Answer: Solar PV Principle and Typical Solar Energy Systems

The Photovoltaic Effect

The photovoltaic effect, discovered by Alexandre-Edmond Becquerel in 1839, is the fundamental principle:

1. Photon Absorption: When sunlight (photons) strikes a PV cell made of semiconductor material (silicon), photons transfer their energy to electrons
2. Electron Excitation: If photon energy exceeds the bandgap energy (~1.1 eV for silicon), electrons are knocked free from their atomic bonds, creating free electrons and holes
3. P-N Junction: The cell consists of two doped silicon layers - N-type (phosphorus-doped, excess electrons) and P-type (boron-doped, excess holes). Their interface creates an electric field
4. Charge Separation: The internal electric field at the p-n junction separates the freed electrons (pushed to N-side) and holes (pushed to P-side)
5. Current Flow: When an external circuit connects the two sides, electrons flow from N to P through the circuit, creating direct current (DC)

Structure of a PV Cell

Component	Material	Function
Anti-Reflective Coating	Silicon Nitride (SiN)	Reduces light reflection, maximizes absorption
Front Contact	Silver grid/fingers	Collects electrons (negative terminal)
N-Type Silicon	Phosphorus-doped Si	Excess electrons (donor layer)
P-N Junction	Depletion region	Creates electric field for charge separation
P-Type Silicon	Boron-doped Si	Excess holes (acceptor layer)
Back Contact	Aluminum	Completes circuit (positive terminal)

Types of PV Cells

Type	Description	Efficiency
Monocrystalline	Single crystal silicon, uniform structure	18-22%
Polycrystalline	Multiple crystals, less uniform	15-18%
Thin-Film	CdTe, CIGS, a-Si on substrate	10-13%
Bifacial	Absorbs light from both sides	20-25%

Key Parameters

• Open Circuit Voltage (Voc): Maximum voltage when no current flows (~0.6V per cell)
• Short Circuit Current (Isc): Maximum current when voltage is zero
• Maximum Power Point (Pmax): Optimal operating point (Vmp × Imp)
• Fill Factor (FF): Ratio of actual to theoretical maximum power
• Efficiency: Power output / Solar irradiance × Area

1. Grid-Tied (On-Grid) System

Most common system that connects to utility grid.

Components:

• Solar panels (array)
• Grid-tied inverter (converts DC to AC, synchronizes with grid)
• Net meter (measures energy exported/imported)
• AC distribution panel

Working:

• Excess solar power exported to grid (net metering credit)
• Grid supplies power when solar is insufficient
• No battery storage - relies on grid as virtual battery
• Shuts down during grid outage (safety - anti-islanding)

Applications:

Urban rooftops, commercial buildings, utility-scale plants

2. Off-Grid (Standalone) System

Independent system not connected to utility grid.

Components:

• Solar panels
• Charge controller (regulates battery charging)
• Battery bank (stores energy)
• Off-grid inverter (DC to AC)

Working:

• Solar charges batteries during day
• Batteries supply power at night/cloudy days
• Must be sized for worst-case scenario (autonomy days)
• Complete energy independence

Applications:

Remote areas, villages without grid, telecom towers

3. Hybrid System

Combines grid connection with battery backup.

Components:

• Solar panels
• Hybrid inverter (manages grid, battery, solar)
• Battery bank
• Net meter

Working:

• Uses solar first, then battery, then grid
• Provides backup during power cuts
• Can export excess to grid
• Best of both worlds - reliability + backup

Applications:

Homes with unreliable grid, critical loads

4. Solar Water Pumping System

• Solar panels power DC or AC water pump
• No battery - pumps when sun shines
• Water storage tank acts as buffer
• PM-KUSUM scheme: 3.5 million solar pumps by 2026

5. Utility-Scale Solar Power Plant

• Large ground-mounted installations (1 MW to GW scale)
• Central inverters, tracking systems (single/dual axis)
• Sells power to grid at agreed tariff (PPA)
• India's largest: Bhadla Solar Park (2,245 MW)