The oblateness effect on the extraterrestrial solar radiation

Calculations of the daily solar radiation incident at the top of the Earth's atmosphere, with and without the effect of the oblateness, are presented in a figure illustrating the seasonal and latitudinal variation of the ratio of both insolations. It is shown that, in summer, the daily insolation of an oblate Earth is slightly increased in two regions symmetric with respect to the summer solstice. In winter, the flattening effect results in a somewhat more extensive polar region, the solar energy input being always reduced (in some cases by more than 2 per cent) when compared to a spherical one. In addition, we also numerically studied the mean daily solar radiation. It is found that the mean summer daily insolation is scarcely increased between the equator and the subsolar point, but decreased poleward of the above mentioned limit. In winter, however, the mean daily insolation is always reduced, the maximum loss of insolation attaining as much as 1 per cent in the 55–85° latitude interval. The partial gain of the mean summertime insolation being much smaller than the reduction during winter season evidently yields a mean annual daily insolation which is decreased, maximally by about 0.3 per cent, at all latitudes.     

The distribution of global solar radiation over the land surfaces of the earth

The history, present world distribution and accuracy of measurements of global solar radiation are briefly reviewed. 221 annual total values measured during 1975 with thermopile pyranometers, were used to map the world distribution of solar irradiation and statistically analyze its variation with latitude, altitude and longitude in the northern and southern hemispheres. The pole-to-pole distribution of irradiation was fitted by cubic splines with seven knots giving a standard deviation that was 12 per cent of the average irradiation over the Earth's land surface. The distribution based on the 1975 data is compared with previously published analyses and current uncertainties and gaps in the world measurement network are discussed.     

The effect of chemical coatings on the performance of solar collector plates

This paper presents the results of the investigation of the effect of chemical coatings on the performance of solar collector plates. The three metallic plates used are aluminium, copper and iron sheets. Their absorptance and reflectance before and after the application of chemical coatings were investigated. Two types of chemicals were used: cupric nitrate (Cu(NO₃)₂·3H₂O) for CuO coating, and carbon black for carbon black coating. The methods used in these coatings are spraying pyrolysis and painting methods, respectively. The results show increases in solar absorptance and decreases in reflectance with chemical coatings.     

The effect of circumsolar radiation on a solar concentrating system

Empirical models for the energy distribution of the sun, as seen after atmospheric scattering, show a strong correlation on an annual or month-by-month basis to observed data. When applied to cases where the requirement is for a real-time solar energy distribution, such as in the optimisation of the flux distributions in imaging concentrators, these models prove insufficient. In this paper we present results illustrating trends in observed solar profiles that are invariant to changes in location that lay the framework to a definitive solar model. We show how using this information, a more complete understanding of the effect of a change in the spatial energy distributions of the sun can have on the size of the spatial energy distribution in the absorber plane of a (linear Fresnel) concentrating collector.     

The effect of time errors on the accuracy of solar radiation measurements

There is a need to focus on the accuracy of the time that is attached to solar radiation measurements. In this paper the effect of time errors on the calculation of the clearness index is investigated. An approximate expression for the error in clearness index due to time errors is derived. Using instantaneous solar radiation measurements from Townsville, Australia, the effect of time errors is illustrated with cumulative probability distribution curves. It is shown that even relatively small time errors (10 min) can substantially change the distribution curves. Time errors can be caused by drifts or inaccuracies of the data recording device's clock.     

发生温度对太阳能喷射式制冷系统性能的影响研究

为了确定发生温度对太阳能喷射式制冷系统性能的影响,基于太阳能喷射式制冷系统试验台,以蒸发温度、冷凝温度及室内环境温度为定量,发生温度为变量进行了试验研究.试验结果表明:当喷射器结构确定时,喷射系数ER、系统性能系数COP和机械性能系数COP_m均不会随着发生温度的升高一直增大,系统必然存在一个最佳的发生温度使其性能达到最佳.研究可为今后最佳发生温度的选择及实际应用中如何维持系统高效运行提供理论指导.     

The method Heliosat-2 for deriving shortwave solar radiation from satellite images

This article presents the method Heliosat-2 that converts observations made by geostationary meteorological satellites into estimates of the global irradiation at ground level. This new version integrates the knowledge gained by various exploitations of the original method Heliosat and its varieties in a coherent and thorough way. It is based upon the same physical principles but the inputs to the method are calibrated radiances, instead of the digital counts output from the sensor. This change opens the possibilities of using known models of the physical processes in atmospheric optics, thus removing the need for empirically defined parameters and of pyranometric measurements to tune them. The ESRA models are used for modeling the clear-sky irradiation. The assessment of the ground albedo and the cloud albedo is based upon explicit formulations of the path radiance and the transmittance of the atmosphere. The method Heliosat-2 is applied to Meteosat images of Europe for the months of January 1995, April 1995 and July 1994. Pyranometric measurements performed by thirty-five meteorological stations are used to assess the performances that are close to those of Heliosat-1 found in the literature. Possible improvements are discussed.     

The temperature behavior of smart windows under direct solar radiation

The purpose of this paper was to investigate the variation in temperature of electrochromic devices under direct solar radiation and to compare the results with double-glazed glass. The devices consisted of a V₂O₅ layer as an ion storage film and a WO₃ layer as an electrochromic layer. The V₂O₅ and WO₃ films were prepared by thermal and electron beam evaporation, respectively. The optical properties and structures of these films were investigated. Both the ion storage film and the electrochromic layer were amorphous. The optical absorption was caused by a direct-forbidden transition in V₂O₅ and by an indirect-allowed transition in WO₃. The maximum temperatures under solar radiation were measured for colored and bleached devices, double glass and air, they were found to be approximately 63, 63, 53 and 36 °C, respectively. The rates of increasing temperature to the incident power density for colored, bleached devices and double glass were 0.051, 0.049 and 0.041 °C/(W/m²), respectively.     

Effect of solar control glazings on human skin temperature

Human skin temperatures were measured under exposure to near-infrared radiation through solar control glazings containing SiWO_x films as solar control materials. It was found that the increase in skin temperature corresponds to the solar transmittance (T_s) value of the glazings. When the hand was exposed to radiation through a glazing containing solar films, the skin temperature after five minutes was less 0.9–1.8°C lower than when the corresponding glass without the films was used. The solar control glazings were effective in reducing the increase in skin temperature.     

Analysis of satellite derived beam and global solar radiation data

Images from the GOES 8 satellite were used along with auxiliary information such as snow cover to produce an hourly solar radiation database on a 0.1° grid for the Pacific Northwest from 1998 through 2002 [Perez, R., Ineichen, P., Moore, K., Kmiecik, M., Chain, C., George, R., Vignola, F. 2002. A new operational satellite-to-irradiance model. Solar Energy 73(5), 307–317]. Both global and beam irradiance values were derived from the satellite images and diffuse values were calculated from the beam and global values. Data from the University of Oregon Solar Radiation Monitoring Network were used to help refine and validate the model used to produce the database from the satellite images.This article presents new and independent tests of this satellite database from one year with high quality data from Kimberly, Idaho that was not used in the original development and testing of the satellite model. The mean bias error of the satellite-derived global and beam irradiance values were 5% and 2% respectively. The standard deviation ranged from 22% for global values to 41% for beam values. The largest discrepancies occur on clear winter days when it is difficult to distinguish between frost or snow on the ground and low lying fog or clouds. It is suggested that ground-based solar or visibility measurements or auxiliary satellite data are needed to augment the satellite cloud cover and snow cover data to reduce errors that can occur during cold winter days.     

Estimation of solar radiation over Cambodia from long-term satellite data

In this work, monthly average daily global solar irradiation over Cambodia was estimated from a long-term satellite data. A 14-year period (1995–2008) of visible channel data from GMS5, GOES9 and MTSAT-1R satellites were used to provide earth-atmospheric reflectivity. A satellite-based solar radiation model developed for a tropical environment was used to estimate surface solar radiation. The model relates the satellite-derived earth-atmospheric reflectivity to absorption and scattering coefficients of various atmospheric constituents. The absorption of solar radiation due to water vapour was calculated from precipitable water derived from ambient relative humidity and temperature. Ozone data from the TOMS and OMI satellite data were employed to compute the solar radiation absorption by ozone. The depletion of radiation due to aerosols was estimated from the visibility data. Five new solar radiation measuring stations were established at Cambodian cities, namely Siem Reap (13.87°N, 103.85°E), Kompong Thom (12.68°N, 104.88°E), Phnom Penh (11.55°N, 104.83°E), Sihanouke Ville (10.67°N, 103.63°E) and Kampot (10.70°N, 104.28°E). Global solar radiation measured at these stations was used to validate the model. The validation was also carried out by using solar radiation measured at four Thai meteorological stations. These stations are situated near the Cambodian border. Monthly average daily global irradiation from these stations was compared with that calculated from the model. The measured and calculated irradiation is in good agreement, with the root mean square difference of 6.3%, with respect to the mean values. After the validation, the model was used to calculate monthly average daily global solar irradiation over Cambodia. Based on this satellite-derived irradiation, solar radiation maps for Cambodia were generated. These maps show that solar radiation climate of this country is strongly influenced by the monsoons. A solar radiation database was also generated for solar energy applications in Cambodia.     

A note on diffuse solar radiation on a tilted surface

We are presenting a note on comparative study of diffuse component of solar radiation on tilted surface with different angle of inclination. The monthly mean daily diffuse radiation have been calculated on inclined surfaces from the data of horizontal surface using the Circumsolar and Isotropic model and the anisotropic model of Klucher and Hay at Lucknow (Latitude 26.75°, Longitude 80.85°), Uttar Pradesh, India. These calculated results are compared with our measured value. The comparison shows that Klucher model who considered the effect of cloudy sky conditions gives comparatively good estimations particularly at low inclination angles.     

The effect of temperature on the power drop in crystalline silicon solar cells

The influence of temperature and wavelength on electrical parameters of crystalline silicon solar cell and a solar module are presented. At the experimental stand a thick copper plate protected the solar cell from overheating, the plate working as a radiation heat sink, or also as the cell temperature stabilizer during heating it up to 80°C. A decrease of the output power (−0.65%/K), of the fill-factor (−0.2%/K) and of the conversion efficiency (−0.08%/K) of the PV module with the temperature increase has been observed. The spectral characteristic of the open-circuit voltage of the single-crystalline silicon solar cell is also presented. It is shown that the radiation-rate coefficient of the short-circuit current-limit of the solar cell at 28°C is 1.2%/(mW/cm²).     

The temperature dependence of the spectral and efficiency behavior of Si solar cell under low concentrated solar radiation

Relative spectral response of monocrystalline silicon solar cell is measured at different cell temperatures. At room temperature, the spectral response is found to have its maximum peak in the infrared (IR) range (800–1100 nm). By increasing the cell temperature, modification in the shape of the spectral response is observed and a shift of the peak towards the IR part of the spectrum is found. This behavior is of special importance that the temperature of highly illuminated thin film coated solar cells will be elevated. Other cell parameters as maximum power, fill factor, and cell efficiency are also studied at five illumination levels, viz., 1154, 1329, 1740, 2812, and 4010 W/m² and temperature ranging from −3 to 90°C.     

The importance of maximum air temperature in the parameterisation of solar radiation in Nigeria

Using existing and new empirical model equations to analyse available data for nine stations located in different geographical and climatic zones in Nigeria, it is clearly demonstrated that maximum air temperature is an important climatological parameter which should be used in solar radiation modelling in Nigeria. It is also shown that seasonal variations in the values of the model parameters are significant.Preliminary investigations are also reported which show that (1) a slight modification of the Swartman-Ogunlade formulae improves their applicability to Nigerian stations, and (2) maximum air temperature and relative humidity can be used together (to the exclusion of sunshine duration) to predict solar radiation with satisfactory accuracy.     

A method for the determination of the global solar radiation from meteorological satellite data

A statistical method is presented for the determination of the global solar radiation at ground level. It makes use of data from the meteorological satellites, which provide extensive coverage as well as adequante ground resolution. In the first step, a reference map of ground albedo is deduced from the time-sequence of satellite images. Then, by comparing the satellite data with the computed albedo map, a cloud coverage index is determined for each ground point of 5 km × 5 km. This index is linearly correlated to the atmospheric transiission factor. The regression parameters are estimated using a training set provided by ground pyranometers. Tests for two different time periods show a good agreement between the actual and model-derived hourly global radiation.     

Incident solar radiation on Argentina from the geostationary satellite GOES: Comparison with ground measurements

Daily values of the solar radiant energy incident on earth's surface constitute a quantity of increasing importance, not only in nonconventional energy development but also for agricultural, weather, and climate monitoring and predictions. In Argentina, the solar global radiation has been measured since 1978 through a network of pyranometers distributed all over the country. Simultaneously, in the area limited by 21° and 41°S latitude and 53° and 67°W longitude, insolation determinations have been made using the geostationary satellite GOES from May 1982 to June 1984. Therefore, it was possible to compare the simultaneous hourly and daily values of solar global radiation corresponding to the northern part of the country during 1982 and 1983. In this article, the results of the comparison are presented. It is shown that the standard error of the satellite-derived hourly and daily insolation values when compared against pyranometers is about 25%, and in the range of 15 to 20% of the mean values, respectively. Therefore, operational estimation of surface insolation in the region using GOES data appears normally feasible.     

The effect of surface thermal radiation on entropy generation in an open cavity with natural convection

In this work, a numerical study on entropy generation in a square open cavity with natural convection and surface thermal radiation is presented. The overall continuity, momentum, and energy equations for the gas phase in the open cavity were solved numerically by means of the finite-volume method. Temperature-dependent fluid properties were considered. During the calculations, the values of the Rayleigh number (Ra) were set in the range of 10⁴–10⁶. The temperature difference between the hot wall and the bulk fluid (ΔT) was varied between 50 and 400 K, and was represented by a dimensionless temperature difference (φ) for the purpose of generalization of the present study. The results of this investigation indicate that surface thermal radiation increases the overall entropy generation rate between 33.52% and 560.87%, and thus cannot be neglected in the analysis of this type of system.     

The effect of solar radiation on dynamic thermal performance of floor heating systems

This paper presents a numerical investigation of transient heat transfer in floor heating systems using a three-dimensional explicit finite difference model. The study focused on the influence of the cover layer and incident solar radiation on floor temperature distribution and on energy consumption. Complete and partial (area) carpets were considered as well as hardwood cover layers over concrete or gypcrete thermal storage. Experimental and simulation results for an outdoor testroom reveal that solar beam radiation can cause a local floor surface temperature in the illuminated area 8°C higher than that in the shaded area. Partial carpet cover further increases floor surface temperature differences up to 15°C when solar radiation is absorbed. Solar radiation stored in the floor thermal mass was found to reduce heating energy consumption significantly (30% or more). Increase of thermal mass thickness from 5 cm to 10 cm did not lead to higher energy savings with conventional proportional-integral control. Advanced control algorithms need to be developed to maximize energy savings while maintaining good thermal comfort.