A new approach to the real-time assessment of the clear-sky direct normal irradiance

Abstract

Highlights• A new approach to the real-time assessment of the clear-sky DNI is described• An existing clear-sky DNI model is part of the approach • A methodology for the computation of atmospheric turbidity is developed• Clear-sky DNI data are detected using a wavelet-based multi-resolution analysis• An evaluation procedure, allowing changes in DNI to be simulated, is proposedA new approach to the real-time assessment of the clear-sky direct normal irradiance AbstractIn a context of sustainable development, interest for concentrating solar power and concentrating photovoltaic technologies is growing rapidly. One of the most challenging topics is to improve solar resource assessment and forecasting in order to optimize power plant operation. Since clear sky defines the nominal operating conditions of the plants, improving their management requires the use in real-time of clear-sky direct normal irradiance (DNI) models. Typically, accuracy is best achieved by considering water vapor and aerosol concentrations in the atmosphere separately. However, measuring such physical quantities is not easy and requires a weather station close to the considered site. When these data are not available, the attenuating effects can be modelled by atmospheric turbidity factors which can be obtained from DNI under clear-sky conditions. So, the main purpose of the present paper is to propose an efficient approach to assess the clear-sky DNI in real time. This approach combines an existing empirical model, proposed by Ineichen and Perez, with a new methodology for the computation of atmospheric turbidity. It takes advantage of the fact that changes in atmospheric turbidity are relatively small throughout the day in comparison to changes in DNI, even when the sky is free of clouds.In the present study, we considered data from two experimental sites (Golden, in the USA, and Perpignan, in France) and used a wavelet-based multi-resolution analysis as a clear-sky DNI detection tool. In addition, we compared the proposed approach with several combinations of empirical models and ways of computing atmospheric turbidity. The first model is a polynomial of the cosine of the solar zenith angle, whereas the two other models use atmospheric turbidity as an additional input. Regarding its calculation, monthly and daily mean values have been considered. Moreover, we defined a procedure in order to evaluate the accuracy of all the considered approaches. This procedure allows changes in DNI caused by clouds to be simulated using a noisy signal applied to clear-sky periods. In both sites, our approach to the real-time assessment of the clear-sky DNI outperforms the other approaches. In the worst case, the mean absolute error is reduced by 8 W m −2 in comparison to the approaches based on monthly mean values of atmospheric turbidity, and reduced by about 30 W m −2 when taking the polynomial-based model as a reference.

Towards the intrahour forecasting of direct normal irradiance using sky-imaging data

“…To overcome this problem, clear-sky data (in that case, there is no disturbance to DNI at all) have been selected using an off-line detection algorithm, i.e. a wavelet-based multi-resolution analysis ( Nou et al, 2016 ). Such a selection will contribute to the better understanding of the transient behavior of atmospheric turbidity during the day.…”

“…In addition, because a new DNI measurement has to be linked to the situation of the sky, a real-time detection algorithm for clear-sky situations has been developed. It makes use of the last-known clear-sky situation and requires the maximum opacification speed of the atmosphere to be evaluated ( Nou et al , 2016 , 2017 ). Clear-sky DNI is then forecasted using the Ineichen and Perez's model, combined with a persistence of atmospheric turbidity.…”

“…Of course, forecasting clear-sky DNI is also needed to forecast DNI for all sky conditions. Forecasts are given by (6) : where , with the forecast horizon, b is a function of the altitude of the considered site, is the extraterrestrial solar irradiance, m is the relative optical air mass, and is the last-known value of atmospheric turbidity given by the real-time detection algorithm developed by Nou et al ( 2016 , 2017 ).…”

“…Regarding the forecasting of clear-sky DNI, the approach described in Section 2.1 is used. Keep in mind that such an approach combines the Ineichen and Perez's model with a persistence of atmospheric turbidity (35) : where , with the forecast horizon, b is a function of the altitude of the site, is the extraterrestrial solar irradiance, m is the relative optical air mass, and is the last-known value of atmospheric turbidity ( Nou et al , 2016 , 2017 ).…”

Mathematical Modeling and Simulation of the Photovoltaic System's Output Power depends on Temperature and Irradiance

“…Mono kristal malzemeden yapılan FV paneller sıcaklığın yüksek olduğu bölgelerde polikristalden yapılan güneş panellerine göre daha fazla enerji üretirler. Küçük alanlarda daha fazla enerji üretirler (Xiao, Edwin, Spagnuolo &GJatsvevich,2013;Hirose, Osaki, Kuroki & Numa, 2010;İzgi, İnan & Ay, 2008;Nou, Chauvin, Thil & Grieu, 2013;Rustemli & Dincer, 2011;Uno & Kukita, 2017) Matlab/Simulink yazılım programı ile FV sistemin eşdeğeri oluşturulmuştur. Ayrıca değişik sıcaklık ve güneş ışınımı değerleri için PV sistemin çıkış gücü ölçülmüştür.…”