1. Prof. Sukumar Mishra Indian Institute of Technology Delhi
ROOF TOP PV
Prof. Sukumar Mishra
Indian Institute of Technology Delhi

2. Non-conventional Energy Sources
Need
Environmental Concerns – Reduces CO2 emissions
Reduces dependence on fossil fules
Solution for meeting fast growing power requirements
Types
Solar Energy
Wind Energy
Geothermal Energy
Biomass
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3. Non-Conventional Energy Scenario in India
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4. Photovoltaic Power Generation
Electricity is generated from the solar energy using photovoltaic (PV) cells.
PV cell is a silicon PN junction device
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5. Operation of PV cell
Typically, a PV cell generates a voltage around to 0.8 volts depending on the semiconductor and the built-up technology.
This voltage is low enough for any use. Therefore tens of PV cells (involving 36 to 72 cells) are connected in series to form a PV module. These modules can be interconnected in series and/or parallel to form a PV panel.
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6. PV cell characteristics
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7. PV Array Formation
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8. PV characteristics
These characteristics depend heavily on insolation and temperature.
Series and parallel resistance also varies the characteristics.
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9. Partial Shading scenario
Shading may occur due to clouds, environmental obstructions such as trees and buildings, self-shading between panels in parallel rows, dirt and dust, bird droppings etc.
These shading effects may be static, i.e. slow due to sun-light angle during the day or may be very dynamic (e.g. moving clouds).
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10. Series-Parallel Connection of PV Cells
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11. Effect of partial shading
Leads to change in characteristics
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12. Effect of partial shading (Cont.)
Leads to heating in the cell due to losses in the current flowing through the shunt resistance.
This leads to hotspot at the nth cell and fire hazard in the PV park
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13. Use of bypass diode
A diode is put in parallel to the cell to bypass the sheded cell and avoid hotspot.
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14. Use of blocking diode
Blocking diode blocks any circulating current flow to the shaded cell in parallel connected PV strings.
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15. Mismatch in PV characteristics due to manufacturing
The electrical characteristics of PV panels have some manufacturing variance (e.g. 5%) at manufacturing and it gets worse with temperature changes and over time (and PV systems are usually installed for many years).
Since the string has the same current for all panels, traditional inverters try to find the optimal current that will fit most of the panels.
However, there will be some energy penalty that may reach 3-5% overall and even more over time.
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16. Uniform Shading
If all sub-strings have the same illumination/shading, then the total current/power will be degraded according to the shading ratio. But if the substrings have different shading, then like partial shading in a serial string, the total power performance will be degraded by more than the shading ratio
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17. Concern for whom
On large solar plants, this is not really a problem as the panels are cleaned regularly and they have been constructed and designed to avoid any shading. Solar fields are also usually away from tall building and other obstructions.
But on a residential or commercial roof top, you can have a lot of ways there can be shade
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18. Benefit of Micro-inverter compared to string inverter
Solution
Micro inverters instead of string inverters
Benefit of Micro-inverter compared to string inverter
The string voltage is typically about V, which means it is enough to seriously electrocute someone.
If you have a fire in your building, you could shut off the AC, but these things [solar panels] are sitting in the sun – if it’s daylight – and they are at open circuit voltage. If you get a fireman spraying water on that roof and he is on a fire truck or a ladder, he would get electrocuted.”
If each panel has an inverter, however, this would not happen as the voltage would be too low to cause any harm.
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19. MPPT tracking
Techniques have been developed to extract maximum power from the PV system.
Perturb and Observe
Incremental Conductance
Fractional open circuit voltage
Fractional short circuit current
Fuzzy logic control
Neural Network
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20. Pros and Cons of Solar Electricity
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21. Growth of PV systems in India
Jawaharlal Nehru National Solar Mission
Target of deploying MW grid connected solar power by 2022.
Grid connected solar power in India (till 31/03/2014) – 2647 MW
Off grid solar power (till 31/03/2014) – MW
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22. Grid Connected PV System
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23. THANK YOU
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