Monthly average daily insolation on tilted collectors in Libya

This work presents monthly average total insolation values calculated for various locations in Libya. The insolation values are for actual sky conditions based on sunshine records. Latitudes ranging from 24 to 32°N and tilt angles from 0 to 90° are considered. The presented insolation values show that the optimum value of tilt angle for space heating systems is about latitude plus 15° while that for cooling systems is 0° A combination of horizontal roof and vertical south wall gives more or less uniform insolation throughout the whole year. The maximum annual insolation occurs for tilt angle nearly equal to the latitude.     

Estimating insolation based on PV-module currents in a cluster of stand-alone solar systems: Introduction of a new method

In order to evaluate the performance of solar home systems (SHS), data on local insolation is a prerequisite. We present the outline of a new method to estimate insolation if direct measurements are unavailable. This method comprises estimation of daily irradiation by correlating photovoltaic (PV)-module currents from a number of solar home systems, located a few kilometres apart. The objective is to obtain reliable daily and monthly insolation figures that are representative for an area of a few square kilometres.     

Estimation of global radiation using clearness index model for sizing photovoltaic system

A methodology for developing a simple theoretical model for calculating global insolation on a horizontal surface is described in this paper. The input parameters to the model are the latitude of the desired location and the amount of total precipitable water content in the vertical column at that location. Both the parameters are easily measurable with inexpensive instrument such as global positioning system (GPS). The principal idea behind the paper is to have a model that could be used for designing a photovoltaic system quickly and within reasonable accuracy. The model in this paper has been developed using measured data from 12 locations in India covering length and breadth of the country over a period of 9–22 years. The model is validated by calculating theoretical global insolation for five locations, one in north (New Delhi), one in south (Thiruvanandapuram), one in east (Kolkata), one in west (Mumbai) and one in central (Nagpur) part of India and comparing them with the measured insolation values for these five locations. The measured values of all these locations had been considered for developing the model. The model is further validated for a location (Goa) whose measured data is not considered for developing the model, by comparing the calculated and measured values of the insolation. Over the range of latitudes covering most parts of India, the error is within 20% of the measured value. This gives the credibility of the model and the methodology used for developing the model for any region in the world.     

A brief review of models for computing solar radiation on inclined surfaces

Modelling of solar energy systems requires estimation of the hourly radiation incident on surfaces of different tilts and orientations. Most meteorological stations report radiation values on a daily rather than hourly basis. Again, the value of the diffuse component may or may not be reported. Hence, different strategies may be encountered, for each location, with the common goal of computing hourly radiation on inclined surfaces.

In this study, five different schemes are presented to achieve this goal. In each scheme, certain correlations are required which are discussed in detail. The study is divided into five sections, each dealing with a specific type of correlation. These sections are: (i) correlations between daily diffuse and global radiation; (ii) correlations between hourly/daily global radiation; (iii) correlations between hourly/daily diffuse radiation; (iv) correlations between hourly diffuse and global radiation; and (v) models for computing diffuse sky radiation on inclined surfaces. The last section deals extensively with the anisotropic nature of sky diffuse radiation.

The important aspects of all correlation studies are highlighted, and the relative merits and demerits of their results are brought to light.

Mathematical expressions, where available, for models/correlations are provided so that the reader will have access to a comprehensive study. This information should be useful for modelling purposes in which computation of radiation on surfaces of different orientations and tilts is required.     

Optimum tilt of solar collector for maximum natural flow

A theoretical treatment to determine the natural flow rate through a flat plate collector having heat losses is made. The governing equations have been expressed in terms of Grashof and Prandtl numbers and a dimensionless heat loss parameter. The method predicts the optimum tilt of the flat plate collector for the maximum draught for a given latitude and insolation. The experimental verification of the theoretical results shows a fair agreement.

A method to calibrate the hot wire anemometer at the desired temperature is evolved for the range of velocities encountered in practice for the flat plate solar collector.     

Design guidelines for appropriate insolation of urban squares

A systematic study of building design around urban squares, while keeping the solar rights, is presented. The assessment was carried out both visually and quantitatively using the model SHADING (Solar Energy. 1994; 52-1, 27-37). The quantitative evaluation is general and allows a fast and efficient evaluation of the Geometrical Insolated Coefficient (GIC), which is the ratio between the insolated area and the total examined area. The calculation of the GIC was carried out year around for different proportions of squares and buildings heights around them. By calculating the Average GIC for Winter for each examined square, recommendations and design guidelines are derived and presented.     

15% Efficiency CdS/CdTe thin film solar cells using CdS layers doped with metal organic compounds

For improving the photovoltaic performance of CdS/CdTe thin film solar cells, the CdS window layer is one of the most crucial factors. Here we demonstrate the photovoltaic performances of the low-environmental-load CdS/CdTe solar cell employing the CdS layer doped with various metal organic (MO) compounds, i.e., (CH₃)₂SnCl₂, (C₆H₅)₃GeCl, (CH₃CO₂)₃In, [(C₂H₅)₂NCS₂]₂Zn. Due to the MO doping, the degree of (1 1 1) preferential orientation of CdTe on the CdS layer is improved remarkably, influencing the increases in V_oc and F.F. Being almost independent of the kind of the MO compounds, the short circuit current increases due to increasing optical transmittance of the MO-doped CdS layers. As a result, utilizing MO-doped CdS, we have achieved the conversion efficiency of 15.1%.     

Hydrogen production characteristics of a bioethanol solar reforming system with solar insolation fluctuations

The development of a bioethanol steam reforming system (FBSR) is considered as a means of distributing energy using PEM fuel cells. Small-scale solar collectors (collection areas on the order of several m²) are installed in a house as a method for applying the FBSR. However, the temperature distribution of a reforming catalyst fluctuates under conditions of unstable solar insolation. Therefore, heat transfer analysis applied in reforming the catalyst layer of the reactor and the temperature distribution and transient response characteristics of the gas composition of the process were investigated. As a case study, meteorological data for representative days in March and August in Sapporo, Japan were recorded, and the hydrogen production speed, power generation output and amount of electricity purchased were analyzed. The results showed that although fluctuations in solar insolation affected the efficiency of the FBSR, the average efficiency of each representative day exceeded 40%. By installing two solar collectors, each with a collection area of 1 m², 21–25% of the average power demand of an individual house can be supplied.     

Performance assessment of a solar still using blackened surface and thermocol insulation

Fluoride contaminated drinking water is a severe problem in many parts of the world because of fluoride-related health hazards, which are considered to be a major environmental problem today. The present work is aimed at utilizing solar energy for removal of fluoride from drinking water by using a “solar still”. Also tests have been conducted with the “solar still” to find out hourly output rate and “still efficiencies” with various test matrixes. It is observed that the distillate from “solar still” showed a fluoride reduction of 92–96%. Further, the efficiency of “solar still” got increased by 11% when capacity of water in the solar basin was raised from 10 to 20 L. Upon suitable modification of the solar basin with appropriate base liner and insulation, this efficiency of the “solar still” is found to be further increased by 6% with a 20 L basin capacity.     

CdS/CdTe solar cell using sputtering technique

Cadmium telluride films were deposited from a CdTe target using rf planar magnetron system on CdS/ITO/Glass substrates. The as grown films were polycrystalline and consisted of closepacked array of preferentially oriented single crystal grains. Chemical heat treatment using cadmium chloride was used to form the junction between cadmium sulphide and cadmium telluride layers. It also improves the structure of the sputtered CdTe film and the lifetime of carriers.     

Experimental investigation on a thermoelectric refrigerator driven by solar cells

Experimental investigation and relevant analysis on a solar cell driven, thermoelectric refrigerator has been conducted. To make the device portable, daytime use and nighttime use of the refrigerator are treated in different ways. Solar cells are applied to power the refrigerator in the day. Storage battery, assisted by an a.c. rectifier, is used to provide electric energy in the night and in cloudy or rainy days. Experiment results demonstrate that the unit can maintain the temperature in the refrigerator at 5–10 °C, and have a COP about 0.3. It is expected that the refrigerator would be potential for cold storage of vaccine, foodstuffs and drink in remote area, or outdoor applications where electric power supply is absent.     

Re-examination of ASHRAE insolation models with reference to Indian locations

Recently, the Indian Meteorological Department has made available measured horizontal global and diffuse radiation data for cloudless days for many Indian locations. A study is undertaken to re-examine the normal incident and diffuse radiation models suggested by ASHRAE using data for seven locations. Both models were found to be valid, but the diffuse radiation model needs refinement. The computed values of A, B and C were found to be different from those of ASHRAE. Comparison of computed insolation using these values of A, B and C with measured data shows good agreement. Calculation performed with ASHRAE values shows disagreement with measured data by some amount for global and by significant amounts for diffuse radiation.     

A study on the intercepted insolation as a function of slope and azimuth of the surface

Monthly average daily global radiation on surfaces tilted towards the equator and also inclined at various azimuth angels are estimated for three locations in the Southern African region from reported global and diffuse radiation on a horizontal surface. The anisotropic model suggested by Hay is used in obtaining daily radiation at various slopes and orientations. Total annual radiation data are also computed for various tilt and azimuth angles and optimum tilt and azimuth angels corresponding to maximum insolation are obtained for winter, summer and annual collection. The optimum tilt and orientation reported here show some variation from those reported earlier by other investigators and the results are discussed.     

Experimental investigation and analysis on a thermoelectric refrigerator driven by solar cells

Experimental investigation and performance analysis on a solar cell driven, thermoelectric refrigerator has been conducted. Research interest focused on testing the system performance under sunshine. Experiment results demonstrated that the unit could maintain the temperature in the refrigerator at 5–10°C, and have a COP about 0.3. Further analysis indicated that the performance of the system is strongly dependent on intensity of solar insolation and temperature difference of hot and cold sides for the thermoelectric module, etc. There exist optimum solar insolation rates, which let the cooling production and COP achieve maximum value, respectively. It was expected that the refrigerator would be potential for cold storage of vaccine, food and drink in remote area, or outdoor conditions where electric power supply is absent.     

Measurement of solar cell parameters using impedance spectroscopy

Measurement of solar cell parameters is important for the design of satellite power systems. These parameters can be measured using impedance spectroscopy and an equivalent circuit model developed. In this study parameters of a Back Surface Reflector Field solar cell (BSFR) have been measured using impedance spectroscopy. The results show high diffusion capacitance of BSFR cells and their exponential relation to the operating voltage. Cell dynamic resistance, diode factor, transition capacitance, and series resistance could also be measured. The minority carrier life time also has been calculated.     

Rectification in CdS/TCO bilayers

The origin of process-induced rectification in CdS/ITO and CdS/SnO₂ bilayers has been investigated. Both pre-treatment of the transparent conducting oxide (TCO) substrates and post-growth treatment of the bilayers were explored for both oxidising and reducing conditions. In/CdS/TCO structures were used for I-V testing, and the CdS layers were verified as being pinhole-free using a test employing a rectifying Au/CdS contact. Whilst neither pre-oxidation nor reduction of any TCO substrate failed to induce rectification in CdS/TCO, oxidation of CdS always induced rectification, regardless of the substrate type. This was attributed to oxidation of CdS (confirmed by Auger electron spectroscopy), and the results are consistent with a band diagram postulated for the CdO/CdS/ITO structure. Recommendations are made for the fabrication of CdTe/CdS/TCO solar cells.     

Influence of CdS growth process on structural and photovoltaic properties of CdTe/CdS solar cells

The morphology of CdS layers grown by chemical bath deposition (CBD) and high vacuum evaporation (HVE) have been investigated. The grains of CBD-CdS are more compact and smooth than those of HVE-CdS. The annealing and CdCl₂ treatment cause grain growth, which is stronger for the CdCl₂ treated samples. The grain-size of the as-deposited CdTe on CBD-CdS is about 5 times larger than of those grown on HVE-CdS. The structural and electrical properties of CdTe/CdS solar cells are strongly dependent on the CdS. The grain size of CdCl₂ treated CdTe layers are similar, irrespective of the transparent conducting oxide substrate and CdS deposition method. The efficiency of solar cells on thin CBD-CdS is low (about 5.6%) because of pin-holes and a large intermixing of CdTe-CdS. The cells on HVE-CdS yield a higher current density despite thicker HVE-CdS as compared to CBD-CdS. The efficiency of solar cells on HVE-CdS is 12.3%.     

EFFECTS OF PROCESSING ON CdTe/CdS MATERIALS AND DEVICES

By analyzing CdTe/CdS devices fabricated by vacuum evaporation, a self consistent picture of the effects of processing on the evolution of CdTe cells is developed which can be applied to other fabrication methods. In fabricating CdTe/CdS solar cells by evaporation, a 400°C CdCI₂ heat treatment is used which recrystallizes the CdTe and interdiffuses the CdS and CdTe layers. The interdiffuson can change the bandgap of both the CdTe and CdS which modifies the spectral response of the solar cell. After this heat treatment a contacting/doping procedure is used which converts the CdTe conductivity to p-type by diffusion from Cu from the contact. Finally, the cell is treated with Br₂CH₃,OH which improves both Voc and FF. Analogous process steps are used in most fabrication processes for CdTe/CdS solar cells.     

Accelerated degradation in amorphous silicon solar cells by a combination of current injection and light insolation

A new degradation technique for amorphous silicon solar cells comprising of a combination of current injection and insolation has been developed. Compared to the conventional light degradation technique, current-induced degradation which involves forward biased current stress, results in a lower stabilized cell parameters including efficiency. This stabilized efficiency is, however, independent of the amount of current injected. The excess degradation is recovered under light illumination. The stabilized value of the conversion efficiency is within experimental error identical to that observed in long term, illumination only degradation. The method is fast, accurate and reliable. Its reliability has also been tested for single junction solar cells having intrinsic layers of different band gaps, as well.     

Characterization of CuInSe2/CdS thin-film solar cells prepared using CBD

CuInSe₂/CdS thin-film heterojunction solar cells were fabricated entirely by chemical bath deposition technique. The illuminated J−V characteristics of the devices prepared with different thicknesses of CdS and CuInSe₂ were studied. The typical solar cell parameters obtained for the best cell are: V_oc = 365 mV, J_sc = 12 mA/cm², FF = 61%, and η = 3.1% under an illumination of 85 mW/cm² on a cell of active area 0.1 cm². The J−V and C−V characteristics under dark condition and the spectral response were also studied for the best cell. The diode quality factor obtained is 1.7.