## 3 Results and Discussion
### Discussion and Major Findings for Sun and Wind Generation over Earth's Zones  
The occurrence of periods of low renewable production is becoming increasingly important in recent years, given the significant increase in the contribution of renewable generation in different areas, coupled with its intrinsic variability and unpredictability. In fact, this phenomenon is often referred to as Dunkelflautte (DF). This DF can become even more significant if the effects of uncertainty in generation sources and systems are considered. These \"dark doldrums\" are periods when renewable resources (specifically wind and/or solar power) produce little or negligible energy. Nowadays, there is no clear definition of the phenomenon, as the inherent characteristics of renewable energy production within a day make it complicated to establish a \"low productivity\" within a day scale. Daily variations in renewable energy production make setting a fixed threshold for low productivity difficult. Night-time periods are a clear but inevitable example of solar power lowering energy production. Many authors state that a DF is defined as a period exceeding 60 min when energy production is below a percentage of the installed capacity. It is below a threshold ranging from 0% to 20% of the installed capacity. This threshold is defined by [PERSON] et al. [52] as 10%; [PERSON] and [PERSON] [53] use more extreme values by establishing the threshold at 2 and 5%, while using 20% as a general standard [54]. Entering into the performance of particular situations, in the case of solar generation, obviously, there will be no generation during the night periods. But coupled with the nighttime periods, there can be very rainy or even just cloudy days leading to low solar generation and even periods of low wind, leading to an extremely low generation of both resources. For example, considering the usual definition of 20% of peak generation to consider the existence of Dunkelflautte, there are prolonged episodes of reduced generation in regions of low solar and wind resources; specifically, in the Sichuan region, there is a period of about 48 h (hours 7479 to 7542) where 20% of solar peak power generation is not reached in any of the hours (Figure 10a). This situation is much more pronounced in the case of wind (Figure 10b), also for this location, where we have had about half a year without exceeding 20% of the peak power (remember that we have the annual hourly performance of 93 simulations randomly sampled through the PDFs found from the historical data used). Therefore, obviously this location would be very unsuitable for the installation of this technology. In this case, given the poor performance of wind generation, the aggregate of the two practically follows the performance of solar generation (Figure 10c). At the complete opposite point is the coastal area of the Sahara Desert, near the Layoune, where sun and wind present optimal conditions (Figure 11). Even in the case of an optimal region of sun and wind resources, where there is practically no appreciable precipitation and wind conditions are very good, there are quite long periods of low generation. Specifically in the case of solar generation, there has not been a single day that has not reached 20% generation, so that the period of low generation is reduced from the last hours of one day in the afternoon to the first hours of sunshine the following day, just only 19 h (Figure 11a). In the case of wind (Figure 11b), as conditions are also very favorable, there is a very short period of time with generation below 20%, in this case of only 20 h (hours 7206 to 7225).