Abstract
Direct Normal Irradiance (DNI) predictions obtained from the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecast (IFS/ECMWF) were compared against ground-based observational data for one location at the south of Portugal (Évora). Hourly and daily DNI values were analyzed for different temporal forecast horizons (1 to 3 days ahead) and results show that the IFS/ECMWF slightly overestimates DNI for the period of analysis (1 August 2018 until 31 July 2019) with a fairly good agreement between model and observations. Hourly basis evaluation shows relatively high errors, independently of the forecast day. Root mean square error increases as the forecast time increases with a relative error of ~45% between the first and the last forecast. Similar patterns are observed in the daily analysis with comparable magnitude errors. The correlation coefficients between forecast and observed data are above 0.7 for both hourly and daily data. A methodology based on a new DNI attenuation Index (DAI) was developed to estimate cloud fraction from hourly values integrated over a day and, with that, to correlate the accuracy of the forecast with sky conditions. This correlation with DAI reveals that in IFS/ECMWF model, the atmosphere as being more transparent than reality since cloud cover is underestimated in the majority of the months of the year, taking the ground-based measurements as a reference. The use of the DAI estimator confirms that the errors in IFS/ECMWF are larger under cloudy skies than under clear sky. The development and application of a post-processing methodology improves the DNI predictions from the IFS/ECMWF outputs, with a decrease of error of the order of ~30%, when compared with raw data.
Highlights
Solar energy is becoming a crucial renewable resource in modern societies, contributing to the sustainability of the planet with the mitigation of greenhouse gas emissions by reducing the consumption of coal or fuel oil for electricity production; the availability of solar resources over time at a given region of interest determines the cost/benefit of solar power plants implementation
Several researchers have estimated the potential of renewable energies like wind or solar radiation for electricity or thermal energy production around the world; for example, in Europe and Africa [1], in Chile [2], in Iberian Peninsula [3], in United Kingdom [4], and Spain [5]
The main objective of this study was to assess the performance of the IFS/ECMWF global model (CY45R1 cycle—released at 5 June 2018) to predict Direct Normal Irradiance (DNI) in the south of Portugal, by comparing its results with observational data of Évora station, on an hourly and daily basis and for various forecasting horizons
Summary
Solar energy is becoming a crucial renewable resource in modern societies, contributing to the sustainability of the planet with the mitigation of greenhouse gas emissions by reducing the consumption of coal or fuel oil for electricity production; the availability of solar resources over time at a given region of interest determines the cost/benefit of solar power plants implementation. Several researchers have estimated the potential of renewable energies like wind or solar radiation for electricity or thermal energy production around the world; for example, in Europe and Africa [1], in Chile [2], in Iberian Peninsula [3], in United Kingdom [4], and Spain [5]. Concerning solar energy, there are two main ways of converting solar energy into electricity: Photovoltaic (PV) and Concentrating Solar Power (CSP). The PV panels convert either direct and diffuse solar irradiance, while the CSP technology only concentrates the Direct Normal Irradiance (DNI). The forecast of global solar radiation (direct + diffuse) for the same region was addressed, for example, in [6] and [9]
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