Measurements Of Diffuse Irradiance Research Articles

Assessment of estimation methods of DNI resource in solar concentrating systems

The DNI (Direct Normal Irradiance) is the resource utilized by solar concentrators, in CSP and CPV applications. However, it is not often measured due to practical reasons, and is derived instead from measurements of global irradiance or satellite-based models. This paper describes a study of different estimation procedures for the assessment of the DNI, using experimental data with a time scale of 1min, taken at two different latitudes in Italy. The analyzed approaches include measuring techniques and models. Accurate estimations of DNI are obtained as derived from measurements of global horizontal irradiance and diffuse horizontal irradiance if the offset in pyranometers are properly corrected and the zenith angle is not too high. The measured and calculated values of DNI are used to estimate the hourly, daily, monthly and seasonal irradiation. The results show that the different estimation methods can lead to quite different conclusions when comparing short-term and long-term solar radiation availability.

New device for the simultaneous measurement of diffuse solar irradiance on several azimuth and tilting angles

A device is proposed for the measurement of diffuse solar irradiance on tilted surfaces pointing-to as well as fixed on the equatorial direction. Most commercial devices are not designed for this purpose. Here, we present a specific design for a system based on a single multi-lobe shadow-ring that allows us to measure diffuse irradiance with several tilted pyranometers – from 60 to 90 degrees from the horizontal plane – and on different azimuth angles. This first prototype allows us to measure in the four cardinal directions. The ring is designed to minimize the covered celestial dome observed by each sensor. Moreover, the complete device needs a very small installation area and its simplicity means it is inexpensive and easy to operate. The measurements have subsequently to be corrected with a geometrical correction factor that takes into account the portion of the sky vault blocked by the shadow-ring. These measurements are essential for estimating solar potential on tilted surfaces – such as building façades – and for improving current anisotropic solar models, among many other applications.

Assessment of aerosol optical and micro-physical features retrieved from direct and diffuse solar irradiance measurements from Skyradiometer at a high altitude station at Merak: Assessment of aerosol optical features from Merak.

Optical and micro-physical features of aerosol are reported using Skyradiometer (POM-01L, Prede, Japan) observations taken from a high-altitude station Merak, located in north-eastern Ladakh of the western trans-Himalayas region during January 2011 to December 2013. The observed daily mean aerosol optical depth (AOD, at 500 nm) at the site varied from 0.01 to 0.14. However, 75 % of the observed AOD lies below 0.05 during the study period. Seasonal peaks of AOD occurred in spring as 0.06 and minimum in winter as 0.03 which represents the aged background aerosols at the site. Yearly mean AOD at 500 nm is found to be around 0.04 and inter-annual variations of AOD is very small (nearly ±0.01). Angstrom exponent (a) varied seasonally from 0.73 in spring to 1.5 in autumn. About 30 % of the observed a lies below 0.8 which are the indicative for the presence of coarse-mode aerosols at the site. The station exhibits absorbing aerosol features which prominently occurred during spring and that may be attributed by the transported anthropogenic aerosol from Indo-Gangatic Plain (IGP). Results were well substantiated with the air mass back-trajectory analysis. Furthermore, seasonal mean of single scattering albedo (SSA at 500 nm) varied from of 0.94 to 0.98 and a general increasing trend is noticed from 400 to 870 nm wavelengths. These features are apparently regional characteristics of the site. Aerosol asymmetry factor (AS) decreases gradually from 400 to 870 nm and varied from 0.66 to 0.69 at 500 nm across the seasons. Dominance of desert-dust aerosols, associated by coarse mode, is indicated by tri-modal features of aerosol volume size distribution over the station during the entire seasons.

Pyranometer Thermal Offset: Measurement and Analysis

AbstractThe reliable estimation of the radiative forcing and trends in radiation requires very accurate measurements of global and diffuse solar irradiance at the earth’s surface. To improve measurement accuracy, error sources such as the pyranometer thermal offset should be thoroughly evaluated. This study focuses on the measurement and analysis of this effect in a widely used type of pyranometer. For this aim, a methodology based on capping the pyranometer has been used and different criteria for determining the thermal offset have been applied and compared. The thermal offset of unventilated pyranometers for global and diffuse irradiance has been measured under a wide range of cloud, ambient temperature, wind speed, and radiation conditions. Significant differences in absolute values and variability have been observed between daytime and nighttime, advising against correcting the thermal offset effect based only on nighttime values. Notable differences in the thermal offset between cloudy and cloud-free conditions have been also observed. The main results show that the ambient temperature, the radiation, and its direct/diffuse partitioning are the variables more related to the daytime thermal offset.

A new PV module performance model based on separation of diffuse and direct light

To increase the energy yield of installed solar photovoltaic (PV) systems, proper operation of all components should be ensured. One way to detect any misbehavior is to estimate the electric power of a PV system using data from meteorological stations or an in-situ integrated weather station to calculate the electric power of a PV system using a PV module performance mathematical model. By comparing the calculated and the actual electric power, the performance of a PV system can be determined. Such a PV system monitoring greatly depends on accuracy of measured weather parameters and precision of a PV module performance model used. Simple heuristic models are easy to implement but lack accuracy in particular under lower irradiances, higher share of diffuse light or higher solar incident angles. To improve accuracy, we developed a new PV module performance model with splitting diffuse and direct light including solar incident angle. As a consequence, the new model requires additional measurement of diffuse irradiance, but simplicity is maintained using simple equations with low number of fitting coefficients that were extracted for a typical multi-crystalline (mc-Si) and thin film copper indium gallium selenide (CIGS) PV module, based on outdoor monitoring data in Ljubljana, Slovenia. The new model was compared to two other heuristic models and a decrease of root mean square error (RMSE) in eight snowless month period (during October 2013 to September 2014) was observed, on average by 1%. Much larger improvement with the new model is obtained at lower irradiances: RMSE at irradiances of 200W/m2 in June 2014 was three times lower compared to the other two models. The new PV module performance model with separated diffuse and direct irradiance component assures higher accuracy of prediction and thus higher reliability of PV system fault detection applications.

Assessment of diffuse radiation models for cloudy atmospheric conditions in the Azores region

Measurements of diffuse, global and direct solar irradiance are key to a proper assessment of the potential of solar energy technologies. Unfortunately, worldwide measured irradiance databases usually consist of only global solar radiation data, often with limited spatial coverage. The estimation of the diffuse fraction from global radiation is therefore essential. Solar radiation models for prediction hourly diffuse fraction from global radiation are reviewed in this paper in order to explore their applicability to the Azorean region. Since models of diffuse fraction exhibit some degree of location dependence, a correction to the best performance model to improve the results for the studied region by considering the zenith angle is also presented.

Estimation of direct normal irradiance from measured global and corrected diffuse horizontal irradiance

DNI (direct normal irradiance) can be calculated from global and diffuse horizontal irradiance measurements. However, the diffuse irradiance values need to be corrected because the pyranometer's shadowband does not obstruct only the solar disk but also a larger part of the sky vault. In this study, we use four diffuse correction models (Drummond [1], LeBaron et al. [2], Batlles et al. [3], Muneer and Zhang [4]) and, considering the importance of DNI for several solar energy projects, we try to assess the performance of the models when the calculated DNI is compared to the measured one by the pyrheliometer. Based on 1-min averaged measurements covering a one-year period in Athens, Greece, it is concluded that the empirical approach of Batlles et al. performs best. It presents the lowest residuals (±1%) with no dependence from the clearness index. The estimated annual DNI, derived from the synergetic use of this model with measurements of horizontal global and diffuse irradiance, is lower by 0.4%, while the calculated values from the global and uncorrected diffuse irradiances are overestimated by 7.7%.

Buildings roofs photovoltaic potential assessment based on LiDAR (Light Detection And Ranging) data

One of the major challenges today is assessing the suitability of PV (photovoltaic) systems' installations on buildings' roofs regarding the received solar irradiance. The availability of aerial laser-scanning, namely LiDAR (Light Detection And Ranging), means that assessment can be performed automatically over large-scale urban areas in high accuracy by considering surfaces' topographies, long-term direct and diffuse irradiance measurements, and influences of shadowing. The solar potential metric was introduced for this purpose, however it fails to provide any insights into the production of electrical energy by a specific PV system. Hence, the PV potential metric can be used that integrates received instantaneous irradiance which is then multiplied by the PV system's efficiency characteristics. Many existing PV potential metrics over LiDAR data consider the PV modules' efficiencies to be constant, when in reality they are nonlinear. This paper presents a novel PV potential estimation over LiDAR data, where the PV modules' and solar inverter's nonlinear efficiency characteristics are approximated by modelled functions. The estimated electrical energy production from buildings' roofs within an urban area was extensively analysed by comparing the constant and nonlinear efficiency characteristics of different PV module types and solar inverters. The obtained results were confirmed through measurements performed on an existing PV system.

Modeling the luminous efficacy of direct and diffuse solar radiation using information on cloud, aerosol and water vapor in the tropics

This paper presents luminous efficacy models for direct and diffuse solar irradiance using information on cloud, aerosol and water vapor in the tropics. The model is based on five years (2007–2011) of diffuse illuminance and irradiance measurements and two years of direct illuminance and irradiance measurements, April 2010–March 2012. Data are taken at four solar radiation monitoring stations in Thailand, specifically Chiang Mai (18.78 °N, 98.98 °E) in the Northern region, Ubon Ratchathani (15.25 °N, 104.87 °E) in the Northeastern region, Nakhon Pathom (13.82 °N, 100.04 °E) in the Central region and Songkhla (7.20 °N, 100.60 °E) in the Southern region. The models express luminous efficacy as functions of the aerosol optical depth and precipitable water, obtained from the AERONET network, and a cloud index for hourly time scales derived from the MTSAT-1R satellite. The model performance is good when validated against independent data from these stations. Root mean square differences (RMSD) of 9.7% and 6.8% for direct normal efficacy and diffuse efficacy, respectively are obtained. The models compared favorably with most existing models when tested against these independent data.

Diffuse illuminance availability on horizontal and vertical surfaces at Madrid, Spain

The overall objective of this work is to provide diffuse illuminance availability at Madrid (Spain) through a statistical analysis of illuminance values corresponding to a long-term data series. The illuminance values are obtained from irradiance measurements by means of different empirical models for luminous efficacy. The values of diffuse illuminance on a horizontal and on vertical surfaces facing the four cardinal points are estimated and the different aspects related to daylight availability in an area with specific climatic conditions are analyzed. The experimental data consist of global and diffuse irradiance measurements on a horizontal surface provided by the National Meteorological Agency in Spain (AEMET) for Madrid. These data consist of hourly values measured in the period of 1980–2005. The statistical results derived correspond to a daylight typical year for the five surfaces considered. This information will be useful to building experts to estimate natural illumination availability when daylighting techniques are applied in building design with the main aim of electric energy savings.

Comparison of shadow‐ring correction models for diffuse solar irradiance

Reliable and accurate measurements of diffuse solar irradiance are needed in order to partition global irradiance into its direct and diffuse components. Diffuse irradiance is commonly measured using sun tracking systems or shadow rings. Data obtained using a shadow ring must be corrected for the portion of diffuse irradiance blocked by the ring. In this paper we have examined and evaluated six of the most widely used correction models. Approaches that account for radiation anisotropy perform notably better than those using only geometric corrections. Our results also argue for the need to adjust empirical models to local conditions. Empirical approaches developed by LeBaron et al. (1990) and by Batlles et al. (1995) perform best when compared with the more theoretical models.

Introducing GRADRAD: The greater Durban radiometric network

Abstract A new solar radiation network is proposed for the city of Durban, South Africa. This initiative aims to develop a local radiometric database that includes a number of broadband ground stations in KwaZulu-Natal. Currently, the network comprises three ground stations located within the metropolitan boundaries of Durban. Stations will use similar thermopile instruments and a common software format to facilitate comparison of data. High quality global, direct and diffuse irradiance measurements will be collected by sampling at 6-s intervals and averaging over a 1-min period. The data will be processed and made available to the global research community on a dedicated website. Future aims of this project are to add further ground stations covering the eastern province of KwaZulu-Natal, and eventually to expand the network beyond provincial boundaries to include South Africa as a whole.

Effects of stratocumulus, cumulus, and cirrus clouds on the UV-B diffuse to global ratio: Experimental and modeling results

Broadband measurements of global and diffuse UV-B irradiance (280–315nm) together with modeled and measured diffuse to global ratios (DGR) have been used to characterize the influence of different types of clouds on irradiance at the surface. Measurements were carried out during 2000–2001 in Córdoba City, Argentina. The Tropospheric Ultraviolet Visible (TUV) model was used to analyze the behavior of the modeled DGRs for different cloud optical depths and at different altitudes and solar zenith angles (SZA). Different cloud altitudes were also tested, although only the results for a cloud placed at 1.5–2.5km of altitude are shown. A total of 16 day with stratocumulus, 12 with cumulus, and 16 with cirrus have been studied and compared among them and also against 21 clear sky days. Different behaviors were clearly detected and also differentiated through the analysis of the averages and the standard deviations of the DGRs: 1.02±0.06 for stratocumulus, 0.74±0.18 for cumulus, 0.63±0.12 for cirrus, and 0.60±0.13 for the clear sky days, respectively. Stratocumulus clouds showed a low variability in the DGR values, which were concentrated close to one at all SZAs. DGR values for cumulus clouds presented a large variability at all SZAs, mostly associated with the different optical depths. Finally, the closeness between the DGR values for cirrus clouds and the DGR values for clear days showed that these clouds generally do not strongly affect the UV-B irradiance at the surface at any SZA. In the opposite side, stratocumulus clouds were identified as those with the largest effects, at all SZAs, on the UV-B irradiance at the surface.

Solar radiation models-A review

Solar radiation models for predicting the average daily and hourly global radiation, beam radiation and diffuse radiation on horizontal surface are reviewed in this article. Estimations of monthly average hourly global radiation from daily summations are discussed. It was observed that Collares-Pereira and Rabl model as modified by Gueymard (CPRG) yielded the best performance for estimating mean hourly global radiation incident on a horizontal surface for Indian regions. Estimations of monthly average hourly beam and diffuse radiation are discussed. It was observed that Singh-Tiwari and Jamil-Tiwari both models generally give better results for climatic conditions of Indian regions. Therefore, their use is recommended for composite climate of Indian regions. Empirical correlations developed to establish a relationship between the hourly diffuse fraction and the hourly clearness index using hourly global and diffuse irradiation measurements on a horizontal surface are discussed. Fifty models using the Angstrom–Prescott equation to predict the average daily global radiation with hours of sunshine are considered. It was reported that Ertekin and Yaldiz model showed the best performance against measured data of Konya, Turkey. Copyright © 2010 John Wiley & Sons, Ltd.

Measurement and modeling of solar irradiance components on horizontal and tilted planes

In this work new measurements of global and diffuse solar irradiance on the horizontal plane and global irradiance on planes tilted at 20° and 30° oriented due South and at 45° and 65° oriented due East are used to discuss the modeling of solar radiation. Irradiance data are collected in Padova (45.4°N, 11.9°E, 12 m above sea level), Italy. Some diffuse fraction correlations have been selected to model the hourly diffuse radiation on the horizontal plane. The comparison with the present experimental data shows that their prediction accuracy strongly depends on the sky characteristics. The hourly irradiance measurements taken on the tilted planes are compared with the estimations given by one isotropic and three anisotropic transposition models. The use of an anisotropic model, based on a physical description of the diffuse radiation, provides a much better accuracy, especially when measurements of the diffuse irradiance on the horizontal plane are not available and thus transposition models have to be applied in combination with a diffuse fraction correlation. This is particularly significant for the planes oriented away from South.

Retrospective cloud determinations from surface solar radiation measurements

Surface solar radiation measurements have been made at many meteorological sites for long periods. These data potentially provide higher temporal resolution cloud amount information than traditional estimations of cloud amount recorded in eighths of the sky obscured (oktas). To utilise existing solar radiation datasets for this, two quantities are derived from the surface measurements of global ( G) and diffuse ( D) solar irradiance, and top of atmosphere solar irradiance ( E) — the Diffuse Fraction (DF = D/ G) and the Opaqueness (Op = 1 - G/ E). These are compared with subjective cloud observations made at Reading daily during 1997 to 2006. This shows that DF measurements are sensitive to cloud amount, particularly for low and moderate cloud coverage. Complimentary information is available in Op, which is particularly sensitive to cloud amount in moderate to overcast cloud coverage. In overcast conditions, DF = 1, during which Op provides a measure of overcast cloud thickness. As well as cloud amount, the variability in DF and Op provides a basis for discriminating between cumuliform and stratiform cloud: large variability indicates convective cloud whereas only small variability occurs under stratiform cloud.

Global and diffuse solar irradiances in urban and rural areas in southeast Brazil

The seasonal evolution of daily and hourly values of global and diffuse solar radiation at the surface are compared for the cities of Sao Paulo and Botucatu, both located in Southeast Brazil and representative of urban and rural areas, respectively. The comparisons are based on measurements of global and diffuse solar irradiance carried out at the surface during a six year simultaneous period in these two cities. Despite the similar latitude and altitude, the seasonal evolution of daily values indicate that Sao Paulo receives, during clear sky days, 7.8% less global irradiance in August and 5.1% less in June than Botucatu. On the other hand, Sao Paulo receives, during clear sky days, 3.6% more diffuse irradiance in August and 15.6% more in June than Botucatu. The seasonal variation of the diurnal cycle confirms these differences and indicates that they are more pronounced during the afternoon. The regional differences are related to the distance from the Atlantic Ocean, systematic penetration of the sea breeze and daytime evolution of the particulate matter in Sao Paulo. An important mechanism controlling the spatial distribution of solar radiation, on a regional scale, is the sea breeze penetration in Sao Paulo, bringing moisture and maritime aerosol that in turn further increases the solar radiation scattering due to pollution and further reduces the intensity of the direct component of solar radiation at the surface. Surprisingly, under clear sky conditions the atmospheric attenuation of solar radiation in Botucatu during winter – the biomass burning period due to the sugar cane harvest – is equivalent to that at Sao Paulo City, indicating that the contamination during sugar cane harvest in Southeast Brazil has a large impact in the solar radiation field at the surface.

A proposed working standard for the measurement of diffuse horizontal shortwave irradiance

Atmospheric radiative transfer model estimates of diffuse horizontal broadband shortwave (solar) irradiance have historically been larger than measurements from a shaded pyranometer. A reference standard for the diffuse horizontal shortwave irradiance does not exist. There are no current efforts to develop an absolute standard that are known to the authors. This paper presents the case for a working standard for this measurement. Four well‐behaved pyranometers from two previous intensive observation periods (IOP) were chosen for this study. The instruments were characterized for spectral and angular response before the IOP and calibrated during the IOP using a shade/unshade technique with reference direct irradiance from an absolute cavity radiometer. The results of the comparison and detailed analyses to explain the differences suggest selecting three of the four for the working standard. The 95% confidence uncertainty in this standard is estimated at 2.2% of reading + 0.2 W/m2. In lieu of a comparison to this trio, a procedure for obtaining low‐uncertainty diffuse horizontal shortwave irradiance is suggested.

Influence of air mass history on the columnar aerosol properties at Valencia, Spain

The physical and radiative properties of atmospheric aerosols have been obtained in Valencia (latitude 39.508°, longitude −0.418°, 60 m a. s. l.), a city of the Spanish Mediterranean coast, by the inversion of direct solar irradiance and diffuse sky irradiance measurements made with a CIMEL CE318 system, from January 2002 to July 2005. The data acquired by the CE318 were used to determine the instantaneous values of the aerosol optical depth (AOD), the columnar water vapor content (w) and the Ångström wavelength exponent (α). The SKYRAD code was used to obtain the size distribution, the asymmetry parameter, the complex refractive index and the single scattering albedo of the aerosols. By dividing the source regions into sectors, and with the help of a simple model that quantified the relative influence of each sector in the final character of the air masses arriving over the measurement site, it was possible to classify and understand the dependence of these physical and radiative properties on the dominant air mass type.

Shortwave radiative closure studies for clear skies during the Atmospheric Radiation Measurement 2003 Aerosol Intensive Observation Period

The Department of Energy's Atmospheric Radiation Measurement (ARM) program sponsored a large aerosol intensive observation period (AIOP) to study aerosol during the month of May 2003 around the Southern Great Plains (SGP) Climate Research Facility (CRF) in north central Oklahoma. Redundant measurements of aerosol optical properties were made using different techniques at the surface as well as in vertical profile with sensors aboard two aircraft. One of the principal motivations for this experiment was to resolve the disagreement between models and measurements of diffuse horizontal broadband shortwave irradiance at the surface, especially for modest aerosol loading. This paper focuses on using the redundant aerosol and radiation measurements during this AIOP to compare direct beam and diffuse horizontal broadband shortwave irradiance measurements and models at the surface for a wide range of aerosol cases that occurred during 30 clear‐sky periods on 13 days of May 2003. Models and measurements are compared over a large range of solar‐zenith angles. Six different models are used to assess the relative agreement among them and the measurements. Better agreement than previously achieved appears to be the result of better specification of input parameters and better measurements of irradiances than in prior studies. Biases between modeled and measured direct irradiances are in the worst case 1%, and biases between modeled and measured diffuse irradiances are less than 1.9%.