In the methodology section we describe the method used to generate multiple synthetic annual solar radiation time series at different locations. This method requires a minimum of 10 annual sets of coupled DNI and GHI in hourly-resolution values as input and provides 100 annual Plausible Solar Years (PSYs) of 1-min coupled DNI and GHI dataset whose annual cumulative values correspond to the annual Probability of Exceedance from 1 to 100. Input data can be obtained from observations, satellite estimates or irradiance reanalysis databases. The size of the sample of the observations should be sufficient to statistically characterize the annual and monthly distribution of the GHI and DNI. For this reason, it’s advisable a minimum period of 10 years, either consecutive or non-consecutive, of observed data sets. The methodology applied for the synthetic generation consists of three steps (Larrañeta et al., 2019).
In the first step, we obtain 100 annual series at monthly scale. We use the probability integral transform method over the observed integrated monthly values, to generate the monthly synthetic solar data.. We constrain the synthetic generation to maintain the relation between GHI and DNI in a given location (for more details see Larrañeta et al., 2019).
The method is applied 10000 times for each month of the year. The monthly values can be concatenated to obtain 10000 annual cumulative sets of synthetic data in a monthly basis.
The observed GHI and the DNI hourly data are also integrated into annual cumulative sums and then fitted to a Normal distribution in order to estimate the theoretical probabilities of exceedance. In this step, we make a modification to the original algorithm that assumed that the annual GHI follows a Normal distribution but the annual DNI follows a Weibull distribution. In this case, we assume that the annual GHI and DNI follows a Normal distribution.
At the end of step 1, we seek and select the 100 closest sets (from P1 to P100) within the 10000 synthetically generated to those estimated fitting a normal distribution to the observed sets. In Fig. 1 we show the results of the first step calculations for the location of Seville. On the left we present the annual values of DNI versus their corresponding values of GHI of the observed data in a scatter plot, the 10000 synthetic data and the Normal distribution values. On the right we present the Normal distribution values and the 100 selected synthetic values closest to them
In the second step, we downscale the synthetic solar irradiance sets from monthly to daily time resolution. Based on (Grantham et al., 2018), the daily model consists of three components: a seasonal, an autoregressive (AR) and a random component. To determine if the set has an ARMA structure, we first perform a standardization of each annual set in the daily resolution. To that end, we estimate the seasonal component from the Fourier model, this component just depends on the day of the year, and then we divide by the standard deviation of the annual set. We analyze the correlations of this set and we observe that it can be represented by a first-order autoregressive progress (AR(1))
We then construct the AR (1) model for the standardized series. For the same locations shown in Fig. 1 the AR(1) coefficients are 0.34 and 0.42 in average for GHI and respectively. The random component or white noise is the difference between the standardized series and the AR(1) model.
We then generate synthetic daily values from the AR(1) model. The synthetic generation procedure for the daily coupled GHI + DNI data follows four main steps:
1. We first calculate the random component. We apply a bootstrapping technique to the white noise series. This technique uses random numbers assumed to be probabilities and calculate the estimated value of that probability from the daily white noise CDFs of each month. We maintain the relation between the GHI and the DNI by using the same random number for each couple of daily solar radiation values.
2. Then we calculate the autoregressive component. The AR component depends on the previous solar radiation value. We calculate this component by multiplying the previous value of a given day by the estimated AR(1) coefficient for a given location.
3. In the last step, we take into to account the seasonality to the synthetic set. We add both random and autoregressive components for each day and then undo the standardization. To that end, we multiply each day by its corresponding standard deviation value and finally add the contribution from the Fourier model.
Finally, for each synthetic day, we search the observed day with the closest value in terms of energy (kt and kb) and select their corresponding values of variability and distribution. The observed datasets have been also labeled with two more daily indexes. The variability index (VI) that provides information about the intra-daily variability of the solar radiation and the morning fraction index (Fm), that provides information about the intra-daily distribution of the solar radiation (Moreno-Tejera et al., 2017 ). We run this procedure for the generation of 1000 annual sets of daily values. We aggregate the daily synthetic sets into monthly cumulates to select those months that more closely match the ones obtained in step 1.
In the last step, we use the synthetic daily quartets of kb, kt, VI and Fm obtained in step 2 to generate the 1-min synthetic data sets. In this step, we use the ND model tool (Larrañeta et al., 2019) for the synthetic generation of 1-min coupled GHI and DNI data. The ND model uses non-dimensional databases from a location with similar climate to the input data; therefore, it can be used in at location without local adaptation (M. Larrañeta et al., 2018). The non-dimensional database daily profiles are labeled with the indexes kt, kb, VI and Fm that provide information about energy, variability and distribution following Moreno-Tejera., 2017.). We generate synthetic daily quartets of kb, kt, VI and Fm in step 2 that are used to seek and find the most similar day among the dimensionless database. The selected dimensionless profiles are “opened” into real values by multiplying the 1-min kt and kb values to the extraterrestrial irradiance and the clear sky DNI of each corresponding day.
In Table 1 we present information of the locations of the dimensionless databases implemented in the algorithm. We have selected seven locations with different climates. The dimensionless databases are composed by 1-min observed GHI and DNI data obtained from the Australian Bureau of Meteorology website (Bureau of Meteorology, 2015) for six different locations in Australia. The 1-min observed GHI and DNI datasets for the location of Seville, Spain, are obtained from GTER meteorological database (S. Moreno-Tejera et al., 2016)..
Table 1
Dimensionless databases available sites and their Köppen-Geiger classification climate.
Location (ID)
|
Country
|
Latitude (ºN)
|
Longitude (ºE)
|
Altitude (m)
|
Köppen-Geiger classification climate
|
Sevilla
|
Spain
|
37.22
|
5.58
|
16
|
Csa
|
Darwin
|
Australia
|
-12.27
|
130.53
|
17
|
Aw
|
Broome
|
Australia
|
-17.57
|
122.14
|
12
|
Bsh
|
Alice Springs
|
Australia
|
-23.41
|
133.53
|
583
|
Bwh
|
Rockhampton
|
Australia
|
-23.37
|
150.51
|
20
|
Cfa
|
Melbourne
|
Australia
|
-37.48
|
144.57
|
25
|
Cfb
|
Adelaide
|
Australia
|
-34.55
|
138.35
|
59
|
Csb
|