is
either consumed immediately by home or business loads, or is sent out
to the general utility grid network. The utility meter spins backwards,
or two meters are used to record incoming and outgoing power. At night,
loads operate and the battery bank is kept trickle charged from utility
power since the solar power system does not produce power. In case of
utility power failure, the direct connection to the utility meter is
shut down for safety. Selected circuits in the home or business that
are connected to a special secondary inverter output continue to operate,
drawing energy from battery bank. The solar array recharges the battery
each day until normal utility power resumes. Applications: Urban residential
and commercial systems
Energy performance ratings for solar modules
The performance of solar modules is measured by several factors, including:
- Peak watt (Wp): the maximum power of a module under
laboratory conditions of relatively high light level, favorable air
mass, and low cell temperature. These conditions are not typical in
the real world.
- Normal operating cell temperature (NOCT): a module's
nominal operating cell temperature after the module first equilibrates
with a specified ambient temperature. It results in a lower watt value
than the peak-watt rating, but it is probably more realistic.
- AMPM standard: the performance of a solar module
under more realistic operating conditions. It considers the whole day
rather than "peak" sunshine hours, based on the description of a standard
solar global-average day (or a practical global average) in terms of
light levels, ambient temperature, and air mass.
ENERGY AND POWER