Solar radiation, solar cell and plant

Solar radiation as an energy source

The sun is a gaseous sphere where nuclear fusion of hydrogen into helium releases energy as electromagnetic radiation. The sun's surface is at about 5778 K. The solar constant is about 1367 W/m² outside the atmosphere. After absorption and scattering, 800–1000 W/m² reaches the ground at noon on a clear day. Daily radiation in Nepal is about 4–5 kWh/m² (average), among the best in South Asia, with about 300 sunny days a year. The potential is estimated at several thousand MW (commonly cited around 2,100 MW for commercial-scale solar). It is free, clean, abundant and renewable but intermittent and dilute.

Solar cell

A solar cell is a p-n junction semiconductor diode (usually silicon) that converts sunlight directly into DC electricity by the photovoltaic effect.

1. A silicon wafer has an n-type layer (doped with phosphorus: extra electrons) on top of a p-type layer (doped with boron: extra holes). At the junction, electrons diffuse into the p side and holes into the n side, leaving a depletion region with a built-in electric field (about 0.6–0.7 V for silicon).
2. When photons with energy greater than the band gap of silicon (1.1 eV) strike the cell, they create electron–hole pairs.
3. The built-in field sweeps electrons towards the n side and holes towards the p side, so charges separate.
4. With metal contacts on the front (grid) and back, and an external load connected, electrons flow through the load, giving DC current. Photons with less energy than the band gap pass through, and extra energy of higher photons becomes heat.

   Sunlight (photons)
     | | | |
  ===========  front grid contact
  |  n-type  |   (electrons)
  |----------|   junction / depletion layer
  |  p-type  |   (holes)
  ===========  back contact
      \___[ Load ]___/
         current flow

A typical cell gives about 0.5–0.6 V and, under full sun, about 30–35 mA/cm²; cells are connected in series and parallel to make modules.

Solar PV plant (grid-connected)

 Solar array --DC--> Inverter --AC--> Transformer --> Grid
      |                  |
   (modules)      MPPT, protection, meter

Optionally, a battery bank with a charge controller serves off-grid systems.

• Modules (panels) are connected in series for voltage and in parallel for current to form an array.
• Maximum power point tracker (MPPT) adjusts the load to extract maximum power.
• The inverter converts DC to AC at 50 Hz.
• A transformer steps up the voltage, and protection and metering connect it to the NEA grid.
• Examples in Nepal: Nepal's utility plants of several MW, for example Devighat, Butwal and Kathmandu valley rooftop solar; the Nepal Electricity Authority also buys power from private solar producers.