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101 renewable - Multi Megawatt PV Power Plants

Article Details

Last Updated
17th of October, 2018

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Take a step back and see how a solar power plant comes together in this 3 min. time lapse video:-


ABB photovoltaic plant erection time lapse video at Stanesti Village, Giurgiu County.

Ghizdaru Solar Park 35 MW x 2 it is located adjacent to Stanesti village, Giurgiu County and covers two secotors of land 17 hectares each, with a long-term concession lease for 25 years. The projects have as energy source 30.888 polycrystalline solar modules. Energy produced it is fed into the national electricity grid through an 220kV OHL transformer station placed in 8km distance from the PV plant. With a total capacity installed of 70 MW the project it is the property of Long Bridge Millennium LLC and was designed and engineered by TIAB as ABB GmbH Mannheim subcontractor. ABB GmbH Mannheim is the main contractor for this project.

Main data of electricity production for  “Ghizdaru Solar Park 35MW x 2”:-

  Total capacity installed: 70MW.
  Connect to Transelectrica 220/110kV transformer station.
  Mean annual electricity production: 82.000 MWh.
  Project total value: EUR 112.000.000 (@ 2013/yr referance).
  Incomeyr (EUR45/MWh): EUR 3.690.000 (@ 2013/yr referance).
  Income/yr (6 Green Certificates/MWh): EUR 27.060.000 (@ 2013/yr referance).
  TOTAL INCOME/yr: EUR 30.750.000 (@ 2013/yr referance).
  Gross Return on investment: 27,4%.

The very basic of an solar power plant:-

The basic of a solar power generation plant it is based on photovoltaic cell current generator principles. A photovoltaic cell can be considered as a current generator and can be represented by the equivalent circuit of Figure 1.

The current I at the outgoing terminals is equal to the current generated through the PV effect Ig by the ideal current generator, decreased by the diode current Id and by the leakage current IL. The resistance series Rs represents the internal resistance to the flow of generated current and depends on the thick of the junction P-N, on the present impurities and on the contact resistances. The leakage conductance Gi takes into account the current to earth under normal operation conditions. In an ideal cell we would have Rs=0 and Gl=0. On the contrary, in a high-quality silicon cell we have Rs=0.05÷0.10Ω and Gi=3÷5mS. The conversion efficiency of the PV cell is greatly affected also by a small variation of Rs, whereas it is much less affected by a variation of Gi.

  Fig. 1

fig-3.3.jpg


The no-load voltage Voc occurs when the load does not absorb any current (I=0) and is given by the relation: Voc =Ii /Gi.

The diode current is given by the classic formula for the direct current:
  Id = ID [Q x Voc / e AxKxT -1]
where:
  ID is the saturation current of the diode;
  Q is the charge of the electron (1.6 . 10-19 C);
  A is the identity factor of the diode and depends on the recombination factors inside the diode itself (for crystalline silicon is about 2);
  k is the Boltzmann constant (1.38 . 10-23J/K);
  T is the absolute temperature in K degree;
  Therefore the current supplied to the load is given by: I =Ig - Id - Ii= Ig - ID x [e QxVoc/ A.k.T -1] - Gi x Voc

In the usual cells, the last term, i.e. the leakage current to earth Ii, is negligible with respect to the other two currents. As a consequence, the saturation current of the diode can be experimentally determined by applying the no-load voltage Voc to a not-illuminated cell and measuring the current flowing inside the cell.

Voltage-Current Characteristics:-

The voltage-current characteristic curve of a PV cell is shown in Figure 2. Under short-circuit conditions the generated current is at the highest (Isc), whereas with the circuit open the voltage (Voc=open circuit voltage) is at the highest. Under the two above mentioned conditions the electric power produced in the cell is null, whereas under all the other conditions, when the voltage increases, the produced power rises too: at first it reaches the maximum power point (Pm) and then it falls suddenly near to the no-load voltage value.

  Fig. 2

fig-2.3.jpg

The characteristic data of a solar cell can be summarized as follows:
  Isc short-circuit current;
  Voc no-load voltage;
  Pm maximum produced power under standard conditions (STC);
  Im current produced at the maximum power point;
  Vm voltage at the maximum power point;
  FF filling factor: it is a parameter which determines the form of the characteristic curve V-I and it is the ratio between the maximum power and the product (Voc . Isc ) of the no-load voltage multiplied by the short-circuit current.

  Fig. 3

fig-4.jpg


If a voltage is applied from the outside to the PV cell in reverse direction with respect to standard operation, the produced current remains constant and the power is absorbed by the cell. When a certain value of inverse voltage (“breakdown” voltage) is exceeded, the junction P-N is perforated, as it occurs in a diode, and the current reaches a high value thus damaging the cell. In absence of light, the generated current is null for reverse voltage up to the “breakdown” voltage, then there is a discharge current similarly to the lightening conditions (Figure 3 – left quadrant).




Grid Connection:-

A PV Plant connected to the grid and supplying a consumer plant can be represented in a simplified way by the scheme of Figure 4.

The supply network (assumed to be at infinite short-circuit power) is schematized by means of an ideal voltage generator the value of which is independent of the load conditions of the consumer plant. On the contrary, the PV generator is represented by an ideal current generator (with constant current and equal insolation) whereas the consumer plant by a resistance Ru.

The currents Ig and Ir, which come from the PV generator and the network respectively, converge in the node N of Figure 2.4 and the current Iu absorbed by the consumer plant comes out from the node:

  Fig. 4

fig-4.4.jpg

  Iu = Ig + Ir
Since the current on the load is also the ratio between the network voltage U and the load resistance Ru:
  Iu = U / Ru - The relation among the currents becomes:
  Ir = U / Ru - Ig
  If in the [2.6] we put Ig = 0, as it occurs during the night hours, the current absorbed from the grid results:
  Ir = U / Ru
On the contrary, if all the current generated by the PV plant is absorbed by the consumer plant, the current supplied by the grid shall be null and consequently the formula [2.6] becomes:
  Ig = U / Ru

When the insulation increases, if the generated current Ig becomes higher then that required by the load Iu, the current Ir becomes negative, that is no more drawn from the grid but put into it. Multiplying the terms of the [2.4] by the network voltage U, the previous considerations can be made also for the powers, assuming as:

  Pu = U . Iu = U2 / Ru the power absorbed by the user plant;
  Pg = U . Ig the power generated by the PV plant;
  Pr = U . Ir the power delivered by the grid

Note:-

The nominal peak power (kWp) is the electric power that a PV plant is able to deliver under standard testing conditions (STC): 1 kW/m2 insolation perpendicular to the panels; 25°C temperature in the cells; Air mass (AM) equal to 1.5;

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