teorie fotovoltaice

7/29/2019 teorie fotovoltaice

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Solar Photovoltaic

Solar PV generation involves the generation of electricity from free and

inexhaustible solar energy.

The major advantaje of a PV systems are:

- sustainable nature of solar energy as fuel

- minimum environmental impact

- drastic reduction in customers electricity bills due free availability of sunlight

- long functional lifetime of over 30 years with minimum maintenance- silent operation no sound pollution (no moving parts)

The major disadvantage of PV systems are:

- Initial cost. The lowest cost of PV panels is around 1EUR/W the other

components of a PV plant (inverter, infrastructure) are not included.- Solar cells produce DC which must be converted to AC (using a grid-tied

inverter when used in currently existing distribution grids. The energy

conversion system produces an energy loss of 4-12%.- The PV energy conversion efficiency is up to 22% (the latest technology), but

usually under 15%.

- Limited power density: approx. 1000 2/mW it strongly depends of the

location.- Solar electricity is not available at night and is less available in cloudy

weather conditions. Therefore, for islanded systems a storage orcomplementary power source is required.

- Solar electricity is almost always more expensive than electricity generated byother sources.

A PV systems consists mainly of:- PV panels that convert solar power into DC electrical power.

- Power convertors that transforms the DC power into AC power.

A single PV panel is made of multiple cells connected in series and parallel on

solid frame. Generally, one PV module has a rated power of 100 200W. The modules

are connected in series and parallel to obtain a certain output voltage and power. PVpanel orientation can be fixed at an optimal angle according to the location (most used),or variable using a sun trackers (electrical or hydraulic).

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Basic of PV energy conversion

PV cell converts sunlight directly into electricity. It is made of semi-conducting

material in two layers: P and N. When radiation from the sun hits the photovoltaic cell,the boundary between P and N acts as a diode: electrons can move from N to P, but notthe other way around.

Photons with sufficient energy hitting the cell cause electrons to move from the P

layer into the N layer. An excess of electrons builds up in the N layer while the P layerbuilds up a deficit. The difference in the amount of electrons is the voltage difference,

which can be used as a power source.

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PV model

sh

ddphshdphPR

VIIIIII ==

Where:

- PI is the output cell current

- phI is the photovoltaic current

- dI is the diode current

- shI is the shunt current

PSdP IRVV =

=1exp0

nkTqVII dd

Where:

- 0I is reverse saturation current

- q is electron charge ( Cq19106,1 = )

- k is Boltzmanns constant (1231038,1 = Kjoulek )

- T is cell temperature [K]

- dV is diode voltage

- n is diode ideality factor (1 for an ideal diode)

( )[ ]251000

+= TKIG

I ISCph

Where:

- G is the solar irradiance in 2/mW

- SCI is the short-circuit current at CO25 and 1000 2/mW

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- IK is the short-circuit current temperature coefficient

CAK OI 0017,0=

PV maximum power

The maximum power extracted from a PV panel is achieved for a certain point onthe current-voltage characteristic.

Maximum power point (MPP) mainly varies with the irradiance and with the celltemperature. PV systems include a maximum power point tracer (MPPT), which controls

the output PV current or voltage in order to bring the operating point at MPMP VI , .

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PV panel parameters

The values of the performance parameters are specified for standard conditions of

irradiance 1000 2/mW and cell temperature CO

25 . The quality standard of crystalline Si

PV modules is IEC-61215.

Main electrical parameters:

- Module Power ( maxP ) [Wp]

- Rated Voltage ( MPPV ) [V]

- Rated Current ( MPPI ) [A]

- Open Circuit Voltage ( OCV ) [V]

- Short Circuit Current ( SCI ) [A]

- Module efficiency [%]

Photovoltaic Power Plants grid connected

The two main components of a PV system connected to the grid are: PV panelsand DC-AC converter (inverter).

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PV converter classification:

1) with DC-DC converter with isolation on the low-frequency side:

2) with DC-DC converter with isolation on the high-frequency side:

3) with DC-DC converter without isolation transformerless:

Example:

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4) without DC-DC converter with isolation:

Example:

5) without DC-DC converter without isolation - transformerless

Example:

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PV plant design

PV modules may be connected to the grid with module inverters, string inverters,

or central inverters:

1) Module inverters with small power ratings, are fixed on the back side of every

module. They can adjust an optimal MPP per device than results in a high total energy

yield of the PV system. Nevertheless, this topology is quite expensive due to large

number of inverters, extended AC-side cabling and maintenance.

2) String inverters convert the DC power of a whole module string. Compared to

the module inverter, the MPP control is less optimal if the incident light is unevenly

distributed or shading aries on some modules.

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3) Central inverters offer the advantage of high efficiency and low specific cost

(only one inverter). The plant panels are arranged in many parallel strings that areconnected to the single central inverter on the DC side. The main drawback is the

dependence of the power generation on a single component, if the central inverter failsthen the generation unit will stop working.

PV stand-alone systems (islanded)

In islanded mode PV power plants feed local consumers with electrical energy.Due to the solar energy intermittent nature, storage devices have to be used in

conjunction with PV plants in order to achieve continuous supply of the loads.

Moreover, most times PV is part of hybrid power systems, where several energysources are used. Wind and solar with energy storage is the most spread configuration

because of the two sources complimentarily (sun in the day time and in the summer, windin the night time and in the winter)

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Configurations:

1)

This configuration is the simplest, but with the lowest performance. The PV andbattery are connected on a common DC-bus that supplies DC loads and the inverter. A

power management system ensure proper charging/discharging conditions for the battery,

by switching on/off thePV/inverter/DC loads in case of over-charging and over-

discharging.The main disadvantage is that the maximum PV power cannot be extracted,

because it is directly connected on the battery thus, the battery impose the PV voltage.

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2)

This configuration includes a DC-DC converter in series with the PV, witch acts

as battery charge controller and MPPT, thus extracting maximum power from the PV.

The system performance are improved, but with the cost of an additional power

converter.

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3)

In this configuration both the battery and the PV are connected in the system

trough DC-AC converters and the power exchange is accomplished on AC bus.It is the most flexible configuration and is suitable for higher power range.

However the complexity and cost are also higher.

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