2 PV Flashcards
Describe a 1st generation PV cell
- bulk silicon devices that use wafers of mono- or poly-crystalline silicon
- produced in large quantites
- dominate the market
Describe a 2nd generation PV cell
- Use either thin film (amorphous silicon) or other semiconductor materials (eg CIGS or CdTe)
- Amorphous silicon is cheaper but less efficient than crystalline silicon and other thin film semiconductor materials
Describe a 3rd generation PV cell
- uses emerging tech (like dye-sensitised and organic/polymer solar cells)
- the principle operation of 3rd gen differs from 1st and 2nd
- yet to be produced in large quantities
Why are 1st gen PVs used the most?
- robust modules that are stable over time and have well-defined operating characteristics
- has already been large investment in large manufacturing plants and the cells are produced in large quantities
- BUT, they use very pure silicon which is expensive and energy intensive to produce
What are the main components of a grid connected PV system?
Module (cost dominating)
Max power point tracking
Inverter
Mains
What is the role of an inverter?
What is the role of Max power point tracking?
What are the components of a stand-alone PV system?
Solar modules
battery (dominate reliability - esp if lead acid)
controller
low voltage disconnect
What is the role of a controller?
What is the role of an battery?
What is the role of low voltage disconnect?
What is the purpose of a blocking diode?
It is connected in series with a module or string of modules to ensure there is no reverse current flow when there is no sun.
A blocking diode is always connected to prevent a battery discharging into a PV array at night.
What is the purpose of a bypass diode?
They’re connected across strings of cells to ensure no current is forced through a reverse biased shaded cell
What happens if current is forced through a reverse biased shaded cell?
Creates hot spots and damage to the cell
Why are Maximum Power Point Trackers used?
Max power is obtained on the knee of the VI curve
This point changes with cell temp and irradiance
Hence need to use MPPTs to actively control the module terminal voltage that’s required
Common methods of Maximum power point tracking
Perturb and observe (hill climbing)
Incremental conductance
Fraction of open circuit voltage (less effective)
What is Perturb and observe as an MPPT?
What is incremental conductance as a MPPT?
What is fraction of open circuit voltage as MPPT?
What components are included within the equiv circuit of a PV cell?
Current Source
Forward Biased Diode
Shunt Resistor
Series Resistor
Explanation of the PV cell equivalent circuit
- When a PV cell is dark, no electrons are excited from the valence to conduction band (into the crystal lattice) and the PV cell acts as a simple diode
- When exposed to light, some photons have enough energy to excite the electrons to the conduction band and then pulled across elec field created by the depleted region
- ∴ the equiv circuit is a current source with a forward biased diode
- the bulk and contact resistances of the semiconductor adds a series resistor
How does open circuit voltage change with irradiance?
No change
How does short circuit current change with irradiance?
Directly proportional
What is the FF when VI curve is of rectangular characteristic?
FF = 1
How does Voc change with cell temperature?
It will rise. Can be calculated using the difference between temps from STC to the given one, as well as the number of cells and 𝛿Voc
What are the units of insolation?
kWh(sun)/day
