Solar Energy Projects - What is Important in Solar Irradiance Measurements

The input for all solar energy generation systems is solar radiation. Determination of solar irradiance is crucial for gaining knowledge that helps make decisions regarding efficiency, performance, future energy yield and maintenance. These factors play a key role in investments.

This brochure illustrates several useful guidelines for photovoltaic and solar thermal projects. It will include practical data on site prospecting and monitoring the efficiency of solar power plants.

Glossary of Terms

The following are some important terminologies related to solar radiation:

Irradiance: - Radiant flux received by a surface per unit area, irradiance is measured in W/m2

Di-use Horizontal Irradiance (DHI): - DHI is the quantity of energy emitted from the sky that is being scattered in the atmosphere. DHI can be measured using a pyranometer that is shaded from direct sunlight.

Direct Normal Irradiance (DNI): - DNI is the quantity of solar radiation received per unit area by a surface held perpendicular to the rays coming in a straight line from the sun at its current position in the sky.

DNI is measured using a pyrheliometer having a field of view of 5°. It has to be mounted on an automatic sun tracker.

Plane of Array (POA) Irradiance: - This is also referred to as global tilted irradiance (GTI) - irradiance incident on a tilted plane such as a PV panel.

POA irradiance is tough to determine using other measurements because of the complex nature of radiation reflected from the ground and shadowing effects. A pyranometer installed at the same tilt angle as the PV panels can be used to precisely measure it.

Global Horizontal Irradiance (GHI): - GHI is the quantity of solar radiation received per unit area by a horizontal surface from the hemisphere above. It consists of DNI corrected for the angle of incidence on the surface, and DHI.

The relationship can be expressed as GHI = DHI + DNI • cos(θ), where θ is the solar zenith angle (vertically above the location is 0°, horizontal is 90°).

A horizontally mounted pyranometer can be used to measure GHI.

Topics Covered in Brochure

The topics covered in Solar Irradiance Monitoring in Solar Energy Projects brochure are listed below:

  • Site Prospecting and Energy Forecast
  • Fixed Photovoltaic (PV) Installations
  • Medium and large commercial PV installations (MW range)
  • Tracking Photovoltaic (PV) Systems
  • Concentrating Photovoltaic (CPV) Systems
  • Concentrating Solar Power (CSP) Systems

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