Solar Irradiance Concepts: DNI, DHI, GHI & GTI
The output of energy that is generated by the system after receiving at an area on the Earth is known as solar irradiance. Solar irradiance is measured as electromagnetic radiation in W/m² (Watts per meter squared).
The energy released from the sun is the primary energy source for Earth; it affects everything from plant metabolism to climate change. Small changes in output can have dramatic effects on Earth. Consequently, it is an important variable to measure for studies in agriculture, climate, and power generation.
The agricultural sector keeps track of received solar energy and historical data to better plan growing cycles. The role of climate modelers is to obtain precise outputs from their models that help make accurate predictions.
The measurement of solar irradiance can be done on Earth or in space. On Earth, measurements are affected by atmospheric conditions; it depends on the position of the sun in the sky and the angle of the measuring surface. In space, primarily distance from the sun affects solar irradiance. There is a significant contribution to the sun’s solar cycle.
To large solar farms, solar irradiance is of particular interest. Solar irradiance data facilitates insights into PV panel performance by comparing the expected outputs with the actual ones. The solar insolation data can determine optimal sites so that the building of new solar farms and optimized panel orientation can occur.
Solar irradiance on Earth is affected by various factors, including atmospheric changes, weather events, and local ‘obstacles’ like mountains or trees. The usage of multiple sensors is done to measure all solar irradiance types. Here below, we have mentioned some of the types of solar irradiance.
Diffuse Horizontal Radiation (DIF)
Diffuse Horizontal Radiation is a type of solar irradiance in which the particles and molecules in the Earth’s atmosphere scatter the sun's radiation.
DIF occurs when radiation interacts with tiny particles in the atmosphere. Shorter wavelengths are scattered, which is why the sky is blue.
Mie scatter, and Non-Selective scatter have a more significant effect on diffuse horizontal irradiance. Mie sprinkles when the radiation wavelength is similar to atmospheric particles, which can be the nature of pollution, dust, and pollen.
Non-Selective scatter into existence when the particles are more significant than the wavelength of the radiation, spreading visible and infrared spectrums of radiation. Clouds are the primary contributor to non-selective scatter.
The usability of Pyranometers is the measurement of DIF. A shading ball or ring fitted to a pyranometer blocks the DNI leaving only diffuse.
After tilting the pyranometer at an angle, it can measure tilted global irradiance. As a result, solar farms will often have pyranometers listed at the panel’s ‘plane of array’ in addition to a GHI pyranometer.
Global Tilted Irradiance (GTI)
GTI is an approximate value for the energy yield calculation of fixed-installed tilted PV panels. GTI generally stands for Global Tilted Irradiance, which represents irradiation that falls on an inclined surface. A horizontal surface is parallel to the ground; a sloped surface also receives a small amount of ground-reflected radiation (REF).
Direct Normal Irradiance (DNI)
Direct normal irradiance represents the quantity of radiation received per unit area on a surface perpendicular to the sun. Consequently, the pyrheliometer measures DNI. Using a sun tracker that can be SOLYS2, a pyrheliometer can track and point directly at the sun at all times. The pyrheliometer does not measure diffuse radiation. DNI Solar Radiation is essential for concentrated solar power stations.
Global Horizontal Irradiance (GHI)
GHI Solar Radiation refers to the total radiation absorbed on a horizontal surface on the Earth. International Horizontal Irradiance supports both Direct Normal Irradiance and Diffuse Horizontal Irradiance. For this, a pyranometer is positioned horizontally to the ground that later measures the Global Horizontal Irradiance Data.
Diffuse horizontal irradiance (DHI)
Diffuse horizontal irradiance (DHI) is the terrestrial irradiance received by a flat surface scattered or diffused by the atmosphere. DHI is the component of global horizontal irradiance that does not come from the beam of the sun. Like GHI, DHI is typically measured with the help of a pyranometer. In this case, the direct light of the sun is blocked to eliminate the beam component of the radiation. The sun may be blocked by a ball or disc, which only removes the 5° cone around the sun and must utilize a tracker to shade only the pyranometer sensor continually. The diffuse light should be measured and corrected by a location and time-dependent correction factor.
What Is Solar Irradiance?
Power refers to the rate of energy transfer over time or, in simple words, irradiance. It measures the amount of solar energy that comes in a particular area in a given moment [Watt/m2]. Irradiance is a measure of solar power. On the other hand, insolation is a measure of solar energy.
How To Measure Solar Irradiance
If you desire to measure solar radiation, keep following the guide in this article. Always make sure to make the right track to get your data flowing. Follow these instructions to get accurate and reliable solar radiation data from your pyranometer.
Introduction
Do they have the desire to measure solar radiation? People have been measuring the energy coming from the sun for centuries, and today more and more people are making an effort to create the measurement of solar radiation than ever before.
The discussion of site selection, instrument mounting, and data logger setup is a vital step to go with. To increase the reliability of the solar radiation data for years, there is a massive requirement to maintain quality control and maintenance of the product.
What needs to be measured?
Before initiating the measurement process, start by asking yourself what you want to measure. The term solar radiation is used in many different applications with different meanings.
Solar radiation is defined as the energy reaching the Earth from the sun. A large part of this is sunlight, but the solar spectrum extends into the UV and the near-infrared.
The sunlight reaches us in many ways: directly from the sun (direct solar radiation), through scattering through the atmosphere (diffuse solar radiation), or via reflections. These quantities can be measured separately, but most of the time, people are interested in the surface's total radiation, termed as the global horizontal irradiance.
Choose an instrument
So that the determining factors can be measured accurately, it’s time to know which tool is required to make the calculations and get accurate data.
In conditions with dew and frost, a ventilation unit can significantly improve the dependability of your measurement.
One can use separate direct and diffuse solar radiation measurements to calculate global horizontal irradiance.
Shading the pyranometer can be done using a shadow ring or mounting it on a solar tracker with a shading mechanism.
Albedo Meters are available that integrate these measurements into one instrument.
Net radiometers integrate these measurements into one instrument.
They are combined with measurements of sensible heat flux, latent heat flux, and ground heat flux.
Accuracy
Not all pyranometers are created equally. Different models have various accuracies, outputs, characteristics, and price tags. As per ISO 9060, to help the user, pyranometers are classified into other accuracy classes.
Several factors, such as location installation, the level of maintenance, and the recalibration interval, determine the total measurement of accuracy.
Solar Irradiance Data By Location
The place where you will install your sensor is a significant factor to consider. Pyranometers have a whole hemispherical field of view. They cover the entire sky. This states that the area above the black sensor surface should be free of any obstruction. At the same moment, the instrument should remain accessible. Generally, rooftops help facilitate good locations to mount solar radiation sensors.
Install your sensor
The next step is to make the installation of the instrument. The device's structure helps fix and level all conditions for years. A sensor that needs to be balanced appropriately or aligned will result in the wrong results generation.
Pyrheliometers are standardized on 38 mm tube diameters. This facilitates mounting on a solar tracker. Pyranometers and pyrometers have 2 x M5 threads for mounting from below on either a 46 mm or a 65 mm pitch. All models have a single M6 line in the center.
Datalogger
Instruments with analog output, whether millivolts, amplified voltages, or currents, need to be connected to a data logger (a voltmeter). Over the market, several types of dataloggers are available; you need to choose a good resolution in the low voltage range rated for your conditions of use. The quality of the datalogger is essential to keep in mind. An inaccurate datalogger will give you an erroneous measurement, no matter how good your solar radiation sensor is.
The output generated by pyranometers, pyrheliometers, and pyrometers is in millivolts. They require no power supplies or batteries.
Grounding
Proper instrument grounding is a significant consideration, especially when working near high-voltage sources such as PV arrays. An instrument that is not adequately grounded can phone to facilitate false readings, fail or become a safety hazard.
Conclusion
Calculating solar irradiance and determining optimal panel placement for PV systems used to be a very math-heavy and time-consuming process. Nowadays, many solutions are available to help solar professionals get the information they need quickly and with less room for human error.