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Solar photovoltaic (SPV) is primarily a semi conductor based
technology used to convert solar radiation into direct electricity. A
PV system basically comprises of PV modules and the balance of systems
(BOS). Balance of systems includes support structure, wiring, storage,
power electronics etc.
When solar radiation strikes the PV module, DC (direct current) electricity
is generated. During generation, power can be used in any DC load directly.
But the generation exists till sun shines. So, some storage device is
needed to run the system at night or in low sunshine hour. Again this
power cannot be used to run any AC (alternating current) load. Inverter
has to be used to convert DC into AC.
Initially photovoltaic was confined to space application but gradually
Success of solar cells in space led to their use in terrestrial applications
- starting from consumer goods to telecommunication. Now it has found
its multi-dimensional ways in building application.
PV is becoming more popular because of the following points
• PV technology is one of the well proven renewable technology
• PV system has successfully been used in different small and large
applications
• PV is a modular technology. Any amount of power, starting from
miliwatt to megawatt can be generated. PV system can be added to the existing
PV power if load demand increases.
• PV system has no moving parts, consuming no conventional fuels
and creating no pollution.
Types of PV system (PV configuration):
There are two basic configuration of SPV system
• Stand-alone system
• Grid interactive system (PV system with a connection to utility
grid)
Stand-alone systems are virtually self sufficient and not interacted with
grid. Such system may have some backup/storage system to run during the
no sun or low sun hour.
• PV system without storage battery (Direct coupled
PV system)
• DC system with storage battery
• DC systems powering AC load (with or without storage)
PV system without storage battery (Direct
coupled PV system):
This is the simplest and least expensive photovoltaic system designed
to be used only during daytime. Here the electricity generated is directly
and simultaneously used by the appliances. Through out the day, the insolation
level is changing continuously and so the output.
Examples of direct use systems include:
• Remote water pumping with a storage tank.
• Stand alone solar powered appliances such as calculators and toys.
DC Systems With Storage Batteries:
Photovoltaic systems must include a means of storing electrical energy
if they are to operate loads at night or during cloudy weather. Batteries
are used to store the electrical energy generated by the photovoltaic
modules. Power can be drawn from the batteries whenever required- during
the day or night, continuously or intermittently. In addition, a battery
bank has the capacity to supply high surge currents for a short time.
This gives the system the flexibility to start large motors or to perform
other “high power” tasks. This system’s basic components
include a photovoltaic module, a charge controller, storage batteries
and appliances that represent the system's electrical load. But here the
type of loads used should be of DC load as battery is capable of running
DC load only.
PV with AC Loads :
Photovoltaic modules produce direct current (DC) electrical power and
batteries store DC energy. However many common appliances require alternating
current (AC) power. Direct current systems which power AC loads must use
an inverter. Inverters provide convenience and flexibility in a photovoltaic
system but also add complexity and cost. The following figure shows PV
system with storage battery, powering AC load. It is also possible to
power the AC load without battery but in that case it would be confined
only to daytime when solar radiation is sufficient to generate required
electricity.
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Utility grid interconnected system:
A utility grid interactive photovoltaic system is connected to the utility
grid. A specially designed inverter is used to transform the PV generated
DC electricity to the grid electricity (which is of AC) at the grid voltage.The
main advantage of this system is that power can be drawn from the utility
grid and when power is not available PV can supplement that power. But
again such grid interactive system is designed with battery or without
battery storage.
Utility grid Interconnected Systems –
without battery:
Addition of battery adds cost to a PV system. Even battery-less grid interactive
system is possible. But here, there will be no back up when the grid power
is interrupted. Without battery storage, surplus power produced by the
PV system can be fed to the grid through an specially designed inverter.
The inverter should be of special kind, as it has to automatically shut
down and not feed solar power into the grid when the utility grid goes
down due to some reason. In such cases, solar power can be diverted to
the load used. But it is possible only during daytime as there is no battery
backup. The inverter specified for this type of system has a feature which
controls the PV array voltage to operate at it’s point of maximum
efficiency.
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Net metering system in grid interactive
PV system:
Net metering system allows utility customers to generate their own electricity
and reduce their electric bill. Here the surplus PV generated power is
fed into the grid and the meter can note the unit generated. Another meter
is used to record the power unit that is drawn from the grid when there
is power demand at the site. In some cases, same meter box is used to
serve both the purposes. It runs forward when power is drawn from the
grid and runs backward when power is fed to the grid. At the end of the
month, electric bill is prepared based on the net unit. In order to support
the production of PV –generated power, some utilities offer a better
price for the unit (kWh) fed into their grid than they charge for the
unit from the grid.
Hybrid Systems
System with more than one source of power is called “Hybrid system”
Since the supply of solar is very unpredictable, it is often desirable
to design a system with additional source of power. The most common type
of hybrid system contains a gas or diesel powered engine generator. Another
hybrid approach is a PV/Wind system. Adding a wind turbine to a PV system
provides complementary power generation. The wind often continues to blow
at night and during low sun conditions. For even greater reliability and
flexibility, an engine generator can be included in a PV/Wind system.
A PV system consists of following components.
1. Solar PV module
2. Battery
3. Charge controller
4. Inverter/converter
5. Mounting structure and tracking device
6. Interconnections and other devices
In every configuration all these components are not used. Components used
depend upon the type of configuration, which in other way depend upon
the application. For example: Storage battery is not used in case of direct
coupled PV system, inverter is not used in case of DC load.
• Solar irradiation: Power
of a solar cell changes with solar radiation. which is different for different
geographical location, tilt and orientation.. The change of power is almost
linear with the solar radiation. There is a very little change in open
circuit voltage (Voc) of the solar cell, but the short circuit current
(Isc) varies almost linearly with the solar intensity.
• Temperature: Power decreases with increasing
solar cell temperature. Voc decreases by a value of approximately 3mV/K
for each degree rise in temperature. A solar cell with Voc of 0.6 V at
250C reaches a value of 0.45V at 750C.Isc increases with rise of temperature
but the reduction in voltage is much greater than the corresponding increase
in current. This affects the power, which decreases at a rate of about
0.45% per degree rise in temp. The operating temperature of the battery
should be nominal (25 –35 degree C). Higher temperature may give
a higher capacity of battery but at the same time it reduces the life
of the battery.
• Aging effect: Solar cells, which are
properly encapsulated, have a very long life and power does not reduce
in any significant manner. The effect of aging is more severe in amorphous
Si solar cells.
• Shading effect: Shading has a very bad
impact on the performance of the PV system. Even a partial shading (on
one or two cells) of the whole module can reduce the output drastically
and if it persists for a longer period, it may damage the whole system.
To protect the modules from such adverse effect, a bypass diode is used.
The effect is more prominent in crystalline silicon solar modules. Amorphous
silicon modules are less affected by shading.
• Other effect: Mismatching of module
in a string, resistance of wires and cables etc can drastically alter
the performance of the PV system.Dust and dirt can reduce the PV output.
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