Solar radiation is the energy powering the Earth. You can use two instruments to measure solar radiation – a solarimeter and a pyranometer. The Sun radiates multiple wavelengths, but only a part of its energy reaches the Earth’s surface.
Before you test your solar panels, you need to appreciate the concept of solar radiation and the factors that affect solar radiation.
What Is Solar Radiation?
Simply put, solar radiation is a fancy name for solar energy or solar resource. It describes the electromagnetic energy the Sun emits. Solar radiation has a solar spectrum comprising visible sunlight, infrared, and UV light.
There are different forms of solar radiation – direct solar radiation or direct beam radiation (direct contact from the Sun), reflected radiation (contact via reflected surfaces), and diffuse solar radiation (solar irradiance dispersing throughout the atmosphere). Interestingly, all forms of solar irradiance can be measured, but the calculation that interests people more is global solar radiation (direct radiation + diffuse radiation).
Some of the elements (most of which are atmospheric components) that influence total radiation include air molecules, dust, water vapor, pollutants, volcanoes, forest fires, and clouds.
Factors Affecting Solar Irradiance
The following factors affect how solar energy gets to the Earth’s surface
The Sun’s Height
The Sun’s height is also referred to as the angle of incidence. The larger this angle, the higher the Sun resulting in more solar radiation reaching the earth’s surface. Inversely, the smaller the angle, the lower the Sun resulting in less solar radiation. The Sun’s height also affects the solar spectrum, which includes UV, infrared, and visible light from the Sun. If the height of the Sun is 9, UV light makes up 4%, visible lighting makes up 46%, and infrared lighting makes up 50%.
If the Sun’s altitude is at 30 degrees, it will result in a solar spectrum comprising infrared light making up 53%, visible lighting making up 44%, and UV light making up 3%. The highest position of the Sun is referred to as the solar zenith angle.
The Earth circulates around the Sun in an elliptical orbit, making it closer to the Sun at a given time of the year. During this period, the Earth gets more global radiation.
Solar radiation in the upper part of the atmosphere is referred to as solar constant because it doesn’t change. However, when it gets to the Earth’s surface, its radiation levels fluctuate. A significant reason this occurs is the level of atmospheric transparency in both environments. Atmospheric transparency characterizes the state of the sky.
If it’s cloudless and clear, the atmospheric transparency is high, and the solar radiation will be stronger. On the other hand, if the weather is dusty and cloudy, the atmospheric transparency is low, equating to less solar radiation reaching the Earth. In essence, atmospheric transparency describes the level of clarity or cloudiness and the amount of impurities in the sky. In summary, more solar radiation passes through the sky if the atmospheric conditions are suitable.
Solar radiation data is proportional to the distance the Sun’s luminosity have to travel to the Earth. This means the shorter the distance solar radiation travels, the more solar energy will reach the Earth. On the flip side, the longer the distance the Sun’s rays travel, the more solar radiation is lost. The distance between the Sun’s position and the ground is called atmospheric mass. The larger the mass, the lower the air quality and the longer it will take for the Sun’s rays to reach the Earth and vice versa.
The higher the altitude, the more transparent the atmosphere will be, and the more solar radiation will reach the Earth. On the other hand, if the altitude is low, significantly less solar energy will get to the Earth’s atmosphere. There will be a temperature difference between the perihelion and aphelion, just as the temperature of the Pingchuan will differ from that of the basin. The same applies to the sunny slope and the shady slope.
Sunshine hours refer to the period the Sun shines in a particular location. Some areas have a longer sunshine time than others, especially in tropical regions. The longer the sunshine time, the more solar radiation that location gets from the Sun. Tropical regions may get up to 8 hours of sunlight, while temperate regions will get less than that.
The lower the latitude, the stronger the solar radiation. Using 1 square meter, for instance, places like Moscow may get about 330kj of solar energy annually because of its high latitude. In contrast, a location like Beijing may get 550kj because of its lower latitude. Both locations will have significantly less solar energy than locations in the Sahara region.
The energy from the Sun varies based on the season. During the summer solstice, the direct normal irradiance and diffuse horizontal irradiance will be on the higher side, especially in the southern hemisphere. This is because that’s when more of the Sun’s rays reach the Earth’s surface. On the other hand, the winter solstice comes with direct solar radiation and diffuse radiation, especially in the northern hemisphere.
How to Measure Solar Radiation?
You can measure solar radiation data with a solarimeter or pyranometer.
With a Solarimeter
This measuring instrument uses the photovoltaic effect of the Sun’s rays to determine the net radiation (the radiation that reached the Earth’s surface). It’s also called a silicon cell pyranometer and works like a solar panel. It converts the Sun’s rays (visible light) to electricity. How much it generates depends on the temperature of the solar cells. The solarimeter can capture a wavelength range of 330-1100nm.
With a Pyranometer
This is the main instrument for measuring the total solar radiation of a surface. It compares the temperature variance between a dark surface and a clear, bright one. It measures solar flux density in units of W/m². The former absorbs more and reflects less solar radiation, while the latter reflects more and absorbs less. The temperature difference is measured with a thermopile.
This instrument is capable of capturing a spectral range of 300-2800nm. This gives it a wider range than the solarimeter. At the same time, it makes the pyranometer an unsuitable instrument for assessing the performance of solar panels under specific environmental conditions. On the other hand, a solarimeter will be ideal for evaluating the performance of solar panels since it works in a similar way.
A pyranometer takes much longer (several seconds) to respond than a solarimeter which takes less than a second. The sunshine recorder is a less expensive but less accurate instrument for measuring solar radiation.
At different periods during a calendar year, scientists measure sunshine hours in various locations. Most of these locations have similar climates and are usually within the same latitude. They measure total radiation on a horizontal surface.
When it comes to photovoltaic modules, solar radiation data is denoted by kWh/m² (kilowatt hours per square meter).
Different Solar Resource Values to Measure and Their Instruments
- Global horizontal irradiance, solar radiation on a tilted plane, solar irradiance in the “plane of array”, and reflected solar radiation- pyranometer
- Direct beam radiation – pyrheliometer on a solar tracker
- Albedo – 2 x pyranometer
- Surface energy balance – 2 x pyranometer + 2 x pyrgeometer
- Diffuse solar radiation – pyranometer shaded
Accuracy of the Measurement Instrument and Data Collected
Not every pyranometer has precise accuracy, and as their models and prices differ, so do their accuracy categories. Three major factors influence the accuracy of the instrument, namely the location of the installation, recalibration level, and maintenance level.
The importance of the location site cannot be emphasized enough. We advise putting your pyranometer where it can have a complete view of the sky, and there will be no obstructions (electric poles, houses, and trees) in the area above the black sensor. The best location will be a rooftop.
Another essential thing to note when installing your pyranometer is to ensure that it isn’t in close proximity to reflective sources such as a light wall and keep the instrument away from artificial light sources such as lamps. While installing your sensor, ensure it’s properly aligned and leveled.
Instruments such as the pyrheliometers are installed on tube diameters with a dimension of 38 mm for easy mounting on a solar tracking system. On the other hand, pyrgeometers and pyranometers have a pair of M5 threads that allows you to mount them from below on a pitch measuring either 46 mm or 65 mm. Then, choose a datalogger for storing the solar resource data, preferably one with good resolution in low voltage.