Download Photovoltaic Fundamentals

DESIGN AND OPTIMATION OF
PHOTOVOLTAIC SYSTEMS
Professor Peter P. Groumpos
Laboratory for Automation
and Robotics
Department of Electrical and
Computer Engineering
1
OUTLINE
Introduction
Definition of the Problem
PV Fundamentals
Components of PV Systems
Design a PV System
Examples
Hybrid Energy Systems
Examples
Conclusions
INTRODUCTION
• Last century main concern regarding the electric energy is its:
a) Production
b) Distribution
c) Meeting the electric loads/demand
• The industrial development, population growth and other
world changes have increase the energy needs.
• Renewable Energy Sources (RES) have been considered to be
valuable candidates in meeting the growing global power
demand.
• Photovoltaic is a promising Renewable Energy Source
What’s wrong with this picture?
• Pollution from burning fossil fuels leads to an increase in
greenhouse gases, acid rain, and the degradation of
public health.

In 2005, the U.S.
emitted 2,513,609
metric tons of carbon
dioxide, 10,340 metric
tons of sulfur dioxide,
and 3,961 metric tons
of nitrogen oxides from
its power plants.
Why Sustainable Energy Matters
• The world’s current energy system is built around fossil
fuels
– Problems:
• Fossil fuel reserves are ultimately finite
• Two-thirds of the world' s proven oil reserves are
locating in the Middle-East and North Africa (which can
lead to political and economic instability)
Why Sustainable Energy Matters
• Detrimental environmental impacts
– Extraction (mining operations)
– Combustion
» Global warming (could lead to significant changes
in the world' s climate system, leading to a rise in
sea level and disruption of agriculture and
ecosystems)
Making the Change to Renewable Energy
•
•
•
•
Photovoltaic
Geothermal
Wind
Hydroelectric
Problem Definition
PROBLEM 1
GIVEN ANY POWER-LOAD DEMAND CAN A PV SYSTEM MEET
THIS POWER DEMAND?
PROBLEM 2
CAN A SINGLE “ENERGY SOURCE” MEET THE TOTAL POWER
DEMAND FOR ANY GIVEN “APPLICATION” ?
History
• 1839: Discovery of the photoelectric effect
by Bequerel
• 1873: Discovery of the photoelectric effect
of Selen (change of electrical resistance)
• 1954: First Silicon Solar Cell as a result of
the upcoming semiconductor technology (
= 5 %)
9
THE PHTOVOLTAIC EFFECT
– photo = light;
voltaic = produces
voltage
-Photovoltaic (PV) systems convert light
directly into electricity (using
semiconductors)
Solar Cell and Photoelectric Effect
h
+
1.
Light absorption
2.
Generation of „free“ charges
3.
effective separation of the
charges
-
Result: wearless generation of electrical Power
by light absorption
11
PV technology basics
Solar cells are semiconductor devices
that produce electricity from sunlight via the
photovoltaic effect.
How PV Cells Work
A typical silicon PV cell is composed of a thin
wafer consisting of an ultra-thin layer of
phosphorus-doped (N-type) silicon on top of a
thicker layer of boron-doped (P-type) silicon.
An electrical field is created near the top surface of
the cell where these two materials are in contact,
called the
P-N junction.
A junction between dissimilarly doped semiconductor
layers sets up a potential barrier in the cell, which
separates the light-generated charge carriers.
This separation induces a fixed electric current and
voltage in the device. The electricity is collected and
transported by metallic contacts on the top and
bottom surfaces of the cell.
When sunlight strikes the surface of a PV cell, this
electrical field provides momentum and direction
to light-stimulated electrons, resulting in a flow of
current when the solar cell is connected to an
electrical load
Diagram of photovoltaic cell.
PV BASICS
• Inside current of PV cell looks like
“Reverse direction.” Why?
P
?
• By Solar Energy, current is pumped up from
N-pole to P-pole.
• In generation, current appears reverse. It is
the same as for battery.
N
P
Current appears
to be in the
reverse direction ?
13
Looks like
reverse
N
I-V Curve of Different loads
The I –V curve of a photovoltaic cell, module, or array does not by
itself tell us anything about just where on that curve the system
will actually be operating. This determination is a function of the
load into which the PV delivers its power.
• Loads do have their own I-V curves: The same voltage is
across both the PV and load, and the same current runs through
the PV and load.
• the intersection point of the two curves is the operating
point.
Equivalent circuit of a solar cell
IPH
current
source
RS
ID
ISG
IL
RP
UD
IPH:
photocurrent of the solar-cell
ID /UD:
current and voltage of the internal p-n diode
USG
RL
UL
RP:
shunt resistor due to inhomogeneity of the surface and loss-current at
the solar-cell edges
RS:
serial resistor due to resistance of the silicon-bulk and contact
material
ISG/USG: Solar-cell current and voltage
RL/IL/UL: Load-Resistance, current and voltage
ISG = IL, USG = UL
16
Solar-Cell characteristics
ID
diodecharacteristic
simplified circuit
ID
ISG
RL UD=USG
RL= 
RL=0
ISG / PSG
MPP
ISG = I0 = IK
Load resistance
ID
IMPP
MPP = Maximum Power Point
UD
solar-cell
characteristics
Power
UMPP
U0
USG
17
Solar-cell characteristics
• Short-current ISC, I0 or IK:
• mostly proportional to irradiation
• Increases by 0,07% per Kelvin
• Open-voltage U0, UOC or VOC:
•
•
•
•
This is the voltage along the internal diode
Increases rapidly with initial irradiation
Typical for Silicon: 0,5...0,9V
decreases by 0,4% per Kelvin
18
Solar cell characteristics
•
The fillfactor (FF) of a solar-cell is the relation of electrical power generated
(PMPP) and the product of short current IK and open-circuit voltage U0
FF = PMPP / U0  IK
•
The solar-cell efficiency  is the relation of the electrical power generated
(PMPP) and the light irradiance (AGG,g) impinging on the solar-cell :
 = PMPP / AGG,g
19
Solar-cell characteristics (cSi)
P = 0,88W, (0,18)
P = 1,05W, (0,26)
P = 0,98W, (0,29)
20
Solar cell characteristics
• Power (MPP, Maximum Power Point)
• UMPP
 (0,75 ... 0,9) UOC
• IMPP
 (0,85 ... 0,95) ISC
• Power decreases by 0,4% per Kelvin
•
The nominal power of a cell is measured at international defined test
conditions
(G0 = 1000 W/m2, Tcell = 25°C, AM 1,5) in WP (Watt peak).
21
Solar-cell characteristics
22
PV Module
A PV-Module usually is assembled by a certain amount of series-connected solarcells
typical open-.circuit Voltage using 36 cells: 36 * 0,7V = 25V
Problem: due to series connection, the failure of one cell
(defective or shadow) reduces the current through all cells!
23
PV Module
in order to avoid this kind of failure, cells or cell strings
are bypassed by diodes which shortcut the defective or
shaded cell(s) :
24
A PHOTOVOLTAIC SYSTEM
• What is a photovoltaic (PV) system
– Cell, Module, and Array
– Balance Of Systems (BOS)
PV Cells, Modules, Panels & Arrays (1)
ARRAY
PANEL
PV Cells, Modules, Panels & Arrays (2)
Photovoltaic cells are connected electrically in series
and/or parallel circuits to produce higher voltages,
currents and power levels. PV MODULES
Photovoltaic modules consist of PV cell circuits sealed in
an environmentally protective laminate, and are the
fundamental building block of PV systems.
Photovoltaic panels include one or more PV modules
assembled as a pre-wired, field-installable unit. A
photovoltaic array is the complete power-generating
unit, consisting of any number of PV modules and panels
Balance of System (BOS)
• The BOS typically contains;
– Structures for mounting the
PV arrays or modules
– Power conditioning
equipment that massages and
converts the do electricity to
the proper form and
magnitude required by an
alternating current (ac) load.
– Sometimes also storage
devices, such as batteries, for
storing PV generated
electricity during cloudy days
and at night.
Solar-cell Technologies
• Materials
• Technologies
• Market shares and development
29
Materials
Definition of semiconductor:
This is a matter of electron configuration
IB
Extract of periodic table:
Silicon (Si)
IIB IIIB IVB VB VIB
Germanium (Ge)
13
14
Al
31
29
Cu
Ga
48
Cd
49
In
Gallium-Arsenide (GaAs)
15
Si
32
Ge
P
33
As
51
Sb
34
Cadmium-Telluride (CdTe)
Se
52
Te
Indium-Phosphorus (InP)
Aluminium-Antimon (AlSb)
Copper, Indium, Gallium, Selenide (CIS)
4.6.07 - 6.6.07
Clemson Summer School 2007
Dr. Karl Molter / FH Trier / [email protected]
30
4.6.07 - 6.6.07
Clemson Summer School 2007
Dr. Karl Molter / FH Trier / [email protected]
31
Arguments for different technologies
•
•
•
•
•
•
Potentially high efficiency
Availability of material
Low material price
Potentially low manufacturing costs
Stability of characteristics for many years
Environment friendly and non toxic Materials
and manufacturing process
32
Evaluation of mono-crystalline Silicon:
+
–
–
+
+
+
+
Mass production efficiency between 15 - 18% (>23% in laboratory)
A lot of raw material needed
Raw silicon costs are strongly varying in time
Well known production process, but consumes much energy, optimization by
EFG and band-Technology
Very good long term stability
material almost pollution free
Second place in market shares
33
Evaluation of multi-crystalline Silicon:
+
–
–
+
+
+
+
Mass production efficiency between 12 - 14%
A lot of raw material needed
Raw silicon costs are strongly varying in time
Well known production process, consumes less energy than mono-Si
very good long term stability
material almost pollution free
First place in market shares
34
Evaluation of amorphous Silicon (a-Si):
–
+
+
+
–
+
Mass production efficiency only 6 – 8%
Thin-Film Technology (<1µm), only few
raw material needed
Well known production process, consumes
far less energy than crystalline Silicon
large area modules can be manufactured in one step
long term stability only for efficiency between 4 – 6%
material almost pollution free
35
Evaluation of Copper, Indium, Diselenide (CIS)
+
+
+
+
–
Mass production efficiency 11 – 14%
Thin-Film Technology (<1µm), only few raw material needed
large area modules can be manufactured in one step
good long term stability
raw material not pollution free (Se, small quantity of Cd)
36
Evaluation of GaAs, CdTe and others
+
–
–
–
–
–
Mass production efficiency up to 18%
some raw materials are rather rare
raw material very expensive
some production processes not suited for mass production
long term stability not well known
raw material not pollution free (esp. As, Cd)
37
Production process
1. Silicon Wafer-technology (mono- or multi-crystalline)
Most purely silicon
99.999999999%
melting /
crystallization
Tile-production
Occurence:
Siliconoxide (SiO2)
= sand
Plate-production
cleaning
typical Wafer-size:
10 x 10 cm2
Quality-control
Wafer
Link to
SiO2 + 2C = Si + 2CO
Producers of Silicon Wafers
38
Production process
1. Silicon Wafer-technology (mono- or multi-crystalline)
Occurence:
Siliconoxide (SiO2)
= sand
typical Wafer-size:
10 x 10 cm2
Link to
SiO2 + 2C = Si + 2CO
Producers of Silicon Wafers
39
Production process
1. Silicon Wafer-technology (mono- or multi-crystalline)
Occurence:
Siliconoxide (SiO2)
= sand
typical Wafer-size:
10 x 10 cm2
Link to
SiO2 + 2C = Si + 2CO
Producers of Silicon Wafers
40
ProductionProcess
mono- or multicrystalline Silicon
crystal growth process
41
Technology -Trends
• Thin-Film Technology
– few raw material needed
– demand of flexible devices
– production of large area cells / modules in one step
• enhancement of cell efficiency
–
–
–
–
Tandem-cell for better utilization of the solar spektrum
Light Trapping, enhancement of the light absorption
Transparent contacts
bifacial cells
• Solar-concentrating photovoltaics
42
PV technology basics
How a PV System Works
PV systems are like any other electrical power generating systems, just the equipment used is different than that used for
conventional electromechanical generating systems.
Depending on the functional and operational requirements of the system,
the specific components required, and may include major components
DC-AC power inverter,
battery bank,
system and battery controller,
auxiliary energy sources
and sometimes the specified electrical load (appliances).
In addition, an assortment of balance of system (BOS) hardware,
Including wiring, overcurrent, surge protection and disconnect
devices, and other power processing equipment.
PV technology basics
Types of PV Systems
How Are Photovoltaic Systems Classified?
Photovoltaic power systems are generally
classified according to:
•
functional and operational
requirements,
•
component configurations,
•
how the equipment is
connected to other power
sources and electrical
loads.
The two principle classifications are
grid-connected or utilityinteractive systems
stand-alone systems.
Photovoltaic systems can be designed to
provide DC and/or AC power service, can
operate interconnected with or independent of
the utility grid, and can be connected with
other energy sources and energy storage
systems.1.7.1 Grid-Connected (UtilityInteractive) PV Systems.
Diagram of grid-connected photovoltaic system
Why Are Batteries Used in Some PV Systems?
Batteries are often used in PV systems for the purpose of
storing energy produced by the PV array during the day, and
to supply it to electrical loads as needed (during the night and
periods of cloudy weather).
Other reasons batteries are used in PV systems are:
 to operate the PV array near its maximum power point,
 to power electrical loads at stable voltages, and
to supply surge currents to electrical loads and inverters.
REMARK: In most cases, a battery charge controller is used in
these systems to protect the battery from overcharge and
over discharge.
Diagram of stand-alone PV system
with battery storage powering DC and AC loads.
PV technology basics
Types of PV Systems
How Are Photovoltaic Systems Classified?
Photovoltaic power systems are generally
classified according to:
•
functional and operational
requirements,
•
component configurations,
•
how the equipment is
connected to other power
sources and electrical
loads.
The two principle classifications are
grid-connected or utilityinteractive systems
stand-alone systems.
Photovoltaic systems can be designed to
provide DC and/or AC power service, can
operate interconnected with or independent of
the utility grid, and can be connected with
other energy sources and energy storage
systems.1.7.1 Grid-Connected (UtilityInteractive) PV Systems.
Diagram of grid-connected photovoltaic system
PV technology basics
photovoltaic hybrid system.
Stand-alone PV systems are designed to
operate independent of the electric
utility grid, and are generally designed
and sized to supply certain DC and/or AC
electrical loads.
These types of systems may be powered
by a PV array only, or may use wind, an
engine-generator or utility power as an
auxiliary power source in what is called a
PV-hybrid system.
PV technology basics
The simplest type of stand-alone PV system is a direct-coupled system, where the DC output of a
PV module or array is directly connected to a DC load
Since there is no electrical energy storage (batteries) in direct-coupled systems, the load only
operates during sunlight hours, making these designs suitable for common applications such as
ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems.
Matching the impedance of the electrical load to the maximum power output of the PV array is a
critical part of designing well-performing direct-coupled system.
For certain loads such as positive-displacement water pumps, a type of electronic DC-DC converter,
called a maximum power point tracker (MPPT) is used between the array and load to help better
utilize the available array maximum power output.
Direct-coupled PV system.
Resistive Load I-V Curve
Ohm’s law:
V = IR or I = V/R
• As R increases, the intersection point
moves along the PV I –V
curve from left to right.
• The value of resistance that will result in
maximum power:
Rm = Vm/Im
• were Vm and Im are the voltage and
current at the maximum power point
(MPP).
Mechanism of generation
• Voltage and Current of PV cell ( I-V Curve )
P
A
(A)
N
Short Circuit
Current(I)
High insolation
•Voltage on normal operation point
0.5V (in case of Silicon PV)
•Current depend on
- Intensity of insolation
- Size of cell
Normal operation point
(Maximum Power point)
P
Low insolation
V
IxV=W
N
(V)
Voltage(V)
52
about 0.5V (Silicon)
Open Circuit
Basic Characteristic
• I / V curve and P-Max control
A
P
V
(A)
N
P1
Current(I)
Ipmax
I/V curve
• To obtain maximum power, current
control (or voltage control) is very
important.
• “Power conditioner” (mentioned later)
will adjusts to be most suitable voltage
PMAX and current automatically.
P- Max control
Power curve
IxV=W
P2
(V)
Voltage(V)
53
Vpmax
Basic Characteristic
• Estimate obtained power by I / V curve
A
P
If the load has 0.05 ohm resistance,
cross point of resistance character and
PV-Character will be following point.
Then power is 10x0.5=5 W
R  0.05()
(A)
N
PV character
( I/V curve )
12
10
R  0.05()
Current(I)
8
6
Ohm’s theory
4
V
I 
R
2
0
I  V / 0.05
(V)
0
54
0.1
0.2
0.3
0.4
Voltage(V)
0.5
0.6
Basic Characteristic
• I / V curve vs. Insolation intensity
•Current is affected largely by change of
insolation intensity.
•Partially shaded serial cell will produce
current mismatch.
P
5A
N
P
Bypass Diode
(A)
Mismatch
1A
High intensity insolation
N
Current(I)
5A
P
Low intensity insolation
5A
1A
N
P
IxV=W
1A
(V)
55
Bypass
Diode
N
4A
Basic Characteristic
• Temperature and efficiency
•When module temperature rises up, efficiency decreases.
•The module must be cooled by natural ventilation, etc.
Effi c ie n cy ( % )
14
2%
down
12
Crystalline cell
10
Amorphous cell
8
6
Summer time
on roof top
(65C)
Typical
(25C)
4
0
56
10
20 30 40 50 60 70
Module Temperature (deg.C)
80
90
100
Resistive Load I-V Curve
=With a fixed resistance the operating point slips off the MPP as the
solar irradiance changes.
=A device called a maximum power point tracker (MPPT) be introduced,
the purpose of which is to keep the PVs operating at their highest
efficiency point at all times.
Various type of PV cell
Types and Conversion Efficiency of Solar Cell
Conversion Efficiency of
Module
Single crystal
10 - 17%
Poly crystalline
10 - 13%
Amorphous
7 - 10%
Crystalline
Silicon
Semiconductor
Non-crystalline
Solar
Cell
Compound
Semiconductor
Organic
Semiconductor
Gallium Arsenide (GaAs)
Dye-sensitized Type
7 - 8%
Organic Thin Layer Type
2 - 3%
Electric Energy Output
Conversion Efficiency =
Energy of Insolation on cell
59
18 - 30%
x 100%
Various type of PV cell
• PV Module (Single crystal, Poly crystalline Silicon)
Single crystal
Poly crystalline
120W
(25.7V ,
4.7A)
128W
(26.5V ,
4.8A)
1200mm
800mm （2.62ft）
Efficiency is higher
60
Same size
800mm (2.62ft)
Efficiency is lower
Various type of PV cell
Hierarchy of PV
Cell
Module
Array
Size
0.5V
5-6A
20-30V
5-6A
200-300V 50A-200A
2-3W
about 10cm
100-200W about 1m
10-50kW
about 30m
Array
10 - 50 kW
Module,Panel
100 - 200 W
Cell
2–3W
61
6x9=54 (cells)
100-300 (modules)
Example Roughly size of PV Power Station
How much PV can we install in a roof?
1 kw PV need 10 m2
Please
remember
20m()
The room has about 200 m2
We can install about
20 kW PV in this room
62
10m
PV SYSTEMS
Pros and Cons of PV
Photovoltaic systems have a number of merits and unique advantages over
conventional power-generating technologies.
PV systems can be designed for a variety of
applications and operational requirements, and
can be used for either centralized or distributed
power generation.
PV systems have no moving parts, are modular, easily expandable and even
transportable in some cases. Energy independence and environmental
compatibility are two attractive features of PV systems.
The fuel (sunlight) is free, and no noise or pollution is created from operating
PV systems. In general, PV systems that are well designed and properly
installed require minimal maintenance and have long service lifetimes.
At present, the high cost of PV modules and equipment (as compared to
conventional energy sources) is the primary limiting factor for the
technology. Consequently, the economic value of PV systems is realized over
many years. In some cases, the surface area requirements for PV arrays may
be a limiting factor. Due to the diffuse nature of sunlight and the existing
sunlight to electrical energy conversion efficiencies of photovoltaic devices,
surface area requirements for PV array installations are on the order of 8 to
12 m^2 (86 to 129 ft^2) per kilowatt of installed peak array capacity.
PV-Module price
experience curve: price per Wp against cumulative production
with Research &
Development
end of 2004
without Research
& Development
cumulative production in MWp
4.6.07 - 6.6.07
Clemson Summer School 2007
Dr. Karl Molter / FH Trier / [email protected]
64
Discussion Questions
• Are PV systems as efficient or economic as
fossil-fueled systems?
• Is PV a viable alternative for all of our power
needs? For homes? For vehicles? For other
needs?
• Is PV the answer to all of the world’s power
needs?
65
PHOTOVOLTACS IS THE FUTURE