Solar PV (Part 2)

Summary

This 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

• 🌞 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.
• 🔋 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.
• 🔗 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.
• 📈 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.
• 🏭 Manufacturers often use the short-circuit current (Isc) to describe the peak current of a PV cell under specific conditions.
• 💡 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.
• 🔬 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.
• 🔌 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.
• ⚠️ 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.
• 🔧 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

00:00

🌞 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.

05:02

🔌 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.

Keywords

💡PV cell

A PV cell, or photovoltaic cell, is a semiconductor device that converts light into electricity. It's the fundamental building block of solar panels. The script explains that PV cells are made of silicon, with one side doped with phosphorus to create an n-type semiconductor and the other with boron to create a p-type. When these are combined, they form a junction that allows the flow of electricity when photons from sunlight are absorbed, freeing electrons and creating a current.

💡Semiconductor

A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. In the context of the video, silicon is used as the semiconductor material in PV cells. The script describes how impurities are added to silicon to create the necessary p-n junction for the photovoltaic effect to occur.

💡Band gap

The band gap is the energy difference between the valence band and the conduction band in a semiconductor. The script mentions that when a photon from the sun provides enough energy to an electron, it can jump from the valence band to the conduction band, which is necessary for the flow of current in a PV cell.

💡Conduction band

The conduction band is the range of energy levels in a solid where electrons can move freely and conduct electricity. The script explains that electrons must be excited from the valence band to the conduction band by absorbing energy from photons to conduct electricity in a PV cell.

💡Valence band

The valence band is the highest range of energy levels in a solid that is normally occupied by electrons. In the script, it's mentioned that electrons in the valence band do not conduct electricity until they gain enough energy to jump to the conduction band.

💡Photons

Photons are elementary particles that are the quantum of light and all other forms of electromagnetic radiation. The script describes how photons from the sun provide the energy necessary for electrons to move from the valence band to the conduction band in a PV cell.

💡PN junction

The pn junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, within a single crystal of semiconductor. The script explains that the pn junction is crucial for the operation of a PV cell, as it allows for the separation of charge carriers that generate the photovoltaic effect.

💡Short circuit current (Isc)

The short circuit current is the current that flows through a solar cell when it is short-circuited, meaning the voltage across the cell is zero. The script mentions isc as one of the key parameters measured in a PV cell, indicating the maximum current that can be generated under illumination.

💡Open circuit voltage (Voc)

Open circuit voltage is the voltage across the terminals of a solar cell when no current is flowing. It is an important parameter in the script, indicating the maximum voltage that can be achieved by a PV cell when it is not connected to any load.

💡Bypass diode

A bypass diode is a diode connected in parallel with each solar cell to prevent the cell from becoming a load when it is shaded or not receiving sunlight. The script discusses how bypass diodes are used to prevent hotspots and to ensure that current can flow around shaded cells, thus protecting the entire PV system.

💡Hotspot

A hotspot is a localized area on a solar cell or panel that becomes excessively hot due to high power dissipation. The script explains that hotspots can be created when one or more cells in a PV panel are shaded, causing a large current to flow through a small area, which can be mitigated by using bypass diodes.

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