How to Read the Live Irradiance Graph

 

The live irradiance graph shows solar resource data from the monitoring stations included in K.A.CARE’s Renewable Resource Monitoring and Mapping (RRMM) Program network around Saudi Arabia, updated every minute.  You can select different monitors with the drop down menu to see current solar radiation levels in the country in real-time.  This page is a short tutorial on what these graphs mean and how to read them. 

 

Solar Resource Concepts

As illustrated in the diagram below, solar radiation interacts with the Earth’s atmosphere to create three types of irradiance at ground-level:

 

1.      Direct (Beam) – Solar radiation available directly from the sun

2.      Diffuse (Sky) – Scattered solar radiation available from the entire sky, but not including the direct radiation

3.      Global (Total) – All solar radiation from the entire sky

 

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Three components of sunlight striking a solar resource monitor

Source: National Renewable Energy Laboratory

Each K.A.CARE solar measurement station is equipped to provide data for each of the solar radiation components, including a radiometer similar to the one shown in the above diagram.  Understanding the amount of solar radiation available at any time from each of these components is important for siting, designing, and operating solar power production facilities.

Typical Sunny Day in Saudi Arabia

This first live irradiance graph below is from a typical clear sunny day at the K.A.CARE City site, and we will use it to explain the key features of the live irradiance graph.  Then, we will show you a few other charts to illustrate different environmental conditions and how they affect the solar resource.

 

The left (vertical) y-axis is irradiance, which is the amount of power (watts) striking a surface area (square meters) at any point in time.  This irradiance is similar to the speed of wind turning the blades of a turbine. The sum of the irradiance over time (such as a day) is the total energy available for that period in watt-hours. 

 

The bottom (horizontal) x-axis shows time from morning to night.  You can see that all three radiation values increase as the sun rises about 5:30 am, peak about noon when the sun is overhead, and decrease as the sun sets in the afternoon. 

 

Irradiance Graph for a Typical Sunny Day in Saudi Arabia

CitySite

Global Horizontal (GHI) | Direct Normal (DNI) | Diffuse Horizontal (DHI)

 

The red line on this graph is global horizontal irradiance (GHI), radiation from the entire sky.  GHI is the key quantity of interest for flat plate (non-concentrating) collectors, such as traditional Photovoltaic (PV) systems.  These systems can effectively use sunlight no matter which directions the photons come from when they reach the solar panel.

 

The green line is direct normal irradiance (DNI), radiation from the direct beam of the sun.  DNI is the key resource (fuel) for all concentrating solar energy conversion technologies, such as Concentrating Solar Power (CSP).  Since these technologies reflect photons onto a separate receiver, they need to be coming in a straight line from the sun so they can be focused and received as concentrated energy in a single location.  Since sunlight can be scattered or reflected by atmospheric constituents, such as aerosols (solid or liquid droplets) in the air, including clouds, dust, and pollution, the DNI resource varies the most over space and time.

 

The blue line is diffuse horizontal irradiance (DHI), radiation that has been scattered or diffused by the atmosphere and is not in the direct beam.  GHI, DNI, and DHI all relate to each other, with GHI being the combination of the other two.  Generally, when DNI is high during clear skies, then DHI is lower because less DNI is scattered to the diffuse.  

 

Exceptionally Clear Sunny Day in Saudi Arabia

This next live irradiance graph is from an exceptionally clear sunny day in the extreme northwest part of Saudi Arabia in Hagl, within the Tabuk province.  Extreme low levels of dust and pollution and very bright skies resulted in DNI being higher than GHI for the entire day.  The intensity of solar energy varies widely with climate, season, and latitude. Some parts of the Kingdom (and elsewhere in the world) see such clear skies more often than other areas.  K.A.CARE’s monitoring program will help scientists and engineers characterize these differences, with the goal of best matching the solar resources with conversion technologies, such as PV and CSP.  Notice that the diffuse light levels were very low because little of the light coming directly from the sun (DNI) was scattered out of the beam.

 

Irradiance Graph for an Exceptionally Clear Sunny Day in Saudi Arabia

Hagl

Global Horizontal (GHI) | Direct Normal (DNI) | Diffuse Horizontal (DHI)

 

Cloudy Afternoon in Saudi Arabia

Our next live irradiance graph is from Al-Ahsa in the Eastern Province.  While the day started clear, at about 10:00 a.m. we can see that clouds, pollution, or dust levels increased so the data began to fluctuate.  The DNI or direct normal radiation decreased as the light scattering and absorption increased.  The DHI or diffuse radiation increased as light was still being received by the monitor, but from different directions other than directly from the sun.  The GHI or total radiation remained relatively unaffected, indicating that the light was just scattered by the clouds, pollution, or dust, but not totally blocked. 

 

Irradiance Graph for a Day with a Cloud Afternoon in Saudi Arabia

Global Horizontal (GHI) | Direct Normal (DNI) | Diffuse Horizontal (DHI)

 

Big Dust Storm Day

In this last live irradiance graph, the day started clear.  However, about 1:00 pm, a huge dust storm enveloped the region.  The direct radiation dropped to zero.  The global radiation decreased significantly, indicating that part of the sunlight was blocked completely, not just scattered.  This plot from KAU Hada Alsham shows the effect of extreme overcast conditions, which produced GHI near zero from 4:30 p.m. for another two hours until sunset.

 

Irradiance Graph for a Day with a Big Dust Storm in Saudi Arabia

HadaAlsham

 

To get a sense of the difference between a relatively clear day (left), and a day with a dust storm (right), consider the difference in the two images below of the Kingdom Tower in Riyadh.  Both images were taken at the same time of day (morning).   The left image shows a relatively clear sunny day with high DNI and GHI, as discussed above.  The right image shows a moderate dust storm, with significantly reduced DNI, with the direct beam partially scattered and partially absorbed or reflected.  Understanding the timing, spatial pattern, and magnitude of differences in conditions such as those shown here provides the foundation for solar power deployment.

 

غبار برج المملكة.png

 

Kingdom Tower in Riyadh, on a Relatively Clear vs. Moderately Dusty Day