Solar radiation absorption in solar ponds

The local rate of absorption of the solar radiation in a solar pond is determined for the direct component at angles of incidence from 0° to 75° with 15° intervals as well as for the diffuse component by the exact treatment of the radiation problem. The effects of bottom reflection, the pond depth, the type of radiation on the thermal performance of the pond are examined, and a new rigorous approach is presented for treating diffuse radiation as a direct beam. The fraction of the solar radiation absorbed within the first 10 cm of water is determined under various conditions. The local rate of solar energy absorption at any depth and at any incidence angle can readily be computed from a fourth-degree polynomial expression, the coefficients of which are tabulated for different incidence angles and bottom reflectivities.     

Some factors affecting the absorption of solar radiation in solar stills

The performance of a solar still can be affected to a significant degree by the build-up of reflecting layers of salt on the water surface and basin liner. This paper presents a method of quantitatively assessing the effect of these salt layers, taking into account the band absorption characteristics of water. A revised curve of the absorption of the solar spectrum in water is given.     

The coefficient of performance for a solar absorption dual cycle

The traditional definition of the coefficient of performance (COP) can create misleading results because this definition does not define the heat input base to be used for estimating the COP. Furthermore, such one-dimensional definitions do not consider other important factors which may affect the system selection.

A new selection criterion for a solar powered dual absorption system will be presented in this paper. The dual cycle, which has the advantage of not using a cooling tower, has often been ignored because of the misleading definition of the COP.

The present paper also describes a lithium bromide dual absorption cycle.     

Photocatalysis and radiation absorption in a solar plant

Recently, many papers have appeared in literature about photocatalytic detoxification. However, progress from laboratory data to the industrial solar reactor is not easy. Kinetic models for heterogeneous catalysis can be used to describe the photocatalytic processes, but luminic steps, related to the radiation, have to be added to the physical and chemical steps considered in heterogeneous catalysis. Thus, the evaluation of the radiation, and its distribution, inside a photocatalytic reactor is essential to extrapolate results from laboratory to outdoor experiments and to compare the efficiency of different installations. This study attempts to validate the experimental set up and theoretical data treatment for this purpose in a Solar Pilot Plant. The procedure consists of the calibration of different sunlight radiometers, the estimation of the radiation inside the reactor, and the validation of the results by actinometric experiments. Finally, a comparison between kinetic constants, for the same reaction in the laboratory (artificial light) and field conditions (sun light), is performed to demonstrate the advantages of knowing the radiation inside a large photochemical reactor.     

Effective absorption coefficient for graded band-gap semiconductors and the expected photocurrent density in solar cells

A simple model for the generation of carriers by photons incident on a (linearly) decreasing band-gap material, such as has been described in recent CIGS solar cells, is developed. The model can be generalized for different cases such as increasing band-gap grading or for having a more complex band-gap profile.The model developed for direct band semiconductors such as CIGS or AlGaAs allows us to define an effective absorption coefficient, so that the ideal photocurrent density can be calculated in a similar manner as for solar cells with non-graded band-gap materials.We show that this model gives completely different results as those expected from intuitive approaches for calculating this ideal photocurrent density. We also show that grading of the band-gap of the absorbing material in solar cells makes the photocurrent less sensitive to the total band-gap change, in such a way that the design of the band-gap variation can be more flexible in order to have other advantages such as higher built-in voltage or higher back surface field in the device structure.     

Broadband coefficient of absorption of solar radiation by water layer in tray water heating collectors with exposed evaporation surfaces

The broadband coefficient of the absorption of solar radiation by the water layer in tray solar water heating collectors with exposed evaporation surfaces is determined.     

Enhancement of solar radiation absorption using nanoparticle suspension

The radiation absorption characteristics of a Ni nanoparticle suspension were investigated by spectroscopic transmission measurement. It was demonstrated that the absorption coefficient of the nanoparticle suspension is much higher than that of the base liquid for visible to near-infrared wavelengths. Radiation characteristics predicted by the Mie theory showed good agreement with the increase of absorption coefficient in wavelengths where the base liquid is transparent. It was also confirmed that a new transmittance measurement technique for a liquid sample using a liquid cell with no spacer was quite useful for evaluating a material possessing an extremely strong absorption band. The proposed measurement method and successive Kramers-Kronig analysis were validated by measuring the optical properties of water. The measurement and prediction process of the thermal radiation properties of nanoparticle suspensions developed here could be used in developing direct absorption solar collectors.     

Study of solid-gas-suspensions used for direct absorption of concentrated solar radiation

Suspensions of solid particles of the appropriate diameter in gases have interesting selective absorption properties, as they absorb solar radiation, whereas the emmissivity in the IR range is low. For the numerical evaluation of the important factors governing this absorption, the absorption cross section obtained by Mie Theory was substituted in Beer-Lambert relation for different conditions. For a given concentration and optical depth, the absorption was found to be dependent on the imaginary part, n₂, of the complex index of refraction, n = n₁ − in₂, of the material used. It is also dependent on the size of the particle d (represented by the diameter in case of spherical particles). For solar spectrum (0,2–3 μ) and spherical particles, the recommended values are n₂ 0,6 and d = 0.5 μ. These values may be satisfied by spherical particles of graphite.     

Design and analysis of Salisbury screens and Jaumann absorbers for solar radiation absorption

Two types of resonance absorbers, i.e., Salisbury screens and Jaumann absorbers are systematically investigated in solar radiation absorption. Salisbury screen is a metal-dielectric-metal structure which overcomes the drawback of bulky thickness for solar spectrum. Such structures have a good spectral selective absorption property, which is also insensitive to incident angles and polarizations. To further broaden absorption bandwidth, more metal and dielectric films are taken in the structure to form Jaumann absorbers. To design optimized structural parameters, the admittance matching equations have been derived in this paper to give good initial structures, which are valuable for the following optimization. Moreover, the analysis of admittance loci has been conducted to directly show the effect of each layer on the spectral absorptivity, and then the effect of thin films is well understood. Since the fabrication of these layered absorbers is much easier than that of other nanostructured absorbers, Salisbury screen and Jaumann absorbers have a great potential in large-area applications.     

The influence of radiation absorption on solar pond stability

This paper presents a study of the gravitational stability of a salty layer of a fluid subject to an adverse temperature gradient as a result of heat absorption. This is intended to model solar ponds where an artificial gradient of salt concentration in water is used to prevent convective motions induced by the absorption of solar radiation. The stability of the Boussinesq approximation of the Navier-Stokes equations is analysed for perturbations of a certain kind imposed on the stationary solution. The marginal states for the onset of convection are obtained using a Galerkin method based on a weak formulation of the governing equations. The analysis considers solar energy absorption in the layer and assumes prescribed heat flux values as boundary conditions for the temperature equation. Results are compared with those obtained earlier by different authors for a layer of fluid, heated from below, with linear profiles of both salt concentration and temperature.     

Evaluation and comparison of hourly solar radiation models

In this paper, an attempt has been made to develop a new model to evaluate the hourly solar radiation for composite climate of New Delhi. The comparison of new model for hourly solar radiation has been carried out by using various model proposed by others. The root mean square error (RMSE) and mean bias error (MBE) have been used to compare the accuracy of new and others model. The results show that the ASHRAE and new proposed model estimate hourly solar radiation better for composite climate of New Delhi in comparison to other models. Hourly solar radiation estimated by constants obtained by new model (modified ASHRAE model) for composite climate of India is fairly comparable with measured data. The percentage mean bias error with new constants for New Delhi was found as low as 0.15 and 0% for hourly beam and diffuse radiation, respectively. There is a 1.9–8.5% RMSE between observed and predicted values of beam radiation using new constants for clear days. The statistical analysis has been used for the present study. Copyright © 2008 John Wiley & Sons, Ltd.     

Transmission coefficient of the transparent insulation of flat diffuse solar radiation heating systems

A method is proposed for calculating the transmission coefficient of the transparent insulation of the diffuse solar radiation heating systems in terms of the equivalent value of the direct solar radiation ray incidence angle.     

Effect of radiation absorption in saline water on the performance of solar ponds

The effect of the solar radiation absorption characteristics of saline water on the performance of salt-gradient solar ponds under quasi-steady conditions is studied. Several models have been proposed to simulate the absorption of radiation in solar ponds, however, it is shown in this work that experimental data published by many authors militate in favour of representing this absorption simply by a single exponential. Pond performance is strongly influenced by the absorption characteristics. The mean pond temperature of the storage zone and the pond efficiency drop considerably as the turbidity of the water increases. The high values of the extinction coefficient result also in damping the pond temperature fluctuations and increase its time lag with respect to the insolation. The importance of using accurate in-site measured absorption data for pond design and performance prediction is emphasized.     

Evaluation of solar radiation at just in Northern Jordan

Monthly averaged total daily global and direct incident solar radiation are presented. Maximum, averaged, and the minimum values of both global and direct incident radiation are given over the measuring period 1990–1996. The global and direct incident radiation at noon hour are also presented. A mathematical model for maximum total daily GR is given as a function of the number of the month of the year. Two other mathematical regressions were also obtained for the monthly averaged total daily global radiation and for monthly averaged total daily direct incident solar radiation.     

Evaluation and establishment of diffuse solar radiation models for Bursa,Turkey

The main objective of the present study is to evaluate the applicability of the diffuse solar radiation models from previous studies and establish new models for Bursa. Therefore, 35 regression models from previous studies in the literature are used and categorized as follows: (1) cloudness index is the function of clearness index; (2) cloudness index is the function of relative sunshine duration; (3) the diffuse coefficient is the function of the clearness index; and (4) the diffuse coefficient is the function of the relative sunshine duration. Also, new four diffuse solar radiation models were developed using 1968–2015 long-term global solar radiation and sunshine duration data. The new models are then compared with 35 empirical equations available in previous studies in terms of different statistical tests. Consequently, Models 6 and 8 are found as the most accurate and high-performance empirical models for estimation of the monthly mean diffuse solar radiation on a horizontal surface for Bursa, Turkey.     

Impact of coefficient of attenuation of solar radiation on thermal losses in translucent covers

The technique for calculating the impact of the attenuation coefficient of the passing total solar radiation of the translucent cover material and the intensity of the total (direct and diffuse) solar radiation incident on their frontal surface on their thermal losses is proposed. The multiple inner reflection of the radiation between the inner (x = δ) and outer (x = 0) interfaces and the mutual opposition of the directions of the solar radiation fluxes reflected at these interfaces, as well as the exponential distribution of the power of absorbed radiation over the thickness of the wall of the considered translucent covers are taken into account. The calculations performed based on the practical implementation of this technique are exemplified.     

Influence of radiation attenuation coefficient of light transparent coating and radiation reflection coefficient of radiation absorbing surface of heat exchange panel on radiative heat exchange coefficient in the “light transparent coating/radiation absorbing heat exchange panel” system of flat solar collectors

An analytic expression is proposed for determining the coefficient of radiative heat exchange between the inner surface of the heat exchange panel as a function of the radiation attenuation coefficient of the light transparent coating and the radiation reflection coefficient of the surface of the radiation absorbing surface of the heat exchange panel of flat solar collectors.     

Thermodynamic analysis of the performance of a solar absorption heat transformer at maximum coefficient of performance

It is proven that a solar absorption heat transformer affected by the irreversibility of finite-rate heat transfer may be modelled as an equivalent combined system consisting of a solar collector and an endoreversible absorption heat transformer, the latter being further treated as a combined cycle having an endoreversible heat pump driven by an endoreversible heat engine. The maximum coefficient of performance of the system is determined, based on the linear heat loss model for solar collectors and the general optimum relation for endoreversible absorption heat transformers. The optimality problems concerning the primary performance parameters of the system are discussed. The results obtained here may serve as a good guide for the evaluation of existing real solar absorption heat transformers or provide some theoretical bases for the optimal design of future solar absorption heat transformers. © 1997 by John Wiley & Sons, Ltd.     

On the use of the coefficient of variation to measure spatial and temporal correlation of global solar radiation

In this work we perform a statistical analysis of global solar radiation measured at two sites, Petit Canal and Pointe a Pitre, 38 km distant from, Guadeloupe, French West Indies. We have established a correlation model based on the coefficient of variation assuming a time scale separation. The coefficient of variation is calculated on 10 min interval with data measured at 1 Hz. This analysis highlights the dynamic correlation that can occur between measurements from two different sites with a time step of 1 s. From these results, knowing the coefficient of variation on a site, we have established a new correlation model on this parameter for another site. A diagram linking the standard deviation for the studied sites, for a given coefficient of variation is proposed for correlated and non-correlated cases. Moreover this analysis evidences the existence of a threshold time under which there is no significant correlation. The methodology and the model can be applied to any other sites to establish diagrams of the coefficient of variation.This model can be useful in choosing new sites of PV production in establishing correlation from on site to another.