Correlations of global and diffuse solar luminous efficacy for all sky conditions and comparisons with experimental data of five localities

Some simplified correlations of mean hourly global and diffuse luminous efficacy on the horizontal plane for all sky conditions have been obtained for Arcavacata di Rende (Italy) and have been compared with other literature models and other experimental data measured in Geneva (Switzerland), Vaulx-en-Velin (France), Bratislava (Slovakia) and Osaka (Japan). The comparisons show that, for global efficacy, the differences among the various models are not significant, and the use of a model with a constant value of efficacy gives good predictions of global illuminance. For the prediction of diffuse illuminance the differences among the models are larger, but the use of a constant diffuse efficacy provides a good first estimate of diffuse illuminance.     

A study on luminous efficacy of global radiation under clear sky conditions in Athens, Greece

The current interest in energy efficient design has led to a demand for methods of predicting daylight availability in buildings. However, basic daylight data, which are the basis of these prediction methods, are not available for most regions of Greece, as well as for many areas worldwide. For that purpose luminous efficacy can be used in order to generate daylight illuminance from solar radiation data, which are much more widely recorded.This paper concentrates on the analysis of global luminous efficacy in Greece under clear sky conditions. The study involves the evaluation of existing models with respect to daylight data recorded in Athens, Greece, as well as the development of new luminous efficacy models for global radiation.The study was based on climatic data provided by the National Observatory of Athens during the years 1996–2000 and tested according to CIE. recommendations. The proposed luminous efficacy models were derived by the analysis, correlation and linear regression of the accepted values of measured data. Statistical analysis techniques were used for the assessment of their performance, as well as for the evaluation of the accuracy in predicting global illumination provided by the proposed and the existing models.The new proposed models can offer engineers and researchers a more reliable and accurate approach for estimating the global illumination levels under clear skies. Nowadays, the knowledge of daylight levels is considered highly important in local architecture, the energy conservation and environmental policies in general, particularly in the case of Mediterranean regions, where clear sky conditions dominate.     

Correlations of direct solar luminous efficacy for all sky,clear sky and intermediate sky conditions and comparisons with experimental data of five localities

Many correlations of mean hourly direct luminous efficacy for all sky, clear sky and intermediate sky conditions have been compared with experimental data measured in Arcavacata di Rende (Italy), Geneva (Switzerland), Vaulx-en-Velin (France), Bratislava (Slovakia) and Osaka (Japan). The comparisons show that, for all sky conditions, the correlations developed for one locality predict the luminous efficacy in the other localities with mean errors between ?13% and 20% and root mean square errors between 15% and 35%; for clear sky conditions, the correlations developed for one locality predict the luminous efficacy in the other localities with mean errors between ?7% and 14% and root mean square errors between 7% and 19%; for intermediate sky conditions, the correlations developed for one locality predict the luminous efficacy in the other localities with mean errors between ?17% and 26% and root mean square errors between 20% and 41%.If the correlations with local coefficients are applied in each locality, the errors become considerably shorter. The paper gives indications on the values of the measured mean luminous efficacies and on the best models.     

A simple clear skies model for the luminous efficacy of diffuse solar radiation on inclined surfaces

In the present work we study the luminous efficacy of diffuse solar radiation incident on vertical surfaces for a clear sky and mean hourly values of diffuse irradiance and diffuse illuminance. We develop a model easy to use, similar to a model previously obtained for horizontal surfaces. To develop the present model for vertical surfaces we assume that the slope of the surface influences diffuse illuminance and diffuse irradiance in the same way. As a consequence of this hypothesis, the luminous efficacy of diffuse solar radiation is assumed to be the same for both horizontal and inclined surfaces.     

Analysis of zenith luminance data for all sky conditions

Two different procedures are explored in the present work to estimate the zenith luminance at the International Daylight Measurement Program station in Madrid, when all the data available are used independently of the sky type. On the basis of the results obtained, correlations for the seasonal dependence of the zenith luminance on solar elevation are reported. However, it is observed that polynomials obtained for different ranges of the luminous clearness index, the ratio of global to extraterrestrial illuminance, can predict the zenith luminance with a more reasonable accuracy. The results show that the luminous clearness index is an important parameter for zenith luminance characterization, to be used when global illuminance values on horizontal surfaces are available, but diffuse illuminance is not measured     

Luminous efficacy of direct solar radiation for all sky types

Three models for the luminous efficacy of direct solar radiation on a horizontal surface, valid for all sky conditions, have been developed in the present work, and two of them are proposed. In all the models a function for the direct luminous efficacy for all sky conditions relative to the direct luminous efficacy for clear skies is obtained, using the function for the direct luminous efficacy for clear skies given in a previous work, and the brightness index as a new independent variable. One of the two models is obtained by fitting the values of the relative luminous efficacy as a function of the brightness index. The other proposed model is developed by obtaining in the first place models for direct illuminance and direct irradiance, both relative to the corresponding clear sky models, and then finding the ratio between the relative direct illuminance and irradiance models. Statistical assessment of the direct illuminance values estimated with the proposed models gives similar values for the MBE and the RMSE. However, the model obtained from illuminance and irradiance models is mathematically coherent.     

Hourly and daily global photometric illuminance and luminous efficacy at a tropical station,Ile-Ife,Nigeria

Diurnal and seasonal patterns of hourly and daily global photometric illuminance and luminous efficacy at Ile-Ife (7.5°N, 4.57°E), Nigeria, were investigated with respect to the prevailing atmospheric conditions over the station. The daylight availability at the station is characterised in terms of the global luminous efficacies. During the cloudy months of July–September and Harmattan (dust haze) months of December–January, when clear sky conditions are not common, the photometric illuminance has values varying from 34.8 to 52.97 Klux hours over the period of 11.00–15.00 LST. For the relatively less cloudy and less hazy months of February and March, also when clear sky events are more likely to occur, the values range from 50.87 to 65.21 Klux. Frequency and cumulative frequency analyses for every 5 lm/W interval of hourly and daily luminous efficacy show that hourly and daily daylight availability at Ile-Ife in terms of luminous efficacy can be as low as 35 lm/W and as high as 115 lm/W and varies seasonally. The probability of having daylight with hourly luminous efficacy more than 100 lm/W is more during the dry (rainy) season than during the dry (Harmattan) season.     

Conceptual differences between the bioclimatic urbanism for Europe and for the tropical humid climate

This article makes part of a series of conceptual papers to continue the discussion about how architecture and urbanism interact with climate, in tropical regions. Students engaged in normal courses of architecture in tropical regions, particularly in South America, develop their knowledge based on concepts generated in the developed countries—usually related to cold environments. Consequently, these students acquire wrong ideas about urban design of open spaces. Integrating urbanism and climate in tropical countries is still very incipient as an approach and many lecturers reject it, since they prefer to continue with a more formal one, dictated by most of the dominant countries. The herein paper underlines several different concepts and perspectives that separate the two conceptions, leading to a reflection about the subject.     

Performance of a photovoltaic solar refrigerator in tropical climate conditions

The problem of preserving vaccines as well as perishable foodstuffs is absolutely conditioning the life of populations living in intertropical areas, that is to say, population deprived of all energy forms other than the one coming from the solar radiation. Vaccines keep their properties if their temperature is maintained between −3°C and 8°C, without taking the ambient temperature into accout. Also some foodstuffs need a temperature of between 0 and 10°C for preservation in a good condition. However, in some areas in the Sudan, day temperature can reach and even exceed 40°C. So a cooling of these materials is imperative. Nevertheless, without having a conventional energy source at one's disposal, the existence of mechanical refrigerators connected to photovoltaic panels could present a possible solution to this difficulty. This paper presents a field test of the performance of photovoltaic (PV) refrigerator in the Sudan tropical climate.     

Theoretical prediction and validation of global horizontal solar irradiance for a tropical climate in India

This paper aims to propose monthly models responsible for the theoretical evaluation of the global horizontal irradiance of a tropical region in India which is Sivagangai situated in Tamilnadu. The actual measured global horizontal irradiance hails from a 5 MW solar power plant station located at Sivagangai in Tamilnadu. The data were monitored from May 2011 to April 2013. The theoretical assessment was conducted differently by employing a programming platform called Microsoft Visual Basic 2010 Express. A graphical user interface was created using Visual Basic 2010 Express, which provided the evaluation of empirical parameters for model formulation such as daily sunshine duration (S), maximum possible sunshine hour duration (S₀), extra terrestrial horizontal global irradiance (H₀) and extra terrestrial direct normal irradiance (G₀). The proposed regression models were validated by the significance of statistical indicators such as mean bias error, root mean square error and mean percentage error from the predicted and the actual values for the region considered. Comparison was made between the proposed monthly models and the existing normalized models for global horizontal irradiance evaluation.     

A passive-heated building for a cool,humid climate in India

This paper describes the design and thermal performance of a passive-heated building for the cool, humid climatic conditions of Srinagar, India. The annual climatic variations of Srinagar are from a cold and very humid winter to a temperate, humid late summer. Thus, the major trend is creating comfort conditions by appropriate heating. The design discussed here therefore uses passive heating concepts. Special care is taken to prevent wasteful heat losses during the winter and unwanted heat gains in the summer. In order to ascertain the performance of the design, a thermal analysis has been carried out. The performance was found to be satisfactory.     

Experiments in a solar simulator on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate

This paper presents an indoor and analytical study to evaluate the performance of a desiccant cooling system that uses silica gel as desiccant, electric light bulbs to simulate solar radiation, and forced flow of air through an IDC (integrated Desiccant/Collector). In the regeneration process, the rate at which water is removed from the desiccant increases with irradiation and decreases with air flowrate. In the air dehumidification process, the adsorption rate decreases with irradiation and increases slightly with flowrate. Comparisons between analytical calculations and experimental data show good agreement, and the calculations show that it should be possible to operate this system in tropical humid climates using the regeneration process in the day and the air dehumidification in the night time.     

Computing global and diffuse solar hourly irradiation on clear sky. Review and testing of 54 models

Fifty-four broad band models for computation of global and diffuse irradiance on horizontal surface are shortly presented and tested. The input data for these models consist of surface meteorological data, atmospheric column integrated data and data derived from satellite measurements. The testing procedure is performed for two meteorological stations in Romania (South-Eastern Europe). The testing procedure consists of forty-two stages intended to provide information about the sensitivity of the models to various sets of input data. There is no model to be ranked “the best” for all sets of input data. Very simple models as well as more complex models may belong to the category of “good models”. The best models for solar global radiation computation are, on equal-footing, ESRA3, Ineichen, METSTAT and REST2 (version 81). The second best models are, on equal-footing, Bird, CEM and Paulescu & Schlett. The best models for solar diffuse radiation computation are, on equal-footing, ASHRAE2005 and King. The second best model is MAC model. The best models for computation of both global and diffuse radiation are, on equal-footing, ASHRAE 1972, Biga, Ineichen and REST2 (version 81). The second best is Paulescu & Schlett model.     

Verification of some very simple clear and cloudy sky models to evaluate global solar irradiance

Several very simple clear sky and cloudy sky global irradiance models were tested under the climate and latitudes of Romania (Eastern Europe). The very simple clear sky models do not require meteorological data while the very simple cloudy sky models need only data on the total cloud amount. Three slightly more complex cloudy sky models which use additional meteorological inputs were also tested. The mean absolute error of the very simple clear sky models varies between 7 and 14% in July and between 12 and 19% in January. The best model originates from Western Europe. The performance of the very simple cloudy sky models is comparable to that of the more complicated ones. This is the main conclusion of the paper. Generally, their root mean square error varies between 35 and 45% in close agreement with results from the literature. Twelve classes of daily average cloudiness were defined. The model accuracy is better for the first six classes (with smaller cloud amount) and worse for the other six classes.     

Importance of the diffuse sky radiation in evaluation of the performance of a solar cell

Representative results of the numerical simulation of responses (viz. photocurrent and optimum power output as well as efficiency) of the conventional and violet Si cells and of the Ga_1−xAl_xAs---GaAs and GaAs cells are presented as a function of the solar zenith angle for seven different models of the terrestrial atmosphere. The atmospheric models used vary from an aerosol-free and cloud-free model with gaseous absorption to several models with moderately thick stratus cloud layer and high concentrations of aerosols. This study, restricted to horizontally situated solar cells, illustrates the manner in which characteristics are significantly affected by position of the sun, turbidity and cloudiness of the atmosphere, as well as reflectivity of the underlying surface.     

Experimental evaluation of insulation material in roofing system under tropical climate

The objective of this study is to determine the influence of radiant barriers on conductive and radiative heat transfers when they are integrated to a building envelope and to compare their efficiency to traditional insulation material (mineral wools, polystyrene). It is also about determining which insulation material and process can lead to a better heat flux reduction through a building roof. For this study four identical small-scale test cells were used. Their respective roof was equipped with the insulation material to be tested: One with polystyrene, the second with a radiant barrier the third one with fibber glass and the last one with no insulation material was considered as the reference cell. Different test were performed with a view to evaluate the influence of parameters such as roof absorptivity and roof air layer ventilation on the heat flux reduction through the roof. With the measured temperature, the conductive and radiative heat fluxes were calculated. With a white corrugated iron roof top the heat flux reduction provided by the radiant barrier is 37%. With a black one this material allows a reduction of 33%. It is shown that whatever the roof absorptivity value, the radiative heat flux is predominant over the conductive one. With no ventilation, the radiant barrier is comparable to polystyrene and fiber glass; when the airspace is ventilated the radiant barrier provides a better insulation.     

The estimation of global and sky radiation in Austria

For obtaining the radiation values necessary for utilization of solar energy at as much possible locations in Austria, the monthly sums of global radiation at the horizontal surface are estimated after Angström from the relative duration of sunshine using the extraterrestrial radiations as maximum values. The thereby needed constants, which were evaluated for 19 stations in Austria with synchronous measurements of global radiation and duration of sunshine, are stated for the seasons as functions of altitude above sea level. The sky radiation is estimated by means of relationships after Liu-Yordan. These relationships were evaluated from the longterm measurements of global- and sky radiation at three stations in Austria for the mean monthly sums and for the mean hourly sums.     

A comparison of luminous efficacy models with illuminance and irradiance measurements

Several luminous efficacy models have been tested against simultaneous illuminance and irradiance measurements in Helsinki, Finland (60°11′N, 24°50′E). When compared with the measured values, the Perez luminous efficacy model had the lowest relative root mean square difference (RMSD) of 6.7%. All tested models had an RMSD within 2% of the constant model which used the measured yearly average of 110 lm/W as the constant. For all models, the error was smaller for high solar altitudes but greater for the low altitudes. The monthly average luminous efficacy was fairly constant between April and October but it was clearly lower during the winter months when the sun is very low.     

A comparison of two models for the angular distribution of diffuse sky radiance for overcast skies

A comparison is presented of two models for the angular distribution of diffuse sky radiance under overcast skies: the generalized Standard Overcast Sky (SOC) model and the Three-Component Continuous Distribution (TCCD) model. The defining equations for the models are seen to be different, but are shown to be similar in form for the case of overcast skies. It is concluded that for overcast skies 1) the SOC and TCCD equations are capable of predicting the diffuse sky radiance equally well, 2) calibrations of the parameters of one model can be used to calibrate parameters of the other, and 3) parameters can be estimated more easily for the TCCD equation than for the SOC equation. These results substantially raise the confidence in the validity of both models for overcast skies, and support the choice of the most convenient for any particular application.