Global radiation transmittance of transparent insulation materials

Solar diffuse radiation transmittance of cellular arrays are investigated by numerical technique. The computational results are reduced in terms of equivalent beam angle of incidence and its variation with tilt angle and aspect ratio are represented by polynomial equations which are tested by the measurements of global radiation transmittance of several TIM samples. The predicted results deviate from the measurements on an average by 2.0%.     

Evaluation of models for predicting insolation on slopes within the Colorado alpine tundra

This empirical study evaluates insolation predictions for the Colorado tundra from models based upon isotropic and anisotropic distribution approximations for diffuse sky and reflected solar radiation. The data set of hourly insolation values was obtained from 40 locations on an alpine ridge by simultaneous measurement of direct beam irradiance and total insolation to the horizontal ridge crest and two nearby sloping surfaces. Six models are used to predict insolation, two based on the isotropic distribution of diffuse solar radiation and four on anisotropic diffuse distribution fields. Three models (one isotropic and two anisotropic) employ measurements of normal direct beam irradiance while the other 3 models incorporate the correlation between the “clearness index” and the ratio of diffuse sky to total insolation. The precision of insolation estimates from models using the correlation method is only slightly less than from other models. Accounting for the increase of diffuse radiation in the circumsolar sky improves insolation predictions. However, inclusion of additional regions of diffuse radiation anisotropy decreases model accuracy. Errors of insolation estimates for the alpine tundra from all models vary in a systematic manner as a function of relative azimuth and ground slope angles.     

Solar transmittance characteristics of evacuated tubular collectors with diffuse back reflectors

The partition matrix between light sources and light sinks is introduced to evaluate the transmittance properties of optically asymmetric collectors constructed with evacuated tubes. A ray tracing model is used to compute its elements. Curve fits to the ray tracing results are provided. The effects of polarization are discussed. Comparisons with previously published results are presented in order to check the accuracy of the proposed model. The beam and diffuse transmittance of a tubular cover are calculated and their dependency on the tube orientation investigated. The biaxial incidence angle modifier product is extended to evacuated tube collector arrays with a diffuse back reflector, providing a simple interpolation process for the beam transmittance. An effective beam direction for diffuse radiation is computed so that transmittances for ground reflected radiation and sky diffuse radiation can be calculated from biaxial incidence angle modifier measurements.     

Experimental thermodynamic cycles and performance analysis of a solar-powered adsorptive icemaker in hot humid climate

This paper analyzes and presents the thermodynamic cycles and the experimental performance parameters of a solar adsorptive icemaker that uses activated-carbon methanol pair. The solar energy technology employed is far less sophisticated than that of collectors using selective surface or evacuated tubes. The collector-adsorber is multi-tubular with an opaque black radiation-absorbing surface, and thermal insulated by means of transparent covers, the so-called transparent insulation material (TIM). The solar radiation hits on both faces of the tubes by means of semi-cylindrical reflectors. It is shown the results of tests carried out in a region of Brazil close to the Equator, on days characterized by the predominant cloud cover degree. Three cycles have been analyzed: one with clear sky, another with partially cloudy sky, and a third under entirely cloudy sky. The maximum regenerating temperatures were 100.1, 87.3 and 92.7 °C, with an ice production of 6.05, 2.10 and 0 kg by square meter of projected area, for cycles of clear sky, partially cloudy and overcast nights, respectively.     

Optimum glazing combination for solar space heating

A study is made for comparing the maximum seasonal energy yield obtainable by solar collectors for space heating application. Different glazing combinations with glass and plastic as glazing materials are considered. The study is made for four different locations. The performance of eight glazing combinations with covers ranging in number from one to three is compared to obtain the optimum combination for each location. The results show that selecting the optimum glazing combination improves the performance significantly. In general, plastic covers give higher yield. The study confirmed that the use of two covers is justified in cold, cloudy climates while a single cover is suitable for temperate climates. In most cases three covers lead to a significant reduction in the yield. Replacing plastic by glass as a top cover for longer life results in a small yield reduction. Some of the other conclusions are that the ratio of average to normal transmittance-absorptance product changes significantly with location and month of the year. However, the seasonal average value of this ratio is almost constant for any number of covers but changes with location.     

Solar radiation modeling and measurements for renewable energy applications: data and model quality

Measurement and modeling of broadband and spectral terrestrial solar radiation is important for the evaluation and deployment of solar renewable energy systems. We discuss recent developments in the calibration of broadband solar radiometric instrumentation and improving broadband solar radiation measurement accuracy. An improved diffuse sky reference and radiometer calibration and characterization software for outdoor pyranometer calibrations are outlined. Several broadband solar radiation model approaches, including some developed at the National Renewable Energy Laboratory, for estimating direct beam, total hemispherical and diffuse sky radiation are briefly reviewed. The latter include the Bird clear sky model for global, direct beam, and diffuse terrestrial solar radiation; the Direct Insolation Simulation Code (DISC) for estimating direct beam radiation from global measurements; and the METSTAT (Meteorological and Statistical) and Climatological Solar Radiation (CSR) models that estimate solar radiation from meteorological data. We conclude that currently the best model uncertainties are representative of the uncertainty in measured data.     

Improved photovoltaic energy output for cloudy conditions with a solar tracking system

This work describes measurements of the solar irradiance made during cloudy periods in order to improve the amount of solar energy captured during such periods. It is well-known that 2-axis tracking, in which solar modules are pointed at the sun, improves the overall capture of solar energy by a given area of modules by 30-50% versus modules with a fixed tilt. On sunny days the direct sunshine accounts for up to 90% of the total solar energy, with the other 10% from diffuse (scattered) solar energy. However, during overcast conditions nearly all of the solar irradiance is diffuse radiation that is isotropically-distributed over the whole sky. An analysis of our data shows that during overcast conditions, tilting a solar module or sensor away from the zenith reduces the irradiance relative to a horizontal configuration, in which the sensor or module is pointed toward the zenith (horizontal module tilt), and thus receives the highest amount of this isotropically-distributed sky radiation. This observation led to an improved tracking algorithm in which a solar array would track the sun during cloud-free periods using 2-axis tracking, when the solar disk is visible, but go to a horizontal configuration when the sky becomes overcast. During cloudy periods we show that a horizontal module orientation increases the solar energy capture by nearly 50% compared to 2-axis solar tracking during the same period. Improving the harvesting of solar energy on cloudy days is important to using solar energy on a daily basis for fueling fuel-cell electric vehicles or charging extended-range electric vehicles because it improves the energy capture on the days with the lowest hydrogen generation, which in turn reduces the system size and cost.     

The assessment of different models to predict the global solar radiation on a surface tilted to the south

Global and diffuse solar radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar conversion systems (both flat plate solar collector and PV) are tilted towards the sun in order to maximize the amount of solar radiation incident on the collector surface. Consequently, the solar radiation incident on a tilted surface must be determined by converting the solar radiation intensities measured on a horizontal surface to that incident on the tilted surface of interest. There exist a large number of models designed to perform such a conversion. 11 such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of hourly global and diffuse solar radiation on a horizontal surface, normal incidence beam and global radiation on a south-oriented surface tilted at 40°. The horizontal diffuse radiation measured using a shadow ring was corrected using four different correction models. This resulted in 44 model permutations. The individual model performance is assessed by an inter-comparison between the calculated and measured solar global radiation on the south-oriented surface tilted at 40° using both graphical and statistical methods. The relative performance of the different models under different sky conditions has been studied. Different grading systems have been applied in an attempt to score the relative performance of the models.     

Infrared sky radiation in Geneva

Continuous measurements of sky radiation have been performed in Geneva for more than two years. We present here some results, and we show that the radiative earth-sky deficit is correlated to the clearness of the day as evaluated from the amount of solar radiation reaching the ground. The corresponding model allows to determine the sky temperature within 2–3°C, with respect to ambient temperature, diffuse and global solar radiation. These results can be of help when computing energy balances involving sky radiation, especially in buildings and solar energy fields.     

Measurements and predictions of solar radiation incident on horizontal surfaces at Santa Fe, Argentina (31° 39′S, 60° 43′W)

Measurements and predictions of solar radiation during a period of 10 years on horizontal surfaces at Santa Fe (31° 39′ S, 60° 43′ W), Argentina, reported as average daily global radiation for each month, are presented. Data are compared to those obtained with a previously published and verified model for computing solar radiation on horizontal planes at the earth's surface for cloudless sky days. Measurements show an important reduction of global radiation with respect to the cloudless sky model predictions for all months of the year. Conversely, averaged daily diffuse solar radiation calculated with Page's formula shows a small increment with respect to the predicted diffuse solar radiation for cloudless sky conditions. When direct solar radiation data, calculated from global and diffuse solar radiation values, are compared to theoretical prediction, a significant decrease is observed. This trend is similar to that obtained for global solar radiation.     

Atmospheric aerosol influence on the Brazilian solar energy assessment: Experiments with different horizontal visibility bases in radiative transfer model

The radiative transfer model BRASIL-SR is used by Brazilian Institute for Space Research for the assessment of the solar irradiation in Brazil. The model parameterizes the influence of aerosols in the solar radiation transmittance using climate averages of horizontal visibility, which does not represent the actual atmospheric condition in Brazil, especially during dry season. In clear sky conditions, aerosols are a major source of bias in solar radiation models. Their concentration have large spatial and temporal variability particularly in the Brazilian Midwestern region from April until October, due to forest fires, and in Southeastern region due to pollution from megacities. In this study, meteorological data from METAR comprising the years of 2006, 2007 and 2008 were analyzed to evaluate the seasonal variability of the horizontal visibility in Brazil to better represent the influence of aerosols on the model estimations of surface solar irradiation. New horizontal visibility values was generated to each month simulated, to provide input data to the BRASIL-SR model and site specific ground data were used to validate the model estimates. The global, direct beam and diffuse solar irradiation estimates obtained by making use of the new horizontal visibility data presented an overall lower BIAS and RMSE deviations.     

The Sun’s apparent position and the optimal tilt angle of a solar collector in the northern hemisphere

In this paper, the Julian dating system is adopted to calculate the Sun’s apparent position. Both the sunshine duration and the optimal installation angle of a fixed solar collector are obtained for different time periods and latitudes in the northern hemisphere. To simulate different operating environments, both kinds of radiation flux are considered, i.e. extraterrestrial radiation and the global radiation calculated using an empirical model. The results show that Julian dating system is precise enough to predict the Sun’s locus according to analyses of the solar declination and the azimuth and elevation angles. The orientation that a solar collector must be installed at can be roughly estimated by determining the sunshine duration. The majority of the Sun’s path lies in the southern sky over a year, and hence the sunshine duration in the southern sky is longer, except for locations with latitudes below 1.5°. The yearly optimal angles are positive (approximately 0.91 and 0.76 multiplied by the latitude for extraterrestrial and global radiation, respectively), for latitudes below 65^o. Above this, the curves are flatter, and the differences between the two types of radiation flux become greater. The ratio of the yearly irradiation captured by a collector installed at its yearly optimal angle to that captured by the ground surface increases with latitude, and reaches a maximum of 1.71 and 1.35 near latitudes of 65° for extraterrestrial and global radiation, respectively. The efficiency of a solar collector decreases when operating in a cloudy environment. The amount of global radiation incident on the ground surface is about 0.51-0.66 times that of extraterrestrial radiation throughout the hemisphere.     

Experimental testing of models for the estimation of hourly solar radiation on vertical surfaces at Arcavacata di Rende

More than 55,000 data of hourly solar radiation on a horizontal surface and on vertical surfaces exposed to the south, west, north and east, measured at Arcavacata di Rende (CS), were compared with hourly radiation data calculated by various calculation models.Erbs, Reindl et al. and Skartveit et al. correlations for the split of hourly global radiation in the diffuse and beam components were used together with the isotropic sky model and three anisotropic sky models.The agreement between experimental and calculated data is generally good.     

Increasing the solar photovoltaic energy capture on sunny and cloudy days

This report analyzes an extensive set of measurements of the solar irradiance made using four identical solar arrays and associated solar sensors (collectively referred to as solar collectors) with different tilt angles relative to the earth’s surface, and thus the position of the sun, in order to determine an optimal tracking algorithm for capturing solar radiation. The study included a variety of ambient conditions including different seasons and both cloudy and cloud-free conditions. One set of solar collectors was always approximately pointed directly toward the sun (DTS) for a period around solar noon. These solar collectors thus captured the direct beam component of the solar radiation that predominates on sunny days. We found that on sunny days, solar collectors with a DTS configuration captured more solar energy in accordance with the well-known cosine dependence for the response of a flat-surfaced solar collector to the angle of incidence with direct beam radiation. In particular, a DTS orientation was found to capture up to twice as much solar energy as a horizontal (H) orientation in which the array is tilted toward the zenith. Another set of solar collectors always had an H orientation, and this best captured the diffuse component of the solar radiation that predominates on cloudy days. The dependence of the H/DTS ratio on the solar-collector tilt angle was in approximate agreement with the Isotropic Diffuse Model derived for heavily overcast conditions. During cloudy periods, we found that an H configuration increased the solar energy capture by nearly 40% compared to a DTS configuration during the same period, and we estimate the solar energy increase of an H configuration over a system that tracks the obscured solar disk could reach 50% over a whole heavily-overcast day. On an annual basis the increase is predicted to be much less, typically only about 1%, because the contribution of cloudy days to the total annual solar energy captured by a photovoltaic system is small. These results are consistent with the solar tracking algorithm optimized for cloudy conditions that we proposed in an earlier report and that was based on a much smaller data set. Improving the harvesting of solar energy on cloudy days deserves wider attention due to increasing efforts to utilize renewable solar energy. In particular, increasing the output of distributed solar power systems on cloudy days is important to developing solar-powered home fueling and charging systems for hydrogen-powered fuel-cell electric and battery-powered vehicles, respectively, because it reduces the system size and cost for solar power systems that are designed to have sufficient energy output on the worst (cloudy) days.     

CLEAR SKY SOLAR INSOLATION DATA FOR ISLAMABAD

Monthly mean values of both integrated and instantaneous clear sky solar radiation components for Islamabad territory are presented and discussed. The components include global normal, direct normal, global horizontal, direct and diffuse horizontal radiations, sunshine duration, number of clear days and ambient temperature for solar energy applications. Direct normal irradiance values are used to get clear sky sunshine duration by ab-initio. The need for replacing the conventional sunshine recorder is discussed.     

Comparative study of transparent insulation materials cover systems for integrated-collector-storage solar water heaters

The thermal performance of transparently insulated integrated-collector-storage solar water heaters is investigated theoretically as well as experimentally for a comparative study of cover systems having transparent insulation materials devices placed between the top glazing and the absorber. The data on solar transmittance, heat loss reduction characteristics and solar collector-storage efficiencies of various configurations is generated for the system performance comparisons. These hot water systems exhibit average (diurnal basis) solar collection and storage efficiencies in the range of 20–40% at a collection temperature of 40–50°C. The performance of water heaters with cover system having absorber-perpendicular configuration of TIM excel over absorber-parallel TIM covers. The effect of variation in the temperature of heat collected and cost of cover systems on the performance comparisons is also discussed.     

Performance study on the east–west oriented single-axis tracked panel

A theoretical study on the performance of an east–west oriented single-axis tracked panel was originally proposed in this paper. Mathematic expressions applicable for calculating the angle that the tracked panel should rotate by to follow the Sun are derived. The incident angle of sunlight upon the panel as well as the instantaneous increments of solar energy captured by the panel relative to a fixed horizontal surface are then demonstrated graphically. To simulate different operation environments, three kinds of radiation sources will be considered, i.e. the extraterrestrial radiation, global radiation predicted by empirical models under clear sky situation and global radiation observed in Taiwan. Simulation results show that the yearly gains correlate positively with the radiation level, i.e. 21.2%, 13.5% and 7.4% for the extraterrestrial, predicted and observed radiations, respectively, which are far less than those obtained from a north–south oriented single-axis tracked panel. The irradiation increases with the maximum rotation angle of the panel, the benefit of increasing the rotation in overcast environment is not as good as in clear sky, for annual energy collection 45° is recommended. The irradiation received decreases with latitude, but it has a greater gain in higher latitude zone.     

A brief review of models for computing solar radiation on inclined surfaces

Modelling of solar energy systems requires estimation of the hourly radiation incident on surfaces of different tilts and orientations. Most meteorological stations report radiation values on a daily rather than hourly basis. Again, the value of the diffuse component may or may not be reported. Hence, different strategies may be encountered, for each location, with the common goal of computing hourly radiation on inclined surfaces.

In this study, five different schemes are presented to achieve this goal. In each scheme, certain correlations are required which are discussed in detail. The study is divided into five sections, each dealing with a specific type of correlation. These sections are: (i) correlations between daily diffuse and global radiation; (ii) correlations between hourly/daily global radiation; (iii) correlations between hourly/daily diffuse radiation; (iv) correlations between hourly diffuse and global radiation; and (v) models for computing diffuse sky radiation on inclined surfaces. The last section deals extensively with the anisotropic nature of sky diffuse radiation.

The important aspects of all correlation studies are highlighted, and the relative merits and demerits of their results are brought to light.

Mathematical expressions, where available, for models/correlations are provided so that the reader will have access to a comprehensive study. This information should be useful for modelling purposes in which computation of radiation on surfaces of different orientations and tilts is required.     

A comparative study of solar irradiation models on various inclined surfaces for India

This paper presents a statistical approach for the estimation of the diffuse/global irradiation on various inclined surfaces from the measured data of horizontal surface. In fact diffuse solar radiation on an inclined plane consists of two components: sky diffuse radiation and reflected radiation from the ground. For analyzing estimation of the daily tilted sky diffuse component from the daily horizontal diffuse irradiance, we have considered six models Badescu, Circumsolar, Skartveit and Olseth, Hay, Klucher and Liu and Jordan (Isotropic). All these models except Badescu adopted the same methodology for estimating the ground-reflected radiation component, therefore, only sky diffuse component was analyzed at Lucknow (latitude 26.75°, longitude 80.50°), India location. Statistical analysis showed that the Skartveit and Olseth model gives good prediction for the low inclination angle however; Klucher model gave better performance for highly inclined south-facing surfaces. The Root Mean Square Errors (% RMSE) value varies from 3.45% to 24.15% except for Badescu and Circumsolar model which predict worse results. In general, Klucher’s model provides close agreement with the measurements.     

带有平面反射板的全玻璃真空集热管表面能流分析

本文用解析方法对带平面反射板的单根太阳能真空集热管表面辐射流分布进行了分析，并得出了相应的结果。太阳直射辐射和散射辐射直接或通过反光板间接散布在真空管吸热体表面。为能更清楚地理解辐射的物理过程，在文中对不同辐射成分，不同反射方式分别进行了分析计算。