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The Importance of Accurate Irradiance Modeling for Solar Contractors: Takeaways from the Aurora Solar Webinar

image credit: ID 137475220 © Kampan Butshi - Dreamstime

Irradiance is one of the key measures for any solar installation.

There are software programs that can do all of the calculations for you, but when you have an understanding of what goes into calculating irradiance for yourself, it will increase your confidence answering customer questions about PV systems at any scale.

What is irradiance?

Irradiance is a measure of the solar power that arrives at a particular area in any given moment (kW/m2).

In practice, irradiance is often used interchangeably with insolation (also known as irradiation). Insolation expresses the amount of solar energy that an area receives during a certain period of time (kWh/m2). Technically these terms are different, and we’ll be focusing on irradiance to describe the amount of solar energy for the given surface area.

Some other key solar terms to be aware of when considering irradiance are Solar Access, Tilt and Orientation Factor, and Total Solar Resource Fraction. Defining these terms helps to give a more complete understanding of irradiance.

Solar Access (also known as Solar Access Value or Solar Access Percentage) refers to how much sunlight is available for a solar array at a particular site. This describes the amount of energy that is actually available compared to the amount of energy that would be available in perfect, shade-free conditions. To determine Solar Access, divide the solar energy that is present by the solar energy that would be available if there was no shade. Knowing the Solar Access makes it clear how much any shade present reduces the amount of available solar energy.

Tilt and orientation factor (TOF) accounts for how the slope and direction of a surface impacts the amount of solar energy that surface receives. TOF is calculated by dividing the irradiance at the actual tilt and orientation by the irradiance that would be available at the optimal tilt and orientation. So, TOF expresses the actual conditions divided by the optimal conditions.  The specific TOF that will maximize solar energy for an installation varies depending on the latitude.  

Total Solar Resource Fraction (TSRF) is a measure of available solar energy that accounts for Solar Access and TOF. TSRF provides a more complete picture of how much solar energy will be available for PV panels to convert into electricity because it takes into account shading, as well as how much solar energy will reach the panels based on their tilt & orientation. TSRF is calculated by multiplying the solar access of the site by the TOF percentage.

Additionally, there are other factors, like stringing configuration and the capabilities of the components used for the system which influence how much electricity will be produced based on how much sunlight the system receives.

Why does irradiance matter?

Irradiance is a way of understanding the amount of energy that will be available for the system installed, while accounting for local weather patterns and for any shade that may be present. Calculating irradiance is a key aspect of accurately quantifying the solar energy available to a PV system, and it’s an essential first step for correctly estimating what the energy output of the system will be.

The components of irradiance

Typically, irradiance is thought of as only being based on the rays of sunlight that directly reach a surface, but there are actually several sources of irradiance to consider.

Direct beam irradiance (DBI) is what we might intuitively associate with irradiance. The DBI determines if there are any objects that would block the rays of the sun from directly reaching a solar panel. Determining the shading from surrounding objects requires calculations based on the location of the sun for every daylight hour of the year (and these calculations can quickly add up!).

Calculating if the rays of the sun will directly hit a particular surface requires an accurate understanding of surrounding objects like trees, neighboring structures, surrounding roof planes and obstructions like skylights, vents, or chimneys. A project site can contain thousands of objects and intersections to account for at every daylight hour of the year (with over 5,000 daylight hours to account for at most locations). Running these calculations can be a very lengthy process—especially for large or complex sites, which can require 100,000 to 500,000 calculations for each point!

There are also two types of indirect or diffuse irradiance to take into account.

  • Sky diffuse irradiance is light that’s reflected from the atmosphere and is separate from direct rays of sunlight.
  • Ground diffuse irradiance is light that’s reflected back up from the ground.

To calculate these three types of irradiance, it’s necessary to account for the angle of the solar array and the direction of the sun relative to the panels.

The impact of shade

Shade can reduce power output by more than you might expect. The impact of shade is non-linear, meaning a single shaded module can reduce the output of an entire string.

Why can shading cause such severe losses? It’s similar to water flowing through a pipe. Shade acts like a clog in a water pipe, the clog restricts the flow of water through the entire pipe. Similarly, when a solar panel is shaded, the shade reduces the current through the entire string. Every panel of a string has to operate at the current set by the shaded panel, which prevents unshaded panels from operating at their maximum power. That’s why a partially shaded panel can have a dramatic effect on the total power generated and can lead to potentially significant losses.

Irradiance modeling in action

Visualizing irradiance on individual PV panels allows for quick evaluations and more informed decisions. Sub-module simulations give more accurate estimates of solar energy production, and these production estimates have a significant impact on the expected financial returns of a PV system.

Another factor to take into account when modeling irradiance is shade from hills or mountains beyond the immediate project site. Horizon shading accounts for topography around the project site to increase the accuracy of energy production estimates.

Calculate irradiance faster than ever

Typically for large multi-megawatt project sites, computing irradiance can take quite a bit of time, but with a deeper understanding the factors that go into calculating irradiance, you can answer your prospective customers’ questions more confidently, and they’ll feel more confident in choosing your company for their PV installation.

Joelle Smolarski's picture

Thank Joelle for the Post!

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