Solar Photovoltaic Technologies

Transcription

1 Solar Photovoltaic Technologies Lecture-32 Prof. C.S. Solanki Energy Systems Engineering IIT Bombay

2 Contents Brief summary of the previous lecture Wafer based solar PV technology Thin film solar cell technologies Different generation of PV Crystlline Si for thin film solar cells 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 2

3 CEP course on Renewable Energy Solutions Solar Photovoltaic Technologies: Present and Future Issues in PV Modules and Arrays By Prof. C. S. Solanki 23 rd April /1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 3

4 Contents Module design Interconnection effects Temperature effects Other issues Lifetime of PV modules 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 4

5 Module structure Module A number of interconnected encapsulated cells Number of cells: typically 33 or 36 A module should be long-lasting, stable unit provides mechanical support to solar cell, relatively thin material protection to electrical interconnection from harsh environment prevent water or water vapour from corroding the electrical contacts Module can be a rigid structure (C-Si modules) or a flexible one (a-si modules, not always) 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 5

6 Module material Most modules consists of: a transparent top cover, glass an encapsulant, EVA (ethyl vinyl acetate) a rear support layer, Tedlar a frame Top cover requirements: should have high transmittance low reflection (ARC-not robust enough texturing will accumulate dust) Impervious to water stable under prolonged UV exposure high thermal conductivity mechanically rigid to provide mechanical strength Material choices: acrylic, polymers, glass Solution: low iron-content glass is most commonly used low cost, strong, stable, highly transparent, impervious to water, good self-cleaning properties 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 6

7 Module material Encapsulant s requirements: Adhesive between solar cells stable at elevated temperatures and high UV exposure. It should also be optically transparent and should have a low thermal resistance. Requirements Solution: EVA of (ethyl rear vinyl surface: acetate) is the most commonly low thermal used resistance encapsulant and material. Cell are sandwiched it must prevent between the ingress thin EVA of water sheets or water vapour Frame: Solution: a conventional In most PV modules, module frame a thin is typically polymer made sheet, of called aluminium Tedlar, is used as the rear surface.. 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 7

8 Packing density Packing density: Percentage of the area of the module covered with solar cells depends on the shape of the solar cells determines the module efficiency and output power and also module temperature Packing density The "zero-depth concentration effect" in modules with sparsely packed cells and a white rear surface 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 8

9 PV Modules Various modules 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 9

10 Module Voltage In a typical module, 36 cells are connected in series to produce a voltage sufficient to charge a 12V battery. An individual silicon solar cell has a voltage of just under 0.6V under 25 C and AM1.5 illumination. Higher temperature reduces the cell voltage, in any case module should provide 15V to charge battery Module Current Current obtained from a solar cell depends on cell area and cell efficiency Under optimum tilt angle current density from a commercial solar cell is approximately between 30 ma/cm 2 to 36 ma/cm 2 Typical current between 3 to 4A Module circuit design 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 10

11 Solar cell data sheet Manufacturer specify Physical, Electrical, Environmental parameters Cells electrical characteristics Effect of temp on cell parameters 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 11

12 PV modules & arrays Cell Module Array 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 12

13 Current Curre nt Obtaining higher electrical o/p Series Connection 3.9 A I sc Parallel Connection 7.8 A I sc 36 cells in series Voltage Voltage 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 13 V oc 21 Volts V oc I tot 63 Volts Usually cell in module exhibits identical characteristics Shape of the I- V curve of the module is same as that of cells with change in ( qnv scale of axis nkt MI L MIo ( e I-V relationship for N cell in series and M cell in parallel: ) 1)

14 Current Current Variation in module output Parameters affecting module output Light intensity or irradiance Module temperature Type of load connected to the module Effect of irradiance current reduces linearly Effect of irradiance Current increases marginally V oc reduces logarithmically Temp. increases reduces voltage by 2.2 mv/ o C Voltage Voltage 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 14

15 Current Power output Effect of load on Module Output Fixed resistive load noon afternoon x morning : Maximum power point X : Operating point x X : X : -- Voltage Hour of the day Battery load 36 cell vs 33 series connected module cells 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 15

16 Maximum power point tracking Physical MPPT tracking: tracking the sun Electrical MPPT tracking: tracking the load line I-V curve Maximum Power 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 16

17 Mismatch effects Mismatch losses are caused by the interconnection of solar cells or modules with different electrical properties or which experience different conditions under mismatch module output is determined by the solar cell with the lowest output power being generated by the "good" solar cells can be dissipated by the lower performance cell excessive heating may results in irreversible damage Shading of one region of a module compared to another is a major cause of mismatch is PV modules 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 17

18 Mismatch effects The impact and power loss due to mismatch depend on: operating point of the PV module; the circuit configuration; and any parameter which are different from the remainder of the solar cells. For mismatch, the greatest difference is when the cell is driven into reverse voltage bias. Cell dissipates power Cell generates power mostly mismatches are caused by differences in either the short-circuit current or open-circuit voltage Cell dissipates power 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 18

19 Current Current Mismatch for Cells Connected in Series mismatch in short-circuit current or mismatch in open-circuit voltage mismatch in short-circuit current is most common easily caused due to shading Open circuit voltage mismatch Cell 1 Cell 2 Cell 1 Cell 1 Cell 2 Cell 2 Voltage Voltage V total = V1 +V2 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 19

20 Current Current Mismatch for Cells Connected in Series Short-circuit current mismatch power loss due to mismatch depends on the operating point of the module and the degree of mismatch Current of the combination is the short circuit current of the poor cell extra power generated by good cell is dissipated in poor cell Could cause irreversible damage In open circuit voltage situation things are not that severe but at low voltages lot of power loss could occur Cell 2 Cell 1 Cell 2 Cell 1 Voltage Voltage 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 20

21 Mismatch for Cells Connected in Series Cell 1 Cell V - Cell 2 Cell 2 Shadow - 0.5V + under matched condition In short circuit current situation V=0 Forward bias injection current is zero under non-matched condition In short circuit current situation Good cell will reverse bias the poor cell power dissipation in the poor cell 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 21

22 Hot-Spot Heating Hot-spot heating occurs when there is one low current solar cell in a string of at least several high short-circuit current solar cells 9 Unshaded cell 1 shaded cell One shaded cell in a string reduces the current through the good cells, causing the good cells to produce higher voltages that can often reverse bias the bad cell Power gets dissipated in the poor cell local overheating, or "hot-spots", leads to destructive effects cell or glass cracking, melting of solder or degradation of the solar cell. 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 22

23 Destructive effect of hot-spot heating can be circumvent with use of bypass diode Bypass diode is connected in parallel but with opposite polarity Cell 1 Cell 2 Shadow Bypass Diodes + 0.5V V + bypass diode conducts when a solar cell is reverse biased due to Bypass diode 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 23

24 Bypass Diodes one bypass diode per solar cell is too expensive option amount mismatch depends on the degree of shading A partial shading will cause a lower forward bias voltage The maximum group size per diode, without causing damage, is about 15 cells/bypass diode, for silicon cells. normally for 36 cell module 2 bypass diodes are used 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 24

25 Mismatch for Cells Connected in Mismatch is parallel connection is not as severe as in series mismatch occur really at module level, in a module cells are connected in series The voltage across the cell combination is always the same and the total current from the combination is the sum of the currents in the individual cells. Parallel Open circuit voltage of the combination due to mismatch in parallel connection 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 25

26 Mismatch Effects in PV Arrays Open circuit problem arises when there is open circuit or shading electrically identical to the case of one shaded solar cell in series with several good cells Bypass diodes are used to protect modules Blocking diodes May also be used to minimize mismatch losses blocks the current to flow into the shaded module from parallel module It also prevents loading of battery during the night 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 26

27 PV Module Temperature encapsulation alters the heat flow into and out of the PV module thereby increasing the operating temperature of the PV module. The operating temperature of a module is determined by the equilibrium between the heat produced by the PV module, the heat lost to the environment and the ambient operating temperature Increase in temperature reduces the open circuit voltage lowers output power higher temperature increases degradation rates by a factor of about two for each 10 C increase 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 27

28 Heat Generation in PV Modules The factors which affect the heating of the module are: the reflection from the top surface of the module; the electrical operating point of the module; absorption of sunlight by the PV module in regions which are not covered by solar cells; absorption of low energy 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 28

29 current Cell test conditions Standard test condition (STC) 1kW/m 2, 25 o C, wind 1m/s Other test conditions Actual output that a user get is less than the STC Lower irradiance, higher temperature Power is reduced by about 25 in NOC as compared to STC. Voltage 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 29

30 Degradation and Failure Modes for Bulk Silicon PV Modules PV module's operating life is largely determined by the stability and resistance to corrosion of the materials Reversible Reductions in Output Power Shading, dust on the modules, Solar Cell Degradation interconnection failures between the modules A gradual degradation in module performance can be caused by: increases in RS due to decreased adherence of contacts or corrosion (usually caused by water vapor); decreases in RSH due to metal migration through the p-n junction; or antireflection coating deterioration. Other modes Module open circuit, module short circuit, module glass breakage, bypass diode failure, encapsulant failure, hot-spot failure 8/1/2008 IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 30