CHAPTER 1 BACKGROUND 1.0 Introduction Over the last 100 years the energy consumption in the world has risen exponentially. Some studies predict that by the middle of this century there will be a rise in the world's energy consumption by a factor of 4. To supply this huge amount of energy economically, safely and without polluting the environment will be extremely difficult. Due to the limitations on conventional sources of energy, few of the current technologies for power production will outlast the 21st century. Even today, renewable energy offers the possibility of covering our energy requirement without relying on fossil fuels. An energy industry could be structured completely on the basis of renewable energies (solar, wind, geo-thermal, etc.). The sun represents by far the largest available energy source. From the sun an energy quantity of 3.9*10^24 J = 1.08*10^18 kWh arrives on the earth's surface every year. This corresponds to about 10,000 times the world primary energy requirement and is far more than all available energy reserves. If we only succeed in use a fraction of this 1
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CHAPTER 1
BACKGROUND
1.0 Introduction
Over the last 100 years the energy consumption in the world has risen exponentially.
Some studies predict that by the middle of this century there will be a rise in the world's
energy consumption by a factor of 4. To supply this huge amount of energy
economically, safely and without polluting the environment will be extremely difficult.
Due to the limitations on conventional sources of energy, few of the current technologies
for power production will outlast the 21st century. Even today, renewable energy offers
the possibility of covering our energy requirement without relying on fossil fuels. An
energy industry could be structured completely on the basis of renewable energies (solar,
wind, geo-thermal, etc.).
The sun represents by far the largest available energy source. From the sun an energy
quantity of 3.9*10^24 J = 1.08*10^18 kWh arrives on the earth's surface every year.
This corresponds to about 10,000 times the world primary energy requirement and is far
more than all available energy reserves. If we only succeed in use a fraction of this
1
owner arriving on earth, the entire current energy requirement of mankind could be
covered.
A promising technique for generating electricity from solar energy is called photovoltaic
(PV) effect (discovered in the 19'th century by Becquerel). Using solar cells made of
doped silicon; electricity is produced directly from sunlight. The current conversion
efficiency is around 12% for commercial PV modules. This electricity is in the form of
DC (direct current). To change it to AC (alternating current) a device called an inverter
is used.
Solar or photovoltaic (PV) cells are a clean renewable source of energy that has been
used in stand-alone applications for many years. However, with the growing concern
over greenhouse gas emissions and other environmental issues, renewable energy
sources such as PV are being increasingly connected to the electricity network. Europe
and Japan are at the forefront of development in grid-connected PV, although use of
such systems in Australia has grown rapidly in recent years.
Grid-connected PV systems can vary greatly in size, but all consist of solar modules,
inverters (which convert the DC output of the solar modules into AC electricity), and
other components such as wiring and module mounting structures. Some of the first
grid-connected systems consisted of several hundred kilowatts of PV modules layed out
in a large centralised array, which fed power into the local high voltage electricity
network in much the same way as a large thermal generator.
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In recent years, small rooftop mounted systems have become increasingly popular, as
improved technology has enabled the advantages of such systems to be exploited. It is
now becoming increasingly common for homeowners to install a small PV system on
their roof to supply some or all of their electricity needs.
For a small grid-connected rooftop PV system as shown in Figure 1-1, the power
produced by the array during the day can be used to supply local loads, with the excess
energy fed into the local grid for use by other customers. At night, the local loads are
simply supplied by the grid. If the PV system is large enough, it can supply more energy
into the grid than is used by local loads. Instead of receiving a bill every month, the
customer would then receive a cheque from their utility for generating this electricity.
Roof mounted solar modules Grid
Metering for import and export of power
Electrical loads
Inverter
Figure 1-1: Grid-connected rooftop photovoltaic system
(Key Center for Photovoltaic Engineering UNSW: Grid Connected Photovoltaic
http://www.pv.unsw.edu.au/info/gridconn.html)
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Distributed grid-connected PV systems offer many benefits to both the owner of the
system and the utility network. For many owners, the main attractions of such a system
are self-sufficiency and the environmental benefits of using renewable energy. The
simplicity of the system also means the owner does not need energy storage in the form
of batteries-essentially the grid is acting as a storage device. Being a modular system, it
can also expand easily as requirements or available capital grow.
The modularity of PV systems offers further benefits. The production costs for some PV
system components are related to volume of production, meaning that a large number of
small identical components can be cheaper to make than one big component. This means
that a small PV system can be as cheap or in some cases cheaper than a large system.
Furthermore, the many small systems can be distributed throughout an electricity
network rather than centralized in one location. This allows the electricity utility to take
advantage of locations where the value of electricity is greater, such as at the end of a
long and inefficient transmission line.
A grid connected PV system offers other potential cost advantages when placed at the
end of a transmission line, since it reduces transmission and distribution losses and helps
stabilize line voltage. PV systems can also be used to improve the quality of supply by
reducing 'noise' or providing reactive power conditioning on a transmission line. When
all these advantages are considered, well-positioned grid-connected PV systems are
already economically viable, even though further cost reductions are required to make
PV systems economic over the entire electricity network.
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Many utilities are developing buy-back policies that ensure private generators are paid
fairly for the electricity they sell, with some opting for rate-based incentive schemes,
while other bodies prefer net metering or avoided cost payments. The NSW electricity
distributor Integral Energy has initiated one of the more promising energy buy-back
schemes in Australia. It uses the net metering process, but if the PV system produces
more electricity than required by the site, Integral Energy will buy back the excess at a
rate marginally lower than the standard electricity retail rate.
The main technical advance that has made grid connection of small PV systems feasible
is the availability of low-cost high-quality inverters. These inverters convert the DC
electricity generated by the PV system into AC grid electricity. Recent developments
have been towards even smaller low-cost units that can be individually incorporated into
PV modules. Built-in electronics would then allow such "AC modules" to be
interconnected and grid-connected with a minimum of costly external circuitry or
protection equipment.
A variety of grid-connected PV systems have been installed throughout the world. In
1990, Germany began its "1000 Rooftops Program" which saw 1-4 kW PV systems
installed on each of 2 250 residences. In 1997, Japan is installing 3 kW systems on 9 400
rooftops, while the USA has gone one better by announcing plans to but PV system on
1 000 000 rooftops. In these and other projects involving commercial buildings, PV cells
are being incorporated into roofing materials, cladding and windows. System cost can be
further reduced in this way by offsetting them against the cost of building materials.
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1.1 Project Overview
In this project, a circuit diagram for grid-connected photovoltaic is design. For a house,
it is sufficient to use a 2 kW inverter. The inverter is simulated as a pulse- width
modulated voltage source operating with bipolar switching. The pulse-width-
modulation technique, which compares the fundamental frequency with the carrier
frequency is used to overcome switching losses.
In order to simulate the circuits and to validate the design process PSCAD simulation
software is used. Power System Computer Aided Design (PSCAD) is graphical based
design software that allows the design and simulation of power systems and power
electronics components. It allows the viewing of output graphs of any features in the
system including internal component parameters.
1.2 Computer Simulation
Traditionally, analogue simulators have been used in the simulation of large power
networks. Analogue simulators use passive components such as inductors, capacitors
and resistors arranged to represent the electrical characteristics of power system
components. These approximate models of power system components are then
interconnected to form a complete model of the system. This type of computer
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simulation operates in real-time mode since the source models operate at real-time
frequency.
In this project, the computer simulation package is based on electromagnetic transient
software. The modelling capabilities of modern electromagnetic transient software such
as EMTDC are capable of representing power systems in much greater detail than
analogue simulators. EMTDC relies on mathematical models to represent power system
components.
1.3 Project Aims
The primary concern of this project is to analyse the grid-connected photovoltaic. The
aims of this project is identified as follows:
Improvements with respect to Solar Energy conversion into Electrical Energy
Computer simulation of the Photovoltaic–Grid system for performance analysis.
Study of Dynamic behavior of Photovoltaic-Grid energy systems under disturbance
Study of Grid-Connected Photovoltaic/Diesel Energy Systems.
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CHAPTER 2
LITERATURE CRITICAL REVIEW
2.0 Introduction
The heart of the solar photovoltaic (PV) energy system is the photovoltaic device. The
photovoltaic device is a high-technology approach to converting electrical energy. The
electricity generated by a PV device is direct current (DC) and it can be used in DC form
or can be converted to alternating current (AC). PV-generated electricity can also be
stored in a storage device for later use.
Conceptually, in its simplest form a PV device is a solar-powered battery whose only
consumable is the light that fuels it. There are no moving parts; operation is
environmentally benign and if the device is correctly encapsulated against the
environment, there us nothing to wear out. Photovoltaic devices have many additional
benefits that make them useable and environmentally acceptable.
Photovoltaic systems are modular and so their electrical power output can be engineered
for virtually any application form from low-powered consumer uses to energy-
significant requirements such as generating power at electric utility central power
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stations. Moreover, incremental power additional are easily accommodated in
photovoltaic systems unlike in more conventional approaches which use fossil or nuclear
fuel and require multi-megawatt plants to be economically feasible.
There are two types of PV technologies commercially available. These are crystalline
silicon and thin film. In crystalline-silicon technologies, individual PV cells are cut from
large single crystals. In thin-film PV technologies, the PV material is deposited on glass
or thin metal that mechanically supports the cell or module. Thin metal mechanically
supports the cell or module. Thin film-based modules are produced in sheets that are
sized for specified electrical outputs.
To understand the many facts of photovoltaic energy, we need to understand the
fundamentals of how the PV devices work. Although photovoltaic cells come in a
variety of forms, the most common structure is a semiconductor material into which a
large –area diode or p-n junction has been formed. The fabrication processes tend to be
traditional semiconductor approaches such as diffusion, ion implantation and so on.
Electrical current is taken from the device through a grid contact structure on the front of
the cell that allows the sunlight to enter the solar cell, a contact on the back that
completes the circuit and an anti-reflection coating that minimizes the amount of
sunlight reflecting from the device. The fabrication of the p-n junction is the key to the
successful operation of the photovoltaic devices.
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2.1 Photovoltaic
Photovoltaic describes a technology, in which radiant energy from the sun is converted
to direct current (dc) electricity as shown in Figure 2-1. Although the scientific basis of
the photovoltaic effect has been known for nearly 150 years, the modern photovoltaic
cell was not developed until 1954. Only four years later the first cells were providing
power for U.S. spacecraft. Some of these early systems are still operating in space today
and attest to the reliability and durability of the technology.
Figure 2-1: Convert energy to dc electricity
Most solar cells are made of silicon semiconductor material treated with special
additives. When the sunlight strikes the cells, a flow of electrons is generated
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proportional to the intensity of the sunlight and the area of the cell. A solar cell 10
centimeter a side will produce about 3.5 amperes in full sunlight. Each solar cell
produces approximately one-half volt. Higher voltages are obtained by connecting the
solar cells in series. The typical photovoltaic module used for terrestrial applications
contains 36 silicon solar cells, connected in series to provide enough voltage to charge a
12-volt battery. The series-connected solar cells are encapsulated and sealed, most with
a tempered glass cover and a soft plastic backing sheet. The laminated module protects
the electrical circuits from the environment and gives the long life that photovoltaic
modules are noted for. Modules may be connected in series to obtain required system
voltages or in parallel to obtain higher currents.
2.2 Cells, Modules and Panels
The photovoltaic hierarchy is shown in Figure 2-2. The Photovoltaic electricity is
produced by an array of individual PV modules electrically connected in series and
parallel to deliver the desired voltage and current. Each PV module, in turn, is
constructed of individual solar cells also connected in series and parallel. A typical
crystalline silicon solar cell is 100 cm2 and produces about 1.75 peak watts (Wp) at 0.5
volt and 3.5 amps under full sun at standard test conditions (STC: 1,000 W/m2 and 25ºC
cell temperature).
11
Dozens of solar cells are connected together to produce a PV module. The number of
cells determines a module's size and power. Cells and modules connected electrically in
series build voltage while cells and modules wired in parallel build current.
Figure 2-2: Photovoltaic hierarchy
(Tomas Markvart and Klaus Bogus: Solar electricity: 2nd Edition)
There are two basic types of PV modules commercially available today: those made
from crystalline silicon and those made from amorphous silicon. Crystalline silicon
modules are presently the dominant commercial product and deliver approximately 100-
120 W / m2 at STC. Amorphous silicon (a-Si) thin-film modules, which are beginning to
enter the market, require less material to produce than the thick crystalline products and
so can be made less expensively. Today's commercial a-Si modules deliver 40-50 W /
m2 under full sun at STC. Other thin-film PV materials such as copper- indium-
diselinide (CIS) and cadmium telluride (CdTe) are currently under development and
hold the promise of lower costs in the future.
12
When designing a PV system, one or more of the following parameters determines the
array size: available aperture area, available resources (both solar and financial), and the
load requirements. The array's operating voltage will determine, or be determined by the
dc input voltage requirement of the inverter. Figure 2-3 illustrates the grid-connected
photovoltaic array.
RETE
CARICO
Figure 2-3: Grid-connected photovoltaic array
2.3 Technical Explanation Of Photovoltaic Cells
13
A single PV cell is a thin semiconductor wafer, generally made of highly purified
silicon. The wafer has been doped on one side with atoms that produce a surplus of
electrons and the other side with atoms producing a deficit of electrons. This establishes
a voltage difference between the two sides of the wafer. In silicon this is just under half
a volt. Metallic contacts are made to both sides of the wafer. When the wafer is
bombarded by the photons in sunlight, electrons are knocked off the silicon atoms and
are drawn to one side of the wafer by the voltage difference. If an external circuit is
attached to the contacts, the electrons have a way to get back to where they came from
and a current flows through the circuit. The PV cell acts like an electron pump. The
amount of current is determined by the number of electrons that the solar photons knock
off the silicon atoms, so by the size of the cell, the amount of light on the cell and the
efficiency of the cell.
A PV module consists of many cells wired in parallel to increase current and in series to
produce a higher voltage. Modules consisting of 36 cells in series have become the
industry standard for large power production. The module is encapsulated with tempered
glass (or some other transparent material) on the front surface, and with a protective and
waterproof material on the back surface. The edges are sealed for weatherproofing, and
there is often an aluminum frame holding everything together in a mountable unit. A
junction box, or wire leads, providing electrical connections is usually found on the
module's back. Although truly weatherproof encapsulation was a problem with the early
modules assembled 15 years ago, we have not seen any encapsulation problems with
glass-faced modules in many years.
14
PV costs are now down to a level that makes them the clear choice for most remote, and
many not so remote, power applications. They are routinely used for roadside
emergency phones and many temporary construction signs, where the cost and trouble of
bringing in utility power outweighs the higher initial expense of PV, and where mobile
generator sets present more fueling and maintenance trouble. More than 100,000 homes
in the United States, largely in rural sites, now depend on PVs as a primary power
source, and this figure is growing rapidly as people begin to understand how clean and
reliable this power source is, and how deeply our current energy practices are borrowing
from our children. Because they don't rely on miles of exposed wires, residential PV
systems are more reliable than utilities, particularly when the weather gets nasty. PV
modules have no moving parts, degrade very, very slowly, and boast a lifespan that isn't
fully known yet, but will be measured in decades. Standard factory warranties are
usually 10 years, with some manufacturers offering up to 25-year warranties. Compare
this to any other consumer goods, or power generation technology.
2.4 How Does Solar Cell Works?
15
Figure 2-4, illustrate the overview of how solar cell works. The photovoltaic effect is the
release of electron from semi-conductors when falls on their surface. A typical solar cell
consists of two layers of treated silicon; P-type and N-type silicon. P-type silicon has
unbound positive charges. N-type silicon has free negative charges. When the sunlight
hits the solar cell, they P-type and N-type silicon move apart. This movement creates a
direct current and generates voltage
Figure 2-4: How solar cell works
(U.S. Department of Energy Photovoltaic Program: Turning Sunlight Into Electricity
http://www.eren.doe.gov/pv/conveff.html (1st December 2001))
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2.5 DESCRIPTION OF PHOTOVOLTAIC ARRAY MODEL
The model of the photovoltaic array is based on the well-known single-diode
representation of a silicon photovoltaic cell as shown in Figure 2-5.
Figure 2-5: Equivalent circuit of a photovoltaic cell
(Renewable Energy 304: Lecture Notes)
Component-specific parameters:
(Note: Model parameters for the BP 280 PV module are shown in brackets)
Ior Inverse diode saturation current at reference temperature
[ Ior = 3.047e-7 A ]
ISCR Short-circuit current under STC [ ISCR = 4.92 A]
It Short-circuit current temperature coefficient [ It = 1.7 e-7 A/°K ]
A Diode ideality factor [ A = 1.403 ]
Tr Cell reference temperature [ Tr = 300 °K ]
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NOCT Normal operation cell temperature [ NOCT = 43 °C ]
EG Bandgap for semiconductor material [EG (Si) = 1.11 eV ]
RSH Cell shunt resistance [ RSH = 50 Ω ]
RS Cell series resistance [RS=50 mΩ]
NS Number of cells in series [NS = 36]
NP Number of cells in parallel [NP = 1]
The governing equations, describing the I-V characteristics of a crystalline silicon
photovoltaic cell are represented in the following. The light-generated current is given
as:
ILG = ISR x GN +It (Tc –Tr) (1)
where the normalized irradiance GN is calculated from
The diode current of the photovoltaic cell is calculated as:
where the inverse saturation current of the pn junction is expressed as:
2/1000 mWGGN = (2)
( )
⎥⎤
⎢⎡
−=+
1PVCSPVC
c
IRV
AkTq
eII (3) ⎥⎦
⎢⎣
oD
⎟⎠
⎜⎝
⎟⎟⎠
⎜⎜⎝
= cr TTAk
r
coro e
TII (4)
⎟⎞
⎜⎛
−⎞⎛ GqET
113
18
The current due to the shunt resistance of the photovoltaic cell can be expressed as:
SH
SPVCPVCRSH R
RIVI
+= (5)
Therefore, the photovoltaic cell current is given as:
IPVC = ISRGN +It (Tc –Tr) - ID –IRSH (6)
(7) ( )
SH
SPVCPVCDrctNPVC R
RIVITTIGII
SCR
+−−−+=
Inspection of equation (7) shows that the photovoltaic cell current is a function of itself,
forming an algebraic loop, which can be solved conveniently using SIMULINK.
Alternatively, it is possible to neglect the influence of the series resistance (RS=0Ω) to
derive a simplified equation for the photovoltaic cell current. The cell temperature is
calculated as :
( )CNOCTGTT oac 20
800−+= (8)
A photovoltaic module can be modeled as a series/parallel connection of cells as
expressed by the following equations for the photovoltaic module voltage and current,
respectively
19
PVCSPVM xVNV = (9)
PVCPPVM xINI = (10)
Similarly, a photovoltaic array is represented by the number of modules connected in
series Ms and the number of modules in parallel MP, where the photovoltaic array
voltage and current are given as:
PVCSSPVMSPVA xVxNMxVMV == (11)
(12) PVCPPPVMPPVA xIxNMxIMI ==
Therefore, the photovoltaic cell voltage is calculated from the photovoltaic array
voltage, which is an input to the photovoltaic array model:
SS
PVAPVC NM
VV = (13)
When calculating the photovoltaic array current, the cell current is multiplied by the
number of strings of cells in parallel for each module as well as the number of module
strings in parallel, as expressed by equation (12).
This model of the photovoltaic array does not account for variations of the performance
of individual cells, shading effects or wiring losses.
20
2.6 What Is Power Point Tracking and Is It Worth the Expense?
The output of a PV module is characterized by a performance curve of voltage versus
current, (I-V curve) as shown in Figure 7. The maximum power point of a PV module is
the point along the I-V curve that corresponds to the maximum output power possible
for the module. This value can be determined by finding the maximum area under the
current versus voltage curve. The maximum power point for standard test conditions of
1000W/m2 and 25C with air mass of 1.5 is shown in Figure 2-6 to have about 17.4 volts
and 2.5Amps.
Typical I-V Curve @ 25°C for Silicon Module
Maximum Power Point, Vmp & Imp
Isc, Short circuit current
Imp
Vdischarge
Volts
Open circuit voltage,VocVmp
Figure 2-6: Typical I-V Curve
21
For crystalline modules, the current remains fairly constant as the voltage moves up and
down throughout the typical battery voltage ranges. In PV systems that charge a battery,
the battery to which it is connected determines the module output voltage. Should the
battery be at a low state-of-charge, the output voltage of the PV module will be reduced
in voltage, and hence the module output wattage is reduced. (See V discharge on Figure
2-6) With a battery discharged to 11.0V, a corresponding module current of 2.6A can be
realized, which is only 66% of the available module power.
Maximum power point tracking enables a PV module, or array, to operate at its
maximum power point while charging a battery at a lower voltage, in this instance the
module can produce 43.5W instead of 28.6W. There are several factors that will
influence the amount of power gain one can expect; these factors are cell temperature,
conversion losses, amount of available sunlight, cell structure, battery voltage, and
blocking diodes. Some power is lost in the conversion from the voltage at the maximum
power point to battery voltage. The efficiency of most maximum power point tracking
units is usually around 93%.
Some maximum power point trackers, like the Fire Wind and Rain unit, will have an
automatic bypass that will allow the charge controller to use the battery voltage as the
module output voltage if the conversion takes more power than is being gained by using
maximum power point tracking.
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For crystalline modules, voltage will drop about 2.4mV per degree C per cell. A 36-cell
module on a typical summer day in Kingston, NY will have a cell temperature of 45C
during peak sun hours. This yields a voltage drop of 1.73V, and shifts the I-V shown in
Figure 7 thus lowering the maximum power point closer to the battery voltage. As
sunlight diminishes from the standard test condition of 1000W/m2, the voltage
corresponding to the maximum power point drops slightly, but the main component in
the decrease of available power is the decrease in available current.
In the case of amorphous silicon modules, the I-V curve will change current more
dramatically as the voltage changes throughout the battery voltage and maximum power
point ranges. This will translate into less gain seen by using the maximum power point
tracker.
Battery voltage will also play a major role in the amount of increased watt-hours one can
expect from a module or array using a maximum power point tracker. If the battery bank
is mostly near a full state-of-charge, then the voltage of the battery bank will be closer to
the maximum power point voltage and very little gain will be seen using the maximum
power point tracker. The use of blocking diodes will also mandate that the module be 0.3
to 0.7 volts higher than the battery voltage, thus lessening the difference between battery
voltage and voltage at the maximum power point.
When does using a maximum power point tracker make sense? The typical wattage gain
using a maximum power point tracker is 10 to 13%. Therefore, for systems under 300W,
23
it is usually more cost effective to buy another module than to buy a maximum power
point tracker. However, for systems above 300W the additional cost of the maximum
power point tracker can more than pay for itself with increased watt-hour output from
the array. Also, the percentage gain is greater in the winter, when the air temperature is
colder (thus colder cell temperature and higher max power point voltage), and this is
when the added array output is most needed. Recently there have been several
manufacturers that are marketing several relatively inexpensive units that will find a
home in many PV designs to come.
2.7 Photovoltaic (PV) System
Solar cells are made of certain semiconductor materials, which produce a voltage when
exposed to light. Small wires are placed on the semiconductor to provide a path for the
flow of direct current (DC) electricity. As more light falls on a cell, more electricity is
generated; therefore, a PV system must not be shaded (i.e. by shadows, snow, or wet
leaves) because such shading can substantially reduce performance.
A typical solar cell made of crystalline silicon is 4 inches in diameter and 0.010 of an
inch thick. In direct sunlight, it generates 2 amperes of direct current at 0.5 volts. By
connecting solar cells in series (to increase the voltage), and in parallel (to increase the
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current), the output of a PV system can match the requirements of the load to be
powered. If more power is required, modules can be appropriately connected in series or
parallel to form what is called a PV array (see Figure 2-7).
Figure 2-7: Residential photovoltaic system
Having the solar cells track the sun as it moves across the sky can increase the total
energy output of a PV system. Concentrating mirrors and lenses can also be used to
increase output. These more complex systems are promising, but the additional cost
must be evaluated on a case-by-case basis.
Most current PV installations are for power requirements in locations remote to existing
power lines. In some instances, such as radio communications equipment on top of
mountains, photovoltaics may be the only reasonable means of supplying power.
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However, distance from a power line is not always the controlling factor. For example,
even if a power line is located in close proximity to a small load (such as an emergency
call box), it is often more economical to use PV power instead of running a special line
to the box.
2.8 Use of Photovoltaic (PV)
Electric power generation options are now starting to be compared on a basis that
includes "externalities." Externalities are the "hidden" costs associated with a power
source that are not accounted for in the price of the power produced. These hidden costs
include damage to the environment caused by the sourcing, processing, transporting,
using, and disposal aspects of a power source. The operational costs and externalities
associated with the conventional fuel mix (coal, oil, nuclear, natural gas) used for
generating electricity are not substantially less than the "full" costs associated with
photovoltaic systems and, in many cases, exceed the costs of PV's. The use of PV's is
much less polluting than other fuel choices. Refer to Figure 2-8 for the comparison of
Commercial Status and Implementation Status.
The primary strategy for use of PV's as the electrical power source for a residence is
reducing the need for electricity. Refrigerators, air conditioners, electric water heaters,
electric ranges, electric dryers, and clothes washers are all large users of electricity.
26
Highly energy conserving alternatives and gas appliances are available to greatly reduce
electrical loads.
Figure 2-8: Comparison of Commercial Status and Implementation Status
27
2.9 Advantages Of Photovoltaic (PV) System
Photovoltaic offers advantages over diesel generators, batteries and conventional utility
power.
High reliability:
Photovoltaic cells were originally developed for use in space where repair is
extremely expensive and difficult if not impossible. Photovoltaic systems still
power nearly every satellite circling the earth because they operate reliably for long
periods of time with virtually no maintenance.
Low operating costs:
Photovoltaic cells use the energy from the sunlight to produce electricity-the fuel is
free. With no moving parts, the cells require little maintenance. These low
maintenance, cost effective photovoltaic systems are ideal for supplying power to
communication stations on mountain tops, navigational buoys at sea or homes far
from utility power lines.
No pollution:
Because they burn no fuel and have no moving parts, photovoltaic systems are clean
and silent. This is especially important where the main alternatives for obtaining
power and light are from diesel generators.
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Modular:
A photovoltaic system can be constructed to any size. Furthermore, the owner of a
photovoltaic system can enlarge it if his or her energy needs increase. For instance,
homeowners can add modules every few years as their energy needs and financial
resources grow.
Low construction costs:
Photovoltaic systems are usually placed close to where the electricity is used. This
means that a much shorter wire is required than if power is brought in from a utility
grid. Fewer wires mean lower costs, shorter construction time and a reduction in
paperwork as permits do not need to be applied for, particularly in urban areas. In
addition, using photovoltaic eliminates the need for a step-down transformer from
the utility line. The photovoltaic system makes the traditional requirements of
building large, expensive power plants and distribution systems unnecessary.
29
CHAPTER 3
GRID-CONNECTED PHOTOVOLTAIC (PV) SYSTEMS
3.0 Introduction
Grid-connected systems are sometimes referred to as cogeneration systems. They
normally do not include batteries. Here, the inverter must be capable of accepting the
full range of solar array voltage and power excursions, and must be capable of operating
at the array peak-power point instantaneously. In this case, the utility network acts as an
infinite energy sink and accepts all available power from the PV system. The simplest
grid-connected system has a PV array and an inverter as in the case of low-voltage
residential grid connection as shown in Figure 3-1. For high-voltage grid-connected
systems (greater than 220 or 380 Vac), transformers and appropriate power switching and
protection devices are included.
Figure 3-1: Grid-Connected PV Configuration, without battery
(Ahmed Zahedi: Solar Photovoltaic Energy Systems: Design and Use)
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Grid-connected systems, power factor correction and harmonic filtering devices are
essential. However, the grid-interface criteria vary with the utility companies and have
yet to be standardized internationally. Most of the inverters now being used for grid-
connected applications incorporate peak-power tracking capability. Those inverter
controls the PV array output to maintain operation at its maximum power point, which
changes rapidly with variations in solar intensity and module temperature.
3.1 Grid-Connected Photovoltaic (PV) Systems
The load in such plants is the utility network, and the usual assumption here is that the
grid is capable of accepting any amount of power from the PV plant. In other words, the
utility grid serves as an infinite energy sink. The utility company dictates the
requirements for the grid-connected system, and each utility may impose a unique set of
requirements. The main grid interface criteria, which should be checked with the utility,
are the following:
Voltage regulation
Frequency regulation (usually 2% of nominal)
Harmonic distortion in the operating load range:
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⇒ Total of all current harmonics (usually 5% maximum)
⇒ Any single current harmonic (usually 3% maximum)
⇒ Total of all voltage harmonics (usually 5% maximum)
Power factor and reactive power consumption:
⇒ Utilities often stipulate a power factor requirement for co generators, from
0.9 lagging to 0.9 leading at full load.
⇒ Reactive power consumption is closely related to the power factor (PP).
Typical residential and industrial loads operate with a lagging PF as low as
0.85. Because of this, the utility requires some power factor correction by
co-generating sources to minimize reactive power being supplied by the
grid. The inverters used in the PV system consume reactive power and thus,
the utility could lose revenue due to real-power line losses.
Protection and operation criteria such as:
⇒ Inverter disconnect criteria in the event of a grid failure (loss of voltage),
inverter failure, or a ground fault on the dc side
⇒ Inverter reconnects criteria
⇒ Adequate safeguard against "islanding" (inability of self-commutated
inverters to detect grid shut-down so that they continue to operate and feed
power into the grid)
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3.2 Performance Calculator For Grid-Connected PV Systems
Fixed or tracking array
The PV array may either be fixed, sun-tracking with one axis of rotation, or sun-
tracking with two axes of rotation. The default value is a fixed PV array.
Figure 3-2: PV Array Orientation
(Stuart R. Wenham, Martin A. Green and Muriel E. Watt: Applied Photovoltaic)
PV array tilt angle (0° to 90°)
For a fixed PV array, the tilt angle is the angle from horizontal of the inclination of
the PV array (0° = horizontal, 90° = vertical). For a sun-tracking PV array with one
axis of rotation, the tilt angle is the angle from horizontal of the inclination of the
tracker axis. The tilt angle is not applicable for sun-tracking PV arrays with two
axes of rotation. The default value is a tilt angle equal to the station's latitude. This
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normally maximizes annual energy production. Increasing the tilt angle favors
energy production in the winter, while decreasing the tilt angle favors energy
production in the summer. For roof-mounted PV arrays, Table 3-1 below gives tilt
angles for various roof pitches (ratio of vertical rise to horizontal run).
Roof Pitch Tilt Angle (°)
4/12 18.4
5/12 22.6
6/12 26.6
7/12 30.3
8/12 33.7
9/12 36.9
10/12 39.8
11/12 42.5
12/12 45.0
Table 3-1: Tilt angles for various roof pitches
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PV array azimuth angle (0° to 360°)
For a fixed PV array, the azimuth angle is the angle clockwise from north of the
direction that the PV array faces. For a sun-tracking PV array with one axis of
rotation, the azimuth angle is the angle clockwise from north of the direction of the
axis of rotation. The azimuth angle is not applicable for sun-tracking PV arrays with
two axes of rotation. The default value is an azimuth angle of 180° (south-facing).
This normally maximizes energy production. Increasing the azimuth angle favors
afternoon energy production, while decreasing the azimuth angle favors morning
energy production. Table 3-2 below provides azimuth angles for various compass
headings.
Compass Heading Azimuth Angle (°)
N 0 or 360
NE 45
E 90
SE 135
S 180
SW 225
W 270
NW 315
Table 3-2: Azimuth angles for various compass headings
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3.3 Grid-Connected Inverter Photovoltaics
Various inverter topologies used Conditioners may produce different degree of quality
of AC power output. While PV/Diesel system can operate in stand-alone mode, it can
also be connected through a line inductor to the Grid as an UPS system. Integrated
Solar/Mains/Diesel system has several desired features such as UPS function, peak
shaving function, Power Conditioning of weak grid supply, Active filtering, Voltage