Standardization method

To monitor different climatic data especially direct beam solar radiation, monthly common statistical studies e.g. average/median values sometimes do not illustrate the most probable trends.

As a few cloudy days in a sunny month or a few sunny days in a cloudy month affect the average values of direct beam radiation considerably, the monthly average values of direct beam radiation are not the best parameters to present the general characteristics of a certain location. To achieve better standardized monthly models of solar radiation, a specific algorithm was developed during the research of the “SOLARCHVISION studies
on Young Cities Project” in 2011. According to this method, the standard values to present an average pattern of each month are calculated by assigning different linear weights to each data point according to the difference between their sorted order and median value within that month.

As the first example considers the five values 100, 750, 900, 950 and 1,000 W/m² as the direct beam radiation at noon on five consecutive days, the mean value for these amounts is 740 W/m², which is lower than four out of five amounts. On the other hand, the SOLARCHVISION standard method calculates the value of 800 W/m² as the average, which is 60 W/m² higher than the mean value and indicates a greater probability for a sunny situation during that period.

The second example concerns the series 100, 150, 200, 350 and 1,000 W/m². The mean value for these amounts is 360 W/m², which is higher than four out of five amounts. The SOLARCHVISION standard method, on the other hand, calculates the value of 300 W/m² as the average which is 60 W/m² lower than the mean value and indicates a greater probability for a cloudy situation during that period.

Unlike the direct beam radiation, in most cases the difference between the trends resulting from this method and the average (mean) is not quite so remarkable.

Below a comparison between monthly trends of hourly values calculated by differnt methods (between Figures 1 & 3) is possible. As presented the SOLARCHVISION method results in a better calculation of trends in each months either mostly sunny or mostly cloudy.

Also you can find more information about the mathematical formulas with more samples inside the BOOK.

Fig. 1: Plot for 30-day average (mean) of direct beam radiation data for Hashtgerd/Tehran

Fig. 2: Daily/hourly plot of direct beam radiation data for Hashtgerd/Tehran (based on Meteonorm 6.0 TMY data)

Fig 3: Plot for 30-day SOLARCHVISION standardization of direct beam radiation data for Hashtgerd/Tehran