In the quest to tap into renewable energy sources, solar thermal technology is a promising contender, converting incident solar radiation into usable heat or electricity. For instance, in a recent report by Solar Heat Worldwide, solar thermal systems produced 479 GWth of energy, equating to 43 million tons of oil saved and 138 million tons of carbon dioxide emissions avoided in 2019.

However, like every other renewable energy technology, solar thermal systems have challenges to overcome. One such challenge, often unnoticed but crucially important, is shading. This phenomenon can play a decisive role in the solar thermal system’s useful thermal energy output and efficiency.

Shading can play a decisive role in the solar thermal system’s useful thermal energy output and efficiency. Source: topten22photo/Adobe Stock

What is shading in solar thermal systems?

Shading refers to the obstruction of sunlight, resulting in a shadow being cast on a surface. In solar thermal systems, shading can be caused by stationary objects around the solar installation, such as nearby buildings, trees and utility poles. This type of shading is called static shading, and its pattern is relatively consistent based on the sun’s position during different times of the day. Another type of shading is called dynamic shading, which is unpredictable and caused by moving objects such as clouds.

Shading, whether static or dynamic, has several negative consequences. First, it causes reduced thermal efficiency since solar collectors rely on capturing as much sunlight as possible. In addition, shading causes temperature differential, resulting in thermal stress and reducing the lifespan of the collector. Finally, shaded conditions can give an erroneous representation of system capabilities, particularly for engineers monitoring the performance of solar thermal installations.

How is shading measured?

Here are the common methods used to measure shading:

#1 Sun path diagrams

Sun path diagrams depict the sun’s trajectory across the sky for a specific location. Engineers combine this trajectory with a site survey, which looks at potential shading objects to predict when and where shading might occur during the day or year.

#2 Shading analysis tools

There are several solar design software systems that feature shading analysis tools. Engineers only need to input the installation’s geographical location and details of the surrounding object, and these tools provide an estimation of shading over different periods. One of the most popular solar design software is the System Advisor Model (SAM), developed by the U.S. National Renewable Energy Laboratory (NREL).

Mitigating shading

#1 Optimal system placement: Before installing solar collectors, engineers typically select an area with minimal obstructions. This is achieved through a series of extensive analyses using sun-path diagrams and shading analysis tools.

#2 Elevating collectors: Engineers typically minimize some form of static shading by mounting solar thermal collectors on elevated structures. This sometimes helps to avoid ground-based shading objects.

#3 Use of tracking systems: Solar tracking systems are technologies that track the position of the sun and ensure that the collectors always face the sun directly. Given that solar collectors produce more power when the angle of incidence is very small, solar tracking not only mitigates shading but also increases overall efficiency.

Learn more about solar tracking devices on GlobalSpec.

#4 Optimize collector arrangement: In solar thermal systems, there is a unique case of self-shading where the collectors shade one another. This usually happens in solar fields with multiple rows of collectors. It is more predominant in the mornings and evenings when the sun is low on the horizon or during the winter when the solar altitude angle is low (meaning the sun is lower in the sky). Engineers typically minimize the effect of self-shading by increasing the row spacing of the collectors.

Conclusion

Shading plays an important role in the thermal energy output and performance of solar systems. Therefore, before proceeding to design and install a solar thermal system in a specific location, it is essential to analyze how static and dynamic shading from objects might affect its performance. Moreover, it is essential to reach out to solar system manufacturers to discuss application requirements.