Efim G. Evseev

Simulation study of a solar collector with a selectively coated polymeric double walled absorber plate

Abstract A simulation model describing a solar collector containing a selectively coated, polymeric, double walled absorber plate with parallel fluid flow channels has been developed and validated utilizing experimental data from such a solar collector module. The simulation model contains a more refined radiative energy balance, which takes into consideration the fact that the glazing may not be totally opaque to long wavelength radiation, and is solved by applying numerical integration to the non-linear differential equations that describe the system. The validated simulation model was then utilized to perform parametric sensitivity studies on the system heat transfer coefficients, collector efficiency factor, outlet temperature of the heat exchange fluid and average daily efficiency with regard to (i) mass flow rate for the heat exchange fluid; (ii) type of glazing, viz., glass, polymer film and double walled polymeric plate; (iii) channel height of double walled structure used as the absorber plate; and (iv) effect of the air gap thickness between the absorber plate and the glazing on the overall heat transfer phenomena. The daily performance tests on the solar collector modules exhibited average daily efficiencies in the range of 50–60% and maximum outlet heat exchange fluid temperatures in excess of 60°C. Such corrosion resistant selectively coated absorber plates would be ideal for heating the feedstock for desalination of seawater in an evaporation–condensation process. The results of these analyzes are reported in detail.

Statistical relationships between solar UVB and UVA radiation and global radiation measurements at two sites in Israel

Statistical relationships between the hourly radiation intensity values of both ultraviolet-B (UVB) and ultraviolet-A (UVA), and of the corresponding global radiation at two sites in Israel, the Dead Sea and Beer Sheva, are presented using radiation data measured from January 1995 to January 1999. It was determined, based upon the square of the correlation coefficient (r 2 ), that in the case of the UVB, a quadratic equation provided a better fit of the database, whereas in the case of UVA, a linear equation was sufficient, i.e. no significant improvement in r 2 was obtained using a quadratic equation. Each monthly empirical equation was tested by utilizing measurements from a single month of independent observations. The accuracy of the individual equations was determined by calculating the mean bias error (MBE), the root mean square error (RSME) and the percent coefficient of variation (CV). The results of this statistical analysis showed that, at both sites, the UVA relationships were characterized by a CV of 20% were observed for more than a third of the individual months at both the Dead Sea and Beer Sheva. Monthly and seasonal relationships from one site were then tested by applying them to the second site. In both cases, the relationships between both UVA and UVB and global radiation were found to be site-specific. On the basis of this statistical analysis, the UVA relationships were found to be far superior predictors than the corresponding UVB ones. In fact, the UVB relationships have limited applicability for predicting UVB from the corresponding global solar radiation intensity.

The analysis of ultraviolet radiation in the Dead Sea Basin, Israel

The Dead Sea basin offers a unique site to study the attenuation of the ultraviolet (UV) radiation, as it is situated at the lowest point on Earth, about 400 m below sea level, and the air above the Dead Sea is characterized by a relatively high aerosol content due to the very high salt content of the Dead Sea. In view of its being an internationally recognized centre for climatotherapy, it is of interest to study both its UV intensity and attenuation as a function of wavelength relative to other sites. In order to provide a basis for intercomparison of the radiation intensity parameters measured at the Dead Sea, a second set of identical parameters were being measured simultaneously at a second site, located at a distance of ca. 65 km and to the west and situated above sea-level (Beer Sheva ata315 m a.s.l.). The ultraviolet radiation, both UV-B and UV-A, were monitored continuously at both sites using Solar Light Co. Inc. broad-band meters. In addition, sporadic measurements utilizing a narrow-band spectroradiometer were performed to ascertain the extent of site-specific spectral selectivity in the ultraviolet spectrum. The monthly average daily attenuation rates were found to vary from 710 2t o 7173 per cent 1000 m 71 and 733 to 787 per cent 1000 m 71 for UV-B and UV-A, respectively. The average monthly values for UV-B and UV-A are 7146 per cent 1000 m 71 and 754 per cent 1000 m 71 , respectively. These values are in the range of values reported previously for studies performed at high altitudes, e.g. in the Alps and the Andes. The relative attenuation in the ultraviolet range as a function of wavelength, i.e. site-specific spectral selectivity, decreases with increasing wavelength. Consequently, the spectral range most effective with regard to erythema undergoes the highest degree of attenuation. These findings are in accordance with radiation scatter theory # 1997 Royal Meteorological Society. Int. J. Climatol., 17, 1697‐1704

A method for characterization and inter-comparison of sites with regard to solar energy utilization by statistical analysis of their solar radiation data as performed for three sites in the Israel Negev region

A method for the characterization and inter-comparison of sites with regard to their suitability for utilization of solar energy based upon the statistical analysis of their solar radiation intensities is presented. In this method each monthly data set of the daily global, horizontal beam and diffuse radiation intensities was analyzed and the following parameters were determined: monthly average daily radiation intensity, coefficient of variation, skewness and kurtosis. The values of the skewness and kurtosis have been applied, using generally accepted rules, to describe the distribution curves for each of the radiation intensity data sets. In addition, the same type of statistical analysis was applied to the monthly average daily ratios of the horizontal beam to global radiation, diffuse to global and the clearness index for the three sites. In this investigation, this statistical analysis method has been applied to the global and beam radiation measured at three sites located in the southern, Negev region of Israel, viz., Beer Sheva, Sde Boker and Eilat. The southern region of Israel is characterized by relatively high average daily irradiation intensities for both global and normal incidence radiation. They have been characterized with regard to the distribution of their intensity levels and a site inter-comparison has also been performed. An inter-comparison of the results of these analyses for the three sites has been performed on a monthly basis. The results of this analysis are used both to characterize and compare the composition of the solar radiation at the three sites under investigation. The results of this study will be presented in detail.

The measurement and analysis of normal incidence solar UVB radiation and its application to the photoclimatherapy protocol for psoriasis at the Dead Sea, Israel.

The broad‐band normal incidence UVB beam radiation has been measured at Neve Zohar, Dead Sea basin, using a prototype tracking instrument composed of a Model 501A UV‐Biometer mounted on an Eppley Solar Tracker Model St‐1. The diffuse and beam fraction of the solar global UVB radiation have been determined using the concurrently measured solar global UVB radiation. The diffuse fraction was observed to exceed 80% throughout the year. The application of the results of these measurements to the possible revision of the photoclimatherapy protocol for psoriasis patients at the Dead Sea medical spas is now under investigation. The suggested revision would enable the sun‐exposure treatment protocol to take advantage of the very high diffuse fraction by allowing the patient to receive the daily dose of UVB radiation without direct exposure to the sun, viz. receive the diffuse UVB radiation under a sunshade. This would require an increase in sun‐exposure time intervals, as the UVB radiation intensity beneath a sunshade is less than that on an exposed surface.

The solar ultraviolet B radiation protection provided by shading devices with regard to its diffuse component

Background: The composition of the incident solar global ultraviolet B (UVB) radiation with regard to its beam and diffuse radiation fractions is highly relevant with regard to outdoor sun protection. This is especially true with respect to sun protection during leisure‐time outdoor sun exposure at the shore and pools, where people tend to escape the sun under shade trees or different types of shading devices, e.g., umbrellas, overhangs, etc., believing they offer protection from the erythemal solar radiation. The degree of sun protection offered by such devices is directly related to the composition of the solar global UVB radiation, i.e., its beam and diffuse fractions.

Coaxial tubular solar collector constructed from polymeric materials: an experimental and transient simulation study

An experimental study and a simulation model describing a coaxial tubular solar collector fabricated from polymeric materials, consisting of an inner black tube as a solar energy absorber in intimate contact with an outer transparent tube as an insulator, having the potential to provide low grade thermal energy at reasonable costs is reported. The simulation model describes the transient performance of the coaxial tubular polymeric solar collector utilizing non-linear equations solved by a difference splitting technique. The simulation model was first validated utilizing the experimental data and was then used to determine the optimal design parameters, viz. the inner, black absorber, and outer, transparent insulator, tube thicknesses. In addition, the effect of an annular air filled gap between the coaxial tubes on system performance was also studied. The results of the experimental and simulations studies are reported together with the optimal design specifications.

Prediction of solar global radiation on a surface tilted to the south

Solar global and diffuse 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 surface tilted to the south (northern hemisphere) 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. Eleven such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of hourly solar global and diffuse radiation on a horizontal surface, normal incidence beam and global radiation on a south-oriented surface tilted at 40°. 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, i.e., clear, partially cloudy and cloudy as defined by the hourly clearness index value, has been studied.

The evaluation of four different diffuse radiation correction models applied to shadow ring measurements for Beer Sheva, Israel

The measurement of the horizontal diffuse radiation, a priori a straightforward task, is fraught with difficulties. It is possible to measure the diffuse radiation by both direct and indirect methods. The most accurate method is probably the indirect one, which utilizes concurrent measurements of the horizontal global and the normal incidence beam radiation. The disadvantage of this method is the relatively expensive tracking system required for measuring the latter. The diffuse radiation can be measured directly with a pyranometer outfitted with either an occulting disk or shadow ring, which prevent the beam radiation from impinging on the pyranometer sensor. The former method can provide accurate measurements of the diffuse radiation but requires a relatively expensive sun tracking system in the east-west axis. The shadow ring is a stationary device with regard to the east-west axis and blocks the beam radiation component by creating a permanent shadow on the pyranometer sensor. The disadvantage of the shadow ring is that it also blocks a portion of the sky, which necessitates a geometrical correction factor. There is also a need to correct for anisotropic sky conditions. Four correction models have been applied to the data and the results evaluated and ranked.

An Inter-Comparison of the Radiation Intensities Measured at Three Sites in the Southern Region of Israel by Regression Analysis

Publisher Summary This chapter explores whether the southern region can be described by a singular set of monthly regression equations. A database consisting of global and normal incidence radiation intensities measured at Beer Sheva, a site in the Negev region of Israel, have been utilized to develop a monthly correlations between Kd, the ratio of the daily diffuse on a horizontal surface to the daily extraterrestrial on a horizontal surface as a function of KD, and the ratio of the daily direct beam on a horizontal surface to the daily extraterrestrial on a horizontal surface. An empirical equation in the form of Kd = a[exp(bKD + cKD2)] has been found to give a very good fit to the experimental data.