Global,direct and diffuse solar radiation on horizontal and tilted surfaces in Jeddah,Saudi Arabia

The measured data of global and diffuse solar radiation on a horizontal surface, the number of bright sunshine hours, mean daily ambient temperature, maximum and minimum ambient temperatures, relative humidity and amount of cloud cover for Jeddah (lat. 21°42′37′′N, long. 39°11′12′′E), Saudi Arabia, during the period (1996–2007) are analyzed. The monthly averages of daily values for these meteorological variables have been calculated. The data are then divided into two sets. The sub-data set I (1996–2004) are employed to develop empirical correlations between the monthly average of daily global solar radiation fraction (H/H₀) and the various weather parameters. The sub-data set II (2005–2007) are then used to evaluate the derived correlations. Furthermore, the total solar radiation on horizontal surfaces is separated into the beam and diffuses components. Empirical correlations for estimating the diffuse solar radiation incident on horizontal surfaces have been proposed. The total solar radiation incident on a tilted surface facing south H_t with different tilt angles is then calculated using both Liu and Jordan isotropic model and Klucher’s anisotropic model. It is inferred that the isotropic model is able to estimate H_t more accurate than the anisotropic one. At the optimum tilt angle, the maximum value of H_t is obtained as ∼36 (MJ/m² day) during January. Comparisons with 22 years average data of NASA SSE Model showed that the proposed correlations are able to predict the total annual energy on horizontal and tilted surfaces in Jeddah with a reasonable accuracy. It is also found that at Jeddah, the solar energy devices have to be tilted to face south with a tilt angle equals the latitude of the place in order to achieve the best performance all year round.     

Solar radiation correlations for the Athens,Greece, area

In the present study, correlations of the solar radiation for the Athens area are provided, based on hourly measurements over the period 1955–1980. These correlations are of two kinds: The first, which provide the average total radiation incident on a horizontal surface in terms of hour, day, and month of the typical year, is suitable for solar energy collection applications. The correlations of the second kind provide the maximum radiation and are suitable for use in the design of air-conditioning installations or other applications requiring knowledge of maximum values. The correlations of this kind are based on the calculation of the solar constants A, B, and C, i.e. the “apparent solar irradiation at zero air mass,” the “atmospheric extinction coefficient,” and the “diffuse radiation factor,” respectively. These solar constants permit the calculation of the maximum direct normal radiation and of the maximum diffuse radiation as well as of the total radiation on surfaces of various orientations and inclinations. The solar heat gain through fenestration of various orientations (needed in air-conditioning applications) is calculated, as an example of the applications of the solar constants A, B, and C.     

A case study to prepare for the utilization of aerosol forecasts in solar energy industries

Precise aerosol information is indispensable in providing accurate clear sky irradiance forecasts, which is a very important aspect in solar facility management as well as in solar and conventional power load prediction. In order to demonstrate the need of detailed aerosol information, direct irradiance derived from Aerosol Robotic Network (AERONET) ground based measurements of aerosol optical depth (AOD) was compared in a case study over Europe to irradiance calculated using a standard aerosol scenario. The analysis shows an underestimation of measurement-derived direct irradiance by the scenario-derived direct irradiance for locations in Northern Europe and an overestimation for the Mediterranean region.Forecasted AOD of the European Dispersion and Deposition Model (EURAD) system was validated against ground based AERONET clear sky AOD measurements for the same test period of February 15th to 22nd, 2004. For the time period analyzed, the modelled AOD forecasts of the EURAD system slightly underestimate ground based AERONET measurements. To quantify the effects of varying AOD forecast quality in their impact on the application in solar energy industry, measured and forecasted AOD were used to calculate and compare direct, diffuse, and global irradiance. All other influencing variables (mainly clouds and water vapour) are assumed to be modelled and measured correctly for this analysis which is dedicated to the specific error introduced by aerosol forecasting. The underestimated AOD results in a mean overestimation of direct irradiance of +28 W/m² (+12%), whereas diffuse irradiance is generally underestimated (−19 W/m² or −14%). Mean global irradiance values where direct and diffuse irradiance errors compensate each other are very well represented (on average +9 W/m² or +2%).     

Monthly mean solar radiation statistics for Australia

Tables of monthly mean solar radiation parameters are computed from detailed cloud cover information. The parameters include direct and global daily total energy inputs to horizontal, inclined and “sun-tracking” surfaces. Comparison with measured global radiation at 12 stations reveals virtually no systematic error in the computation scheme, and an error of 2MJ m⁻² day⁻¹ in the worst case month of any station.     

Empirical regression models for weather data measured in Kuwait during the years 1985, 1986, and 1987

Monthly average daily measurements over a three-year period (1985–1987) for eight weather parameters are reported. The computed three-year averages of total yearly global radiation, diffuse radiation, hemispherical global infrared radiation, and global UV radiation were 1,939.08, 696.82, 3,144.30, and 87.78 kWh/m², respectively. Further, employing least-squares linear regression analysis, empirical fits expressing the monthly average daily measurements as a function of month of the year were developed for seven weather parameters.     

Site-independent algorithm for obtaining the direct beam insolation from a multipyranometer instrument

The possibility of using the Perez irradiance model for reconstructing the beam and diffuse components of solar radiation from measurements with a fixed multipyranometer system is investigated. It is shown that an iterative algorithm can be derived that enables determination of the direct beam component to about ±50 W/m² without requiring any site-dependent adjustments. The standard error on the derived global irradiance on inclined surfaces of arbitrary orientation is at the ±10 W/m² level. It is concluded that the present degree of precision renders the multipyranometer method an attractive alternative to standard instrumentation for radiation surveys in remote regions. Possibilities for further improvements are discussed.     

Modelling and analysis of air-cooled reciprocating chiller and demand energy savings using passive cooling

The study presents modelling and analysis of air-cooled chiller system in an office building at Central Queensland University in Rockhampton, Australia. EnergyPlus, building energy simulation software, has been used to model and to simulate the energy savings. Base case cooling energy has been compared with measured data. The simulated results show a reasonable agreement with the measured data. As a passive cooling means, the effect of economiser usages and pre-cooling have been simulated and analysed to assess annual demand savings for an energy intensive office building at Rockhampton, Australia. It was found that implementation of the pre-cooling and economiser system could save 115 kW/m²/month and 72 kW/m²/month total cooling energy and 26 kW/m²/month and 42 kW/m²/month chiller energy, respectively.     

Performance characteristics of a perforated shadow band under clear sky conditions

A perforated, non-rotating shadow band is described for separating global solar irradiance into its diffuse and direct normal components using a single pyranometer. Whereas shadow bands are normally solid so as to occult the sensor of a pyranometer throughout the day, the proposed band has apertures cut from its circumference to intermittently expose the instrument sensor at preset intervals. Under clear sky conditions the device produces a saw tooth waveform of irradiance data from which it is possible to reconstruct separate global and diffuse curves. The direct normal irradiance may then be calculated giving a complete breakdown of the irradiance curves without need of a second instrument or rotating shadow band. This paper describes the principle of operation of the band and gives a mathematical model of its shading mask based on the results of an optical ray tracing study. An algorithm for processing the data from the perforated band system is described and evaluated. In an extended trial conducted at NREL’s Solar Radiation Research Laboratory, the band coupled with a thermally corrected Eppley PSP produced independent curves for diffuse, global and direct normal irradiance with low mean bias errors of 5.6 W/m², 0.3 W/m² and −2.6 W/m² respectively, relative to collocated reference instruments. Random uncertainties were 9.7 W/m² (diffuse), 17.3 W/m² (global) and 19.0 W/m² (direct). When the data processing algorithm was modified to include the ray trace model of sensor exposure, uncertainties increased only marginally, confirming the effectiveness of the model. Deployment of the perforated band system can potentially increase the accuracy of data from ground stations in predominantly sunny areas where instrumentation is limited to a single pyranometer.     

A multipyranometer instrument for obtaining the solar beam and diffuse components, and the irradiance on inclined planes

We discuss some of the practical issues associated with the use of a stationary multipyranometer system for determining both the beam and diffuse components of solar radiation, and the global insolation on arbitrary inclined planes. Results from a full year of validation measurements are presented and compared with (a) the values obtained using conventional, clock-driven, instrumentation (in the case of the direct beam component); and (b) with model-derived results (in the case of total insolation on inclined planes). The yearly average rms error for the beam component is found to be 50 W/m² (using 10 min data) or 47 W/m² (using hourly totals). The corresponding rms error for the horizontal global radiation is 18 W/m² (for 10 min data). In the case of total insolation on arbitrary inclined planes, the rms errors are found to be smaller than 30 W/m², which is comparable to the accuracy given by the best anisotropic-diffuse models currently available.     

Thermal conversion with fluorescent concentrators

Fluorescent planar collector-concentrators are a new possibility for the conversion of solar energy into thermal energy. The collection and concentration of direct and diffuse radiation is feasible, using a transparent sheet of material doped with a fluorescent dye. The collector offers the advantage of separating the global irradiation into different spectral regions. This geometrical and spectral concentrated light can be converted with adapted highly selective absorbers into high temperature heat. Intensity and spectral region of the sunlight and the selectivity of the absorber determine the thermodynamically possible maximum absorber temperature. A test collector with a fluorescent concentrator area of 0.8 m² with an absorber pipe of 3 mm diameter in an evacuated glass tube was built. At a total irradiation of 850 W/m² on the fluorescent collector surface, a maximum stagnation temperature of 555°C (828 K) was reached. Under diffuse light conditions (150 W/m²), stagnation temperatures above 250°C (523 K) were measured. Thermodynamic calculation, experimental setup and results are given.     

Experimental and theoretical investigation of global and diffuse solar radiation in the United Arab Emirates

In the present study, the global, direct and diffuse components of solar radiation as well as temperature, relative humidity and wind speed have been continuously monitored and analysed on an hourly, daily and monthly basis. The monthly average daily total solar radiation varies from 2700 W h/m² in December to 8000 W h/m² in June with an average clearness index of 0.65. Experimental data are compared to the predictions of different theoretical models as functions of declination and hour angles. Correlations are obtained describing the variation of hourly, daily and monthly averages of total and diffuse solar radiation using polynomial expressions. Empirical correlations describing the dependence of the daily average diffuse to total radiation ratio on the clearness index are also obtained. Data for the daily diffuse to total radiation ratio are compared to correlations obtained by other investigators. The comparison shows a reasonable agreement with some scatter due to the seasonal dependence of the correlation. Comparison of calculations with experimental measurements under clear sky conditions show excellent agreement with a maximum error of 8%.     

Investigation of solar radiation in Botswana and some anomalous phenomena observed

Daily global insolation on a horizontal surface in Botswana is recorded continuously at several synoptic stations and at the University of Botswana's Physics Department. Over a number of years, daily total insolation on a tilted surface (β = −30°) was recorded at the Botswana Technology Centre. Hourly, and instantaneous direct normal, global, diffuse and UV-components are continuously recorded at the University of Botswana. All these measurements are done with standard EPLAB equipment.It is found out that the instantaneous direct normal radiation at Solar noon can be as high as 1150 W·m⁻²; and that at 30 min before sunset it can be above 600 W·m⁻²; and it can also be as high as 100 W·m⁻² at sunset or sunrise moments (i.e. with half of the solar disk under the horizon).Daily direct normal solar radiation can exceed 45 MJ·m⁻². Mean daily global radiation varies from 31 MJ·m⁻² in December to 16 MJ·m⁻² in June. Such big values of daily direct normal and global radiation are explained by low humidity and low turbidity.Cases of an anomalous phenomenon which lead to an abnormally big phase shift when direct normal radiation is increasing greatly after Solar noon are observed, and discussed. It is also found that when humidity is low and visibility is high, hourly I_g values recorded with a pyranometer can be less than I_bn (cosθ_z) + I_d-values. This discrepancy could be quite common for regions where humidity and turbidity are low. The trend in the behaviour of the UV-component during the last five years is also analyzed and discussed. The conclusion is made that the ozone layer over Botswana is continuously being depleted.     

Numerical and Optical Analysis of Solar Power Level Adaptable Solar Reactor

Solar energy is an abundant renewable energy resource that can be used to provide high process heat necessary to run thermochemical processes for production of various solar fuels and commodities. In a solar reactor, sunlight is concentrated into a receiver through a small opening called the aperture. However, obtaining and maintaining semiconstant high temperatures inside a solar reactor is a challenge. This is because the incident solar radiation can fluctuate depending on the position of the sun and the weather conditions. For fixed aperture size reactors, changes in incident solar flux directly affect the temperature inside the reactor. This paper presents a novel solar reactor with variable aperture mechanism that is designed and manufactured at our lab. Radiation heat transfer analysis of this reactor concept is studied via Monte Carlo (MC) ray tracing. MC ray tracing module is coupled to a steady-state one-dimensional energy equation solver. Energy equation is solved for the wall and gas, accounting for the absorption, emission, and convection. Incoming direct flux values for a typical day are obtained from National Renewable Energy Lab database. Results show that for a perfectly insulated reactor, the average temperature of the working fluid may be kept appreciably constant throughout the day if aperture diameter is varied between 3 cm and 1.5 cm for incoming fluxes starting with 400 W/m² at 05:12 a.m., reaching peak value of 981 W/m² at noon, and eventually receiving 400 W/m² at 6:58 p.m., which can make the solar reactor run about 13 hr continuously at 1500 K semiconstant temperature.     

Analysis of the solar radiation data for Beer Sheva, Israel, and its environs

The solar radiation climate of Beer Sheva, Israel, is reported upon in detail. The database utilized in this analysis consisted of global radiation on a horizontal surface, normal incidence beam radiation, and global radiation on a south-facing surface tilted at 40°. Monthly-average hourly and daily values are reported for each of these three types of measured radiations, together with the calculated monthly-average daily values for the components of the global radiation, viz. the horizontal beam and diffuse radiations. The monthly-average hourly and daily clearness index values have also been calculated and analyzed. Monthly-average daily frequency distributions of the clearness index values are reported for each month. The solar radiation climate of Beer Sheva has also been compared to those reported for a number of countries in this region. The annual-average daily global radiation incident on a horizontal surface is 18.91 MJ/m² and that for normal incidence beam radiation is 21.17 MJ/m². The annual-average daily fraction of the horizontal global radiation that is beam is 0.72. The annual-average daily value for the clearness index is 0.587 and the average frequency of clear days annually is 58.6%. We conclude, based upon the above analysis, that Beer Sheva and its environs are characterized by relatively high, average-daily irradiation rates, both global and beam, and a relatively high frequency of clear days.     

Technical note: Simple models for estimating the total, diffuse, direct and normal solar irradiation in Bahrain

Very simple models have been made to estimate the monthly and daily average total, diffuse and beam solar irradiation on a horizontal surface in Bahrain. The direct normal solar irradiation has also been measured and modeled using the rotating shadow band pyrometer. The monthly average estimation was found to be more accurate than the daily average, and modeling the direct solar irradiation is the most inaccurate followed by the diffuse; since these two are very susceptible to atmospheric pollution and weather conditions. The highest actually measured monthly average total for direct, diffuse and direct normal solar irradiation was found to be 585, 383, 343 and 716 W m⁻², respectively, while the lowest actually measured monthly average values were 373, 242, 96 and 342 W m⁻², respectively. The results of the total solar irradiation in four different sites in Bahrain were found to have similar values (the maximum hourly values ranged from 820 to 1000 W m⁻² at mid-day in June) which is expected since the area of Bahrain is nearly 700 km².     

Optical loss of photovoltaic modules under diffuse light

The optical behaviour of photovoltaic (PV) modules from different fabrication technologies was investigated under diffuse light by a novel characterization method. The optical apparatus, hemispherical/hemispherical reflectometer, allows both reflectance and transmittance measurements under an incident diffuse light which simulates the distributed outdoor irradiation from the sky or from the albedo. This paper presents only the configuration for reflectance measurements. The apparatus is provided with a single integrating sphere of 40 cm diameter, which acts as a lambertian source of diffuse light and spontaneously collects the diffuse light reflected by the sample in the front hemisphere. The hemispherical/hemispherical reflectance, R^hh, expresses the optical loss of the PV device under diffuse light, and is obtained by measurements of light irradiance inside the sphere in correspondence with the sample and with a selected number of standards of diffuse reflectance. The best optical performances, in terms of optical loss, were achieved by modules realized with blue mono-Si cells and having textured front surfaces in the cover sheets.     

Calculation of monthly average insolation on tilted surfaces

Several simplified design procedures for solar energy systems require monthly average meteorological data. Monthly average daily totals of the solar radiation incident on a horizontal surface are available. However, radiation data on tilted surfaces, required by the design procedures, are generally not available. A simple method of estimating the average daily radiation for each calendar month on surfaces facing directly towards the equator has been presented by Liu and Jordan [1]. This method is verified with experimental measurements and extended to allow calculation of monthly average radiation on surfaces of a wide range of orientations.     

Comments on “Calculations of monthly average insolation on tilted surfaces” by S. A. Klein

The amount of solar energy that is intercepted by surfaces of any orientation is estimated from a new model of the clear sky, spatial distribution of solar radiation. The model was developed from measurements made during clear sky conditions and uses direct, isotropic reflected, and anisotropic diffuse radiation. The effects of azimuth, tilt, season, latitude, atmospheric turbidity, and reflectivity of the surroundings were computed using hourly measurements of normal beam and horizontal total radiation at four stations in the United States. A transformation of the co-ordinates of orientation produced a general relationship between orientation and intercepted energy. The general relationship was tested against measurements from six locations in the Northern Hemisphere and was found to be valid. The model is also a better estimator of energy intercepted by a tilted surface than are the more commonly used models.     

The estimation of daily, clear sky, solar radiation intercepted by a tilted surface

The amount of solar energy that is intercepted by surfaces of any orientation is estimated from a new model of the clear sky, spatial distribution of solar radiation. The model was developed from measurements made during clear sky conditions and uses direct, isotropic reflected, and anisotropic diffuse radiation. The effects of azimuth, tilt, season, latitude, atmospheric turbidity, and reflectivity of the surroundings were computed using hourly measurements of normal beam and horizontal total radiation at four stations in the United States. A transformation of the co-ordinates of orientation produced a general relationship between orientation and intercepted energy. The general relationship was tested against measurements from six locations in the Northern Hemisphere and was found to be valid. The model is also a better estimator of energy intercepted by a tilted surface than are the more commonly used models.     

Performance of a concentrated photovoltaic energy system with static linear Fresnel lenses

A new type of greenhouse with linear Fresnel lenses in the cover performing as a concentrated photovoltaic (CPV) system is presented. The CPV system retains all direct solar radiation, while diffuse solar radiation passes through and enters into the greenhouse cultivation system. The removal of all direct radiation will block up to 77% of the solar energy from entering the greenhouse in summer, reducing the required cooling capacity by about a factor 4. This drastically reduce the need for cooling in the summer and reduce the use of screens or lime coating to reflect or block radiation.All of the direct radiation is concentrated by a factor of 25 on a photovoltaic/thermal (PV/T) module and converted to electrical and thermal (hot water) energy. The PV/T module is kept in position by a tracking system based on two electric motors and steel cables. The energy consumption of the tracking system, ca. 0.51 W m⁻², is less than 2% of the generated electric power yield. A peak power of 38 W m⁻² electrical output was measured at 792 W m⁻² incoming radiation and a peak power of 170 W m⁻² thermal output was measured at 630 W m⁻² incoming radiation of. Incoming direct radiation resulted in a thermal yield of 56% and an electric yield of 11%: a combined efficiency of 67%. The annual electrical energy production of the prototype system is estimated to be 29 kW h m⁻² and the thermal yield at 518 MJ m⁻². The collected thermal energy can be stored and used for winter heating. The generated electrical energy can be supplied to the grid, extra cooling with a pad and fan system and/or a desalination system. The obtained results show a promising system for the lighting and temperature control of a greenhouse system and building roofs, providing simultaneous electricity and heat. It is shown that the energy contribution is sufficient for the heating demand of well-isolated greenhouses located in north European countries.