Geometric Concentration Factor Research Articles

Turning-induced surface integrity for a fillet radius in a 316L austenitic stainless steel

Turning is a machining process extensively applied to produce revolution parts. Durability of these parts are known to depend on the turning process signature that is often referred as surface integrity. The surface integrity generated in a fillet radius has been barely studied in the literature so far, despite the well-known geometrical stress concentration factor of such singularities. Therefore this paper deals with the investigation of machining-induced surface integrity when turning a fillet radius in a 316L austenitic stainless steel. Different characterization methods are used for that purpose - SEM, EBSD, nanoindentation and X-Ray diffraction. It points out that the turning-induced consequences are not homogeneous along the machined profile. Residual stresses are strongly affected and microstructure is highly modified over a depth of 80 μm that leads to a mechanical properties gradient. It is evidenced that the average uncut chip thickness is the main governing parameter regarding surface integrity. It is also reported that deformation twins appear in the affected zone. It highlights that turning-induced microstructure evolution at a given depth is rather a consequence of severe plastic deformation at high strain rate than dynamic recrystallization. • For the first time, turning-induced surface integrity in 316L is studied along a fillet radius. • Turning-induced affected depth depends on angular position in the fillet radius. • Affected depth is twice larger for longitudinal turning than for face turning. • Microstructure evolution is correlated with average uncut chip thickness parameter and cutting edge radius. • Severe plastic deformation at room temperature or/and high strain rate governs microstructural evolution (deformation twins).

Energy modeling of a solar dish/Stirling by artificial intelligence approach

Solar energy is a renewable energy resources that is available across the world. A solar dish/Stirling system means a parabolic dish concentrator and a Stirling engine combined to generate mechanical and/or electrical output power. In this system, the input energy of Stirling engine is provided by sunlight as a source of heat. This study presents the effect of different variables on the power generation and efficiency of the system. In addition, artificial intelligence approach is employed to model a solar dish/Stirling system. For this target, a huge dataset was provided by considering a wide range of input variables. The intelligent methods are group method of data handling (GMDH) type neural network, adaptive neuro-fuzzy inference system (ANFIS) and multilayer perceptron (MLP) neural network (ANN). The MLP and ANFIS are optimized with particle swarm optimization (PSO) and genetic algorithm (GA). The intelligent methods are trained with inputs and targets. The considered input parameters are the ratio of focal point to dish diameter, hour of day, solar radiation, geometric concentration factor and working gas specific constant. The power generation, global efficiency, heat used to run the Stirling cycle, hot Stirling chamber temperature and engine speed are selected to be the targets. The results depict that the intelligent methods operate successfully for energy modeling of the solar dish/Stirling system and the statistical indicators illustrate that the ANFIS-PSO method performs better than the other developed methods.

Performance comparison of CPCs with and without exit angle restriction for concentrating radiation on solar cells

Abstract To perform this comparison, the compound parabolic concentrator with a restricted exit angle of 65° (CPC-65) and the one without exit angle restriction (CPC-90) were fabricated and tested for concentrating radiation on multi-crystalline solar cells. Both CPC-65 and CPC-90 are identical in the acceptance half-angle (20°) and geometrical concentration factor (2×). Theoretical calculations showed that CPC-90 based PV system (CPV-90) annually concentrated about 3–5% more radiation on solar cells as compared to CPC-65 based PV system (CPV-65). For CPV-65, all radiation would arrive on the solar cells at the incidence angle less than 65°, but for CPV-90, about 8–10% of annual collectible radiation would arrive on solar cells at the incidence angle larger than 65°. Measurements at outdoor conditions showed that the CPV-65 performed slightly better than CPV-90 in terms of short-circuit current and power output as the projection incidence angle of solar rays on the cross-section of CPC-troughs (θp) less than the acceptance half-angle, otherwise the CPV-90 did better. Compared to CPV-90, the power output at maximum power points from CPV-65 were slightly higher, and increases of 2.1%, 5.4% and 8.17% were measured for θp = 0°, 10° and 16°, respectively. Analysis indicated that effect of solar flux distribution over solar cells on power output of both CPVs was almost identical and insignificant, and the CPV-65 performed slightly but insignificantly better than the CPV-90 in terms of annual power output except in areas with poor solar resources where the annual power output from both systems was almost identical.

Optical Performance Comparison of Six ACPCs for Concentrating Radiation on All-Glass Evacuated Solar Tubes

In this work, six asymmetric compound parabolic concentrators (ACPC) were designed for concentrating radiation on all-glass evacuated solar tubes (EST). The ACPCs are required to be horizontally oriented in the east-west direction and to collect direct sunlight for at least 6 hrs in any day of a year. The angular dependence of optical efficiency of six ACPCs, identical in the height of both reflectors after the higher reflector being truncated, was investigated by ray-tracing method, and the annual radiation on EST concentrated by ACPCs was estimated based on solar geometry and monthly horizontally radiation. Results shows that the ACPC designed based on “hat shaped” virtual absorber with a V-groove at the bottom of reflectors is the best in terms of the optical efficiency averaged over the acceptance angle, followed by the one designed based on “hat-shaped” absorber without a V-groove at the bottom, and the one designed based on the cover tube of EST is the worst. However, from the point of annual radiation on EST, the ACPC designed based on the cover tube is the best solution, followed by the one designed based on “ice-cream shaped” absorber and the one designed based on “hat shaped” absorber is the worst due to the smallest geometric concentration factor.

Calculations of notch stress factor of a thin-walled spreader bracket fillet weld with the use of a local stress approach

Presence of geometric notches in welded joints causes concentration of strains and stresses, therefore reducing fatigue strength of such joints. This article presents an analysis of stress concentrations in a fillet weld of a spreader mounting bracket on a small sailing yacht. The aim of this article is to direct the attention of designers, manufacturers and regulatory bodies to issues of fatigue cracks that form in brackets fastening spreaders to masts. The issue concerns mainly yachts with overall length of a hull below 10m. Despite frequent defects the solution is still being chosen by designers, mainly for technological reasons. Current design regulations of classification societies and other regulatory bodies for small recreational yachts do not specify a technical solution for spreader fastening. With the use of finite element analysis it was shown that for short term design loads structural strength of a bracket is sufficient. Then, with the use of the notch stress approach, it was proven that the spreader bracket welded of thin plates is a structural solution that runs a risk of fatigue cracks. The below presented calculations of the geometric concentration factor of weld section in the locations of fatigue cracks were performed for different geometric cases of joints. The analysis results indicate the need for greater attention that should be paid to the problem of fatigue cracks that form in welds of brackets fastening spreaders to masts. The determined values of effective stress concentration coefficients show the need for conducting fatigue calculations of brackets welded of thin plates, or choosing an alternative solution for fastening spreaders to masts. In both cases it seems appropriate to conduct fatigue calculations for a structural element which is “a weak link” of the main drive on a sailing yacht.

Experimental investigation on paraboloid solar concentrator with a conical secondary concentrator

Material removal through ablation caused by concentrated solar thermal power is investigated. Experiments are conducted using a mirror-finish electroplated paraboloid along with a conical secondary concentrator in both fall and summer months in Al Ain city, UAE. Data collection for melting of aluminum shims facilitated the assessment of the feasibility of utilizing the available solar irradiance after the two-stage concentration. A geometric concentration factor of over 2800 is achieved for the first stage of power concentration with the current installation that also has a two-axis sun tracking system and a built-in pyrheliometer. The results confirm the feasibility of the approach, as the available power for ablation around solar noon is order of magnitudes higher than the power needed for the task, for the aluminum shim used. The main aim of the study is to hone the process to the power and precision of continuous pulse laser material removal, notably CO2 lasers, as challenges further reduction of the concentrated solar beams to acceptable tolerances remain to be successfully resolved. Copyright © 2014 John Wiley & Sons, Ltd.

Nonimaging optical concentrators using graded-index dielectric

A new generation of inhomogeneous nonimaging optical concentrators is proposed, able to achieve simultaneously high optical efficiency and acceptance solid angle at a given geometrical concentration factor. General design methods are given, and concentrators are numerically investigated and optimized.

Optical performance of CPCs for concentrating solar radiation on flat receivers with a restricted incidence angle

In some applications of compound parabolic concentrators (CPCs), the incidence angle of solar rays on the absorber is restricted and must be less than a specified value (θe) for efficient energy conversion or transfer. For a flat receiver with a restricted incidence angle (RWARIA, in short), two ideal concentrators designed based on one-sided flat absorber can be employed for radiation concentration: one is the CPC without exit angle restriction (CPC-1), and another is the CPC with a restricted exit angle (CPC-2). In this work, the angular dependence of optical efficiency factor of both CPC-1 and CPC-2 for concentrating radiation on the RWARIA was derived, and a mathematical procedure to estimate daily radiation accepted by the RWARIA by using east-west oriented CPC-1 and CPC-2 was suggested based on the solar geometry and monthly horizontal radiation. Results by numerical calculations show that, for fixed full CPC-1 and CPC-2 with identical acceptance half-angle (θa), the CPC-2 is slightly more efficient than CPC-1 for concentration radiation on the RWARIA except periods of about 30 days before and after both equinoxes; whereas for fixed truncated CPC-1 and CPC-2 with identical geometric concentration factor (Ct) and θa, the CPC-2 is always more efficient. Results also indicate that, for the case of the tilt-angle of the aperture of CPCs being yearly adjusted four times at three tilts, full CPC-2 is less but truncated CPC-2 is more efficient than CPC-1 for concentrating radiation. In practical applications, CPCs are usually truncated due to less efficient of top portion of a CPC reflector for radiation concentration and less reflector material use, therefore, the CPC-2 is more favorable and advisable for concentrating radiation on the RWARIA.

Comparison of multi-crystalline silicon PV modules’ performance under augmented solar irradiation

ABSTRACTIn developing photovoltaic (PV) systems with reliable lifetime performances, it is critical to have quantitative knowledge of not just initial properties and performances, but also their performance over the warrantied 25 year lifetime. In 2010, the Science for Energy Technology Workshop, convened by U.S Department of Energy (DOE) Basic Energy Science, prioritized photovoltaic module lifetime and degradation science (L&DS), which serve as the basis for quantitative and mechanistic understanding of lifetime performance. In order to better understand degradation rates and mechanisms of PV systems in the real-world environment, the SDLE SunFarm at Case Western Reserve University has been created, which is a highly instrumented outdoor test facility with 148 PV modules and > 8000 samples on sun for weathering and degradation studies of materials components and systems designed for long-lived energy systems. I-V and power performance of 10 multi-crystalline silicon PV modules from different manufacturers, using baseline and continuous power monitoring and comprehensive weather and solar resource monitoring, to enable time series analysis for insights into performance characteristics and initial degradation.Five modules from each manufacturer were exposed using mirror augmentation in typical (Cleveland, OH) climatic conditions. The mirror augmentation used geometric concentration factors of 1X, 1.5X and 1.9X of the nominal 1 sun. The effect of mirror augmentation on the modules' performance is reported. A Daystar multi-tracer was used to measure I-V curves of individual modules every 15 minutes while power output under maximum power point tracking was monitored continuously. Monitoring environmental factors (wind speed, wind direction, rainfall, and humidity), solar resource, and module temperatures allow for determination of the effects of these conditions on module power production. Power data was corrected to standard test condition (STC) according to climatic and solar irradiance. Changes in fill factor, short circuit current, open circuit voltage and maximum power are reported for each module. With time series analysis, a better understanding of the module's performance over time and under environmental conditions can be developed.

Irradiation Distribution on Solar Cells inside CPCs with a Restricted Exit Angle

In this work, irradiation distribution on the base of ideal two-dimensional CPCs with and without a restricted exit angle, to which solar cells are attached, are analyzed by the ray-tracing technique. Results show that, given an acceptance half-angle (Θa), for CPCs without a restricted exit angle (CPC-1), radiation flux distribution on the base depends on the incidence angle of solar rays (Θ ); whereas for CPCs with a restricted exit angle (CPC-2), the flux distribution depends on and the restricted exit angle (Θe). For both CPC-1 and 2 with identical geometric concentration factor (Ct=2) and acceptance half-angle ( Θa=20), the peak flux concentration ratio on the base of CPC-2 with =65o decreases about 43%, 30% and 49% for the radiation at the incident angle of 0o, 10o and 18o, respectively, as compared to CPC-1. This implies that solar flux distribution on the base of CPC-2 is more uniform, thus, more efficient for photovoltaic conversion as compared with CPC-1.

UP SCALING AND TEST RESULTS OF AN ADVANCED FRESNEL GREENHOUSE

A greenhouse with Fresnel lenses in the south facing roof and a receiver for concentrated Photovoltaics with water cooling (CPVT system) will result in electrical and thermal energy output from the solar energy excess entering a greenhouse. The PV system converts about half of the direct radiation into heat and electricity. During periods with direct radiation this will significantly reduce the heat load on the greenhouse. For an optimal performance the roof elements must be asymmetric with a steep inclination at the north side (the exact angle of course depends on the latitude of the building site). The Fresnel lens structure is best oriented in upwards direction. In the current design, two lenses are placed in the inner space of a double glass. This prevents pollution and condensation on the lenses. By the upward facing of the lens structure, the focus quality is preserved over a much broader range of angles of incidence compared to a lens with downward facing structures. Each PMMA lens with a size of 1.20×1.52 m is composed of 12 ‘tiles’ for easy production. The focal distance of the lens is 1,875 m and the geometrical concentration factor is 50×. This means that in most cases the focus line is thinner than 3 cm. The performance of the lens with respect to the shape of the focal area and the position of the focal line has been analyzed with ray tracing techniques. From this analyses and by the development of a smart tracking system only two motors can bring the receivers in the required positions. One motor controls the distance between lens and receiver and the other controls the translocation of the receivers parallel to the lens. The second conclusion was that the positions of the focal line are within the bounds of the greenhouse construction for almost the whole year. Only in winter, in the early morning and at the end of the day, the focal line will be unreachable. The 480 m2 greenhouse, with the LCPVT system based on Static Fresnel lenses and a 40 m CPVT-module and a 200 m CT-module, is designed by Bode Project Engineering and constructed by Technokas in Bleiswijk the Netherlands.

DESIGN OF A NIR-CONCENTRATOR SYSTEM INTEGRATED IN A GREENHOUSE

In this paper the design and development of a new type of greenhouse with an integrated filter for reflecting near infrared radiation (NIR) and a solar energy delivery system is described. Especially the optical parts as the spectral selective film, the properties of the circular reflector and the efficiencies of photo voltaic cells are studied. As a first measure, the spectral selective cover material, which prevents the entrance of NIR radiation, is investigated. It has to block up to 35% of the solar energy outside the greenhouse, which will reduce the needed cooling capacity. The second measure is the integration with a solar energy system. When the NIR reflecting coating is designed as a circular shaped reflector integrated in the greenhouse, the reflected solar energy of a PhotoVoltaic (PV) cell in the focus point delivers electric energy. With a ray tracing computer program the optimal geometry of the reflector was designed with respect to the collecting efficiency. The PV cells mounted in the focal point require cooling due to the high heat load of the concentrated radiation (geometric concentration factor of 30). The properties of different PV materials were investigated to find the optimal cell for this application. Cooled greenhouses are an important issue to cope with the combination of high global radiation and high outdoor temperatures. All parts are integrated in a 100m2 prototype greenhouse which will be applied for the proof of principle.

Optical performance and design optimization of V-trough concentrators for photovoltaic applications

Abstract In this work, a detailed mathematical procedure to estimate collectible radiation on the base of a V-trough concentrator, to which solar cells are attached, was developed based on the imaging principle of planar mirrors, solar geometry and monthly horizontal radiation. This model allows predicting the optical performance of V-troughs with any structural and installation parameters, and optimizing the design of such concentrator. Calculation results showed that for fixed east–west aligned V-troughs (EW-V troughs, in short) with given geometric concentration factor ( C g ) and side wall reflectivity ( ρ ), the annual solar gain depended on its opening angle, tilt-angle of the aperture and climatic conditions in sites, and a set of optimal opening angle and aperture tilt-angle for maximizing annual solar gain could be obtained by iterative calculations for different opening angles and tilt-angles. It was also found that for fixed EW-V troughs with C g = 2 and ρ = 0.9, taking the site latitude as quasi optimal tilt-angle was reasonable over all area of China, 15° deviation of azimuth angle from the due south would resulted in reduction of the annual collectible radiation less than 1%, and 10° deviation of the geometric axial line from the crossing line between the horizon and the extended aperture of V-troughs resulted in the reduction of annual solar gain less than 1%. For EW-V troughs with the tilt-angle of the aperture being yearly adjusted four times at three fixed tilt-angle (3T-EW-V troughs), the annual solar gain captured by 3T-EW-V troughs with given C g and ρ in a specific site was approximately a function of the opening angle, and the optimal opening angle for maximizing annual solar gain could be simply estimated by iterative calculations for different opening angles. Compared with fixed EW-V troughs, the annual solar gain on the base of 3T-EW-V troughs increased significantly, especially for those with a large C g , and the monthly average daily solar gain was more stable over a year. This implied that several tilt-angle adjustments in a year were required to make EW-V troughs with C g > 2 efficiently concentrate radiation onto their coupling solar cells in all days of a year.

Numerical modelling of shot peening process and corresponding products: Residual stress, surface roughness and cold work prediction

An investigation of the effects of controlled shot peening (CSP) process parameters on the treated material is presented. For this purpose, a three dimensional numerical model is developed, comprising the target plate and a number of shot impacts; their number is defined as the minimum required for a realistic simulation and minimum computational cost. The numerical model is verified by comparing the predicted residual stress (RS) fields to experimental. A parametric study of the shot velocity and impinging angle on the CSP products is performed for 4 shot types, i.e. S110, S230, S330 and S550. The main advantages of the present numerical model are: a) the relatively high number of shots introduced in the simulation compared to other publications that use only one shot, b) the number of shots dependency on the desired coverage, c) the thorough selection of numerical parameters, d) the high-strain rate material behaviour used for the target plate, e) the capability to calculate CSP effects on the target plate as function of coverage, f) the computed data which include RS field, surface roughness, cold work and geometrical stress concentration factor (Kt) and finally g) the computed results which are validated by experimental measurements.

Luminescent spectral splitting: Efficient spatial division of solar spectrum at low concentration

The purpose of this study was to investigate the potential performance of a novel concept for dividing solar radiation into spectral components that separately illuminate photovoltaic (PV) cells of different band gaps using an optical design that (1) is simple, easily manufactured, and extensible to many spectral channels and (2) does not achieve high geometric concentration factors. The concept that we explore leverages the approach of stacked luminescent solar concentrators (LSCs) for dividing the solar spectrum using fluorophores that are tuned to different spectral bands. However, whereas multicolor LSCs must perform two functions using the same optical component—spectral division and concentration—we consider the performance of a similar design when only one demand—spectral division—is placed on it. We find that the optical quantum and power efficiencies can be quite high (QE>90%, PE>80%) compared to what one might intuitively expect. When we couple the light output to a simple detailed balance model of a solar cell using experimental performance parameters we find that solar-to-electric conversion could exceed 30% with four junctions, using existing PV materials. While this does not exceed what can be achieved by HCPV designs on multijunction epitaxially grown stacks, the concept presented here has the major advantage of being easily extensible to an arbitrarily large number of spectral channels. Because of this extensibility, the number of junctions in the system is limited only by the availability of PV cells with appropriate band gaps, so significantly higher system efficiencies should be accessible without major revision to the basic design presented here.

Fatigue damage assessment of various welded joints under uniaxial loading based on energy methods

The present study intends to evaluate fatigue damage of different welded joints under uniaxial loading condition and its response on fatigue lifetime. The main variables influencing the fatigue life of a welded joint are: applied stress amplitude, material properties, geometrical stress concentration factor. Energy approaches were employed to evaluate the fatigue damage of various weld joints under uniaxial loading conditions. Energy-fatigue life ( W– N) curves were further discussed and compared for their capabilities in assessing fatigue life of various joints through different parameters including curve slope, life data scatter, and how readily coefficients/constants are determined and employed in the energy methods. The critical plane/energy approach was found to be the most suitable energy-based approach for fatigue damage and life assessment of welded joints by offering sharper W– N curves and less life scatter. This approach also allowed employing readily available material coefficients/properties as compared with notch stress-intensity energy approach.

A comparative study on fatigue damage assessment of welded joints under uniaxial loading based on energy methods

The present study intends to evaluate fatigue damage of different welded joints under uniaxial loading condition and its response on fatigue lifetime. The main variables influencing the fatigue life of a welded joint are: applied stress amplitude, material properties, geometrical stress concentration factor. Energy approaches were employed to evaluate the fatigue damage of various weld joints under uniaxial loading conditions. Energy-fatigue life (W-N) curves were further discussed and comported for their capabilities in assessing fatigue life of various joints through different parameters including curve slope, life data scatter, and how readily coefficients/constants are determined and employed in the energy methods. The critical plane/energy approach was found to be the most suitable energy-based approach for fatigue damage and life assessment of welded joints by offering sharper W-N curves and less life scatter. This approach also allowed employing readily available material coefficients/properties as compared with the notch stress-intensity energy approach.

DESIGN OF A SOLAR GREENHOUSE WITH ENERGY DELIVERY BY THE CONVERSION OF NEAR INFRARED RADIATION - PART 1 OPTICS AND PV-CELLS

In this paper the design and development of a new type of greenhouse with an integrated filter for reflecting near infrared radiation (NIR) and a solar energy delivery system is described. Especially the optical parts as the spectral selective film, the properties of the circular reflector and the efficiencies of photo voltaic cells are studied. As a first measure, the spectral selective cover material, which prevents the entrance of NIR radiation, is investigated. It has to block up to 35% of the solar energy outside the greenhouse, which will reduce the needed cooling capacity. The second measure is the integration with a solar energy system. When the NIR reflecting coating is designed as a circular shaped reflector integrated in the greenhouse, the reflected solar energy of a PhotoVoltaic (PV) cell in the focus point delivers electric energy. With a ray tracing computer program the optimal geometry of the reflector was designed with respect to the collecting efficiency. The PV cells mounted in the focal point require cooling due to the high heat load of the concentrated radiation (geometric concentration factor of 30). The properties of different PV materials were investigated to find the optimal cell for this application. Cooled greenhouses are an important issue to cope with the combination of high global radiation and high outdoor temperatures. All parts are integrated in a 100m2 prototype greenhouse which will be applied for the proof of principle.

Maximum concentration for ideal asymmetrical radiation concentrators

A new relation between the maximum geometric concentration factor C and the angular acceptance interval for asymmetrical ideal non-imaging concentrators is proposed. A generalization of the well-known relation for the two-dimensional case, sin θ c = 1/ C where θ c is the acceptance half-angle, results in the proposed relation sin θ 2 − sin θ 1 = 2/ C, where θ 1 and θ 2 are the angles of the acceptance interval limits relative to the normal of the entrance aperture. The proposed relation is valid for any ideal 2D concentrator, symmetrical or asymmetrical. A proof based on conservation of phase space is provided.

An analytical method for reflected flux density calculations

Conception, evaluation and real time control of solar “power tower” systems require the use of fast and accurate computer programs for calculating the flux density distributions on the receiver. Since the classical methods of “cone optics” and “hermite polynomial expansion” have some limitations of speed and accuracy, we have built an analytical model for calculating the convolution of the solar brightness distribution with the principal image of a heliostat (i.e. the fictive image for a “point sun”). We first characterize a principal image of a focusing heliostat by its shape and its geometrical concentration factor. Then this image is projected back onto the central plane (which passes through the center of the mirror), and considered as a flat reflecting surface. And the problem is reduced to density calculation for a flat heliostat. For each point of the receiver, the density of flux reflected by a heliostat is obtained by direct resolution of a convolution integral. The different formulations used to express the density function correspond to the various types of intersections between the image of the solar disk for the considered point and the principal image of the heliostat. Confrontation of this method with a program based on “cone optics” shows a good concordance of results and a strong decrease of computation time. We want to apply this method to the existing “THEMIS” solar plant built in France and to compare our results with real observations. Our density calculation programs will help conceiving fields of focusing heliostats for a new generation of power systems (gaz turbine systems).