Solar energy availability in greenhouses as affected by condensation on cladding materials

To investigate the solar radiation transmission through cladding materials covered with condensation on their inner surface a new experimental set-up was constructed. Transmittance experiments were carried out on vertical glass plates and polyethylene films covered with condensation without and with runoff for several incidence angles in the horizontal plane using perpendicularly polarized radiation. It was observed that condensation on a glass plate slightly decreased the solar radiation transmittance when compared with the dry state. The reduction in transmittance increased from 0.8% at normal incidence to 8.5% at an incidence angle of 60°. Results for polyethylene pointed out that condensation affected the solar radiation transmission and so the available solar energy in the greenhouse much more than in a glasshouse. At normal incidence, a transmittance reduction of 20.8% with respect to the dry state was obtained. For higher incidence angles, the transmittance reduction decreased gradually to 7.2% at an incidence angle of 60°. For both glass and polyethylene, it was observed that the start of runoff gave rise to a transmittance increase of up to 2% when compared with the phase of condensation without runoff.     

The determination of hourly insolation on an inclined plane using a diffuse irradiance model based on hourly measured global horizontal insolation

Using only measured hourly values of global insolation on a horizontal surface, a method has been developed for computing the corresponding hourly values of insolation on a surface inclined at any angle and oriented in any direction. The method uses a solar radiation model in which the diffuse component is calculated from global horizontal radiation using three different relationships; the appropriate equation is selected according to the value of the ratio of measured hourly global insolation to hourly global insolation computed for clear sky conditions. The method has been checked using measured hourly values in Melbourne over a 5-yr period of insolation on both a horizontal surface and a plane inclined at 38° to the horizontal facing north. The differences between the computed hourly values and the measured hourly values are found to be approximately normally distributed about zero with a standard deviation of 0.16 MJ m⁻². This method is particularly useful for predicting the heat output of inclined solar flat plate collectors when only measured global horizontal insolation is available, which is often the case. Good agreement was found between the predicted output of a typical collector using measured 38° insolation and the computed hourly values using this method. Since the method has been checked only against Melbourne data it should be applied elsewhere with caution, but it is believed to have general application.     

Goniospectrometer measurements of the optical performance of a holographic optical element

The optical properties of a thin holographic optical element (HOE) have been investigated. The Cardiff goniospectrometer was used to measure the spatial and spectral distributions of light transmitted by a HOE in the wavelength range 300 to 2100 nm. The spectrally resolved bi-directional reflection and transmission function (BRTF) was obtained for different angles of incidence. The angular resolution of the measurements is ±0.5°. The large amount of raw data was analysed and selected data are presented. The light transmitted through a HOE is split into a forward-scattered component and a colour image that is deflected through about 45°. The two components each contain different colours of the light spectrum depending on the angle of incidence. For a given angle of incidence, the spectral distribution of the transmitted light varies rapidly with the angle at which it is observed, especially in the colour image. The possible inaccuracy in the quantitative estimates of transmittance, caused by polarisation and second order effects are discussed. The results indicate that HOEs provide a method of separating the bulk of the visible light from the infrared part of the solar spectrum, thus providing a means of controlling daylighting and solar gain separately.     

On the effects of tilted light in a global prediction of AlGaAs/GaAs solar cell performance

A global optimisation of AlGaAs/GaAs solar cells under light with different angles of incidence is carried out in this work. Not only are the optimum internal and external device structures calculated, but also the thicknesses of the anti-reflecting coatings layers. For this reason, a study of the whole structure is carried out from a global point of view taking state-of-the-art technology into account. In fact, this kind of work has not been carried out up till now. The best structure, working at 1000 suns (100 W/cm²), reaches an efficiency of 28.8% for normal incidence of light. This efficiency is nearly constant up to an incident angle of around 60° and it then sharply decreases for higher angles. Variations of parameters that can be optimised (thicknesses and doping levels) as a function of angle of incidence ranging from 0° to 90° are presented. Deviations from the optimum structure are also analysed with the purpose of determining their influence in the solar cell manufacturing process.     

Performance analysis of non-isothermal solar reactors for methanol decomposition

A procedure for analyzing the performance of non-isothermal solar reactors for methanol decomposition was developed, based on a model of thermal loss from direct steam generation collector and a comprehensive kinetic model of methanol decomposition employing BASF K3-110 catalyst. It was found that catalytic bed temperature tends towards a certain value, which depends on the chemical reaction type, radiation intensity and collector structure mainly. For a beam incidence angle of 0°, system efficiency increases from 56% at a radiation intensity of 400 W m⁻² to almost 58% at a radiation intensity of 1000 W m⁻². For a radiation intensity of 400 W m⁻², beam incidence angle of 20°, absorber length of 10 m, feed temperature of 373 K and ratio of reaction section of 0.9, the mole flow rate of feed in the range of 0.21–0.23 mol s⁻¹ results in a maximum quantity of reacted methanol of 0.146 mol s⁻¹, while a mole flow rate of feed of 0.15 mol s⁻¹ leads to a maximum system efficiency of 54.2%. The research indicates that the pre-heating section should be as short as possible for effective use of solar energy.     

Enhanced solar energy collection using reflector-solar thermal collector combinations

The amount of direct light gathered by a combination of reflector plus flat-plate collector has been analyzed. The calculations were done allowing variable reflector and collector orientation angles, variables latitude, and arbitrary sun hour angle away from solar noon. The effects of reflection and transmission losses and of polarization of the incident light were included. Correction was also made for the finite size of the reflector. It was found that the optimum orientation has the collector plane almost perpendicular to the plane of the reflector. This optimum orientation is approximately independent of the sun's azimuthal dependence. The optimum reflector angle is found to be between 0° and 10° above the horizon for winter solar conditions. For typical winter operating conditions the enhancement in light gathering power for direct solar radiation is about a factor of 1·4–1·7. This results in an effective increase of 100% in the useful winter heat output from a practical reflector-collector combination with a reflector angle of 0°, over the useful heat output obtained with an optimally oriented simple flat-plate collector. An approximate calculation was also made of the overall enhancement in useful heat output for diffuse solar radiation; an increase by a factor of about 1·5 is predicted. Comparison with the preliminary analysis of the performance of the Coos Bay, Oregon solar house shows substantial agreement with the predictions of the present analysis.     

Solar position algorithm for solar radiation applications

There have been many published articles describing solar position algorithms for solar radiation applications. The best uncertainty achieved in most of these articles is greater than ±0.01° in calculating the solar zenith and azimuth angles. For some, the algorithm is valid for a limited number of years varying from 15 years to a hundred years. This report is a step by step procedure for implementing an algorithm to calculate the solar zenith and azimuth angles in the period from the year −2000 to 6000, with uncertainties of ±0.0003°. The algorithm is described in a book written by Jean Meeus in 1998. This report is written in a step by step format to simplify the complicated steps described in the book, with a focus on the sun instead of the planets and stars in general. It also introduces some changes to accommodate for solar radiation applications. The changes include changing the direction of measuring azimuth angles to be measured from north and eastward instead of being measured from south and eastward, and the direction of measuring the observer’s geographical longitude to be measured as positive eastward from Greenwich meridian instead of negative. This report also includes the calculation of incidence angle for a surface that is tilted to any horizontal and vertical angle, as described by Iqbals in 1983.     

Correlation between global ultraviolet (290–385nm) and global irradiation in Valencia and Cordoba (Spain)

This paper analyses hourly data of global ultraviolet (290–385nm) and global irradiation over a horizontal surface. The data was taken in Valencia (39.5°N, 0.341W) and Córdoba (37.51°N, 4°48W), (Spain) during the years 1996–1999. In order to estimate the ultraviolet solar radiation using global solar radiation values over a horizontal surface, the method of Foyo-Moreno et al. (Foyo-Moreno, I., J. Vida and L. Alados-Arboleda. J. Appl. Meteor. 1998, 38, 1020–1026) has been used. The results obtained with this method are satisfactory with a determination ratio r²=0.98, RMSE=9.3%, MBE=0.5% for Valencia and r²=0.99, RMSE=6.2%, MBE=0.3% for Córdoba.     

Solar radiation transposition models applied to a plane tracking the sun

Many solar radiation transposition models, from a horizontal plane to any fixed inclined surface, have been compared with measurements from different sites. The tests here were carried out for four top-ranked models and with measurements taken on a plane tracking the sun. The best precision obtained in this study is ±7% on the global solar radiation and ±20% on the diffuse solar radiation with a zero bias.     

Determination of atmospheric turbidity in Bangladesh

Atmospheric turbidity parameters have been determined following Angstrom's method at three locations in Bangladesh namely, Dhaka city (23.7°N) and two rural locations, Haripur (26.03°N) and Sripur (24.11°N). The parameters were obtained from direct solar radiation data for specific spectral regions and for the whole spectrum as measured using a normal incidence pyrheliometer provided with cut-off glass filters. A considerable variation of Angstrom's turbidity parameters for Dhaka over the year was observed with a maximum value in March. The value for Sripur for the month of March was somewhat lower and the value for Haripur for the month of April was not much different from that of Dhaka. Linke turbidity factor T_L was also determined using the new value of solar constant 1367 W/m² for all three locations.     

Effect of inclination angle on the light transmittance of condensate covered homogeneous thin transparent materials

The inclination angle of a transparent material exposed to solar radiation strongly varies depending on the application. It is well known that the inclination angle can highly influence the shape of condensation drops—which are sometimes unavoidable in solar thermal applications—via its effect on the receding and advancing contact angles of the drops. By means of a laboratory set-up, the effect of condensate on the transmittance of 25° and 90° (i.e., vertical) inclined single glass, polyethylene and anti-drop condensation polyethylene was measured for incidence angles up to 45°. No significant impact of the inclination angle on the transmittance of the three transparent materials in the wet state was observed. This means that transmittance results obtained on wet materials can be applied irrespective of the inclination angle of the material.     

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

An empirical diagnostic method for the evaluation of solar cell grade CuInS₂ absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS₂ absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS₂ absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS₂ absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS₂ surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS₂ absorbers ranges between 53° and 63°. For low conversion efficiency solar cells (<6%), it is between 48° and 50°. Therefore, it is seen that contact angle measurements on CuInS₂ absorbers can be used to assess the quality of CuInS₂ absorbers prior to solar cell fabrication.     

Performance analysis of new-design solar air collectors for drying applications

This paper presents a performance analysis of four types of air heating flat plate solar collectors: a finned collector with an angle of 75°, a finned collector with an angle of 70°, a collector with tubes, and a base collector. In this study, the first and second laws of efficiencies were determined for the collectors and comparisons were made among them. The results showed that the efficiency depends on the solar radiation and the construction of the solar air collectors. The temperature rise varied almost linearly with the incident radiation. The first law of efficiency changed between 26% and 80% for collector-I, between 26% and 42% for collector-II, between 70% and 60% for collector-III, and between 26% and 64% for collector-IV. The values of second law efficiency varied from 0.27 to 0.64 for all collectors? The highest collector efficiency and air temperature rise were achieved by the finned collector with angle of 75°, whereas the lowest values were obtained for the base collector. The effectiveness order of the collectors was determined as the finned collector with angle of 75°, the finned collector with angle of 70°, the collector with tubes, and the base collector.     

Investigations of solar radiation detectors using a laboratory test facility for solar radiation meteorological instruments

A laboratory test facility for solar radiation detectors has been built and is in operation at the Aerological Station of the Swiss Meteorological Institute (SAP/SMI). This installation is conceived as a universal test bed for solar radiation exposed meteorological instruments, and consists of a commercially available solar simulator, a laser alignment system, a translation mechanism with instrument mounts, and an adjustable projection mirror. The solar simulator produces a well characterized radiation beam which can be filtered to match the terrestrial or outer space solar spectrum with an irradiance of up to one solar constant (1367 Wm⁻²). The instrument mounts and a HeNe laser beam provide a precise and easy alignment of the reference and the test instruments in the radiation beam, allowing for incident angles in the range of 15°–75°. The measurement is based on a comparison of the response of an active cavity absolute radiometer PMO6 with the signal of the test instrument.Detailed investigations of the Haenni Solar 111B type heliometer have revealed important irregularities in the sunshine threshold irradiance angular distribution. Measurements performed with and without the protection glass cover prove the exceedingly high threshold values at large declination angles to be a consequence of enhanced reflections due to the incident angle and inhomogeneities in the glass cover.Very satisfactory results have also been obtained on characterization measurements of pyranometers showing the mean values of the responsivity to be within 0.8% of the calibration values measured at the world radiation center (WRC) at Davos.     

Simultaneous optical losses and current measurements in photovoltaic devices at variable angle of the incident light

In this work, the apparatus and the method for a simultaneous measurement of the optical losses and short-circuit current in a solar cell, at variable incidence angle of the light, is presented. The method has been applied to an n⁺/p c-Si cell with a polished surface. The investigation has been performed over an angular range from 8° to 80°, using a linearly polarized laser beam, either normally or parallelly polarized with respect to the incidence plane. The experimental curves of reflectance seem to be in a good agreement with the theoretical ones derived from the Fresnel equations. Since the measurements are performed inside an integrating sphere, a procedure has been developed to derive, from the total current I^tot, the calculated direct one, I_dirCal excluding contributions from the incoming light back-diffused to the cell under investigation; the results are compared with real direct-current measurement. Since with the proposed method both reflectance and current are measured for the same surface region and under identical illumination conditions, the results have been combined to get the internal spectral response of the n⁺/p c-Si polished surface solar cell.     

Performance of a non-metallic unglazed solar water heater with integrated storage system

The performance of a new design of non-metallic unglazed solar water heater integrated with a storage system has been studied. In this system, the collector and storage were installed in one unit. All parts of the system have been fabricated from fiberglass reinforced polyester (GFRP) using a special resin composition that provides good thermal conductivity and absorptivity. The storage tank has a capacity of 329 l. The design of the storage system was sandwich construction, with the core material made out of polyurethane foam, which combines stiffness and lightness of structure with very good thermal insulation. The width and length of the absorber plat were 1.4 and 1.8 m, respectively. The performance of the system has been investigated by two methods. In the first method, the storage tank was filled up with water the night before the test. The tank was then drained during the night, refilled and made ready for the next day’s test. The tests were repeated under varied environmental conditions for several days. The maximum water temperature in the storage tank of 63 °C has been achieved for a clear day operation at an average solar radiation level of 700 W m⁻² and ambient temperature of 30 °C. The decrease of water temperature with and without the thermal diode is 10 and 20 °C, respectively. In the second method, the testing was of the same way, but in this case without draw-off or draining of the hot water from the storage tank. All data readings were recorded from sunrise to sunset over the same period. The temperature was recorded for several days and ranges of 60–63 °C were obtained in the storage tank. A system efficiency of 45% was achieved at an average solar radiation level of 635 W m⁻² and ambient temperature of 31 °C.     

Evaluation of 12 models to estimate hourly diffuse irradiation on inclined surfaces

This study evaluates the performance of 12 models to estimate hourly diffuse solar irradiation on inclined surfaces from those measured on horizontal surfaces. Total solar irradiation incident on a tilted surface consists of three components including: beam, diffuse and reflected from the ground. On a semi-hourly basis, the beam component can be calculated by the ratio of the incidence angle to the solar zenith angle. The reflected component has a small effect on calculations and may be calculated with an isotropic model. In contrast, models for estimating the diffuse component show major differences, which justify the validation study that this paper discusses. Twelve models were tested against recorded south- and west-facing slope irradiances at Karaj (35°55^′N; 50°56^′E), Iran. The following models were included: Badescu [Ba], Tian et al. [Ti], Perez et al. [P9], Reindl et al. [Re], Koronakis [Kr], Perez et al. [P8], Skartveit and Olseth [SO], Steven and Unsworth [SU], Hay [Ha], Klucher [Kl], Temps and Coulson [TC], and Liu and Jordan [LJ].The relative root mean square error (RMSE), for the south-facing surface ranges from 10.16% to 54.89% for the SO and TC models, respectively. For the west-facing surface, RMSE ranges from 30.71% for the P9 model to 63.53% for the TC model. Statistical indices show that all models produce large errors for the west-facing surface. Statistical indices for the south-facing surface show reasonably good agreement with measured data.     

Determining optimum tilt angles and orientations of photovoltaic panels in Sanliurfa, Turkey

The performance of a photovoltaic (PV) panel is affected by its orientation and its tilt angle with the horizontal plane. This is because both of these parameters change the amount of solar energy received by the surface of the PV panel. A mathematical model was used to estimate the total solar radiation on the tilted PV surface, and to determine optimum tilt angles for a PV panel installed in Sanliurfa, Turkey. The optimum tilt angles were determined by searching for the values of angles for which the total radiation on the PV surface was maximum for the period studied. The study also investigated the effect of two-axis solar tracking on energy gain compared to a fixed PV panel. This study determined that the monthly optimum tilt angle for a PV panel changes throughout the year with its minimum value as 13° in June and maximum value as 61° in December. The results showed that the gains in the amount of solar radiation throughout the year received by the PV panel mounted at monthly optimum tilt angles with respect to seasonal optimum angles and tilt angel equal to latitude were 1.1% and 3.9%, respectively. Furthermore, daily average of 29.3% gain in total solar radiation results in an daily average of 34.6% gain in generated power with two-axis solar tracking compared to a south facing PV panel fixed at 14° tilt angle on a particular day in July in Sanliurfa, Turkey.     

Yearly distributed insolation model and optimum design of a two dimensional compound parabolic concentrator

Optimum acceptance angle of a compound parabolic concentrator (CPC) is studied by the use of an insolation model proposed in this paper. The insolation consists of two components; diffuse and direct. The direct radiation is supposed to be distributed in the field within ±23.5° of declination on the celestial hemisphere and the diffuse radiation is assumed to have uniform irradiance. This yearly insolation model suggests that the optimum half-acceptance angle at the two-dimensional CPC becomes 26° irrespective of the change of the diffuse radiation fraction. This result leads us to the conclusion that, almost all over the world, a common CPC could be used as an optimum concentration for many solar radiation collecting systems.     

Technical note On the development of spatialtemporal solar radiation maps: a Brazilian case study

This work summarizes recently published information on the solar resource of Brazil. We describe the spatial distribution of solar radiation and its relationship with climatic and geographical conditions. In order to harmonize the information in terms of type of instruments, time recording period and data processing methods, a careful selection of records from the data base was made. Density of recording stations is reasonable in the south, southeast and northeast regions of the country, while in the west center and north regions the density of stations is rather poor. The procedure to elaborate the maps of daily solar radiation, monthly and annual average is described. Consideration of the measuring period of the monthly averages, used to elaborate the contour maps, shows that they meet the requirement that 90% of averages are inside the strip of ±7.5%, centralized on the average of very long period measurements. We present one map with the localization of the recording stations and one annual and 12 monthly contour maps, describing daily solar radiation levels over the whole territory. Spacing among the contour lines is (±2 MJm² day). Annual average of solar radiation lies within the interval of ((18±2) MJm² day), except in the northeast region where values higher than (20 MJm² day) are found. Two regions with levels of (16 MJm² day) are also observed. The highest monthly average values (24 MJm² day) are observed in the state of Rio Grande do Sul, southern end of the country, in the summer season (December and January). The lowest values in the country (8 MJm² day) are observed in June and July (winter in the southern hemisphere), on the extreme south coastline of the same state, Rio Grande do Sul, below 32° south latitude.