17-08-2013, 03:54 PM
PHOTOVOLTAIC SYSTEMS
[attachment=57213]
Components of a PV system
The solar energy conversion into electricity takes place in a semiconductor device that
is called a solar cell. A solar cell is a unit that delivers only a certain amount of electrical
power. In order to use solar electricity for practical devices, which require a particular voltage
or current for their operation, a number of solar cells have to be connected together to form a
solar panel, also called a PV module. For large-scale generation of solar electricity the solar
panels are connected together into a solar array.
The solar panels are only a part of a complete PV solar system. Solar modules are the
heart of the system and are usually called the power generators. One must have also
mounting structures to which PV modules are fixed and directed towards the sun. For PV
systems that have to operate at night or during the period of bad weather the storage of energy
is required, the batteries for electricity storage are needed. The output of a PV module
depends on sunlight intensity and cell temperature; therefore components that condition the
DC (direct current) output and deliver it to batteries, grid, and/or load are required for a
smooth operation of the PV system. These components are referred to as charge regulators.
For applications requiring AC (alternating current) the DC/AC inverters are implemented in
PV systems. These additional components form that part of a PV system that is called balance
of system (BOS). Finally, the household appliances, such as radio or TV set, lights and
equipment being powered by the PV solar system are called electrical load. The elements of a
PV system are schematically presented in Figure 9.1.
PV modules
The solar cell is the basic unit of a PV system. An individual solar cell produces direct
current and power typically between 1 and 2 W, hardly enough to power most applications.
For example, in case of crystalline silicon solar cells with a typical area of 10 × 10 cm2 an
output power is typically around 1.5 Wp, with Voc ≈ 0.6 V and Isc ≈ 3.5 A. For actual usage,
the solar cells are interconnected in series/parallel combinations to form a PV module.
In the outdoor environment the magnitude of the current output from a PV module
directly depends on the solar irradiance and can be increased by connecting solar cells in
parallel. The voltage of a solar cell does not depend strongly on the solar irradiance but
depends primarily on the cell temperature. PV modules can be designed to operate at different
voltages by connecting solar cells in series. Table 9.1 contains typical parameters that are
used in module specification sheets to characterize PV modules.
Balance of system
Mounting structures
The principal aim of the mounting structures is to hold the PV modules securely in
place, which usually means that they have to resist local wind forces. When placed in a public
area the structures should prevent stealing the modules. The further common requirements are
not to cause shading of the modules and to be arranged so that there is an easy access to the
modules for the maintenance or repair. The cost of the structures should be low. For
integration in buildings, special mounting structures are being developed that together with
the modules serve as building elements. Typical examples are PV modules in the facades of
buildings, on the roofs of houses, on the roofs of telephone boxes, outdoor lights and warning
signs, and in the noise barriers on motorways. The additional cost of placing PV modules on a
sun-tracking system makes this configuration not profitable in most PV applications.
Energy storage
The simplest means of electricity storage is to use the electric rechargeable batteries,
especially when PV modules produce the DC current required for charging the batteries. Most
of batteries used in PV systems are lead-acid batteries. In some applications, for example
when used in locations with extreme climate conditions or where high reliability is essential,
nickel-cadmium batteries are used. The major difficulty with this form of storage is the
relative high cost of the batteries and a large amount required for large-scale application.
Charge regulators
Charge regulators are the link between the PV modules, battery and load. They protect
the battery from overcharge or excessive discharge. Charge and discharge voltage limits
should be carefully selected to suit the battery type and the operating temperature. These
settings can significantly affect maximum operational life of a battery. High temperatures
tend to reduce battery life because they accelerate corrosion and self-discharge. High
temperatures may also increase out gassing during charging and therefore should be
controlled. The resistance of lead-acid batteries to freezing is reduced when they are
discharged, so batteries should be kept charged when they are left in low temperature
conditions during the winter.
PV system design
Sizing of PV system
Sizing of a PV system means determining how much energy is required to run the
system and how many PV modules are needed to generate it. A PV system has to generate
enough energy to cover the energy consumption of the loads (lights, appliances, equipment)
and energy used by the system itself. The accurate design of a PV system is usually carried
out using a computer model, which calculates the energy yield of the PV system for a
particular location. The size and configuration of solar array is then optimised in order to
match the energy yield of the system to the energy consumption of the system. The energy
yield of a PV system depends on the type of PV modules, the characteristics of a PV inverter,
the orientation of the modules, and meteorological conditions.
Operation
PV systems can generate high voltages. Safety is therefore very important in order to
avoid accidents and damage of expensive components and equipment. For safety reasons,
solar arrays are normally earthed, either by placing a matrix of metal in the ground under the
array, or by using conventional earth rods.
It is normally not necessary to protect solar array from direct lightning strikes,
provided that their mounting structure is well earthed. However, inverters or other electronics
controls connected to the array should be protected. Blocking diodes are installed in solar
arrays to prevent reverse current flows into the modules, which may damage the modules and
cause energy losses. By-pass diodes are incorporated into modules to prevent damage of
arrays when some cells or modules become shaded.
[attachment=57213]
Components of a PV system
The solar energy conversion into electricity takes place in a semiconductor device that
is called a solar cell. A solar cell is a unit that delivers only a certain amount of electrical
power. In order to use solar electricity for practical devices, which require a particular voltage
or current for their operation, a number of solar cells have to be connected together to form a
solar panel, also called a PV module. For large-scale generation of solar electricity the solar
panels are connected together into a solar array.
The solar panels are only a part of a complete PV solar system. Solar modules are the
heart of the system and are usually called the power generators. One must have also
mounting structures to which PV modules are fixed and directed towards the sun. For PV
systems that have to operate at night or during the period of bad weather the storage of energy
is required, the batteries for electricity storage are needed. The output of a PV module
depends on sunlight intensity and cell temperature; therefore components that condition the
DC (direct current) output and deliver it to batteries, grid, and/or load are required for a
smooth operation of the PV system. These components are referred to as charge regulators.
For applications requiring AC (alternating current) the DC/AC inverters are implemented in
PV systems. These additional components form that part of a PV system that is called balance
of system (BOS). Finally, the household appliances, such as radio or TV set, lights and
equipment being powered by the PV solar system are called electrical load. The elements of a
PV system are schematically presented in Figure 9.1.
PV modules
The solar cell is the basic unit of a PV system. An individual solar cell produces direct
current and power typically between 1 and 2 W, hardly enough to power most applications.
For example, in case of crystalline silicon solar cells with a typical area of 10 × 10 cm2 an
output power is typically around 1.5 Wp, with Voc ≈ 0.6 V and Isc ≈ 3.5 A. For actual usage,
the solar cells are interconnected in series/parallel combinations to form a PV module.
In the outdoor environment the magnitude of the current output from a PV module
directly depends on the solar irradiance and can be increased by connecting solar cells in
parallel. The voltage of a solar cell does not depend strongly on the solar irradiance but
depends primarily on the cell temperature. PV modules can be designed to operate at different
voltages by connecting solar cells in series. Table 9.1 contains typical parameters that are
used in module specification sheets to characterize PV modules.
Balance of system
Mounting structures
The principal aim of the mounting structures is to hold the PV modules securely in
place, which usually means that they have to resist local wind forces. When placed in a public
area the structures should prevent stealing the modules. The further common requirements are
not to cause shading of the modules and to be arranged so that there is an easy access to the
modules for the maintenance or repair. The cost of the structures should be low. For
integration in buildings, special mounting structures are being developed that together with
the modules serve as building elements. Typical examples are PV modules in the facades of
buildings, on the roofs of houses, on the roofs of telephone boxes, outdoor lights and warning
signs, and in the noise barriers on motorways. The additional cost of placing PV modules on a
sun-tracking system makes this configuration not profitable in most PV applications.
Energy storage
The simplest means of electricity storage is to use the electric rechargeable batteries,
especially when PV modules produce the DC current required for charging the batteries. Most
of batteries used in PV systems are lead-acid batteries. In some applications, for example
when used in locations with extreme climate conditions or where high reliability is essential,
nickel-cadmium batteries are used. The major difficulty with this form of storage is the
relative high cost of the batteries and a large amount required for large-scale application.
Charge regulators
Charge regulators are the link between the PV modules, battery and load. They protect
the battery from overcharge or excessive discharge. Charge and discharge voltage limits
should be carefully selected to suit the battery type and the operating temperature. These
settings can significantly affect maximum operational life of a battery. High temperatures
tend to reduce battery life because they accelerate corrosion and self-discharge. High
temperatures may also increase out gassing during charging and therefore should be
controlled. The resistance of lead-acid batteries to freezing is reduced when they are
discharged, so batteries should be kept charged when they are left in low temperature
conditions during the winter.
PV system design
Sizing of PV system
Sizing of a PV system means determining how much energy is required to run the
system and how many PV modules are needed to generate it. A PV system has to generate
enough energy to cover the energy consumption of the loads (lights, appliances, equipment)
and energy used by the system itself. The accurate design of a PV system is usually carried
out using a computer model, which calculates the energy yield of the PV system for a
particular location. The size and configuration of solar array is then optimised in order to
match the energy yield of the system to the energy consumption of the system. The energy
yield of a PV system depends on the type of PV modules, the characteristics of a PV inverter,
the orientation of the modules, and meteorological conditions.
Operation
PV systems can generate high voltages. Safety is therefore very important in order to
avoid accidents and damage of expensive components and equipment. For safety reasons,
solar arrays are normally earthed, either by placing a matrix of metal in the ground under the
array, or by using conventional earth rods.
It is normally not necessary to protect solar array from direct lightning strikes,
provided that their mounting structure is well earthed. However, inverters or other electronics
controls connected to the array should be protected. Blocking diodes are installed in solar
arrays to prevent reverse current flows into the modules, which may damage the modules and
cause energy losses. By-pass diodes are incorporated into modules to prevent damage of
arrays when some cells or modules become shaded.