Solar PV (Part 2)
Summary
TLDRThis script discusses the operation of photovoltaic (PV) cells, which are semiconductor devices made of silicon. One side is doped with P-type impurities, and the other with N-type, creating a diffusion region known as the band gap. When photons from the sun provide energy, electrons jump to the conduction band, generating current. The script explains the IV curve, showing how current varies with voltage, and the importance of operating at the maximum power point. It also covers the impact of temperature on solar panel efficiency and the use of bypass diodes to prevent hotspots in shaded cells, ensuring system reliability and efficiency.
Takeaways
- đ Photovoltaic (PV) cells are semiconductor devices made of silicon, with one side doped with P-type impurities and the other with N-type impurities to create a PN junction.
- đ When photons from the sun hit the PV cell, electrons receive energy and jump from the valence band to the conduction band, creating an electric current.
- đ Connecting a wire across the two junctions of a PV cell allows the current to flow, with electrons moving back to recombine with holes, generating electricity.
- đ The current-voltage (I-V) curve for a PV cell is non-linear, showing different behaviors under illumination compared to when it's in darkness.
- đ Manufacturers often use the short-circuit current (Isc) to describe the peak current of a PV cell under specific conditions.
- đĄ The maximum power point is the most efficient operating point for a PV system, where the power output is optimized for a given solar intensity.
- đŹ Research centers, like UNSW led by Professor Martin Green, are working on making PV cells more efficient by using different materials to absorb a broader spectrum of wavelengths.
- đ The electrical characteristics of a PV cell can be modeled with a current source and resistances, including a shunt resistance to account for leakage current.
- â ïž Temperature affects the performance of PV cells; as temperature increases, the short-circuit current may increase, but the open-circuit voltage decreases, leading to a reduction in power output.
- đ§ Designing a PV power system involves connecting cells in series to achieve higher voltage and using bypass diodes to prevent hotspots and inefficiencies caused by partial shading.
Q & A
What is a photovoltaic (PV) cell and how does it function?
-A photovoltaic cell is a semiconductor device made of silicon. One side is doped with P-type impurities and the other with N-type impurities, creating a diffusion region known as the PN junction. When photons from the sun provide energy to electrons, they jump from the valence band to the conduction band, creating a flow of current when connected by a wire.
What is the significance of the energy gap or band gap in a PV cell?
-The energy gap or band gap in a PV cell is the energy difference between the valence band and the conduction band. It is crucial because electrons need to receive enough energy from photons to jump across this gap to conduct electricity.
How does the illumination of a PV cell affect its performance?
-Illumination, particularly from the sun, provides the energy needed for electrons to move to the conduction band. When a PV cell is illuminated, the current-voltage (I-V) curve changes, showing an increase in the short-circuit current (Isc), which is the maximum current the cell can produce.
What is the role of the bypass diode in a PV cell?
-A bypass diode is used to mitigate issues caused by uneven illumination or shading in a PV cell. It diverts current around shaded or malfunctioning cells, preventing hotspots and reducing power dissipation, thus protecting the cell from damage.
What is the purpose of connecting PV cells in series?
-Connecting PV cells in series is done to achieve a higher voltage output. This is necessary for many applications where a higher voltage is required, such as charging batteries or powering electrical devices.
How does temperature affect the performance of a PV cell?
-Temperature has a significant impact on PV cell performance. As temperature increases, the short-circuit current may increase, but the open-circuit voltage decreases. This results in a net power loss, with approximately a 0.5% decrease in power for every 1-degree Celsius increase in temperature.
What is the significance of the open-circuit voltage in a PV system?
-The open-circuit voltage is important because it represents the maximum voltage that a PV cell or panel can produce when no current is being drawn. It is crucial for system design to ensure that components can withstand this voltage, especially when cells are connected in series.
What is the maximum power point of a PV cell and why is it important?
-The maximum power point is the point on the I-V curve where the product of current and voltage is the highest, indicating the most efficient operation of the cell. It is important for system design to operate at this point to extract the maximum power from the solar panel.
How can the efficiency of a PV cell be improved according to the script?
-Efficiency can be improved by using different materials to absorb various wavelengths of the solar spectrum, as invented by a team from UNSW led by Professor Martin Green. This approach allows for capturing more energy from the sun.
What is the significance of the temperature coefficient of power in PV cells?
-The temperature coefficient of power indicates how sensitive the power output of a PV cell is to temperature changes. A typical coefficient suggests that for every 1-degree Celsius increase in temperature, there is a power loss of around 0.5%, highlighting the importance of cooling for optimal performance.
How does shading affect a PV cell and what measures can be taken to address it?
-Shading can cause a significant drop in performance and create hotspots in PV cells by forcing current to bypass the shaded cell. Using bypass diodes can help by diverting current away from shaded cells, reducing power dissipation and preventing damage.
Outlines
đ Understanding PV Cell Operation
The first paragraph explains the working principle of a photovoltaic (PV) cell. It highlights that a PV cell is a semiconductor made of silicon, with one side doped with phosphorus (n-type) and the other with boron (p-type) to create a p-n junction. When photons from the sun hit the cell, electrons in the valence band absorb energy and jump to the conduction band, creating a flow of current when connected by a wire. The paragraph also discusses the IV curve of a PV cell under illumination, noting the short-circuit current (Isc) and the peak power point. It emphasizes the importance of operating at the maximum power point for efficiency and mentions research efforts to improve PV cell efficiency, such as using different materials to absorb a broader range of the solar spectrum.
đ Designing and Operating PV Power Systems
The second paragraph delves into the design and operation of PV power systems. It discusses the significance of the short-circuit current, open-circuit voltage, and the temperature sensitivity of these parameters. As temperature increases, the short-circuit current rises, but the open-circuit voltage decreases, leading to a reduction in power output. The paragraph also addresses the issue of non-uniform illumination on PV cells, which can cause 'hotspots' due to shading. To mitigate this, bypass diodes are used to prevent excessive power dissipation in shaded cells. The text explains how bypass diodes work by clamping the voltage and diverting current away from shaded cells, thus preventing overheating and potential damage. The paragraph concludes by suggesting further discussion on the topic in the following week.
Mindmap
Keywords
đĄPV cell
đĄSemiconductor
đĄBand gap
đĄConduction band
đĄValence band
đĄPhotons
đĄPN junction
đĄShort circuit current (Isc)
đĄOpen circuit voltage (Voc)
đĄBypass diode
đĄHotspot
Highlights
A photovoltaic (PV) cell is a semiconductor device made of silicon.
One side of the cell is doped with phosphorus (n-type) and the other with boron (p-type) to create a p-n junction.
The p-n junction forms a diffusion region that generates an energy gap known as the band gap.
Photons from the sun provide energy to electrons, promoting them to the conduction band where they can conduct electricity.
Connecting a wire across the two junctions allows current to flow as electrons move to recombine with holes.
The process repeats as long as photons provide energy, making PV cells capable of continuous energy generation.
The I-V curve for a p-n junction is non-linear, showing different behaviors under illumination and in the dark.
The short circuit current (Isc) is a key parameter, indicating the maximum current a cell can produce under no load.
The maximum power point is the optimal operating point for a PV system to achieve the highest efficiency.
Research centers, including UNSW led by Professor Martin Green, are working on making PV cells more efficient by using materials to absorb different wavelengths.
The electrical characteristics of a PV cell can be modeled with a current source and diode equation.
The open-circuit voltage (Voc) is crucial as it represents the maximum voltage a cell can produce under no load.
Temperature affects PV cell performance, with increased temperature leading to higher short-circuit current but lower open-circuit voltage.
The power temperature coefficient indicates a decrease in power efficiency as temperature increases.
PV panels are more efficient when cooler, which is important for their operation in hot environments.
Designing a PV power system involves connecting cells in series to achieve higher voltages for various applications.
Bypass diodes are used to mitigate issues caused by uneven illumination or shading of cells.
Without bypass diodes, shaded cells can become hotspots due to excessive power dissipation.
Bypass diodes help by diverting current away from shaded cells, reducing power consumption and preventing hotspots.
Transcripts
third question um
how does pv cell work so in a nutshell
um in the previous cell is actually a
semiconductor so
um there
it is basically a silicon um
device but one side is due by
uh it's makes it some impurity like a p3
chemicals and the other side is
mixed with um first five impurities so
they put together
and it will form a diffusion region
so as uh generate energy gap
so we call it band gap so once the
electron
uh received energy from the photons that
means from the sun
it will be promoted to the conduction
band so when it was in the valence band
it doesn't
connect conduct electricity but once it
gets energy enough
to jump to that band and if we connect a
wire
across you know the two junctions then
the current will flow and the electron
will flow back to the
to the other side and then to
reconvulve the hole so this process
repeat repeat again
as long as they receive the photons
energy
and this is a curve for the pn junction
actually is a dial
when it is not illuminated
well it's under dark area um
area but once it is illuminated so you
will um
the curve will look like this and i am
which is the short circuit current or
the peak current
or sometimes in the manufacturer they
use isc
so sc stands for short circuit
so we can also flip that around by you
know
reverse the direction of our
current definition so you see that um
this is the current to voltage curve it
is a non-linear
and though upon where from here if you
look at the power
to voltage curve you see for
a given solar intensity
there will be one maximum
power point and the job
of a system uh in order to get
the maximum powerpoint is to operate
that electrical
pawn at this point
and different um institutes research
centers they try to make the previous
set more efficient
lately one of two of our
a team from unsw led by
professor martin green they invent
a way to absorb more
energy from the solar spectrum so the
way
is actually to use different materials
to absorb different
wavelength the fourth question
was electrical characteristic of a pv
cell
so we can use this simple model so this
current source to
to show you the maximum current was the
short circuit
using this dial we can show the um
the iov iv non-linear curve
and this s without hessage is actually
the period cell to the ground like
leakage current
leakage current and this hour is
actually the the connection or the lump
sum
resistance between cells
so we can use that this is a normal dot
current and if you use ampere law
il equals to id plus i
then you can rearrange the terms to get
this equation
so one of the and from this equation
again you remember terms
we can also work out the open circuit
voltage
of the curve so why is it important
because
if you go back to the curve well if you
put many cells in series
this voltage will be very high so when
um the system
is like open low as not processing any
power
so it has to make sure you have to
withstand this
maximum voltage which is the open
circuit voltage of the panel or the
array
so i also refer to the example three in
the hand
written notes so the result for that is
we can see that based on the equation
based on the area and also insulation
so we we can work out the whole curve
and for different
um solar intensity we also can work our
different curves
so it is a real manufacturing data
of a solar panel by bp
so we can see that um apart from the
maximum power
maximum current uh short circuit open
circuit current
so this are the last three of three
parameters is very important because
this tells you that
when how they are temperature sensitive
so a more visualized uh
approach to look at this is actually
when you see the temperature increases
actually the short circuit increases but
however
the maximum or the open circuit volume
decreases
and even though short circuit current
increases
the open circuit voltage decreases well
it looks like the same but actually it's
not in fact if you look at the
temperature coefficient of power is is
when
one degrees one
decrease cells increase in degree
celsius
there will be around minus 0.5 percent
of power loss
so in other words the solar panel is
more efficient when
it is cooler
and the next question is how to design
and operate a pv power system so once we
know
one cell will put together uh the cells
in series to make it
a higher voltage so we can use it
for other purposes
but however not all the cell solar cells
will
receive the same illumination so
in this case uh with one cell or several
cells
they are shaded so that
that will create a lot of problems and
that's why
the manufacturer usually use a down or
call by passed on
to mitigate this problem
so in this example we show that
how this animal cells and this is that
the last one and this nma minus one
and we should recover this one all the
current will pass through
the shunt resistor
or ground or the pv to ground
leakage in a resistance so
and this current is very high and um
multiplied by this resistance so a lot
of voltage drops across here and it
becomes a hotspot
so refer to it please refer to the
example
mean the solution of sample to look at
how it is calculated
but the main point is without
that bypass diode and just connect every
cell together
if one cell is shaded and that cell will
become very hot because a lot of power
is being dissipated
on that side so if we
use the bypass dial um the key thing of
a
dot is that it would clamp the voltage
so regardless of how much current you
pass for it
so the um so what it does is actually
most of current will actually divert
and and and to the dials instead of
going through
lp and ls so it will reduce the
consumption or dissipation of power
through the cell
a lot so we just stop here and then
next week we're going to continue to
discuss
5.0 / 5 (0 votes)