A single day of the solar energy received by the Earth exceeds the entire amount of energy the planet uses in one year. This untapped resource could meet our energy needs for years to come, but investing in this inexhaustible power supply will require precise solar radiation measurements, specifically for performance assessment of solar cells and resource forecasting.
Collecting solar irradiance data is essential at every stage of a solar energy project. For example, information could be used to pick an ideal location for cells before constructing a power plant. Knowing exactly how much energy can be utilized from the Sun will lead to optimal solar energy systems. Reliable solar irradiance information is necessary for feasibility research and also to address concerns about project performance - tracking solar irradiance is critical for the appraisal of a solar project's financial viability.
Solar radiation on Earth is normally described as the total radiation across the 280 to 4000 nm wavelengths, also known as shortwave radiation. Total solar radiation, either direct beam or diffuse, incident on a horizontal surface is described as global shortwave radiation, or shortwave irradiance, and is indicated in watts per square meter.
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Pyranometers are solar irradiance sensors that gauge global shortwave radiation. Irradiance data (such as the ratio between the direct and diffuse radiation on a location) is also essential when selecting solar power technology or racking technologies like fixed or tracking panels.
Direct solar irradiance is the amount of solar energy reaching the Earth’s exterior from the Sun’s direct beam on a plane perpendicular to it and is assessed by a device attached to a solar tracker known as a pyrheliometer. This tracker follows the sun across the sky to ensure the beam is aimed into the device. Diffuse solar irradiance is the amount of inbound solar energy on a horizontal plane at the Earth’s exterior as the Sun’s beams are dispersed by the atmosphere.
In some situations (more typically for scientific research), reference solar cells may be used alongside pyranometers. However, reference cells raise the issue of spectral selectivity: a pyranometer measures inbound solar radiation homogeneously, but reference cells are limited to their band-gap specific sensitivity, so that reference cells will not gauge solar radiation homogeneously.
At a solar plant, a pyranometer determines the solar energy that is entering the system while a power meter determines what electrical power it generates. These two values can be used to determine the performance ratio (PR) of a solar plant. PR is a crucial parameter that can show if the solar plant is functioning well or if there are any issues like shading, short-circuits or degradation.
Types of Pyranometers
There are two kinds of pyranometers: thermopile pyranometers and semiconductor pyranometers. A thermopile pyranometer is the “true” pyranometer since it measures the total quantity of radiation on a surface. It has a thermopile detector, a device that transforms thermal energy into electrical energy, with light-absorbing black paint that takes in all radiation from the sun evenly. This produces a temperature difference between the black exterior of the sensors and the housing of the instrument, resulting in a small voltage at the sensor that can be measured and translated into watts per meter squared.
A silicon pyranometer (which is a semiconductor sensor) uses a photodiode to transform light into an electrical current. The disadvantage of the silicon pyranometer is that its spectral sensitivity is restricted, meaning it won’t detect the entire spectrum of the sun. Needlessly to say, this can lead to measurement errors.
A solar irradiance sensor can be a stand-alone device or used as part of a meteorological station. Often, a large solar plant has one or more meteorological stations that track key weather parameters like relative humidity, wind speed, and solar radiation. The data gathered by pyranometers can be used with mathematical models that determine irradiance on the array from the sky under various conditions. This step is essential for modelling the performance of a solar energy installation effectively.