Solar Panels
What are Solar Panels?
Solar panels
are devices that are used to convert light energy into electricity. They are
made from semiconductors and they accomplish this through the photoelectric
effect. Today, solar energy is one of the fastest-growing energy sources in the
last few years in terms of global capacity and most of it is due to
advancements in solar panel technology and continuous reductions in its cost.
How do They Work?
Solar panels are made from
semiconductors and this gives them their distinct characteristic of being able
to convert light into electricity. Semiconductors are a class of substances
that are in between conductors and insulators. This means that they have a
conductivity that is in between the two.
Semiconductors
also have a property of being able to make use of the photoelectric
effect. This phenomenon was first observed in 1887 by Heinrich
Hertz. He had a device that is called the spark gap generator where you have
two small metal spheres working as a transmitter and another two small metal
spheres that work as a receiver. The transmitter can induce sparks between the
metal spheres of the receiver. The two metal spheres of the receiver are
separated by a small air gap and this air gap must be made to be very little to
be able to for it to be effective. Hertz saw that he could increase the capacity
of the receiver to faithfully reproduce the sparks from the transmitter by
illuminating it with visible or ultraviolet light. He and all the other
physicists at that time, however, we're unable to understand why and how it
happens.
It was only more than 100 years
later when the photoelectric effect was explained by none other than Albert
Einstein. To understand it, we must first understand the basic model of an
atom. Electrons inside the atom are arranged into shells with the nucleus in
the center. These shells can be thought of as concentric spheres with larger
spheres corresponding to higher energy levels. Electrons with the highest
energy level are located in the outermost shell which is called the valence
shell. Einstein proposed that light behaves as discrete particles when it hits
the atom. It is then absorbed by the electron, allowing it to have a higher
energy and escape its parent atom. This “free” electron can then be conducted
throughout the material as electricity.
This is the exact same phenomenon
that solar panels use to convert light into electricity. When solar panels are
exposed to light, it creates a large number of free electrons in the
semiconductor material. These free electrons are then collected at the solar
panel terminals to create voltage and current.
Types of Solar Panels: Mono, Poly & Thin Film
There are 3
types
of solar panels that are available in the market today and these are: 1.)
Monocrystalline, 2.) Polycrystalline and 3.) Thin-film solar panels. The first
two types are both made from crystalline silicon. Inside a crystalline silicon,
the silicon molecules form themselves into a neat crystal arrangement all
throughout the material. The only difference between the two being the purity
of the crystalline silicon used. This simply means the absence of other types
of atoms inside the material. The purity of the used silicon material is
important as a higher purity also corresponds to a higher efficiency.
Monocrystalline Solar Panels
Monocrystalline
solar panels can easily be distinguished from their black color. If you will
also take a closer look, you will see that it has an even coloring and uniform
look. This is because monocrystalline solar panels are made from silicon of that
only has one crystal structure throughout the material, hence, the name
monocrystalline.
Because they
are made from a higher grade of silicon, they also have a higher efficiency.
They also have a lower temperature coefficient, which means that they perform
better at higher temperatures. They are also more expensive, however, since the
processes that are used to enhance the purity of silicon are expensive.
For actual
solar PV installations, the added cost of using monocrystalline solar panels
can be worth it as they let you conserve space. Because of its higher
efficiency, you will need to install less solar panels to produce the same
amount of energy. This is very important when installing for buildings or
houses with a limited roof space.
Polycrystalline Solar Panels
Polycrystalline
solar panels usually come in color blue. It also has an uneven coloring which
shows the different crystal structures inside the material. Polycrystalline
silicon is also simpler to manufacture and therefore, costs less.
This type of
solar panel tends to have a lower efficiency because of the lower silicon
purity. They also have a higher temperature coefficient compared to
monocrystalline solar panels. This means that they perform worse than
monocrystalline solar panels at higher temperatures. However, in actual solar
PV installations, the difference between the two is minor and there are more
important things that homeowners need to take into account like brand, model,
logistics, etc.
Thin
Film Solar Panels
Thin film
solar panels are made by depositing one or several thin layers of semiconductor
material onto a substrate. They have the lowest efficiency compared to the
first two types and because of this, they are not commonly used in residential,
commercial or utility-scale applications. But they have a distinct advantage of
being flexible, allowing them to be used in applications where the first two
types can’t be used.
Types of Solar Panels: 60-cell & 72-cell
There are generally two types of
monocrystalline and polycrystalline solar panels available in the market today.
These are 60-cell and 72-cell solar panels. These numbers correspond to the
number of solar cells connected in series inside the solar panel. Solar panels
are composed of smaller units connected in series, which are called solar
cells. Solar cells typically produce a very low voltage and they are connected
in series to produce a useable voltage level.
60-cell solar panels usually have
power ratings of 230W-270 while 72-cell solar panels are usually in the range
of 280W-320W. Because of having more cells in series, 72-cell solar panels are
bigger in size, but most manufacturers just offer them at the same price per
Watt. The choice on which type of solar panels to use, therefore, lies solely
on the design of the solar PV system to be installed.
Solar Module Properties: IV Curve
The
IV
curve is the set of all points where the solar panels can operate
on. These points correspond to the current and voltage values that they can
produce at any given input values. The IV curve is very important as it
describes how the solar panel will operate at any given input irradiance and
temperature.
The IV curve is where we derive
the most important operating parameters of the solar panels which are the:
open-circuit voltage (Voc), the voltage at maximum power point (Vmp), short-circuit
voltage (Isc) and current at maximum power point (Imp). These are the main
parameters used in the design of solar PV systems. The IV curve of a solar
panel is given by the equation:
This equation is used in some solar PV simulation software
to determine the output parameters of the solar panels at any given point in
time.
Solar
Module Properties: Power Curve & Maximum Power Point
From the IV
curve, we can derive another curve that tells us the
power
output of the solar panel at any given input parameters. We can see
that the power output of a solar panel is 0 at both the open-circuit voltage
and the short circuit current. We also see the power output be at its peak at a
point called the maximum power point. The voltage and current values at this
point are called voltage at maximum power point and current at the maximum power
point, respectively.
On a PV array, each solar panel
is “forced” by the inverter to operate at its maximum power point because this
is where the solar panel is at its highest efficiency. This is the main reason
why different brands and models of solar panels are never mixed together in a
PV array. They may have different maximum power points which means that they
must operate at different voltages and currents to produce maximum power. The
inverter, however, can only choose one operating point for all solar panels and
it chooses the point where PV array production is at its maximum. This means
that each solar panel will have to operate outside of its maximum operating
point and will, therefore, be less efficient.
Even in a PV array with a uniform
brand and model of solar panels, imperfections in the manufacturing process may
also introduce differences in the maximum power points in each solar panel. And
similar to the situation explained in the above paragraph, each solar panel will
operate at a slightly different point that its maximum power point. Because of
this, the total maximum output of the PV array is effectively reduced. This is
called mismatch loss and it is also considered by the top PV simulation
programs when calculating energy yield.
Solar Module Properties: Efficiency
The efficiencies of commercially
available solar panels are usually in the range of 15-20%. As with any other
energy source, it is the ratio of input energy to its output. For solar panels,
the input is the insolation that the solar panel receives.
Compared to other energy sources,
solar energy has the lowest efficiency. But comparing energy sources through
their efficiencies is comparing apples and oranges because each energy source
has a different input. To make an accurate comparison, it makes sense to use
LCOE instead. LCOE stands for levelized cost of
energy. It simply means the cost of power produced by any energy source over
its lifetime. The simple way to calculate the LCOE is to divide the total cost
of constructing the power plant with the total amount of energy that it will
produce over its lifetime.
According to the 2017 edition of
Lazard’s annual Levelized Cost of Electricity (LCOE)
study,
solar and wind already has a lower LCOE than all the other energy sources
because of their continuously plummeting costs.
The main reason why this is the
case is because solar energy is technology-based rather than fuel-based like
other fossil fuel sources. For fuel-based sources, improvements can only come
through how efficiently we can extract energy from our fuel source. For a
technology-based energy source like solar, however, its improvement is
exponential This is similar to Moore’s Law for integrated circuits (IC). Not
only that, with improvements in the manufacturing processes used, the total
cost is also continuously going down.
Solar Module Properties: Temperature Characteristic
Semiconductors have a negative temperature coefficient which
means that its resistance increases with an increase in temperature. For solar
panels, this corresponds to a decrease in total
power
output. If we look at the IV curve at different operating
temperatures, we can see that the curve moves slightly to the left with
increases in temperature. This means that the solar panel’s operating voltage
decreases slightly as the cell temperature increases. This is contrary to
popular belief that solar panels convert heat to electricity and therefore,
will produce more energy during hotter ambient temperatures.
There are basically only two
input parameters that affect the solar panel’s output, and these are irradiance
and cell temperature. This is why for solar farms and some commercial solar PV
systems, they install weather sensors like pyranometers (measures irradiance)
and module temperature sensors. The data collected from these sensors can then
be used to get the expected energy production and compared to its actual
production to measure its performance.
Effect of Shading / Solar Module String Characteristics
Solar panels
are connected in series to form a string to increase its voltage to a level
that is compatible with the inverter input. And since they are in series, they
can only produce a single value of current. If one solar panel in the string is
subjected to
shading,
it effectively reduces the amount of irradiance on its input. The IV curve of
that solar panel is moved downward, corresponding to a decrease in output
current. When this happens, all other solar panels in the string are also
forced to decrease their output current to match the shaded solar panel’s
output.
Therefore, solar panels that are
connected in a string only perform as well as the least performing solar panel
on the string. In short, shading on one solar panel effectively affects every
other solar panel on the string.
The same thing happens for the
solar cells that are also connected in series to form the solar panel. Even
when only one portion of the solar panel is subjected to shading, it also
affects all the other solar cells. And because of this, shading on a small area
on a solar panel results to a disproportionate amount of reduction in power
output.
Solar Module Physical
Characteristics
Solar modules are exposed to the
sun, which means that it has to withstand extreme weather conditions like hot
and cold ambient temperatures, rain, snow and hail for its lifetime of 25
years. To be able to do this, the solar
cells inside the solar panels come with several
layers
of protection against the outside elements. These are:
· Glass – the glass at the very front of the solar
modules make them weatherproof and protects them from impact from falling
debris. These are made from 3 to 4mm thick tempered glass. Tempered glass is
used because it breaks only in tiny fragments rather than large, sharp and
jagged sections for standard glass. The IEC minimum standard is that the glass
must be able to withstand an impact from 1 inch wide hailstones traveling at
60 miles per hour.
· Aluminum Frame – The aluminum frame protects the
edge of the laminate section containing the solar cells while also providing a
solid structure to mount the solar cells. The frame is made from aluminum
because of its lightweight characteristic. The solar modules are mounted in
position by clamps installed on the frame. The frame is also grounded in the
installation for protection.
· EVA film – EVA (ethylene vinyl acetate) is a
highly transparent plastic used to encapsulate the solar cells. It has 2
functions, as shock absorption from external impact and as another layer of
protection from temperature and moisture and dirt ingress.
· Backsheet – the back sheet is located on the
rearmost part of the solar module for mechanical protection and electrical
insulation.
