Experimental Evaluation of a Spectral Index to Characterize Temporal Variations in the Direct Normal Irradiance Spectrum

Gustavo Nofuentes,Guilherme Marques-Neves,Jorge Aguilera,José A Caballero,Christian A Gueymard

doi:10.3390/app11030897

Abstract

A simple index is desirable to assess the effects on both flat-plate and concentrating photovoltaics of natural changes in the solar spectrum. Some studies have suggested that the relationship between the Average Photon Energy (APE) and the shape of individual global tilted irradiance, global horizontal irradiance, or direct normal irradiance (DNI) spectra is bijective and can therefore be used as a single number to unequivocally replace a complete spectral distribution. This paper reevaluates these studies with a modified methodology to assess whether a one-to-one relationship really exists between APE and spectral DNI. A 12-month dataset collected in Jaén (Spain) using a sun-tracking spectroradiometer provides the necessary spectral DNI data between 350 and 1050 nm. After quality control and filtering, 78,772 valid spectra were analyzed. The methodology is based on a statistical analysis of the spectral distributions binned in 0.02-eV APE intervals, from 1.74 to 1.90 eV. For each interval, both the standard deviation and coefficient of variation (CV) are determined across all 50-nm bands into which the 350–1050-nm waveband is divided. CV stays below 3.5% within the 450–900-nm interval but rises up to 13% outside of it. It is concluded that APE may be approximately assumed to uniquely characterize the DNI spectrum distribution for Jaén (and presumably for locations with similar climates) only over the limited 450–900-nm waveband.

Highlights

The accurate estimation of both the expected and predicted electricity yields of photovoltaic (PV) systems is normally carried out by following a standard procedure [1]
This dispersion of percentage contributions within the 350–450 nm and 900–1050 nm wavebands was noted in local global tilted irradiance (GTI) spectral distributions for average photon energy (APE) and air mass (AM) around 1.88 eV and 1.5, respectively values obtained at both Jaén and Kusatsu are not negligible when compared to the values across the 50-nm band intervals used here—or 10-nm bands for the Japanese site— that lie outside the central wavelength range (450–900 nm)
This study investigated whether the average photon energy (APE) uniquely represents the direct normal irradiance (DNI) spectrum over the 350–1050 nm band at Jaén, a Spanish sunny site with Mediterranean-Continental climate

Summary

Introduction

The accurate estimation of both the expected and predicted electricity yields of photovoltaic (PV) systems is normally carried out by following a standard procedure [1]. This step is crucial to ascertain the production of electricity that such systems may deliver, enabling both the bankability of PV projects [2] and the quality assurance for PV plants [3], among other desirable outcomes. Such a higher sensitivity of the latter technology to spectral effects is especially noticeable when concentrated sunlight of 1000 suns or more is used in High Concentrating Photovoltaics (HCPV) [6], where multi-junction solar cells based on seriesconnected subcells with different bands of absorption are used in conjunction with optical elements [7,8]. The present contribution is mostly related to the latter type of application

Methods

Results

Conclusion