Direct Normal Irradiance (DNI)
When the direct radiation from the sun falls on a plane surface at 90° (normal) to the beam the radiative flux is the direct normal irradiance. On a clear day up to 95% of the energy received at the Earth’s surface is DNI, but on a cloudy day it is close to zero.
DNI is of most importance to solar energy technologies that rely on focusing the light from the sun; Concentrating Solar Power (CSP) thermal systems and Concentrating Photovoltaic (CPV). It is measured with a pyrheliometer that has a field of view of 5° and is mounted on an automatic sun tracker that moves to keep the instrument pointed accurately at the sun from sunrise to sunset.
Diffuse Horizontal Irradiance (DHI)
The solar radiation scattered by the atmosphere is generally taken to be diffuse and of approximately equal distribution across the sky above the measurement location. On a clear day it is about 5% of the total energy received at the Earth’s surface, but almost 100% on a cloudy day. PV panels respond to light from a wide range of incident angles, so they can utilize this diffuse radiation to produce energy on cloudy days.
When the diffuse radiation from the hemisphere of sky falls on a horizontal plane surface the radiative flux is the diffuse horizontal irradiance. DHI is measured with a horizontal pyrometer mounted on a sun tracker and continuously shaded from the direct sun beam throughout the day. The 5° of sky that is obscured matches the 5° seen by a pyrheliometer.
Global Horizontal Irradiance (GHI)
When all the radiation from the sun (DNI) and sky (DHI) falls on a horizontal plane surface the radiative flux is the global (total) horizontal irradiance. However, GHI is not simply DHI + DNI.
If the sun is directly overhead it makes a circular beam on the horizontal surface, but as it moves down in the sky the beam spreads out into an ellipse – in the same way that shadows get longer in the evening. The DNI is the same in W/m2, but spread over a larger area so the irradiance on the horizontal surface decreases.
GHI is important because it is the parameter measured in weather and climate networks, derived from satellite instruments and calculated with clear sky energy models. It is measured with a horizontal pyranometer.
Local pyranometer GHI measurements allow comparison of the available solar energy between sites and between data sets and the validation of satellite and model estimates for the specific location.
Plane of Array (POA) Irradiance
When a pyranometer is mounted at an angle it measures the tilted global irradiance. If the azimuth and zenith tilt angles are the same as the adjacent PV panels it is in the same plane and measures all the solar radiation available to that array. It also includes reflections from the ground and from the structure of PV panels and frames in front of the pyranometer. This varies with the panel tilt angle, the row spacing and the surface albedo.
Accurate measurement of POA irradiance is critical to calculating plant efficiencies, performance ratios and return on investment.
Back of Array (POA) Irradiance
There is growing interest in the solar energy market for photovoltaic modules with bifacial technologies, in which both sides of a solar cell can absorb sunlight and contribute to energy production.
Bifacial modules require more space between the arrays to allow the direct beam and diff use sky radiation to reach the ground and be scattered back to the rear faces. Depending on the type and height of the modules, row-to-row spacing, support structures and how reflective are the surfaces visible to the rear of the module (including the ground albedo), the energy gain can be from 10% to 25%.