L. Alados-Arboledas

On shadowband correction methods for diffuse irradiance measurements

Abstract Diffuse irradiance, G d , is an important variable in solar resource assessment. The diffuse irradiance can be worked out from global, G , and direct, G b , irradiance measurements, but this method involves the use of relatively expensive tracking mechanisms. Alternatively, a widely accepted technique uses a pyranometer with a shadowband. Because the shadowband screens the sensor from part of the diffuse radiation coming in from the sky, a correction must be made to the measurements. However, because of the anisotropy of diffuse radiation it is difficult to compute an exact theoretical correction. In this study we use two data sets registered in two locations in Spain. The first one consists in coincident hourly values of global, direct, and diffuse irradiance; the latter by means of shadowband. The other data set includes the same variables but as 5-minute values. Our goal is to study the necessary correction factor for diffuse irradiance measurements obtained by means of shadowband. After testing several well-known correction methods, we have developed two different correction models, using two-thirds of the hourly data set, while the remaining one-third and the whole 5-minute data set have been used for validation purposes. The last validation test suggests that our anisotropic models provide reliable corrections for conditions different than the ones where they have been developed. The results obtained by the developed models show a negligible mean bias deviation. Approximately 55% of cases present deviations lower than 5% over the mean value of diffuse irradiance.

Development of a sky imager for cloud cover assessment

Based on a CCD camera, we have developed an in-house sky imager system for the purpose of cloud cover estimation and characterization. The system captures a multispectral image every 5 min, and the analysis is done with a method based on an optimized neural network classification procedure and a genetic algorithm. The method discriminates between clear sky and two cloud classes: opaque and thin clouds. It also divides the image into sectors and finds the percentage of clouds in those different regions. We have validated the classification algorithm on two levels: image level, using the cloud observations included in the METAR register performed at the closest meteorological station, and pixel level, determining whether the final classification is correct.

Photosynthetically active radiation: measurements and modelling

Photosynthetically-active radiation is a necessary input in applications dealing with plant physiology, biomass production and natural illumination in greenhouses. Unfortunately, a worldwide routine network for the measurement of photosynthetically-active radiation is not yet established and it is often calculated as a constant ratio of the broadband solar radiation. This ratio is affected by many parameters. This work describes the variations of the ratio of photosynthetically-active radiation to broadband solar radiation with sky conditions, characterized by means of variables that could be accessible in common radiometric networks. For this purpose, data recorded at a radiometric station located in the University of Almeria, a seashore location (36.83° N, 2.41° W) are used. The data base includes hourly values of the relevant variables that cover the period from June 1990 to December 1992. We have studied the seasonal and daily patterns of the ratio photosynthetically-active radiation to broadband solar radiation. We explained these patterns through the influence of sky conditions, represented by means of broadband solar radiation ratios, solar elevation and dewpoint temperature. Additionally, we have developed weather-dependant functions of this ratio in order to obtain an empirical model that could be used to estimate the photosynthetically-active radiation in those radiometric networks that register solar broadband irradiance. This model can explain over 65% of the variability exhibited by the ratio of photosynthetically-active radiation to broadband solar radiation, avoiding the necessity of local calibration of this ratio.

Empirical modeling of hourly direct irradiance by means of hourly global irradiance

A very important factor in the assessment of solar energy resources is the availability of direct irradiance data of high quality. Nevertheless, this quantity is seldom measured and thus must be estimated from measures of global solar irradiance, a quantity that is registered in most radiometric stations. In this work we analyze the results provided by different models in the estimation of hourly direct irradiance values. We have selected several models proposed by Orgill and Hollands, Erbs et al., Reindl et al., Skarveit and Olseth, Maxwell, and Louche et al. With the exception of the model from Louche et al. that estimates direct irradiance values from direct transmittance values, all of the models estimate direct irradiance from the diffuse fraction. The data set used in this study comprises 25 000 hourly values of global and diffuse irradiance. These values were registered in six Spanish locations with different climatic conditions. The results provided by the model depend on the clearness index, kt, and the solar elevation. The best results are obtained for cloudless skies and higher solar elevation. In those conditions we can estimate the direct irradiance with a root square mean error close to 14% of the average measured value. We have estimated the direct irradiance under cloudless sky conditions using a parametric model proposed by Iqbal. In order to analyze the effect of turbidity on the estimation of direct irradiance we have compared the results obtained by the parametric model when using hourly values of the Angstrom turbidity parameter β with those obtained when using monthly means of hourly values of β.

A Simple All Weather Model to Estimate Ultraviolet Solar Radiation (290–385 nm)

Abstract A new expression to estimate the solar ultraviolet irradiance from parameters usually available in radiometric networks is presented. The authors have analyzed the relation between solar ultraviolet global irradiance (290–385 nm), UV, and broadband global irradiance, G, studying as well the ratio UV/G. The analysis of the dependence between the ultraviolet global irradiance and the broadband global irradiance, both normalized to their respective extraterrestrial values, has been used in order to develop a functional dependence between both global hemispherical transmittances. The cloud radiative effect on both radiometric fluxes has also been studied. After this analysis, the authors propose a new expression to obtain ultraviolet global irradiance from only two input variables: clear-sky ultraviolet irradiance and the global broadband hemispherical transmittance. The dataset used in this paper has been registered in two-radiometric stations: one located in the outskirts of Granada (37.18°N, 3.58°...

One-minute global irradiance probability density distributions conditioned to the optical air mass

Abstract Knowledge of the temporal variability of the solar irradiance is important to study solar energy systems involving thermal and photovoltaic processes. The differences between hourly and instantaneous values of the clearness index considerably affect the utilizability of photovoltaic systems. In this work, we analyzed the probability density distributions of one-minute values of global irradiance, conditioned to the optical air mass, considering those as an approximation to the instantaneous distributions. The study reveals that the bimodality that characterizes these distributions increases with optical air mass. We propose the use of a functional form based on Boltzmanns statistics in order to describe these distributions. This function can be used for the generation of synthetic radiation data. Expressing the distribution as a sum of two functions provides an appropriate modeling of the bimodality feature that can be associated with the existence of two levels of irradiation corresponding to two extreme atmospheric situations, cloudless and cloudy conditions.

The influence of clouds on surface UV erythemal irradiance

The purpose of this study is to examine the effect of clouds on the ultraviolet erythemal irradiance. The study was developed at three stations in the Iberian Peninsula: Madrid and Murcia, using data recorded in the period 2000–2001, and Zaragoza, using data recorded in 2001. In order to determine the cloud effect on ultraviolet erythemal irradiance, we considered a cloud modification factor defined as the ratio between the measured values of ultraviolet erythemal irradiance and the corresponding clear-sky ultraviolet erythemal irradiance, which would be expected for the same time period and atmospheric conditions. The dependence of this cloud modification factor on total cloud amount, cloud type and solar elevation angle was investigated. The results suggest that the effect of cloud on ultraviolet erythemal irradiance can be parameterized in a simple way in terms of the cloud amount. Our results suggest that the same cloud modification factor model can be used at the three analysed locations estimating the ultraviolet erythemal irradiance with mean bias deviation (MBD) in the range of the expected experimental errors. This cloud modification factor is lower than that associated to the whole solar spectral range, indicating that the attenuation for the ultraviolet erythemal irradiance is lower than that associated to other solar spectral ranges. The cloud modification factor for ultraviolet erythemal irradiance presents dependence with solar elevation, with opposite dependencies with solar elevation for overcast and partial cloud cover conditions, a fact that can be explained in terms of the influence of reflection-enhancement of the ultraviolet irradiance in the last case. Concerning the influence of cloud type, a limited study of two cloud categories, low and medium level and high level, indicated that for overcast conditions, lower clouds presents an attenuation of ultraviolet erythemal irradiance 20% greater than that associated to high level clouds. D 2003 Elsevier Science B.V. All rights reserved.

Parametric models to estimate photosynthetically active radiation in Spain

Different applications dealing with plant physiology, biomass production and natural illumination in greenhouses require knowledge of the photosynthetically active radiation. In absence of measurements of this flux, one must rely on parametric approaches. In this way, the radiant energy is computed using available atmospheric parameters. In the present work, we have developed a comparison among several cloudless sky parameterization schemes. For this purpose, data recorded at two

Estimation of photosynthetically active radiation under cloudy conditions

Clouds are the largest modulators of the solar radiative flux reaching the Earth’s surface. The amount and type of cloud cover prevailing at a given time and location largely determines the amount and type of solar radiation received at the Earth’s surface. This cloud radiative forcing is different for the different solar spectral bands. In this work, we analysed the influence of cloud radiative forcing over the photosynthetically active radiation. Knowledge of the photosynthetically active radiation is necessary in different applications, but due to the absence of widespread measurements of this radiometric flux, it must be estimated from available variables. Cloudless sky parametric models compute the global photosynthetically active radiation at surface level by addition of its direct beam and diffuse components. To compute this flux under all sky conditions one must consider the influence of clouds. This could be done by defining a cloud transmittance function. We have developed such a cloud transmittance function considering three different types of clouds. The efficacy of the cloud radiative forcing scheme has been tested in combination with a cloudless sky parametric model using independent data sets. For this purpose, data recorded at two radiometric stations are used. The combination of an appropriate cloudless sky parametric model with the cloud transmittance scheme provides estimates of photosynthetically active radiation with mean bias deviation about 4% that is close to experimental errors. Comparisons with similar formulations of the cloud radiative effect over the whole solar spectrum shows the spectral dependency of the cloud radiative effect.

Evaluating the long-term water balance of arid zone stream bed vegetation using evapotranspiration modelling and hillslope runoff measurements

The difference between long-term actual evapotranspiration (AET) and precipitation (P) provides a useful indication of the extent to which a site retains or loses water resources, and therefore of the likely occurrence of specific land degradation processes. Sink areas OAET q PU receive lateral water inputs from other parts of the catchment. In arid and semi-arid environments these areas are frequently found in, or next to, the stream beds of ephemeral rivers and are often characterised by intensive land use or high conservation values. For both types of land use it is important to know if, and how much, AET exceeds P, and where the lateral water inputs come from. Thick sedimentary fills in the stream bed, variable climate conditions and ephemeral flow conditions pose specific difficulties to the evaluation of the water balance of these sites. The objective of this study was to develop an approach to explore the relative importance of lateral water inputs to shrub stands growing in thick sedimentary fills of semi-arid ephemeral rivers. The approach is based on (i) estimating long-term AET2 P balances in the channel sediments and (ii) assessing whether these inflows originate mainly from surrounding hillslopes or from the upstream part of the catchment. A physically based evapotranspiration model for sparse vegetation was used to estimate the long-term AET rates. The relative importance of hillslope runoff and channel flow was evaluated in a semi-quantitative fashion from a combination of surface area estimates and mostly published values of soil hydrological parameters. The approach was developed and tested in a selected stand of Retama sphaerocarpashrubs in a stream bed at the Rambla Honda field site (Tabernas,