Estimation Of Diffuse Irradiance Research Articles

Correction methods for shadow-band diffuse irradiance measurements: assessing the impact of local adaptation

Shadow-bands are a low cost alternative when a precision solar tracker is not available. Adequate precision may be achieved if the measured diffuse irradiance is corrected to account for the sky portion blocked by the shadow-band. The isotropic sky assumption leads to a systematic under estimation of diffuse irradiance. Several correction methods have been proposed to take into account the anisotropic effects. However, their performance at a given site depends on the dominant local climate. In this work, it is shown that the local adaptation of shadow band correction methods results in a significant improvement in the diffuse irradiance measurement's accuracy. Nine well-known correction methods are implemented and tested (both in their original and locally adapted versions) for the Pampa Húmeda region of southeastern South America. In absence of local adaptation, only one of the pre-existing methods improves the simple isotropic model. All locally adapted versions perform similarly well and outperform significantly the original methods. A new model based on the parametrization of Battle's model is proposed. It provides the best performance compared to all locally adapted pre-existing models, under all-sky and discriminated sky conditions.

New approaches in multi-angular proximal sensing of vegetation: Accounting for spatial heterogeneity and diffuse radiation in directional reflectance distribution models

The development of tower-mounted automated multi-angular hyperspectral systems has brought new opportunities and challenges for the characterization of the Bidirectional Reflectance Distribution Function (BRDF) on a continuous basis. This study describes the deployment of one of these systems in a Mediterranean savanna ecosystem (AMSPEC-MED), and proposes new approaches for modeling of directional effects. In this study, a Hemispherical-Directional Reflectance Distribution Function (HDRDF) was introduced in order to quantify the effect of diffuse radiation on the estimation of BRDF. The HDRDFs of the two covers of the ecosystem - trees and grasses - were un-mixed using a 3-Dimensional (3-D) model of the observed scene. Up-scaling HDRDF estimates to MODIS BRDF product (r2∈[0.74, 0.86]) and down-scaling to hand held spectral measurements (r2=0.88) achieved a reasonable accuracy (RMSE∈[1.81, 3.14]). Despite the uncertainties in the estimation of diffuse irradiance and the 3-D representation of the scene, HDRDF un-mixing demonstrates the potential of automated multi-angular proximal sensing to study vegetation properties in heterogeneous ecosystems and the correction of directional effects of different sources.

SIERRA: A new approach to atmospheric and topographic corrections for hyperspectral imagery

In mountainous areas, slope and altitude variations modulate the airborne sensed hyperspectral radiance image. A new algorithm, SIERRA, has been developed for atmospheric, relief and BRDF corrections in order to extract the surface reflectance in the form of bi-hemispherical albedo that does not depend on solar incidence and observation angles. The forward modeling efforts focus on the estimation of diffuse irradiance and upwelling diffuse radiance, and on the formulation of BRDF effects. The inversion scheme consists of four steps, that go deeper and deeper into the phenomena's complexity. To validate the model, reflectance images are assessed from radiance images simulated with different radiative transfer codes or forward models: MODTRAN4 in the case of homogeneous and flat ground, AMARTIS and SIERRA forward models for heterogeneous and mountainous cases. The surface reflectance is retrieved with a 5% relative error under standard acquisition conditions. SIERRA is applied to HyMap data acquired over the hilly landscape near Calanas, Spain. The hypercube reflectances are compared with those obtained using ATCOR4 and COCHISE. The benefit of relief correction is clearly demonstrated.

Irradiation and PV array energy output, cost, and optimal positioning estimation for South Africa

Simulation results of an irradiation and PV array performance software package (SunSim) are pre-sented. South African irradiation data availability is discussed, an irradiation data classification system proposed, and the estimation of diffuse irradiation on tilted surfaces analysed. Estimation of PV array energy output using King’s performance model is explained, and the influence of the irradiation and temperature data set measurement interval on PV energy output estimation investigated. Simulation results are presented, aimed at identifying optimal fixed PV panel tilt angles and solar-tracking config-urations for different locations in South Africa. Lastly, the cost of PV-based generation in South Africa is investigated.

A numerical method to estimate time-varying values of diffuse irradiance on surfaces in complex geometrical environments

The estimation of diffuse irradiance that impinges on a window or photovoltaic panel is of major importance for the determination of their performance. The present paper introduces the concept of angular shading factors (ASF) for the calculation of time-varying diffuse irradiances to be performed without the need to repeat time consuming intereflection calculations at every time step. For the calculation of ASF, a Monte Carlo backward ray tracing technique was used. As a result, geometrically complex scenes consisting of surfaces with diffuse and specular reflectances, while transparent and translucent surfaces can be modeled as well. After that, the calculation of diffuse irradiance can be performed in a dynamic way, by taking into consideration the sky radiance distribution variability.

Estimation of global, direct and diffuse solar radiation in Athens under clear sky conditions

In this paper the theoretical models proposed by Davies-Hay, Hoyt and Bird for the estimation of the various components of solar radiation under clear sky conditions are applied to Athens. The results produced by the models have been compared to measured hourly values of global irradiation and to instantaneous values of direct and diffuse irradiance. From the comparison it has been found that global solar irradiance estimated by all three models is higher than that of the actual measurements. From the examined three models Davies-Hay model gives the highest hourly values of global solar irradiance. Bird model gives very good results for global solar irradiance and for direct irradiance for most of the months of the year while the estimation of the diffuse irradiance is considered satisfactory. Hoyt model underestimates the direct solar irradiance and over-estimates the diffuse. Davies-Hay estimates of diffuse irradiance are comparable to those produced by Bird model. It is concluded that Bird model produces the best estimates of solar irradiance in Athens.

Estimation of diffuse irradiance on sloping, obstructed surfaces: An error analysis

In several branches of climatology, and in related disciplines, estimates of shortwave and/or longwave diffuse irradiance are required. In this study, these irradiances are obtained by numerical integration of the fundamental radiance and source-receptor geometry equations for a variety of receptor slopes, receptor orientations, skyline patterns and sky radiance distributions. The angular increment used in the numerical integration over zenith and azimuth angle is varied to determine the optimal value in the trade-off between computing expense and accuracy. It is recommended that an angle ofπ/36 radians (5°) or smaller is required to evaluate the diffuse irradiance to 5 percent; for 10 percent error, the angle should not exceedπ/18 radians (10°).