Solar energy

07 A/femafive energy sources (solar enegy) primary importance in the evaluation of geothermal reservoirs since both the area1 extent and the vertical thickness can not be accurately determined, particularly in the initial development stages of the reservoirs. Applications of the method to geothermal fields are discussed. 02/53532 Fumaroiic gas chemistry at Wairakei, New Zealand, 1925-1998 Glover, R. B. el al. Geofhermics, 2001, 30, (5), 51I-525. The Wairakei geothermal field fumarolic discharges are at their greatest intensity from the Karapiti Thermal Area. This part of the geothermal field contains numerous steamdominated features in an area of approximately 1 km’. Since 1952 there have been many changes to the surface features and thermal activity at Karapiti related to the development-induced pressure drawdown over most of the field. A greatly expanded steam cap fed by a large low-pressure steam zone in the Wairakei reservoir has replaced the hot chloride water originally underlying Karapiti. There have been intermittent chemical surveys of the steam vents at Karapiti since 1951, with major chemical surveys undertaken in 1961 and 1990. In 1990, the concentrations of COz were found to be about 200 mmol/lOO mol HzO, double the 1961 values. Of particular interest is the change between 1936 and 1987 in gas chemistry of the main Karapiti feature, the Karapiti Blowhole (F712), which follows the change in heatflow from the Karapiti Thermal Area. Since 1990, gas concentrations appear to be dropping to low, pre-development levels, most likely due to the decreasing pressures in the lower pressure steam zone. Underground processes leading to a high gas content in a large fumarole formed in 1967 and other fumaroles in different parts of the Wairakei field are discussed. 02/02533 Geochemistry of the surface and deep fiulds of the Miravaiies volcano geothermal system (Costa Rica) Gherardi, F. ef al. Georhermics, 2002, 31, (I), 91-128. The Miravalles high-temperature geothermal reservoir, located in the northwestern part of Costa Rica, is liquid-dominated. Reservoir temperatures generally range between 230 and 240°C. The highest measured value is 255°C. Bottom-hole measurements and solute geothermometry indicate that thermal conditions within the reservoir are very stable over time. The waters discharged from the wells have a neutral or slightly alkaline pH and are of the sodium-chloride type. Based on isotope data, the main recharge zone appears to be located on the northeastern side of the Guanacaste Cordillera. Several mixing trends have been identified between reservoir fluids and regional groundwaters. Gas discharges are dominated by COz, with minor amounts of HzS and Nz. Relative Nz, Ar and He contents reveal a typical arc-type signature and significant inflow of meteoric-derived gases. Cl-SiOz-enthalpy and 61s0-6ZH-CI relationships suggest the existence of a maturation trend that is the result of both natural (i.e. direct drainage of deeper fluids) and anthropogenic causes (reinjection of Cl-rich waste waters). Acid fluids with Sod-acidity (pH ranging between 2.4 and 3.7) have been encountered in three wells at the eastern border of the well field. Preliminary data assessment indicates two possible sources, either superficial Hz.? oxidation or inflow of “immature” volcanic waters. 03/02534 Mixing of thermal and non-thermal waters in the Steamboat Hills area, Nevada, USA Skalbeck, J. D. er al. Geofhermfcs, 2002, 31, (I), 69-90. Groundwater monitoring began in 1985 at two geothermal facilities in the Steamboat Hills area, Nevada. Wells representing non-thermal, thermal, and mixed waters are evaluated by assessing temporal variations in B and Cl concentrations, water levels, and temperature. The objective is to assess the hydrologic and geochemical connection between the fractured bedrock geothermal reservoir and the alluvial aquifer. Results suggest that fault-controlled groundwater flow between the geothermal system and the alluvial aquifer is the dominant hydrologic process. Temporal trends suggest that the thermal water component in the alluvial aquifer has increased in most areas but decreased in at least one area.

method also gives a representative (order of magnitude) estimate of the total heat discharged by fumarole clouds if measurements from a single, large fumarole can be used in conjunction with a recent airphoto.

Solar energy 02lO2525 A contactless hotoconductance technique to evaluate the quantum eff Pciency of solar ceil emitters Cuevas, A. el al. Solar Energy Materials di Solar Cells, 2002, 71, 295312.

A new technique, the spectral response of the steady-state photoconductance, is roposed for solar cell characterization in research and development. T1 e emphasis of this paper is on the evaluation of the carrier collection efficiency of the emitter region based on a simple, two-wavelength approach. It is shown that in addition to the wellestablished determination of the wafer recombination properties that results from a long-wavelength photoconductance measurement, detailed emitter quantum-efficiency information can be obtained by performing a second measurement with short-wavelength light. The method is experimentally demonstrated with silicon solar cell precursors having emitters with markedly different levels of surface and bulk recombination losses. The main advantages of the spectral photoconductance technique are that it is fast, contactless, and can be used immediately after junction formation before metaiiization; these properties make it very appropriate for routine monitoring of the emitter region, including in-line process control. 03/02537 A high-efficiency triple cycle for solar power generation Kribus, A. Solar Energy, 2002, 72, (l), 1-l 1. The last three decades have witnessed a trend in solar thermal electricity generation of increasing the concentration of sunlight, the operating temperature, and subsequently the efficiency of conversion from sunlight to electricity. The current state of the art concept is a solar-driven combined cycle, with sunlight concentration ratio of a few thousands, temperatures of about lOOO-1300°C and overall annual average conversion efficiency of about 20%. A possible next step in this trend is presented: a solar triple cycle, with a high-temperature MHD generator and two additional cycles in series. This tripe cycle is powered by solar heat at temperatures around 2000°C and solar concentration of about 10,000. The overall peak conversion efficiency of the solar triple cycle is shown to be significantly higher than the solar combined cycle scheme. The sensitivity of this result to several system parameters and the technological feasibility of the solar triple cycle are also discussed. 03/02538 A new heat-pipe type solar domestic hot water system Mathioulakis, E. and Belessiotis, V. Solar Energy, 2002, 72, (I), 13-20. The performance of a new solar hot water system with an integrated heat-pipe is investigated theoretically and experimentally. The new system uses a wickless gravity assisted loop heat-pipe for the heat transfer from the collector-evaporator to the tank through a heat exchanger-condenser. A detailed heat transfer experimental study is performed, focused on the thermal behaviour of the different parts of the system. The results show that the system can reach satisfactory efficiencies which, in combination with manufacturing simplicity, absence of moving parts and good behaviour under freeze conditions, make it an interesting technological solution. Furthermore, a theoretical model for the collector is proposed and comparison with experimental results is performed. This model can be used for the optimization of the system design.

03lO2535 Steam cloud characteristics and heat output of fumaroies Hochstein, M. P. and Bromley, C. J. Georhermics, 2001, 30, (5), 547-

03/02539 A photovoitaiclthermai (PWT) collector with a po;$n$r absorber plate. Experimental study and analytical

559.

Sandnes, B. and Rekstad, J. Solar Energy, 2002, 72, (I), 63-73. A polymer solar heat collector was combined with single-crystal silicon PV cells in a hybrid energy-generating unit that simultaneously produced low temperature heat and electricity. The PV/T unit was tested experimentally to determine its thermal and photovoltaic performance, in addition to the interaction mechanisms between the PV and thermal energy systems. Thermal efficiency measurements for different collector configurations are compared, and PV performance and temperature readings are presented and discussed. An analytical model for the PVfl system simulated the temperature development and the performance of both the thermal and photovoltaic units.

The heat transferred to the atmosphere by fumarole clouds is linearly proportional to the cloud area that is visible in vertical air-photos. Using measured outputs of a few accessible fumaroles, the total heat output of all fumaroles in a fumarole field can be assessed from the total area of the steam clouds. Normalized heat outputs, Q,, and normalized cloud areas, A., were used as variables. Two surveys of the large Karapiti fumarole field (Wairakei, New Zealand) show that the total heat discharged by fumaroles with cloud areas > IO m* was 99 MW and 88 MW in 1999 and 2000, respectively. The output of vents with smaller cloud areas has to be obtained

336

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surveys.

The

07 Alternative energy sources (solar energy) 02/02540 A stud of electrical uniformity for monolithic polycrystalline si Ticon solar cells Yang, H. ef al. Solar Energy Materials & Solar Cells, 2002, 71, (3) 407412. The aim of this work is to investigate the electrical uniformity of monolithic polycrystalline silicon solar cells prepared by various process techniques. By a series of experiments such as P and Al impurity gettering and silicon nitride passivation, a new conclusion is that the application of P and Al gettering as well as silicon nitride passivation enhances the electrical uniformity of small area solar cells diced from the same polycrystalline silicon solar cells, even if impurity gettering is not effective when the dislocation density is above a threshold value of about lo6 cm-‘. The experiments gives some hints that when large area polycrystalline silicon solar cells are cut into small pieces needed for application, the production process should be modified slightly. 02/02541

by

Cd-free buffer layers for GIGS solar cells prepared

a dry process

Siebentritt, S. er al. Solar Energy Materials & Solar Cells, 2002, 70, (41, 447-457. ZnSe buffer layers for Cu(ln,Ga)Sez/buffer/ZnO solar cells have been prepared by metal organic chemical vapor deposition (MOCVD). Using photoassisted MOCVD, deposition temperatures down to 265°C are possible. It is shown that deposition temperatures well below 300°C are essential as well as deposition times not much longer than 3 min. Higher temperatures and longer deposition times lead to absorber degradation. With optimized buffer deposition efficiencies of 11% have been obtained on CIGS absorbers from the Siemens pilot production line.

Correlation models of diffuse solar radiation applied to the city of Slo Paulo, Brazil 02/02542

Oliveira, A. P. el al. Applied Energy, 2002, 71, (I), 59-73. Measurements of global and diffuse solar-radiation, at the Earth’s surface, carried out from May 1994 to June 1999 in Slo Paulo City, Brazil, were used to develop correlation models to estimate hourly, daily and monthly values of diffuse solar-radiation on horizontal surfaces. The polynomials derived by linear regression fitting were able to model satisfactorily the daily and monthly values of diffuse radiation. The comparison with models derived for other places demonstrates some differences related mainly to altitude effects.

Evaluation of overcast-sky luminance models against measured Hong Kong data

02fO2543

preparation. The incorporation of Ru in the thin oxide film leads to the improvement of the solar conversion efficiency by improving the fill factor of the photovoltaic or photoelectrochemical cells. Such improvement enables the use of the prepared cells as clean energy converters. The fabricated solar cells have an average open-circuit potential of 0.44-0.62 V and a short circuit current of 28-30 mA/cm*. A conversion efficiency up to 14% was achieved.

Modelling and simulation of an absorption solar cooling system for Cyprus 02/02545

Florides, G. A. ef al. Solar Energy, 2002, 72, (I), 43-51. In this paper a modelling and simulation of an absorption solar cooling system is presented. The system is modelled with the TRNSYS simulation program and the typical meteorological year file containing the weather parameters of Nicosia, Cyprus. Initially a system optimization is carried out in order to select the appropriate type of collector, the optimum size of storage tank, the optimum collector slope and area, and the optimum thermostat setting of the auxiliary boiler. The final optimized system consists of a 15-m’ compound parabolic collector tilted 30” from the horizontal and a 600-I hot water storage tank. The collector area is determined by performing the life cycle analysis of the system. The optimum solar system selected gives life cycle savings of Cf1376 when a non-subsidized fuel cost is considered. The system operates with maximum performance when the auxiliary boiler thermostat is set at 87°C. The system long-term integrated performance shows that 84,240 MJ required for cooling and 41,263 MJ for hot water production are supplied with solar energy.

Natural convection solar dryer with biomass back-up heater 02/02546

Bena, B. and Fuller, R. J. Solar Energy, 2002, 72, (l), 75-83. A direct-type natural convection solar dryer and a simple biomass burner have been combined to demonstrate a drying technology suitable for small-scale processors of dried fruits and vegetables in nonelectrified areas of developing countries. From a series of evaluation trials of the system, the capacity of the dryer was found to be 20-22 kg of fresh pineapple arranged in a single layer of O.Ol-m-thick slices. The overall drying efficiency of the unit was calculated to be ~9%. During the same trial, the drying efficiency of the solar component alone was found to be 22%. Other trials estimated the efficiency of the burner in producing useful heat for drying to be 27%. Key features of the biomass burner were found to be the addition of thermal mass on the upper surface, an internal baffle plate to lengthen the exhaust gas exit path and a variable air inlet valve. Further modifications to further improve the performance of both the solar and biomass components of the dryer are suggested.

Li, D. H. W. ef al. Applied Energy, 2001, 70, (4). 321-331. Sky luminance distribution is one of the most important quantities for predicting indoor daylight illuminance levels. Overcast-sky types are essential because they are used in more general sky models and appear quite frequent in some places. This paper presents the work on the evaluation of six worldwide overcast-sky models against 2-year (19992000) measured Hong Kong sky luminance data. Overcast-sky conditions were identified using cloud cover (CLD) and a subsequent interpretation the overcast skies into thin and heavy overcast types was conducted in conjunction with the clearness index (K,). A statistical analysis of the models has indicated that the International Commission on Illumination (CIE) standard overcast sky model performed the best, in particular for the heavy overcast-sky condition. The Building Research Establishment (BRE) quasi-overcast-sky model showed a good agreement with the thin overcast distributions which may include a circumstance component and the sky luminance patterns being orientation dependent.

Rowlands, S. F. et at. Solar Energy Mareriah & Solar Cells, 2002, 71, (3). 399-405. It has long been known that the use of finely textured transparent conducting oxide layers substantially improves the performance of thin film amorphous silicon (a-Si:H) solar cells. Major efforts to understand the nature of this effect and to fully capture its potential have been made by researchers using advanced modeling techniques. In this work, modeling the oblique angle optical performance and use of an effective medium approximation to simulate micro-rough interfaces suggests that effective interface grading makes a significant contribution to optical enhancement.

02tO2544 Improved n-U/oxide junctions for environmentally safe solar energy conversion

02/02545

Badawy, W. Solar Energy Materials & Solar Cells, 2002, 71, (3), 281294. The considerable interest in the practical use of solar energy has increased the importance of photovoltaic and photoelectrochemical systems. Metal oxide films like tin dioxide (SnOs), titanium oxide (TiOs) or indium tin oxide (ITO) are known to form stable photovoltaic junction with semiconductors of practical relevance like silicon (Si). Thin films of SnOa and TiOa were prepared easily and conveniently on the surface of silicon wafers by the spray pyrolysis technique. The prepared heterojunctions, i.e. the Si/oxide junction represent the main part of stable and efficient solar energy converter. In these systems, the solid/solid junction (n-Si/oxide) is separated from the site of the environmental interaction by the stable oxide film (SnOs or TiOs) that protect the conventional semiconductor from photocorrosion. Besides its use in the fabrication of photovoltaic cells, the nSi/oxide was used in the preparation of photoelectrochemical cells. The characteristics of the oxide film were subjected to a series of improvements either in the surface conductivity or the band gap energy, i.e. the position of the Fermi level of the semiconducting oxide by incorporating foreign atoms in the oxide film matrix during its

Optical modeling of finely textured amorphous silicon solar cells

02lO2547

Optimal optical generation profiles in a-Si:H p-i-n

solar cells

Prentice, J. S. C. Solar Energy Marerials & Solar Cells, 2002, 71, (I), 85-101. The dependence of the maximum power output P,,, and short-circuit current Jrc on the form (relative variation with position) of the optical generation rate profile in an a-Si:H p-i-n solar cell has been investigated computationally. It was found that there was an optimal form for the generation profile, and that P,,, increased from 4.64 to 5.29 mW cm- , an Increase of about 14%, when this optimal generation profile was used in the simulation. Optimal doping of the i-layer of the cell with phosphorous led to a PM of 5.60 mW cm-2, and when the optimal generation profile for this P-profiled cell was found, it yielded a Pw of 7.86 mW cmw2, an increase of about 40%. This suggests that the combustion of P-profiling and optimal generation could lead to significant improvements in cell performance. Moreover, it was found that for both cells the form of the optimal generation profile could be associated with the position of the peak in the external quantum efficiency, obtained from the spectral response. The possibility of using band-gap grading to achieve an optimal generation rate profile has been suggested. Fuel and Energy Abstracts

September 2002

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07

Alternative

energy

sources

(wind

energy)

02/02549

Solar photovoltaic in Mali: potential and constraints Diarra, D. C. and Akuffo, F. 0. Energy Conversion and Management, 2001, 43, (2), 1.51-163. Located in the heart of sub-Saharan Africa, between latitudes 10” and 25”N, Mali has a land area of 1.204 million km2. The population is presently estimated to be 12 million with a per capita energy consumption or 227 ktoe per inhabitant per year. Wood fuel still represents 90% of the total energy consumed. Mali does not have any economicallv exnloitable oetroleum deoosits. but it receives an averaee solar insolaiion’of 6.3 k+ h/m2/day. +he &view of the potential aid problems of solar photovoitaic (PV) technology in Mali shows that high gmbient temperatures affecting the performance of PV systems, high cost of installed PV systems, poor after-sale service and passive participation of local communities in implementation of the programmes remain the main constraints and challenges. It is recommended that given Mali’s meagre conventional energy resources, research should be intensified on low cost technology that minimizes the effect of high temperatures on the performance of PV systems. Efforts should be made to avoid duplication of unsuccessful experiences, improve coordination of activities in this area and promote private sector participation. 02102550

Spectral effects on amorphous silicon solar module fill factors Riither, R. et al. Solar Energy Materials & Solar Cells, 2002, 71, (3), 375-385.

The outdoor operation and monitoring of amorphous silicon (a-Si) solar modules present unique features when compared to the more traditional and quite well-understood operation of the crystalline silicon (c-Si) technology. The peculiarities of a-Si contrast to such an extent with those of c-Si solar cells that in the field, while the former performs better during summer, the latter is more efficient in winter. Concepts usually applied to describe phenomena in c-Si devices are often inadequate to describe the performance of a-Si cells. When looking at module performance, the fill factor (FF) can be regarded as one of the characteristic photovoltaic quantities of major interest. Under outdoor illumination, cells are seasonally exposed to different solar spectral contents and intensities, which vary considerably from summer to winter. The FF depends on both the quality (spectrum) and quantity (irradiation) of the incident light. In this context, results are reported showing spectral effects on the FF of amorphous silicon solar modules deployed outdoors. While “blue” spectra improved the FF of a-Si devices, the contrary was observed for “red” spectra. The voltagedeoendent soectral resnonse of a-Si devices is also described and qu’antified. dur results ieveal that a-Si modules can perform quite well at low irradiations and mainly diffuse spectra. Thus, it is concluded that in system sizing programme!, the perfbrmance of a-Si modules should be treated more precisely with respect to spectra, to reveal their true operational characteristics and advantages. 02/02551 Study of the electrochemical synthesis of selective black coatings absorbing solar energy Moise, V. et al. International Journal of Inorganic Materials, 2001, 3, (8), 1323-1329.

To set up a reproducible electrochemical preparation process of a black, adhesive and homogeneous film on a metallic substrate with well-defined optical and mechanical properties, it is necessary to control all the electrochemical parameters and to understand their influence. For so doing, different coatings have been prepared in various conditions and their microstructures have been correlated with the synthesis parameters as well as the physical properties. Chromium oxide films on nickel-coated copper metallic substrates have been deposited from a chromate bath. In this paper, the evolution of the microstructure of the deposits is presented according to some experimental parameters, more specifically the concentration of the hexafluorosilicic acid and trivalent chromium ions present in the electrochemical bath used in our experiments. A marked difference in the particle morphology has been put into evidence by modifying the source of the trivalent chromium ions: (i) from the addition of sucrose to the solution containing hexavalent chromium ions or (ii) from the addition of a trivalent chromium salt to the bath. 02/02552 The effect of electric and magnetic fields on the operation of a photovoltaic cell Erel, S. Solar Energy Materials & Solar Cells, 2002, 71, (Z), 273-280. In this work, the effects of electric and magnetic fields on the operation of a CdS/CulnSez photovoltaic cell are investigated. Various electric field intensities changing from 0 to 35000 V&II, were applied to the sample while it was irradiated by a He-Ne laser with a wavelength X 3: 670 nm. As a result, the typical values for the open circuit voltage of the uhotovoltaic cell sinnificantlv changed with various intensities of the blectric field Edc. hiagnetic &Ids varying from 0.003 to 0.079 T were annlied usine a solenoid with an inductance of 10.55mH and the response

338

if-the

sample

was

observed.

Fuel and Energy Abstracts

In the

third

September 2002

step

of the

experiment, instead of the laser beam, various intensities of white light of 50, 100, 150 and also 250 lux were utilized. A total of 250 lux was measured to be equivalent to the radiation power of He-Ne laser beam on the surface of the photovoltaic cell. The effect of electric fields from 0 up to 3 x lo5 Vdc was applied and some signiticant experimental results were obtained. As a result of the illumination of the photovoltaic cell by the stimulated and spontaneous light emission sources under the effect of various intensities of electric field Edc, different electrical behaviours were observed. Wind characterlstlcs of Oman Sulaiman, M. Y. ef al. Energy, 2002, 27, (1), 35-46. Wind data from four stations in Oman are analysed. The sites are located at latitudes from 17”N to 23”N and longitudes from 54”E to 59”E with nearly equal elevation (18 m) from sea level. Data are fitted to the Weibull distribution function. Weibull parameters are derived from the empirical cumulative function and used to calculate the mean wind speed and variance of the theoretical distribution. The goodness of representing the observed distribution with the Weibull distribution is determined using the Kolmogorov-Smimov (K-S) test. At the 1% and 5% levels of confidence the observed data are well represented by the Weibull distribution. The annual mean values of the wind speed of the observed and theoretical distributions are 2.57 m s-l and 2.53 m s-l for Seeb, 2.85 m s-’ and 2.68 m s-l and 2.68 m s-l for Salalah, 5.06 -’ and 5.03 m s-l for Masirah and 5.52 m s-l and 5.40 m s-’ for Sur respectively. In general wind speed is higher during the summer months, notably June, July and August, and is lower during the winter months of October and November. The monthly mean wind power density varies from 9.71 W me2 for Seeb in the month of November to 520.85 W rnA2for Sur in the month of August. Both Sur and Masirah have good wind energy potential. 02RJ2553

Wind energy 02102554 Optimal wind-hydro solution for Aegean Sea islands’ electricity-demand fulfilment K..;lellis, J. K. and Kavadias, K. A. Applied Energy, 2001, 70, (4), 333Energy shortage and clean-water deficit, especially during the summer, are among the main factors delaying the economic development of Aegean Sea islands. All these islands possess an outstanding wind potential. However, the stochastic behaviour of wind speed leads to significant disharmony between wind energy production and electricity demand. Hence, the prospect of creating a combined wind-hydro energy production station is found to be a vital solution for all these islands, under the preconditions of maximum energy autonomy and limited first installation cost. Accordingly, a methodology of optimal wind-hydro solution estimation is developed and subsequently applied to several typical Aegean Sea island cases, in order to define the most beneficial configuration of the proposed renewable station. All numerical calculations are based on real data, like long-term wind speed measurements, demanded electrical-load and operational characteristics of the system components. In all cases analysed, the renewable energy sources penetration exceeds 85%, while a significant part of the system’s wind energy surplus is forwarded to a desalination plant for clean-water production. 02/02555 Performance analysis of a variables structure controller for Dower reoulation of WECS ooeratino In the stall region * De Battista, H. er al. Internarional Journal of Energy Research, 2001,

25, (15), 1345-1357.

The power regulation in variable speed wind energy conversion systems is examined in this paper. The importance of power control in the stall reeion is stressed. This mode of oaeration is characterized bv a nonm&mum phase behaviour. A variable structure controller is dkscribed that provides stability by means of speed feedback and is robust to grid disturbances and model uncertainties. Performance of the controller is investigated. A compromise arises in the design of the speed feedback gain between high and low frequency wind components rejection. Furthermore, a cut-off frequency of the wind velocity measurement is obtained that minimizes the effect of turbulence on power regulation. Simulation results are presented, corroborating the features of the control strategy. 02/02556 Simulation of wind-driven ventilative cooling systems for an apartment building In Beijing and Shanghai da Graqa, G. C. et of. Energy and Buildings, 2002, 34, (I), l-l 1. This paper presents a performance evaluation of two passive cooling strategies, daytime ventilation and night cooling, for a generic, sixstorey suburban apartment building in Beijing and Shanghai. The