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02740 Thermohydraulic performance of air heating solar collectors with variable width, flat absorber plates
07
Alternative
energy
sources
(solar energy)
as compared to the positive photovoltages produced by the darker films. Spectral response measurements revealed that most of the light darker films produced positive photovoltages in the shorter wavelengths and negative photovoltages in the longer wavelengths. However, some of the light darker films produced only the negative photovoltage for the entire spectral range and their photovoltaic properties were comparatively better. Darker films resulted in only the positive photovoltages in the entire spectral range. As a result of the sulphidation, the bulk crystal structure of the cuprous oxide thin films was not changed, however, the interfacial characteristics of the solar cell structure were modified. 00/02736 The effect of NaF on Cu(ln, Ga)Sel thin film solar cells Granath, K. Solar Energy Mareriah & Solor Cells, 2000, 60, (3), 279-293. This work investigates NaF, on molybdenum coated sodium barrier glass, as a sodium precursor for the growth of Cu(In, Ga)Sez for thin film solar cells. These precursor layers are investigated by X-ray photoelectron spectroscopy (XPS) before and after annealing and after exposure to selenium. XPS is also performed on the Cu(In, Ga)Sez layer, deposited with NaF. The influence of the NaF on the absorber growth is studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrical properties are investigated by cell fabrication and characterization. Cell results are comparable when NaF or out-diffusion of sodium from the soda lime glass are used. 00/02737 The influence of porous silicon coating on silicon solar cells with different emitter thicknesses Vitanov, P. S&r Energy Materials & Solar Cells, 2000, 61, (3). 2 13-22 1. The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n+p silicon junction whose emitter depth was varied between 0.5 and 2.2 pm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n+ emitter. The electrical properties of the samples with PS were improved with decrease of the n+p junction depth. Our results demonstrate short-circuit current values of about 35-37 mA/cm* using n+ region with 0.5 pm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modelling. The spectral response measurements were performed at a wavelength range of 0.4-1.1 pm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400-500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19-20% efficiency by the proposed simple technology. 00/02738 Thermochemical methane reforming using a reactive WOJW redox system Kodama, T. Energy, 2000, 25, (5), 411-425. The methane reforming process combined with metal oxide reduction was evaluated, for the purpose of converting solar high-temperature heat to chemicals below 1273 K. The metal oxide was endothermically reacted with methane, to produce CO, hydrogen and the component metal in the temperature range of 1173-1273 K. Of the metal oxide candidates, WO3 and V,Os were found to be reactive and selective metal oxides for the purposes of methane reforming. The metallic tungsten produced by methane reforming could be used to split water and to generate hydrogen at a lower temperature of 1073 K. To improve the reactivities of W03 for methane reforming and the-subsequent splitting of water, supported tungsten oxides were examined in the temperature range of 1073-1273 K. The reactivities were much improved with the ZrOz-supported W03, giving a methane conversion of 70% and a CO selectivity of 86%. Our findings indicate the possibility that the proposed two-step process using a WO,/W redox system may be a potentially new thermo-chemical path that produces useful energy carriers of processed metal, syngas and methanol for storing and transporting solar energy from the sun belt to remote population centres. 00102739 Thermodynamic analysis of two-component, twophase flow in solar collectors with application to a direct expansion solar-assisted heat pump Aziz, W. er al. Energy (O.r/ord), 1999, 24, (3). 247-259. In the search to replace chlorofluorocarbons in future direct-expansion, solar-assisted heat pump (DX-SAHP) systems, the present work extends the thermodynamic analysis of solar collectors to the multi-component and multi-phase domain to cover newly proposed refrigerant mixtures as potential candidates. A computational methodology is developed to determine the size of a solar collector of a DX-SAHP that uses a binary refrigerant mixture whose thermodynamic and transport properties are predicted from a computer code. The energy equation for the elemental collector tube control volume, incorporating the local thermodynamic and heat transfer characteristics, is integrated to determine the tube length for a given set of inlet and exit thermodynamic states of the refrigerant mixture. Also considered are the effects of various parameters such as the collector mass-flow rate and operating pressure, tube diameter and absorbed solar radiation on the collector tube length, heat transfer coefficient and the local refrigerant temperature in the tube.
304
Fuel and Energy Abstracts
September
2000
00102740 Thermohydraulic performance of air heating solar collectors with variable width, flat absorber plates Hegazy, A. A. Energy Convers. Manage., 2000, 41, (l3), 1361-1378. The study investigates the more general and fundamental case when variable width absorbers are utilized instead of conventional rectangular plates in collecting incident solar radiation. The geometries considered include trapezoidal and concave parabolic absorbers having the same surface area. The effect of absorber shape on collector thermohydraulic performance has been examined for three common models with air flowing over the flat absorber, or under it and on both sides of the absorber. The results are compared with those of rectangular absorber models over a wide range of mass flow rate and solar energy flux. Under the same operating conditions, the comparisons indicate that variable width collectors exhibit performance behaviour similar to those of the constant width corresponding models, but it is accompanied by a marginal decrease in air temperature rise and collection efficiency along with a slight increase in fan power consumption. The comparisons do, thus, support the idea of employing variable width collectors in solar air heating applications, especially those that have design constraints on the investment costs and on the space available to install air collectors. For such applications, trapezoidal shape collectors are highly recommended. 00/02741 Three-layer organic solar cell with high-power conversion efficiency of 3.5% Takahashi, K. So/err Energy Materials & So/or Cells, 2000, 61, (4). 403416. To prepare organic solar cells with practical levels of energy-conversion efficiency, the following strategies were adopted. By using HD as the photosensitizer, which is heterodimer consisting of a weak electrondonating 5, 10, 15, 20-tetra(2, 5-dimethoxyphenyl)porphyrinatozinc and a weak electron-accepting 5, 10, 15striphenyl-20-(3-pyridyl)porphyrin, intramolecular photoinduced electron transfer is promoted resulting in effective charge separation. To create an energetically well-arranged system, the HD was placed between an electronacceptor layer of PV (perylene-3. 4, 9, lotetracarboxyl-bis-benzimidazole) and strong electrondonor layer of MC (3carboxymethyl-5-((3-ethyl-2(3H)-benzothiazolylidine)ethylidene]-2-thioxo~ 4-thiazolidinone), where photoinduced intermolecular electron transfer from HD to PV and rapid injection of electrons from MC to HD suppress back electron transfer in the charge-separated HD. As a result of this, the three-layer solar cell Al/PV/HD/MC/Au showed fairly good photovoltaic properties, short-circuit photocurrent quantum yield of 49.2%, open circuit photovoltage of 0.39 V, fill factor of 0.51 and energy conversion yield of 3.51% when irradiated with 445 nm monochromatic light of 12 1W crn~~’ intensity transmitted through the AI/PV interface. Since the photocurrent hardly decreased with age, the photocurrent observed here is really from energy conversion and not from photocorrosion of the aluminium electrode being occasionally responsible for the photocurrent. 00102742 Transport properties of n-type CuGaSez SchSn, J. H. Solar Energy Materials & Solar Cells, 2000, 61, (4), 417-426. Hall effect, electrical conductivity and charge carrier mobility of n-type CuGaSe* single crystals are studied in the temperature range between 2 and 300 K. The experimental data, analysed within the framework of a simple model of a semiconductor with two types of carriers of one sign, indicate the existence of an impurity band. The values of the gap between the main and impurity band, the concentration of donors and the concentration of the compensating acceptors are calculated. The mobility above 100 K is described by scattering of conduction band electrons at phonons and impurities. The sharp drop at low temperatures is explained by a crossover to Mott-type variable-range hopping in the impurity band. 00/02743 Univariate and multivariate forecasting of hourly solar radiation with artificial intelligence techniques Sfetsos, A. Solar Energy, 2000, 68, (2), 169-178. This paper introduces a new approach for the forecasting of mean hourly global solar radiation received by a horizontal surface. In addition to the traditional linear methods, several artificial-intelligence based t&hniques are studied. These include linear, feed-forward, recurrent Elman and Radial Basis neural networks alongside the adaptive neuro-fuzzy inference scheme. The problem is examined initially for the univariate case and is extended to include additional meteorological parameters in the process of estimating the optimum model. The results indicate that the developed artificial intelligence models predict the solar radiation time series more effectively compared to the conventional procedures based on the clearness index. The forecasting ability of some models can be further enhanced with the use of additional meteorological parameters. 00102744 Use of radial basis functions for estimating monthly mean daily solar radiation Mohandes, M. Solar Energy, 2000, 68, (2). 161-168. The present study utilizes the radial basis functions technique for the estimation of monthly mean daily values of solar radiation falling on horizontal surfaces and compares its performance with that of the multilayer perceptrons network and a classical regression model. In this work, solar radiation data from 41 stations that are spread over the Kingdom of Saudi Arabia is used. The solar radiation data from 31 locations are used for training the neural networks and the data from the remaining 10 locations are used for testing the estimated values. However, the testing data were not used in the modelling or training of the networks
Alternative
energy
sources
(solar energy)
as compared to the positive photovoltages produced by the darker films. Spectral response measurements revealed that most of the light darker films produced positive photovoltages in the shorter wavelengths and negative photovoltages in the longer wavelengths. However, some of the light darker films produced only the negative photovoltage for the entire spectral range and their photovoltaic properties were comparatively better. Darker films resulted in only the positive photovoltages in the entire spectral range. As a result of the sulphidation, the bulk crystal structure of the cuprous oxide thin films was not changed, however, the interfacial characteristics of the solar cell structure were modified. 00/02736 The effect of NaF on Cu(ln, Ga)Sel thin film solar cells Granath, K. Solar Energy Mareriah & Solor Cells, 2000, 60, (3), 279-293. This work investigates NaF, on molybdenum coated sodium barrier glass, as a sodium precursor for the growth of Cu(In, Ga)Sez for thin film solar cells. These precursor layers are investigated by X-ray photoelectron spectroscopy (XPS) before and after annealing and after exposure to selenium. XPS is also performed on the Cu(In, Ga)Sez layer, deposited with NaF. The influence of the NaF on the absorber growth is studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrical properties are investigated by cell fabrication and characterization. Cell results are comparable when NaF or out-diffusion of sodium from the soda lime glass are used. 00/02737 The influence of porous silicon coating on silicon solar cells with different emitter thicknesses Vitanov, P. S&r Energy Materials & Solar Cells, 2000, 61, (3). 2 13-22 1. The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n+p silicon junction whose emitter depth was varied between 0.5 and 2.2 pm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n+ emitter. The electrical properties of the samples with PS were improved with decrease of the n+p junction depth. Our results demonstrate short-circuit current values of about 35-37 mA/cm* using n+ region with 0.5 pm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modelling. The spectral response measurements were performed at a wavelength range of 0.4-1.1 pm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400-500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19-20% efficiency by the proposed simple technology. 00/02738 Thermochemical methane reforming using a reactive WOJW redox system Kodama, T. Energy, 2000, 25, (5), 411-425. The methane reforming process combined with metal oxide reduction was evaluated, for the purpose of converting solar high-temperature heat to chemicals below 1273 K. The metal oxide was endothermically reacted with methane, to produce CO, hydrogen and the component metal in the temperature range of 1173-1273 K. Of the metal oxide candidates, WO3 and V,Os were found to be reactive and selective metal oxides for the purposes of methane reforming. The metallic tungsten produced by methane reforming could be used to split water and to generate hydrogen at a lower temperature of 1073 K. To improve the reactivities of W03 for methane reforming and the-subsequent splitting of water, supported tungsten oxides were examined in the temperature range of 1073-1273 K. The reactivities were much improved with the ZrOz-supported W03, giving a methane conversion of 70% and a CO selectivity of 86%. Our findings indicate the possibility that the proposed two-step process using a WO,/W redox system may be a potentially new thermo-chemical path that produces useful energy carriers of processed metal, syngas and methanol for storing and transporting solar energy from the sun belt to remote population centres. 00102739 Thermodynamic analysis of two-component, twophase flow in solar collectors with application to a direct expansion solar-assisted heat pump Aziz, W. er al. Energy (O.r/ord), 1999, 24, (3). 247-259. In the search to replace chlorofluorocarbons in future direct-expansion, solar-assisted heat pump (DX-SAHP) systems, the present work extends the thermodynamic analysis of solar collectors to the multi-component and multi-phase domain to cover newly proposed refrigerant mixtures as potential candidates. A computational methodology is developed to determine the size of a solar collector of a DX-SAHP that uses a binary refrigerant mixture whose thermodynamic and transport properties are predicted from a computer code. The energy equation for the elemental collector tube control volume, incorporating the local thermodynamic and heat transfer characteristics, is integrated to determine the tube length for a given set of inlet and exit thermodynamic states of the refrigerant mixture. Also considered are the effects of various parameters such as the collector mass-flow rate and operating pressure, tube diameter and absorbed solar radiation on the collector tube length, heat transfer coefficient and the local refrigerant temperature in the tube.
304
Fuel and Energy Abstracts
September
2000
00102740 Thermohydraulic performance of air heating solar collectors with variable width, flat absorber plates Hegazy, A. A. Energy Convers. Manage., 2000, 41, (l3), 1361-1378. The study investigates the more general and fundamental case when variable width absorbers are utilized instead of conventional rectangular plates in collecting incident solar radiation. The geometries considered include trapezoidal and concave parabolic absorbers having the same surface area. The effect of absorber shape on collector thermohydraulic performance has been examined for three common models with air flowing over the flat absorber, or under it and on both sides of the absorber. The results are compared with those of rectangular absorber models over a wide range of mass flow rate and solar energy flux. Under the same operating conditions, the comparisons indicate that variable width collectors exhibit performance behaviour similar to those of the constant width corresponding models, but it is accompanied by a marginal decrease in air temperature rise and collection efficiency along with a slight increase in fan power consumption. The comparisons do, thus, support the idea of employing variable width collectors in solar air heating applications, especially those that have design constraints on the investment costs and on the space available to install air collectors. For such applications, trapezoidal shape collectors are highly recommended. 00/02741 Three-layer organic solar cell with high-power conversion efficiency of 3.5% Takahashi, K. So/err Energy Materials & So/or Cells, 2000, 61, (4). 403416. To prepare organic solar cells with practical levels of energy-conversion efficiency, the following strategies were adopted. By using HD as the photosensitizer, which is heterodimer consisting of a weak electrondonating 5, 10, 15, 20-tetra(2, 5-dimethoxyphenyl)porphyrinatozinc and a weak electron-accepting 5, 10, 15striphenyl-20-(3-pyridyl)porphyrin, intramolecular photoinduced electron transfer is promoted resulting in effective charge separation. To create an energetically well-arranged system, the HD was placed between an electronacceptor layer of PV (perylene-3. 4, 9, lotetracarboxyl-bis-benzimidazole) and strong electrondonor layer of MC (3carboxymethyl-5-((3-ethyl-2(3H)-benzothiazolylidine)ethylidene]-2-thioxo~ 4-thiazolidinone), where photoinduced intermolecular electron transfer from HD to PV and rapid injection of electrons from MC to HD suppress back electron transfer in the charge-separated HD. As a result of this, the three-layer solar cell Al/PV/HD/MC/Au showed fairly good photovoltaic properties, short-circuit photocurrent quantum yield of 49.2%, open circuit photovoltage of 0.39 V, fill factor of 0.51 and energy conversion yield of 3.51% when irradiated with 445 nm monochromatic light of 12 1W crn~~’ intensity transmitted through the AI/PV interface. Since the photocurrent hardly decreased with age, the photocurrent observed here is really from energy conversion and not from photocorrosion of the aluminium electrode being occasionally responsible for the photocurrent. 00102742 Transport properties of n-type CuGaSez SchSn, J. H. Solar Energy Materials & Solar Cells, 2000, 61, (4), 417-426. Hall effect, electrical conductivity and charge carrier mobility of n-type CuGaSe* single crystals are studied in the temperature range between 2 and 300 K. The experimental data, analysed within the framework of a simple model of a semiconductor with two types of carriers of one sign, indicate the existence of an impurity band. The values of the gap between the main and impurity band, the concentration of donors and the concentration of the compensating acceptors are calculated. The mobility above 100 K is described by scattering of conduction band electrons at phonons and impurities. The sharp drop at low temperatures is explained by a crossover to Mott-type variable-range hopping in the impurity band. 00/02743 Univariate and multivariate forecasting of hourly solar radiation with artificial intelligence techniques Sfetsos, A. Solar Energy, 2000, 68, (2), 169-178. This paper introduces a new approach for the forecasting of mean hourly global solar radiation received by a horizontal surface. In addition to the traditional linear methods, several artificial-intelligence based t&hniques are studied. These include linear, feed-forward, recurrent Elman and Radial Basis neural networks alongside the adaptive neuro-fuzzy inference scheme. The problem is examined initially for the univariate case and is extended to include additional meteorological parameters in the process of estimating the optimum model. The results indicate that the developed artificial intelligence models predict the solar radiation time series more effectively compared to the conventional procedures based on the clearness index. The forecasting ability of some models can be further enhanced with the use of additional meteorological parameters. 00102744 Use of radial basis functions for estimating monthly mean daily solar radiation Mohandes, M. Solar Energy, 2000, 68, (2). 161-168. The present study utilizes the radial basis functions technique for the estimation of monthly mean daily values of solar radiation falling on horizontal surfaces and compares its performance with that of the multilayer perceptrons network and a classical regression model. In this work, solar radiation data from 41 stations that are spread over the Kingdom of Saudi Arabia is used. The solar radiation data from 31 locations are used for training the neural networks and the data from the remaining 10 locations are used for testing the estimated values. However, the testing data were not used in the modelling or training of the networks