Monday, September 15, 2014

Measuring Intercepted Solar Radiation


     Many factors influence the true amount of solar radiation that actually reaches the planet's surface. The factors can include the orientation of the Earth's surface, sky obstructions, and surrounding topographic features. Because there are so many factors that can alter how much radiation is being intercepted, P.M. Rich, W.A. Hetrick, S.C. Saving, and R.O. Dubayah developed an algorithm that allows for rapid calculation of the amount of intercepted radiation. Each of the possible factors that affect interception were given formulas to account for there role in the interception. 

Two different projections for intercepted solar radiation. 
Top: hemispherical Coordinate System
Bottom: Equiangular Coordinate System


     In order to measure radiation, the sky is projected onto a plane and then divided into sections. By dividing the sky into sections, it was easier to formulate a table of values that would be easier to compare and analyze. Each section of the sky, depending on the amount of radiation absorbed, will have a calculated "irradiance" value. Irradiance is the amount of electromagnetic radiation per unit area that can be found on a specific surface. Having the value for the full amount of radiation (irradiance) allows for influential factors to be accounted for with the help of cosine algorithms. After accounting for influencing factors, the values are put into a table to analyze. 

Examples of how the sky can be divided. 
Top: A more evenly divided sky provides calculations from all sky directions.
Bottom: The unevenly divided sky produces better calculations when dealing with one specific spot and one particular sky direction.

Evaluation & Conclusion

     The values that were calculated were compared to topographic maps in order to find out what obstructions were causing some areas to collect less radiation that others. The division of the sky played a key role in the results. For example, the more finely and equally divided the sky provided better results when measuring radiation from all directions in the sky while the unequally divided sections produced better results when trying to determine radiation collected in one particular spot from a particular direction.

This process is one that is viable, flexible, and very applicable to a wide array of uses. This algorithm could help decide which areas could make the best use of solar panels for better energy yields. Because the Sun provides such a large amount of Earth's energy and also due to the fact that different influential factors can always be given their own proprietary cosine equations, this algorithmic technique to measure solar radiation exhibits enough flexibility to make it a contender in future environmental energy problems and endeavors.

Rich, P.M., R. Dubayah, W.A. Hetrick, and S.C. Saving. 1994. Using viewshed models to calculate intercepted
solar radiation: applications in ecology. American Society for Photogrammetry and Remote Sensing Technical
Papers. pp 524–529.

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