“…Such a discrepancy has also been identified by Markou et al. 36 in a case of analysing the illuminance levels over Athens and named it ‘opposite case’. Figure 5Distribution of the F index over Athens in the period 1992–2017 among all months (top) and all-sky situations (bottom right).…”
Section: Resultsmentioning
confidence: 59%
“…This observation is confirmed by Figure 5bottom where the highest frequency occurs in the F3-index category followed by F4 for allcloudiness situations. Such a discrepancy has also been identified by Markou et al 36 in a case of analysing the illuminance levels over Athens and named it 'opposite case'.…”
Though numerous daylight studies have been conducted worldwide, works for the daylight climate and/or the daylight variations/trends are rare. This gap is treated here. Therefore, the innovative trend analysis and the month–hour diagrams are applied to the illuminance time series at Athens over the period of 1992–2017 under all-/clear-sky conditions. For all skies, the global illuminance shows a broad maximum at summer noon, while the diffuse counterpart is in spring. Under clear skies, the global illuminance maximum becomes broader, covering spring/summer. However, the diffuse illuminance maximum splits into minor ones in April and June. The innovative trend analysis method finds that the identified trends are + 2.83%/decade (−1.89%/decade) for the global (diffuse) illuminance, under all skies. In clear skies, the above values are modified to +2.66%/decade (−2.00%/decade). These results confirm a recovery in the Athens daylight levels as in the solar radiation ones found by many researchers. Another outcome of the study concerns the observed decline in diffuse illuminance levels in the examined period, which may be attributed to a decline in cloudiness and an increase of sunshine over the area. Also, the variation of the global and diffuse illuminance efficacies on a month–hour basis is examined using the month–hour diagram method.
“…Such a discrepancy has also been identified by Markou et al. 36 in a case of analysing the illuminance levels over Athens and named it ‘opposite case’. Figure 5Distribution of the F index over Athens in the period 1992–2017 among all months (top) and all-sky situations (bottom right).…”
Section: Resultsmentioning
confidence: 59%
“…This observation is confirmed by Figure 5bottom where the highest frequency occurs in the F3-index category followed by F4 for allcloudiness situations. Such a discrepancy has also been identified by Markou et al 36 in a case of analysing the illuminance levels over Athens and named it 'opposite case'.…”
Though numerous daylight studies have been conducted worldwide, works for the daylight climate and/or the daylight variations/trends are rare. This gap is treated here. Therefore, the innovative trend analysis and the month–hour diagrams are applied to the illuminance time series at Athens over the period of 1992–2017 under all-/clear-sky conditions. For all skies, the global illuminance shows a broad maximum at summer noon, while the diffuse counterpart is in spring. Under clear skies, the global illuminance maximum becomes broader, covering spring/summer. However, the diffuse illuminance maximum splits into minor ones in April and June. The innovative trend analysis method finds that the identified trends are + 2.83%/decade (−1.89%/decade) for the global (diffuse) illuminance, under all skies. In clear skies, the above values are modified to +2.66%/decade (−2.00%/decade). These results confirm a recovery in the Athens daylight levels as in the solar radiation ones found by many researchers. Another outcome of the study concerns the observed decline in diffuse illuminance levels in the examined period, which may be attributed to a decline in cloudiness and an increase of sunshine over the area. Also, the variation of the global and diffuse illuminance efficacies on a month–hour basis is examined using the month–hour diagram method.
“…The typical meteorological year (TMY) method aims to address this issue, as shown in Table 1. The method consists of two steps: generation of a TMY that could characterize the representative weather conditions and assessment of solar energy resources based on the generated TMY [7][8][9][10][11][12][13][14].…”
The National Standard of China has recommended the typical meteorological year (TMY) method for assessing solar energy resources. Compared with the widely adopted multi-year averaging (MYA) methods, the TMY method can consider the year-to-year variations of weather conditions and characterize solar radiation under climatological weather conditions. However, there are very few TMY-based solar energy assessments on the scale of China. On the national scale, the difference between the TMY and MYA methods, the requirement of the data record length, and the impacts of the selection of meteorological variables on the TMY-based assessment are still unclear. This study aims to fill these gaps by assessing mainland China’s solar energy resources using the TMY method and China Meteorological Forcing Dataset. The results show that the data record length could significantly influence annual total solar radiation estimation when the record length is shorter than 30 years. Whereas, the estimation becomes stable when the length is greater or equal to 30 years, suggesting a thirty-year data record is preferred. The difference between the MYA and TMY methods is exhibited primarily in places with modest or low abundance of solar radiation. The difference is nearly independent of the examined data record lengths, hinting at the role of regional-specific weather characteristics. The TMY and MYA methods differ more pronounced when assessing the seasonal stability grade. A total of 7.4% of the area of China experiences a downgrade from the TMY relative to the MYA methods, while a 3.15% area experiences an upgrade. The selection of the meteorological variables has a notable impact on the TMY-based assessment. Among the three meteorological variables examined, wind speed has the most considerable impact on both the annual total and seasonal stability, dew point has the second most significant impact, and air temperature has the least. The results are useful for guiding future research on solar energy assessment in China and could be helpful for solar energy development planning.
“…In accordance with the aforementioned DRY proposal, Markou et al (2007) applied 3 methods (the Festa-Ratto method, the modified Danish method and the modified Sandia method) with the objective of establishing typical occurrence frequencies of CIE Standard sky types in Athens and Bratislava. Although the length of the historical data series was shorter than 10 years in both locations, the authors concluded that the proposed variations of the Sandia method, which included a number of atmospheric parameters related to irradiance and illuminance, were the best for all skies analyzed and for the skies considered as normal.…”
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