Solar Radiation Forecasting , Accounting for Daily Variability

Radiation forecast accounting for daily and instantaneous variability was pursued by means of a new bi-parametric statistical model that builds on a model previously proposed by the same authors. The statistical model is developed with direct reference to the Liu-Jordan clear sky theoretical expression but is not bound by a specific clear sky model; it accounts separately for the mean daily variability and for the variation of solar irradiance during the day by means of two corrective parameters. This new proposal allows for a better understanding of the physical phenomena and improves the effectiveness of statistical characterization and subsequent simulation of the introduced parameters to generate a synthetic solar irradiance time series. Furthermore, the analysis of the experimental distributions of the two parameters’ data was developed, obtaining opportune fittings by means of parametric analytical distributions or mixtures of more than one distribution. Finally, the model was further improved toward the inclusion of weather prediction information in the solar irradiance forecasting stage, from the perspective of overcoming the limitations of purely statistical approaches and implementing a new tool in the frame of solar irradiance prediction accounting for weather predictions over different time horizons.


Introduction
Many photovoltaic (PV) applications, such as the sizing of stand-alone microgrids or sizing of energy storage systems for their inclusion in standalone (or grid connected) systems or day-ahead market offering, require forecasting PV production variability along different time scenarios, often also short and very short terms.For this purpose, it is necessary to forecast solar radiation on these time scenarios to account for weather variations during the day and even within each hour of the day.
Significant research is currently being devoted to the development of models to predict solar radiation and PV power.Common operational approaches to solar radiation forecasting include: (1) numerical weather prediction (NWP) models that infer local cloud information through the dynamic modeling of the atmosphere up to several days ahead [1]; (2) models using satellite remote sensing or ground-based sky measurements to infer the motion of clouds and project their impact in the future; (3) statistical time series models based on measured irradiance data applied for very short term forecasting in the range of minutes to hours [2][3][4][5][6][7][8][9][10][11][12][13].
In particular, in [14][15][16] the variability of solar irradiance is captured by observing the changes in clear sky index for a considered time interval.The primary usefulness of the clear sky index is the removal of diurnal and seasonal signals from a given set of radiation data to compute fluctuation power content ( [17]).
Papers [3][4][5][6][7][8][9][10][11][12][13] refer to the clearness index and emphasize that the probability density distributions of the clearness index on short-term intervals, unlike those considered in longer intervals, present a multimodal nature [3,12].In [13] it is stated that the instantaneous solar irradiance (in terms of clearness index) has bimodal probability distributions with peaks corresponding to clear and cloudy conditions.Regarding the statistical characterization of solar quantities, several attempts were proposed to identify the distribution functions that best fit solar data on a short time basis, some based on the Boltzmann statistics ( [3,4]), others referring to the mixture of two normal distributions ( [5,6]) or two bi-exponential probability density functions ( [7,8]).These aspects make such methods mainly suitable for very short time forecasting with no exogenous inputs, as shown in [2], where five different techniques were assessed for one hour ahead and two hours ahead power forecasting of PV plants.
A common characteristic of the aforementioned models is that they refer to a unique parameter accounting for solar irradiance variations.
In [18] a statistical model referring to the clear sky index was proposed to characterize solar radiation, taking into account the daily variability in terms of two proper parameters.More specifically, solar irradiance was described as the sum of two components, characterized by specific quantities that take into account mean daily weather conditions and instantaneous variations of the solar irradiance, respectively.It was demonstrated that experimental characterization of the distributions of the two introduced quantities was suitable for a statistical characterization and for generating synthetic time series characterized by both daily and instantaneous variability starting from experimental probability distributions.
This paper introduces an evolution of the model proposed in [18].The statistical model is developed with direct reference to the Liu-Jordan clear sky theoretical expression but is not bound by specific clear sky models; it accounts separately for the mean daily variability and for variation of solar irradiance during the day by means of two corrective parameters.The first parameter accounts for the mean daily variability, while the second accounts for the variation in solar irradiance during the day.This new proposal allows for a better understanding of the physical phenomena and improves the effectiveness of statistical characterization and subsequent simulation of the introduced parameters to generate synthetic solar irradiance time series, also starting from parametric analytical distributions.The analysis of the parametric analytical distributions that best fit the experimental distributions of the two-parameter data was also developed with reference to field measurements [19], obtaining opportune single parametric distributions or mixtures of more than one distribution.Moreover, the model's ability to combine with weather predictions was investigated, starting from the consideration that changing the variability time scale (e.g., introducing two or more sub-parts of the day) seems to allow for using it for both long-term and short-term scenarios.Thus, a further model improvement was developed by including weather prediction information in the forecasting stage, thus overcoming the limitation of a purely statistical approach.In the application proposed in this paper, the model was tested only for day-ahead forecasting, even if the prospect of utilizing it in other time scenarios, with various forecasting techniques, seems promising.
The remainder of the paper is organized as follows.Section 2 includes a description of the model; Section 3 describes the statistical analysis stage.Simulation and forecasting are developed in Sections 4 and 5 while Section 6 reports the results of field measurement analyses.Finally, conclusions are drawn in Section 7.

Proposed Model
The proposed model is an evolution of that proposed in [18], introducing a new expression decomposing the measured solar irradiance at time n∆t (i.e., the nth sample of the jth day of the mth month), R j,m pn∆tq , in two components: R j,m pn∆tq " S LJ j,m pn∆tq ¨rC m ¨kj,m `ε1j ,m pn∆tqs " S LJ j,m pn∆tq ¨Cm ¨rk j,m `ε2 j,m pn∆tqs where: -S LJ j,m pn∆tq is the "clear sky" theoretical model developed by Liu-Jordan at ground level [20,21]; -C m is the monthly clear sky index for the mth month; -k j,m , mean daily variability for the jth day of the mth month, is a daily parameter, which plays the role of correction factor of C m accounting for specific daily weather conditions; -ε 1j ,m pn∆tq, instantaneous variability for the nth time interval of the jth day of the mth month, is an additive "noise" component accounting for variations of the solar irradiance during a day; -ε 2 j,m pn∆tq is equal to ε 1j ,m pn∆tq {C m .It is worth noting that in Equation ( 1) any clear sky model (other than that proposed by Liu & Jordan) can be used without compromising the validity of the proposed model.
The second expression in Equation ( 1) is a useful alternative to the first and allows for establishing correspondences with popular simplified forms as it will appear clearly in the following sections (e.g., in Section 4, Equation ( 14)).
It is worth noting that the following constraints apply for k j,m and ε 1j ,m pn∆tq values: ´Cm ¨kj,m ď ε 1j ,m pn∆tq ď 1 ´Cm ¨kj,m The parameter C m is calculated for the mth month as: where W o m is the historical mth month average value of the daily solar radiation issued by meteorological services [19,22]; W LJ m is the mth month average value of the daily theoretical Liu-Jordan radiation, that is: being N m the number of days of the mth month and N n the number of ∆t of a day.The formulation corresponding to Equation (1) given in [18] is: and is based on: -S W j,m pn∆tq, which is the historically expected irradiance at ground level for nth sample of the jth day of the mth month, in absence of daily variability and instantaneous weather variability respect to historical behavior, that is: S W j,m pn∆tq " S LJ j,m pn∆tq ¨Cm k j,m as in Equation ( 1), -ε j,m pn∆tq given by: ε j,m pn∆tq " S LJ j,m pn∆tq ¨rε 1j ,m pn∆tqs (8) Figure 1a,b show, for the 11th and 12th of August, respectively, of 2004 for a site in the south of Italy, the profiles of the measures [19] of solar irradiance, R j,m pn∆tq, of its components k j,m ¨SW j,m pn∆tq and ε j,m pn∆tq and of the expected irradiance at ground level, S W j,m pn∆tq according to Equation (6).Of course, a similar interpretation of the same figure can be effected in terms of the components given by Equation (1). Figure 1a,b show, for the 11th and 12th of August, respectively, of 2004 for a site in the south of Italy, the profiles of the measures [19] of solar irradiance,  , (), of its components  , •   , () and ε , () and of the expected irradiance at ground level,   , () according to Equation (6).Of course, a similar interpretation of the same figure can be effected in terms of the components given by Equation (1).In the figure, the regularity of the red and dotted black curves can be observed; they have the same shape as the theoretical Liu-Jordan profile multiplied by Cm; the red curve derives from the product of the black curve and  , .The measured solar irradiance,  , (), (blue curve), is affected by irregularities deriving from the additional "noise" ε , () (green curve).
The 11th of August was characterized by a daily radiation (area of the red curve) almost equal to that expected (area of the black curve); the irregularities appear concentrated in some specific parts of the day.The 12th of August was characterized by a daily radiation (area of the red curve) sensibly lower than that expected (area of the black curve); the irregularities appear to be scattered along the day with morning hours better than those of the afternoon.
The application of the proposed model includes two stages: (1) statistical analysis of irradiance quantities summarized in the flow chart of Figure 2    In the figure, the regularity of the red and dotted black curves can be observed; they have the same shape as the theoretical Liu-Jordan profile multiplied by C m ; the red curve derives from the product of the black curve and k j,m .The measured solar irradiance, R j,m pn∆tq , (blue curve), is affected by irregularities deriving from the additional "noise" ε j,m pn∆tq (green curve).
The 11th of August was characterized by a daily radiation (area of the red curve) almost equal to that expected (area of the black curve); the irregularities appear concentrated in some specific parts of the day.The 12th of August was characterized by a daily radiation (area of the red curve) sensibly lower than that expected (area of the black curve); the irregularities appear to be scattered along the day with morning hours better than those of the afternoon.
The application of the proposed model includes two stages: (1) statistical analysis of irradiance quantities summarized in the flow chart of Figure 2  Figure 1a,b show, for the 11th and 12th of August, respectively, of 2004 for a site in the south of Italy, the profiles of the measures [19] of solar irradiance,  , (), of its components  , •   , () and ε , () and of the expected irradiance at ground level,   , () according to Equation (6).Of course, a similar interpretation of the same figure can be effected in terms of the components given by Equation (1).In the figure, the regularity of the red and dotted black curves can be observed; they have the same shape as the theoretical Liu-Jordan profile multiplied by Cm; the red curve derives from the product of the black curve and  , .The measured solar irradiance,  , (), (blue curve), is affected by irregularities deriving from the additional "noise" ε , () (green curve).
The 11th of August was characterized by a daily radiation (area of the red curve) almost equal to that expected (area of the black curve); the irregularities appear concentrated in some specific parts of the day.The 12th of August was characterized by a daily radiation (area of the red curve) sensibly lower than that expected (area of the black curve); the irregularities appear to be scattered along the day with morning hours better than those of the afternoon.
The application of the proposed model includes two stages: (1) statistical analysis of irradiance quantities summarized in the flow chart of Figure 2

Statistical Analysis
The statistical characterization of the parameters  , and ε , * (), which refer to the jth day of the mth month, is performed by identifying their distributions at each month m.This stage includes the derivation of the distributions of the parameters and their statistical characterization.Inputs are the historical data of solar irradiance measured at the specific location where the procedure is applied.

Experimental Distributions
Starting from the measurements of solar irradiance  , (), the  , parameter can be evaluated from Equation (1) as follows: with   number of samples in a day and ∑ ε , ′ * () •  LJ , () The parameter  , -which is different from the clearness index typically used for characterizing the solar radiation-assumes values lower than 1 on cloudy days, when the total energy registered is lower than the average energy expected in that month according to historical data, and assumes values greater than 1 on sunny days.Its monthly expected value, for a huge number of observations, is equal to 1.
With reference to ε , * (), the following relation can be derived from (1): Note that the values of all the Nn + 1 parameters for each day can be evaluated only at the end of the day-that is, when the measurements  , () for all the Nn samples of the day have been carried out.Once the data for  , and ε , ′ * () have been collected, a statistical analysis can be performed with reference to the mth month of the year, obtaining the experimental joint distribution  K  ,E  ′ * or the experimental marginal distributions  K  and  E  ′ * .Details on the statistical behavior of these quantities are provided in Section 3.2.
In order to statistically characterize the above parameters, the procedure was tested in [18] to derive the daily joint probability density function (jpdf)  k , ,ℰ , or, as an alternative, the marginal pdfs ( K  ,  Ε  ).Considering the marginal is preferable due to the scarcity of available data and to practical reasons without relevant effects on the accuracy.For both the parameters,  , and  , (), the analysis was limited to the case of experimental probability mass functions (pmf) as obtainable from the data observed.

Statistical Analysis
The statistical characterization of the parameters k j,m and ε j,m pn∆tq, which refer to the jth day of the mth month, is performed by identifying their distributions at each month m.This stage includes the derivation of the distributions of the parameters and their statistical characterization.
Inputs are the historical data of solar irradiance measured at the specific location where the procedure is applied.

Experimental Distributions
Starting from the measurements of solar irradiance R j,m pn∆tq , the k j,m parameter can be evaluated from Equation (1) as follows: with N n number of samples in a day and ř N n 1 ε 1j ,m pn∆tq ¨SLJ j,m pn∆tq " 0. The parameter k j,m -which is different from the clearness index typically used for characterizing the solar radiation-assumes values lower than 1 on cloudy days, when the total energy registered is lower than the average energy expected in that month according to historical data, and assumes values greater than 1 on sunny days.Its monthly expected value, for a huge number of observations, is equal to 1.
With reference to ε j,m pn∆tq, the following relation can be derived from Equation (1): Note that the values of all the N n + 1 parameters for each day can be evaluated only at the end of the day-that is, when the measurements R j,m pn∆tq for all the N n samples of the day have been carried out.Once the data for k j,m and ε 1j ,m pn∆tq have been collected, a statistical analysis can be performed with reference to the mth month of the year, obtaining the experimental joint distribution f K m ,E 1m or the experimental marginal distributions f K m and f E 1m .Details on the statistical behavior of these quantities are provided in Section 3.2.
In order to statistically characterize the above parameters, the procedure was tested in [18] to derive the daily joint probability density function (jpdf ) f k j,m , ε j,m or, as an alternative, the marginal pdfs ( f K m , f E m ).Considering the marginal is preferable due to the scarcity of available data and to practical reasons without relevant effects on the accuracy.For both the parameters, k j,m and ε j,m pk∆tq, the analysis was limited to the case of experimental probability mass functions (pmf ) as obtainable from the data observed.

Parametric Distributions
We focused our attention on the analysis of the parametric pdf s that best fit the distributions of the data experimentally obtained.This approach, in fact, has the advantage of simply referring to specific values of the parameters of the distributions of the random variables K m and E 1m , thus making the data storage very efficient and the simulation/forecast procedure extremely straightforward.
In the following subsections, the parametrical characterizations of the distributions of k j,m and ε 1j ,m pk∆tq are shown separately.

Daily Variability Parameter
From the observation of the pmf s of K m at each month of the year, it emerged that they always exhibited a multimodal nature.It also emerged that the different seasons were typically characterized by very different behaviors of the pmf s.Some months of the year, for example, are characterized by scattered distributions in correspondence of low values of k j,m which appear almost uniformly distributed, whereas an increased concentration of the occurrences can be observed for higher values of k j,m , with the majority of occurrences around a specific value, analogous to Gaussian behavior.In other cases, the multimodality is characterized by shapes that are more similar to either Gaussian or Weibull distributions, with modes appearing for both the low and high values of k j,m .In the most general case, the heterogeneity of the distributions of k j,m at each month could be represented by the mixture of two or more distributions that can be expressed as weighted sum of pdf s: where f i pxq are proper pdf s, each characterized by specific values of its parameters and w i are opportune weighting factors.
In our analysis, several options were considered for fitting the distributions of data with probability density curves.For each of them, the root mean square error (RMSE) was evaluated between f(x) and the experimental pmf in order to choose the most suitable parametric distribution to reproduce the selected data.In all the cases, it emerged that a mixture of two pdf s could be an acceptable trade-off between accuracy and simplicity.
The distributions used in this paper to compose the mixtures were Gaussian, Weibull, and uniform, whose analytical expressions are reported in Table 1.

Instantaneous Variability Parameter
A mono-modal nature of the distribution of the instantaneous variability parameter, ε 1j ,m pn∆tq, was observed for all months of the year.The behavior was found to be very well approximated by using a T-location scale distribution: where p 1 is the location parameter, p 2 is the scale parameter, and p 3 is the degree of freedom.
The Gaussian distribution was also tested but the results always appeared to be worse than those obtained with the T-location scale.

Simulation of Solar Radiation
Simulation results are illustrated for the purely statistical case.Starting from the pdf s of the two variability parameters, k j,m and ε 1j ,m pn∆tq, it is possible to simulate the solar irradiance with reference to each n∆t of the jth day of the mth month as: where k j,m and ε 1j ,m pn∆tq assume the values obtained by means of opportune extractions from the marginal parametric distributions f K m and f E 1m ; C m and S LJ j,m pn∆tq assume values deriving from their definitions.Rj,m pn∆tq , evaluated according to Equation ( 13), is able to reproduce synthetic time series of solar irradiance whose variability is similar to that observed in the building stage of f K m and f E 1m .
It is useful to recall that the expression typically used in the design stage to evaluate the power producibility is based on the simple expression: Rj,m pn∆tq " S LJ j,m pn∆tq ¨Cm (14) able to reproduce the expected value of the irradiance without taking into account the phenomena of daily and instantaneous variability.

Forecasting Solar Radiation Including Weather Predictions
Forecasting solar radiation including weather predictions requires a more complex procedure than the simple use of Equation ( 13), as illustrated in Figure 4.
Energies 2016, 9, 200 7 of 17 where  1 is the location parameter,  2 is the scale parameter, and  3 is the degree of freedom.
The Gaussian distribution was also tested but the results always appeared to be worse than those obtained with the T-location scale.

Simulation of Solar Radiation
Simulation results are illustrated for the purely statistical case.Starting from the pdfs of the two variability parameters,  , and ε , ′ * (), it is possible to simulate the solar irradiance with reference to each  of the jth day of the mth month as: where  , and ε , ′ * () assume the values obtained by means of opportune extractions from the marginal parametric distributions  K  and  Ε  ′ * ;   and  LJ , () assume values deriving from their definitions. ̂, () , evaluated according to Equation ( 13), is able to reproduce synthetic time series of solar irradiance whose variability is similar to that observed in the building stage of  K  and  Ε  ′ * .
It is useful to recall that the expression typically used in the design stage to evaluate the power producibility is based on the simple expression:  ̂, () =  LJ , () •   (14) able to reproduce the expected value of the irradiance without taking into account the phenomena of daily and instantaneous variability.

Forecasting Solar Radiation Including Weather Predictions
Forecasting solar radiation including weather predictions requires a more complex procedure than the simple use of Equation ( 13), as illustrated in Figure 4.Here reference is made to day-ahead forecasting, but the procedure can be implemented in different time scenarios depending on the specific weather prediction model used.For this purpose, it has to be noted that in principle any weather prediction model can be used; of course, the critical aspect is to provide an interface procedure to convert the information coming from weather prediction into corresponding proper conditioning of the statistical distributions of k j,m and ε 1j ,m that our statistical approach utilizes.
In what follows, the aforementioned critical aspect is considered to be already solved and the availability of properly conditioned pdfs is assumed.It is worth noting that, in this framework, the statistical approach proposed in this paper could play the role of representing the variability characteristic observed in the past, improving, as a statistical post-processing, the outputs of NWP models.
First of all, k j,m values should be extracted according to day-ahead weather predictions with reference to the conditioned pdf s expressed, for instance, as: where r k j,m is the weather prediction for k j,m and α j and β j the edges of the interval of confidence of r k j,m .The conditioned pdf s can be easily derived from the original unconditioned f K m .More complex is the inclusion of weather prediction for ε 1j ,m pn∆tq values.A possible procedure can be: 1.
division of the daylight hours in N i subintervals; 2.
expression of weather prediction for each ith subinterval of duration ∆i as an opportune value of r k i,j,m , α i,j and β i,j ; 3.
evaluation of r k j,m as opportune weighted mean of r k i,j,m : 4. evaluation of opportune intervals of variability for ε 1j ,m pn∆tq for each ith subinterval, as: ´γi,j,m pC m r k i,j,m q ď ε 1i ,j,m pn∆tq ď δ i,j,m p1 ´Cm r k i,j,m q with γ i,j,m and δ i,j,m assuming values minor or equal to 1 deriving from weather prediction; 5.
Extraction of the k i,j,m values for each of the N i subintervals of the jth day according to the day-ahead weather prediction from conditioned distributions f pk m |α i,j ă r k i,j,m ăβ i,j q analogously to Equation (15); 6.
Extraction of the ε 1i ,j,m pn∆tq values according to day-ahead weather predictions, for instance, with reference to the conditioned pdf s expressed as: with G i,j,m " ´γi,j,m pC m r k i,j,m q and D i,j,m " δ i,j,m p1 ´Cm r k i,j,m q For the ith subinterval the forecasted irradiance is given by: Ri,j,m pn∆tq " S LJ j,m pn∆tq ¨rC m ¨ki,j,m `ε1i ,j,m pn∆tqs.

Field Measurement Analysis
In order to test the proposed model, the data, related to six-year observations (years 2004-2009) from a location in the south of Italy whose latitude and longitude are 40 ˝48.8 1 , 14 ˝20.3 1 [19], were analyzed.The years 2004-2008 were used to calibrate the model (identification period) while the year 2009 was used to validate the model (validation period).The data used represent the global horizontal solar irradiance and are available on a 10 min basis, already filtered in order to clean values deriving from measurements errors or absence of measurements.Due to the characteristics of the proposed method, the data were utilized without any ad hoc time series pre-processing finalized to extract trends or seasonality.In the following subsections, the results of the fitting of the aforementioned data (i.e., K m and E 1m ) are reported first.Then, the results of the application of the model for forecasting purposes are shown together with an evaluation of the method's performance.

Parametric Distribution Fitting
The results are reported separately for mean daily variability and instantaneous variability.

Mean Daily Variability
Table 2 reports the parameters of the distributions and weights of their mixture chosen to fit the K m data with reference to all months of the year.The values of the corresponding RMSEs are also reported in the table.The analysis was effected for all months of the year, each characterized by a proper combination of distributions.Several attempts were made to find the best combination of the above parametric distributions.The choice fell on those characterized by the minor value of the RMSE.In order to provide a graphical example, Figure 5 shows the pmf s and their fitting parametric distributions for the months of January, April, July, and October (representatives of the four seasons) where the experimental pmf s were fitted by the mixture of (1) Uniform and Gaussian distribution in the case of January; (2) two Gaussian distributions in the case of April; and (3) Weibull and Gaussian distributions in the cases of both July and October.
From the analysis of the figure, the bimodal nature of f K m clearly appears.It can also be observed how the selected mixtures of parametric distributions quite accurately approximate the pmf s. Figure 5 also reveals that, in some months, more than two modes could be identified for the pmf s.The results of the analysis performed, however, revealed that mixing more than two parametric distributions doesn't significantly improve the statistical characterization of K m .
In order to provide a graphical example, Figure 5 shows the pmfs and their fitting parametric distributions for the months of January, April, July, and October (representatives of the four seasons) where the experimental pmfs were fitted by the mixture of (1) Uniform and Gaussian distribution in the case of January; (2) two Gaussian distributions in the case of April; and (3) Weibull and Gaussian distributions in the cases of both July and October.From the analysis of the figure, the bimodal nature of    clearly appears.It can also be observed how the selected mixtures of parametric distributions quite accurately approximate the pmfs.Figure 5 also reveals that, in some months, more than two modes could be identified for the pmfs.The results of the analysis performed, however, revealed that mixing more than two parametric distributions doesn't significantly improve the statistical characterization of   .

Instantaneous Daily Variability
The analysis performed for   ′ * , for all months of the year, allowed us to identify the parameter values associated with the t location-scale distribution of each month.Table 3 synthetically reports the results of the analysis.
The pmfs and their fitting parametric distributions for the months of January, April, July, and October are reported in Figure 6, showing the suitability of this distribution for   ′ * .

Instantaneous Daily Variability
The analysis performed for E 1m , for all months of the year, allowed us to identify the parameter values associated with the t location-scale distribution of each month.Table 3 synthetically reports the results of the analysis.The pmf s and their fitting parametric distributions for the months of January, April, July, and October are reported in Figure 6, showing the suitability of this distribution for E 1m .

Forecasting Application
Starting from the parametric pdfs of   and   ′ * , the forecasting procedure described in Section 4 was applied to several tests performed with reference to all months of the year.For the sake of brevity, in what follows, only the results referring to the day-ahead forecasts for the months of January, April, July, and October are reported.
The results of the application of Parametric Marginal distribution functions (PARAMETRIC) have been compared with those obtained by means of Experimental Marginal distribution functions (EXPERIMENTAL) [18] and the Persistence method (PERS, one of the most commonly used reference methods [23]).
In order to compare the forecast methods performances, the mean average % error, MAEj,m, the mean bias % error, MBEj,m, and the root mean square % error, RMSEj,m, have been calculated for each day of the mth month:

Forecasting Application
Starting from the parametric pdf s of K m and E 1m , the forecasting procedure described in Section 4 was applied to several performed with reference to all months of the year.For the sake of brevity, in what follows, only the results referring to the day-ahead forecasts for the months of January, April, July, and October are reported.
The results of the application of Parametric Marginal distribution functions (PARAMETRIC) have been compared with those obtained by means of Experimental Marginal distribution functions (EXPERIMENTAL) [18] and the Persistence method (PERS, one of the most commonly used reference methods [23]).
In order to compare the forecast methods performances, the mean average % error, MAE j,m , the mean bias % error, MBE j,m , and the root mean square % error, RMSE j,m , have been calculated for each day of the mth month: with MR j,m being the daily mean value of the measured solar irradiance evaluated over the N s,j,m samples of the day with solar irradiance different from zero: Skill Scores, evaluated with reference to the Persistence method of MAE and RMSE, have been also evaluated by means of the following expressions where METH refers either to EXPERIMENTAL or PARAMETRIC forecasting methods: Figure 7 reports MAE j,m values for the two different forecast methods with reference to each day of: (a) January, (b) April, (c) July, and (d) October.
It is possible to observe that: ‚ For all the months considered, the daily error varies in a quite wide range (e.g., in October from less than 10% on the 20th to almost 50% on the 13th);

‚
The performances of the methods are very close to each other.
Monthly mean values of the normalized MAE, normalized MBE, and normalized RMSE for the two methods are reported in Table 4, together with the corresponding values for the Persistence method, for the same months considered in Figure 7.
Table 5 reports Skill Scores obtained using the Persistence method as reference of the monthly mean of MAE j,m and of RMSE j,m for the months given in Figure 7. EXPERIMENTAL refers to the use of the experimental marginal distribution functions proposed in [18] and PARAMETRIC refers to the use of the parametric distribution functions proposed in this paper.
It is possible to observe that: ‚ Both methods presented by the authors always have better performance than the Persistence method (Table 4);

‚
The use of parametric distributions gives almost the same performance as the experimental distributions (Table 4); ‚ Skill Scores for both MAE and RMSE quantify the improvement of the performance with respect to the Persistence method (Table 5);

‚
The performance of the proposed methods is also better for the month of July, when more stable weather conditions allow the Persistence method to have good performance (Table 5).Monthly mean values of the normalized MAE, normalized MBE, and normalized RMSE for the two methods are reported in Table 4, together with the corresponding values for the Persistence method, for the same months considered in Figure 7.
Table 5 reports Skill Scores obtained using the Persistence method as reference of the monthly mean of MAEj,m and of RMSEj,m for the months given in Figure 7. EXPERIMENTAL refers to the use of the experimental marginal distribution functions proposed in [18] and PARAMETRIC refers to the use of the parametric distribution functions proposed in this paper.
It is possible to observe that:  Both methods presented by the authors always have better performance than the Persistence method (Table 4);  The use of parametric distributions gives almost the same performance as the experimental distributions (Table 4);  Skill Scores for both MAE and RMSE quantify the improvement of the performance with respect to the Persistence method (Table 5);  The performance of the proposed methods is also better for the month of July, when more stable weather conditions allow the Persistence method to have good performance (Table 5).Going back to Figure 7, it is possible to observe that only some days of the month show MAE values appreciably higher than the rest of the month.In fact, looking at the monthly mode values of the MAE metric reported in Table 6 for the four months analyzed, it is evident that the mode values are appreciably lower than the mean values (see Table 4).Going back to Figure 7, it is possible to observe that only some days of the month show MAE values appreciably higher than the rest of the month.In fact, looking at the monthly mode values of the MAE metric reported in Table 6 for the four months analyzed, it is evident that the mode values are appreciably lower than the mean values (see Table 4).In order to understand the reason why high error values appear in some specific days (see Figure 7), the solar irradiance on such days has been further analyzed.
Energies 2016, 9, 200 14 of 17 As an example, Figure 8a reports, with reference to the 22nd of October (that is, one of the days characterized by a high error value), the measured solar irradiance, R 22,10 , the forecasted solar irradiance, R22,10 , the expected clear sky theoretical solar irradiance, S LJ22,10 , and the product of the factor, k 22,10 by S W22,10 versus the time.It is evident that the daily radiation is lower than that expected from the historical data available for the specific site (k 22,10 is lower than 1); furthermore, 2/3 of daylight hours are characterized by very low irradiance values while the remaining 1/3 of the time is characterized by high values.So this particular day is characterized by a very strong correlation between the k and ε values.This highlights an intrinsic limitation of purely statistical forecasting, which is able to follow the average behavior of the solar irradiance but randomly generates statistically independent epsilon values through the whole daylight period.It is worth noting that the behavior of the analyzed 22nd of October helps us understand the reason why the values of the monthly modes are lower than those of the monthly means (Table 4).In fact, looking at Figure 7, it is possible to note that the majority of the days for all months are characterized by low error (resulting in a low value of the monthly mode) while only some days of all the months are characterized by very high error, resulting in an increased value of the monthly mean.
Results with a sensibly higher level of accuracy were obtained by applying the procedures described in Section 4.2, which is able to take into account weather predictions.
Figure 8b reports the results referring to the 22nd of October obtained from the study of weather predictions able to predict the two-time behavior of the day, with a level of approximation of  ̃ of ±10%.
The figure gives an immediate visual idea of the improvement in forecasting solar irradiance through the hours of the day; the corresponding MAE22,10 results reduced to 10% from the initial value 47% of Figure 8d).As for the other two metrics introduced, MBE22,10 remains almost constant around 5% while RMSE22,10 results reduced to 13% from 58%.By analysis of the figure, the strength of the procedures shown in Section 5 clearly appears to be predicting the different behavior during the day, not only in terms of radiation level but also in terms of variability of instantaneous solar irradiance.The first part of the day, in fact, is characterized by low values of mean solar irradiance (low  ̃1,22,10, see Equation ( 16)) and a significant variability value.The second part of the day, instead, is characterized by a regular profile of the solar irradiance with a high value of mean solar irradiance (high  ̃2,22,10 , see Equation ( 16)).This behavior is quite well captured by the forecasting procedure, of which the accuracy is evident, especially in case of accurate weather predictions.
This shows the difference between a purely statistical method (without taking into account any weather predictions) and the procedure completed with information on weather conditions.The former seems to be a practical solution for simulating synthetic time series in the long run (e.g., for isolated microgrids design purposes).The last seems to be a practical solution especially for those applications where information on instantaneous variations of solar irradiance is required (e.g., for day-ahead forecasts).
For comparative purposes, the results of the proposed model have been compared with the evaluations performed in [24], which refer to a benchmarking exercise organized within the framework of the European project "WIRE" [25] with the purpose of evaluating the performance of It is worth noting that the behavior of the analyzed 22nd of October helps us understand the reason why the values of the monthly modes are lower than those of the monthly means (Table 4).In fact, looking at Figure 7, it is possible to note that the majority of the days for all months are characterized by low error (resulting in a low value of the monthly mode) while only some days of all the months are characterized by very high error, resulting in an increased value of the monthly mean.
Results with a sensibly higher level of accuracy were obtained by applying the procedures described in Section 5, which is able to take into account weather predictions.
Figure 8b reports the results referring to the 22nd of October obtained from the study of weather predictions able to predict the two-time behavior of the day, with a level of approximation of r k of ˘10%.
The figure gives an immediate visual idea of the improvement in forecasting solar irradiance through the hours of the day; the corresponding MAE 22,10 results reduced to 10% from the initial value 47% of Figure 8d).As for the other two metrics introduced, MBE 22,10 remains almost constant around 5% while RMSE 22,10 results reduced to 13% from 58%.By analysis of the figure, the strength of the procedures shown in Section 5 clearly appears to be predicting the different behavior during the day, not only in terms of radiation level but also in terms of variability of instantaneous solar irradiance.The first part of the day, in fact, is characterized by low values of mean solar irradiance (low r k 1,22,10 , see Equation ( 16)) and a significant variability value.The second part of the day, instead, is characterized by a regular profile of the solar irradiance with a high value of mean solar irradiance (high r k 2,22,10 , see Equation ( 16)).This behavior is quite well captured by the forecasting procedure, of which the accuracy is evident, especially in case of accurate weather predictions.
This shows the difference between a purely statistical method (without taking into account any weather predictions) and the procedure completed with information on weather conditions.The former seems to be a practical solution for simulating synthetic time series in the long run (e.g., for isolated Energies 2016, 9, 200 15 of 17 microgrids design purposes).The last seems to be a practical solution especially for those applications where information on instantaneous variations of solar irradiance is required (e.g., for day-ahead forecasts).
For comparative purposes, the results of the proposed model have been compared with the evaluations performed in [24], which refer to a benchmarking exercise organized within the framework of the European project "WIRE" [25] with the purpose of evaluating the performance of state-of-the-art models for short-term renewable energy forecasting.More specifically, 10 different solar power forecasting methods were applied to historical data for 2010 and 2011 and with reference to the suburbs of the city of Milan in Northern Italy and to the suburban area of Catania in Southern Italy.The location analyzed in this paper (Portici, Italy) is geographically situated between these two realities.Hence, though based on different data, this non-customized comparison can give an idea of the performance of the proposed model.For the evaluation in [24], among the metrics adopted, the MAE, normalized by the mean power (MP) measured during the test period, was reported.Table 7 reports the forecasting methods used by the participants in the project [25] and their error ranges in terms of MAE (%MP) for Milano (IT) and Catania (IT).Error ranges refer to minimum and maximum values regardless of the location of the best scores reported in Figures 7 and 8 of [24].Mean normalized MAE (%MR) for the methods reported in Table 4 (evaluated for the city of Portici, IT) are reported in Table 8.The commensurateness of the proposed method with all of the analyzed approaches clearly appears.

Conclusions
Radiation forecasting, accounting for daily and instantaneous variability, was pursued by means of a new bi-parametric model that builds on a model previously proposed by the same authors.The statistical model is developed with direct reference to the Liu-Jordan clear sky theoretical expression but is not bound by specific clear sky models; it accounts separately for the mean daily variability and for the variation of solar irradiance during the day by means of two corrective parameters.This new proposal allows for a better understanding of the physical phenomena and improves the effectiveness of statistical characterization and subsequent simulation of the introduced parameters to generate a synthetic solar irradiance time series.Furthermore, the analysis of the parametric distributions that best fit the experimental distributions of the two-parameter data was developed by obtaining opportune parametric distributions or mixtures of more than one distribution.Finally, the model was further improved by including weather prediction information in the simulation and forecasting stage, thereby overcoming the limitations of a purely statistical approach.
The main outcomes are: -The introduction of single parametric distributions and of mixtures of parametric distributions seems to offer, with reference to specific geographical areas, general models easy to handle in both data acquisition and subsequent simulation stages.-The model is suitable for inclusion of weather prediction in the solar radiation forecast stage.
and detailed in Section 3; (2) simulation and forecast of the solar irradiance summarized in the flow chart of Figure 3 and detailed in Section 4.

Figure 2 .
Figure 2. Flow chart of the "Statistical Analysis" stage.
and detailed in Section 3; (2) simulation and forecast of the solar irradiance summarized in the flow chart of Figure 3 and detailed in Section 4. Energies 2016, 9, 200 4 of 17
and detailed in Section 3; (2) simulation and forecast of the solar irradiance summarized in the flow chart of Figure 3 and detailed in Section 4.

Figure 2 .
Figure 2. Flow chart of the "Statistical Analysis" stage.Figure 2. Flow chart of the "Statistical Analysis" stage.

Figure 2 .
Figure 2. Flow chart of the "Statistical Analysis" stage.Figure 2. Flow chart of the "Statistical Analysis" stage.

Figure 3 .
Figure 3. Flow chart of the "Simulation of solar irradiance" stage.

Figure 3 .
Figure 3. Flow chart of the "Simulation of solar irradiance" stage.

Figure 4 .
Figure 4. Flow chart of the "Forecasting of solar irradiance including weather predictions" stage.Figure 4. Flow chart of the "Forecasting of solar irradiance including weather predictions" stage.

Figure 4 .
Figure 4. Flow chart of the "Forecasting of solar irradiance including weather predictions" stage.Figure 4. Flow chart of the "Forecasting of solar irradiance including weather predictions" stage.

Figure 5 .
Figure 5. Discrete probability distributions of   and their fitting probability density functions (pdfs) with reference to the months of (a) January; (b) April; (c) July; and (d) October.

Figure 5 .
Figure 5. Discrete probability distributions of K m and their fitting probability density functions (pdfs) with reference to the months of (a) January; (b) April; (c) July; and (d) October.

Figure 6 .
Figure 6.Discrete probability distributions of   ′ * , and their fitting pdfs with reference to the months of (a) January; (b) April; (c) July; and (d) October.

Figure 6 .
Figure 6.Discrete probability distributions of E 1m , and their fitting pdfs with reference to the months of (a) January; (b) April; (c) July; and (d) October.

Figure 7 .
Figure 7. MAEj,m values versus the day of the month for the two different forecast methods: EXPERIMENTAL and PARAMETRIC.(a) January, (b) April, (c) July, and (d) October.

Figure 7 .
Figure 7. MAE j,m values versus the day of the month for the two different forecast methods: EXPERIMENTAL and PARAMETRIC.(a) January, (b) April, (c) July, and (d) October.

Table 1 .
Density functions of the distributions chosen for the analysis.

Table 2 .
Mixtures of distributions resulting from the analysis of K m (p 1 1 , p 1 2 , p" 1 and p" 2 are defined in Table1and w 1 and w 2 in Equation (11)).Root mean square error: RMSE.

Table 3 .
Parameters of the t location-scale distribution resulting from the analysis of E 1m .

t Location-Scale Parameters Location (p1) Scale (p2) Degree of Freedom (p3)
̂, () −  , ()| with MRj,m being the daily mean value of the measured solar irradiance evaluated over the Ns,j,m samples of the day with solar irradiance different from zero:MR .= ∑  , () N s,, =1N s,,

Table 4 .
Monthly mean values of MAE j,m , MBE j,m and of RMSE j,m for the months in Figure7.

Table 5 .
Skill Scores, referring to the Persistence method, of the monthly mean of MAE j,m , and of RMSE j,m for the months in Figure7.

Table 4 .
Monthly mean values of MAEj,m, MBEj,m and of RMSEj,m for the months in Figure7.

Table 5 .
Skill Scores, referring to the Persistence method, of the monthly mean of MAEj,m, and of RMSEj,m for the months in Figure7.

Table 6 .
Monthly mode values of MAE j,m for the months in Figure7.