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Showing papers in "Journal of Solar Energy Engineering-transactions of The Asme in 2001"


Journal ArticleDOI
TL;DR: In this paper, the development of a thermocline system that uses molten-nitrate salt as the heat transfer fluid is described and compared to a two-tank molten salt system.
Abstract: Thermal storage improves the dispatchability and marketability of parabolic trough power plants allowing them to produce electricity on demand independent of solar collection. One such thermal storage system, a thermocline, uses a single tank containing a fluid with a thermal gradient running vertically through the tank, where hotter fluid (lower density) is at the top of the tank and colder fluid is at the base of the tank. The thermal gradient separates the two temperature potentials. A low-cost filler material provides the bulk of the thermal capacitance of the thermal storage, prevents convective mixing, and reduces the amount of fluid required. In this paper, development of a thermocline system that uses molten-nitrate salt as the heat transfer fluid is described and compared to a two-tank molten salt system. Results of isothermal and thermal cycling tests on candidate materials and salt safety tests are presented as well as results from a small pilot-scale (2.3 MWh) thermocline.

519 citations


Journal ArticleDOI
TL;DR: In this paper, a solar-hybrid gas turbine combined cycle plant with a secondary concentrator and a volumetric air receiver was tested at the Plataforma Solar de Almeria, Spain.
Abstract: Solar hybrid power plants have a significant potential for cost reduction when the solar energy is introduced into a gas turbine system. The introduction into gas turbine systems could be realized with pressurized volumetric air receivers heating the compressed air of the gas turbine before it enters the combustor. A receiver module, consisting of a secondary concentrator and a volumetric receiver unit, was tested at the Plataforma Solar de Almeria, Spain. Air exit temperatures up to 815°C and power levels of 410 kW were achieved. Total solar test time summed up to 400 hours. Receiver efficiencies were in the range of 70%. A new secondary concentrator with improved efficiency was designed and built. Based on an inexpensive manufacturing technology, the secondary concentrator geometry was optimized to reduce the optical losses: Performance tests with this new secondary concentrator and a cold-water calorimeter proved the expected increase in efficiency of about 10%. Maximum operation power was 450 kW at the exit aperture. The dependency of performance on the incidence-angle showed good agreement with the predictions, as well as the results of a special photographic measurement campaign. Several configurations of solar-hybrid gas turbine cycles in the low to medium power range are examined for performance and costs. The results confirm the promising potential of this technology to reach competitiveness in certain power markets; a comparison between a 30 MW solar-hybrid combined cycle plant and an ISCCS power plant are presented. Future developments for system improvement and cost reduction are discussed.

217 citations


Journal ArticleDOI
TL;DR: Research on solarthermal processing and the need for alternative energy sources have reached the point where efforts to develop some industrial processes and expand research to suggest others are at least desirable, if not imperative as discussed by the authors.
Abstract: Research on solarthermal processing and the need for alternative energy sources have reached the point where efforts to develop some industrial processes and expand research to suggest others are at least desirable, if not imperative. This paper presents a rationale for such an effort, describes the underlying thermodynamics, and summarizes much of the research which has been conducted in the years since the end of World War II. Major emphasis is placed on the work that has been done since the imposition of the 1973 oil embargo and the present.

167 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented a new technique to compute the operating temperature of cells within building integrated photovoltaic modules using a one-dimensional transient heat transfer model, which can be used in conjunction with a calculation procedure to predict the module's temperature for various environmental conditions.
Abstract: A barrier to the widespread application of building integrated photovoltaics (BIPV) is the lack of validated predictive performance tools Architects and building owners need these tools in order to determine if the potential energy savings realized from building integrated photovoltaics justifies the additional capital expenditure The National Institute of Standards and Technology (NIST) seeks to provide high quality experimental data that can be used to develop and validate these predictive performance tools The temperature of a photovoltaic module affects its electrical output characteristics and efficiency Traditionally, the temperature of solar cells has been characterized using the nominal operating cell temperature (NOCT), which can be used in conjunction with a calculation procedure to predict the module's temperature for various environmental conditions The NOCT procedure provides a representative prediction of the cell temperature, specifically for the ubiquitous rack-mounted installation The procedure estimates the cell temperature based on the ambient temperature and the solar irradiance It makes the approximation that the overall heat loss coefficient is constant In other words, the temperature difference between the panel and the environment is linearly related to the heat flux on the panels (solar irradiance) The heat transfer characteristics of a rack-mounted PV module and a BIPV module can be quite different The manner in which the module is installed within the building envelope influences the cell's operating temperature Unlike rack-mounted modules, the two sides of the modules may be subjected to significantly different environmental conditions This paper presents a new technique to compute the operating temperature of cells within building integrated photovoltaic modules using a one-dimensional transient heat transfer model The resulting predictions are compared to measured BIPV cell temperatures for two single crystalline BIPV panels (one insulated panel and one uninsulated panel) Finally, the results are compared to predictions using the NOCT technique

145 citations


Journal ArticleDOI
TL;DR: The neural network model is found to possess better performance than the regression model for turbine power curve estimation under complicated influence factors.
Abstract: This paper examines and compares regression and artificial neural network models used for the estimation of wind turbine power curves. First, characteristics of wind turbine power generation are investigated. Then, models for turbine power curve estimation using both regression and neural network methods are presented and compared. The parameter estimates for the regression model and training of the neural network are completed with the wind farm data, and the performances of the two models are studied. The regression model is shown to be function dependent, and the neural network model obtains its power curve estimation through learning. The neural network model is found to possess better performance than the regression model for turbine power curve estimation under complicated influence factors.

125 citations


Journal ArticleDOI
TL;DR: The DISS project as discussed by the authors is a complete R +TD program aimed at developing a new generation of solar thermal power plants with direct steam generation (DSG) in the absorber tubes of parabolic trough collectors.
Abstract: The DISS (Direct Solar Steam) project is a complete R +TD program aimed at developing a new generation of solar thermal power plants with direct steam generation (DSG) in the absorber tubes of parabolic trough collectors. During the first phase of the project (1996-1998), a life-size test facility was implemented at the Plataforma Solar de Almeria (PSA) to investigate the basic DSG processes under real solar conditions and evaluate the unanswered technical questions concerning this new technology. This paper updates DISS project status and explains O&M-related experience (e.g., main problems faced and solutions applied) with the PSA DISS test facility since January 1999.

124 citations


Journal ArticleDOI
TL;DR: The Directly Irradiated Annular Pressurized Receiver (DIAPR) as discussed by the authors is a volumetric (directlyirradiated) windowed cavity receiver, designed for operation at a pressure of 10 ‐30 bar, exit gas temperature of up to 1,300°C, and aperture radiation flux up to 10 M W/m 2.
Abstract: The Directly Irradiated Annular Pressurized Receiver (DIAPR) is a volumetric (directlyirradiated) windowed cavity receiver, designed for operation at a pressure of 10 ‐30 bar, exit gas temperature of up to 1,300°C, and aperture radiation flux of up to 10 M W/m 2 . This paper presents test results obtained under various irradiation conditions and flow rates. Inlet aperture flux was up to 5 M W/m 2 ; exit air temperatures of up to 1,200°C were obtained, while operating at pressures of 17 ‐20 bar. Estimated receiver efficiency in these tests was in the range of 0.7‐0.9. The absorber and window temperatures were 200‐400°C below the permitted maximum, indicating that higher air exit temperatures are possible. @DOI: 10.1115/1.1345844#

122 citations


Journal ArticleDOI
TL;DR: In this article, a proof-of-concept microtab design and the multi-disciplinary techniques used to fabricate and test the tabs are compared, and the results demonstrate the significant potential for using microtabs for active load control.
Abstract: Micro-electro-mechanical (MEM) translational tabs are introduced for active load control on aerodynamic surfaces such as wind turbine rotor blades. Microtabs are mounted near the trailing edge of rotor blades, deploy approximately normal to the surface, and have a maximum deployment height on the order of the boundary-layer thickness. Deployment of the tab effectively changes the sectional chamber of the rotor blade, thereby changing its aerodynamic characteristics. A tab with tab height to blade section chord ratio, h/c, of 0.01 causes an increase in the section lift coefficient, C 1 , of approximately 0.3, with minimal drag penalty. This paper presents a proof of concept microtab design and the multi-disciplinary techniques used to fabricate and test the tabs. Computational and experimental wind tunnel results for a representative airfoil using fixed as well as remotely actuated tabs are compared. Although the specifics of load control limitations, including actuation and response times will require further research, the results presented demonstrate the significant potential for using microtabs for active load control.

111 citations


Journal ArticleDOI
TL;DR: A comparison of the currently available storage technologies shows that the most cost effective and environmentally acceptable is a compressed air energy storage (CAES) system and especially its more advanced derivatives (CASH, CAES with Humidification and CAESSI,CAES with Steam Injection) as mentioned in this paper.
Abstract: In current conventional utility systems, fossil fuel or nuclear power plants must back up intermittent renewable electricity generators if the utility grid is to have acceptable reliability. For low levels of intermittent power, this can be achieved without difficulty or added expense. For high levels this reliance on other generators becomes explicit and increasingly expensive, and transmission capacity also becomes an issue. An attractive alternative to the conventional approach is one that relies on bulk or utility scale storage of intermittent electricity to provide system reliability. A comparison of the currently available storage technologies shows that the most cost effective and environmentally acceptable is a compressed air energy storage (CAES) system and especially its more advanced derivatives (CASH, CAES with Humidification and CAESSI, CAES with Steam Injection). For wind energy systems, both short term and seasonal storage are technically and economically feasible. Background It is increasingly obvious that anthropogenic global climate change must be taken seriously, and that carbon emissions must be reduced drastically if serious harm to the Earth’s biosphere is to be avoided. This certainly will involve dramatically reducing or eliminating the use of carbon based fuels to generate electricity and utilizing nuclear or solar energy systems instead. While nuclear power can technically be used to replace fossil fuels, it has important disadvantages, most significantly the irreducible finite possibility of a catastrophic accident. It would thus be desirable to limit its use to a minimum and satisfy most or all of our electricity needs using solar power. However, if solar generated electricity is to be a credible alternative it must have technical characteristics equal to those of fossil or nuclear power; that is, it must be easily utilized in modern industrial state, and its cost must be reasonable. Since renewable resources are generally diffuse, remote from major demand centers, and intermittent, the issues of transmission and storage must be addressed; fortunately, it is easy to demonstrate that an affordable technical solution to the challenge of electricity storage already exists. The need for utility scale storage is illustrated by the current status of wind energy, which now supplies about 15 percent of Denmark’s electricity. Wind turbines are by far the lowest cost and most successful source of renewable electrical energy available today. This is due both to the superb quality of the turbines developed over the past decade, as well as to far-sighted and effective public policy that mandates a justifiably high price for wind electricity. These same policies do have some negative effects, however, which up to now have not impeded the rapid increase of wind electricity onto the grid. Utilities, in most cases, are forced to absorb the costs of transmission line and substation reinforcement and of insuring overall system reliability. Given low wind turbine capacity factor (25-30 percent), transmission has already become an issue in some areas, while system reliability is increasingly a problem as wind penetration grows above 10 percent of average electricity demand, as it has in Denmark. In addition, a new system of balancing charges designed to insure that supply balances demand in deregulated markets threatens to penalize wind quite strongly [1]. One way to resolve these issues to the advantage of wind and other intermittent energy is to include storage on the system in a way that recognizes the wind/storage plant as a unified entity: that is, the output of the total system should be classified as renewable energy. This will resolve immediate transmission and reliability issues as well as allowing wind in the not too distant future to supply up to about 80 percent of total electricity demand [2]. Comparison Of Storage Technologies Pumped storage, batteries, superconducting magnet energy storage, flywheel energy storage, regenerative fuel cell storage and compressed air energy storage could be considered for bulk power storage; a cost comparison [3,4 ] of these is listed in Table 1. The critical parameters for these systems are

93 citations


Journal ArticleDOI
TL;DR: In this paper, the experimental results of the European DISS (DIrect Solar Steam) project are presented, where the experiments are subdivided into steady state and transient tests, with the goal of the steady state tests being the investigation of the thermohydraulic phenomena of the occurring two phase flow.
Abstract: This article presents the latest experimental results of the European DISS (DIrect Solar Steam) project. The experiments are subdivided into steady state and transient tests. The goal of the steady state tests is the investigation of the thermohydraulic phenomena of the occurring two phase flow, whereas the transient tests are needed for the controller design. The experimental results are compared to simulation studies. Implications for the plant operation will be discussed.

90 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented a novel process comprising solar upgrading of hydrocarbons by steam reforming in solar specific receiver-reactors and utilizing the upgraded, hydrogenrich fuel in high efficiency conversion systems, such as gas turbines or fuel cells.
Abstract: This paper presents a novel process comprising solar upgrading of hydrocarbons by steam reforming in solar specific receiver-reactors and utilizing the upgraded, hydrogenrich fuel in high efficiency conversion systems, such as gas turbines or fuel cells. In comparison to conventionally heated processes about 30% of fuel can be saved with respect to the same specific output. Such processes can be used in small scale as a stand-alone system for off-grid markets as well as in large scale to be operated in connection with conventional combined-cycle plants. The complete reforming process will be demonstrated in the SOLASYS project, supported by the European Commission in the JOULE/THERMIE framework. The project has been started in June 1998. The SOLASYS plant is designed for 300 kW el output, it consists of the solar field, the solar reformer and a gas turbine, adjusted to operate with the reformed gas. The SOLASYS plant will be operated at the experimental solar test facility of the Weizmann Institute of Science in Israel. Start-up of the pilot plant is scheduled in April 2001. The midterm goal is to replace fossil fuels by renewable or non-conventional feedstock in order to increase the share of renewable energy and to establish processes with only minor or no CO 2 emission. Examples might be upgrading of bio-gas from municipal solid waste as well as upgrading of weak gas resources.

Journal ArticleDOI
TL;DR: In this article, the authors proposed an optical system to reduce the peak flux to a nominally acceptable value in a cost effective manner using a real-time processor which selects specific heliostats for removal from track.
Abstract: In the 1980s, the Utility Study [Hilesland, T., Jr., and Weber, E. R., 1988, Utilities' Study of Solar Central Receivers, Fourth Int. Symp. on Research, Development, and Applications of Solar Thermal Technology, Santa Fe, NM] identified the external cylindrical molten-salt-in-tube receiver with a surround heliostat field as the most cost effective and practical design for commercial applications. Such designs typically require 50-1000 MW of design-point thermal power at outlet temperatures around 1050°F (565°C). Using computer codes such as RCELL [Lipps, F. W., and Vant-Hull, L. L., 1978, A Cellwise Method for the Optimization of Large Central Receiver Systems, Solar Energy, 20(6) pp. 505-516.] or DELSOL [Kistler, B. L., A Users Manual for Delsol 3, Sandia National Laboratories Livermore, SAND86-8018, 1987.] it is straightforward to design an optical system to meet these requirements, defining the smallest receiver (lower cost and thermal losses) and the most cost effective heliostat field. As the performance of heliostats in the anti-sun locations is better such fields tend to be biased (in the northern hemisphere) to the north side of the receiver and produce very high flux densities there; typically 2-5 MW/m 2 . However, the receiver is typicaly limited to a salt velocity and temperature dependent allowable flux density (AFD) of about 1 MW/m 2 . Design methods to reduce this peak flux to a nominally acceptable value in a cost effective manner are presented. Residual excess flux events under non-nominal conditions are handled by a real-time processor which selects specific heliostats for removal from track. This same processor is used to preheat the receiver using a special algorithm to define the required flux density.

Journal ArticleDOI
TL;DR: The measurement and instrumentation team within the Distributed Energy Resources Center at the National Renewable Energy Laboratory, NREL, calibrates pyranometers for outdoor testing solar energy conversion systems as mentioned in this paper.
Abstract: The Measurements and Instrumentation Team within the Distributed Energy Resources Center at the National Renewable Energy Laboratory, NREL, calibrates pyranometers for outdoor testing solar energy conversion systems. The team also supports climate change research programs. These activities led NREL to improve pyranometer calibrations. Low thermal-offset radiometers measuring the sky diffuse component of the reference solar irradiance removes bias errors on the order of 20 Watts per square meter (W/m 2 ) in the calibration reference irradiance. Zenith angle dependent corrections to responsivities of pyranometers removes 15 to 30 W/m 2 bias errors from field measurements. Detailed uncertainty analysis of our outdoor calibration process shows a 20% reduction in the uncertainty in the responsivity of pyranometers. These improvements affect photovoltaic module and array performance characterization, assessment of solar resources for design, sizing, and deployment of solar renewable energy systems, and ground-based validation of satellite-derived solar radiation fluxes.


Journal ArticleDOI
TL;DR: In this paper, a numerical optimization algorithm was used together with an aero-elastic load prediction code and a cost model to minimize the cost of energy from manufacture and installation for the offshore site.
Abstract: A method is presented for site-specific design of wind turbines where cost of energy is minimized. A numerical optimization algorithm was used together with an aeroelastic load prediction code and a cost model. The wind climate was modeled in detail including simulated turbulence. Response time series were calculated for relevant load cases, and lifetime equivalent fatigue loads were derived. For the fatigue loads, an intelligent sensitivity analysis was used to reduce computational costs. Extreme loads were derived from statistical response calculations of the Davenport type. A comparison of a 1.5 MW stall regulated wind turbine in normal onshore flat terrain and in an offshore wind farm showed a potential increase in energy production of 28% for the offshore wind farm, but also significant increases in most fatigue loads and in cost of energy. Overall design variables were optimized for both sites. Compared to an onshore optimization, the offshore optimization increased swept area and rated power whereas hub height was reduced. Cost of energy from manufacture and installation for the offshore site was reduced by 10.6% to 4.6 O. This reduction makes offshore wind power competitive compared with today's onshore wind turbines. The presented study was made for one wind turbine concept only, and many of the involved sub models were based on simplified assumptions. Thus there is a need for further studies of these models.

Journal ArticleDOI
TL;DR: In this paper, the performance of a model-based periodic gain controller is investigated for a two-bladed, variable-speed, horizontal-axis wind turbine with five degrees-of-freedom; tower fore-aft bending, nacelle yaw, rotor position and flapwise bending of each blade.
Abstract: An investigation of the performance of a model-based periodic gain controller is presented for a two-bladed, variable-speed, horizontal-axis wind turbine. Performance is based on speed regulation using full-span collective blade pitch. The turbine is modeled with five degrees-of-freedom; tower fore-aft bending, nacelle yaw, rotor position, and flapwise bending of each blade. An attempt is made to quantify what model degrees-of-freedom make the system most periodic, using Floquet modal properties. This justifies the inclusion of yaw motion in the model. Optimal control ideas are adopted in the design of both periodic and constant gain full-state feedback controllers, based on a linearized periodic model. Upon comparison, no significant difference in performance is observed between the two types of control in speed regulation.

Journal ArticleDOI
TL;DR: In this article, a computerized method has been developed to aid preliminary design of composite wind turbine blades, which allows for arbitrary specification of the chord, twist, and airfoil geometry along the blade and an arbitrary number of shear webs.
Abstract: A computerized method has been developed to aid preliminary design of composite wind turbine blades. The method allows for arbitrary specification of the chord, twist, and airfoil geometry along the blade and an arbitrary number of shear webs. Given the blade external geometry description and its design load distribution, the Fortran code uses ultimate-strength and buckling-resistance criteria to compute the design thickness of load-bearing composite laminates. The code also includes an analysis option to obtain blade properties if a composite laminates schedule is prescribed. These properties include bending stiffness, torsion stiffness, mass, moments of inertia, elastic-axis offset, and center-of-mass offset along the blade. Nonstructural materials-gelcoat, nexus, and bonding adhesive-are also included for computation of mass. This paper describes the assumed structural layout of composite laminates within the blade, the design approach, and the computational process. Finally, an example illustrates the application of the code to the preliminary design of a hypothetical blade and computation of its structural properties.

Journal ArticleDOI
TL;DR: In this paper, a vortex-flow chemical reactor was used to produce Zn and synthesis gas by the combined reduction of ZnO and reforming of CH 4 in a high-flux solar furnace.
Abstract: The co-production of Zn and synthesis gas by the combined reduction of ZnO and reforming of CH 4 has been performed using a vortex-flow chemical reactor in a high-flux solar furnace. The reactor operating temperature ranged between 1221 and 1481 K for an input solar power of 2.3 to 4.6 kW and mean solar flux intensities of 810 to 1609 kW/m 2 . The performance of the reactor was determined by conducting a complete mass and energy balance for the chemical process. The chemical conversion ranged between 83-100 percent. The thermal efficiency, defined as the portion of input solar power absorbed as sensible and process heat, was in the range 11-28 percent. The exergy efficiency for the closed cycle, defined as the ratio of the maximum amount of work that the products leaving the reactor could produce if were re-combined to the input solar power, was in the range 0.3-3.1 percent. Major sources of energy loss are re-radiation heat transfer through the reactor aperture, conduction heat transfer through the reactor walls, and the quenching of the reaction products.

Journal ArticleDOI
TL;DR: In this article, a solar chemical reactor was designed, constructed and tested for the direct thermal decomposition of zinc oxide at temperatures as high as 2250 K using concentrated sunlight, and a 1-dimensional numerical model was developed to predict the reactor's thermal performance under various solar flux levels and to identify the physio-chemical properties of ZnO that are critical for designing the reactor.
Abstract: A solar chemical reactor was designed, constructed and tested for the direct thermal decomposition of zinc oxide at temperatures as high as 2250 K using concentrated sunlight. Along with the reactor, a 1-dimensional numerical model was developed to predict the reactor's thermal performance under various solar flux levels and to identify the physio-chemical properties of ZnO that are critical for designing the reactor. An experimental study was also conducted to ascertain how best to employ a curtain of inert gas to keep the reactor's window clean of Zn and ZnO. The reactor proved to be a reliable research tool for effecting the decomposition reaction and it possesses many features characteristic of a reactor scale-able to an industrial level: it is resilient to thermal shock; it has a low effective thermal inertia, and it can operate in a continuous mode when ZnO as a powder is fed to the reactor. Furthermore, experimental work led to insight on how best to keep the window clean in the course of an experiment. Also, comparisons between output from the numerical model and experimental results show that the solar flux and the ZnO's thermal conductivity and emissivity are the most critical variables affecting the exergy efficiency of the reactor and the mass flux of product gases. The comparison further reveals the need to investigate whether or not the magnitude of the published pre-exponential term in the decomposition rate equation used in the numerical model should be reduced for improving agreement between the model and the experimental results.

Journal ArticleDOI
TL;DR: In this article, the authors document and comprehend the vortex kinematics for three-dimensional, unsteady, vortex dominated flows occurring on horizontal axis wind turbine blades during nonzero yaw conditions.
Abstract: Horizontal axis wind turbines routinely suffer significant time varying aerodynamic loads that adversely impact structures, mechanical components, and power production. As lighter and more flexible wind turbines are designed to reduce overall cost of energy, greater accuracy and reliability will become even more crucial in future aerodynamics models. However to render calculations tractable, current modeling approaches admit various approximations that can degrade model predictive accuracy. To help understand the impact of these modeling approximations and improve future models, the current effort seeks to document and comprehend the vortex kinematics for three-dimensional, unsteady, vortex dominated flows occurring on horizontal axis wind turbine blades during non-zero yaw conditions. To experimentally characterize these flows, the National Renewable Energy Laboratory Unsteady Aerodynamics Experiment turbine was erected in the NASA Ames 80 ft×120 ft wind tunnel. Then, under strictly-controlled inflow conditions, turbine blade surface pressures and local inflow velocities were acquired at multiple radial locations. Surface pressure histories and normal force records were used to characterize dynamic stall vortex kinematics and normal forces. Stall vortices occupied approximately two-thirds of the aerodynamically active blade span and persisted for nearly one-fourth of the blade rotation cycle. Stall vortex convection varied dramatically along the blade radius, yielding pronounced dynamic stall vortex deformation. Analysis of these data revealed systematic alterations to vortex kinematics due to changes in test section speed, yaw error, and blade span location.

Journal ArticleDOI
TL;DR: In this paper, the 3-moment random peak model and a 3- or 4-moments random process model are compared for both flap and edge bending loads in both parked and operating turbine conditions.
Abstract: This paper considers two distinct topics that arise in reliability-based wind turbine design. First, it illustrates how general probability models can be used to predict long-term design loads from a set of limited-duration, short-term load histories. Second, it considers in detail the precise choice of probability model to be adopted, for both flap and edge bending loads in both parked and operating turbine conditions. In particular, a 3-moment random peak model and a 3- or 4-moment random process model are applied and compared. For a parked turbine, all models are found to be virtually unbiased and to notably reduce uncertainty in estimating extreme loads (e.g., by roughly 50 percent). For an operating turbine, however, only the random peak model is found to retain these beneficial features. This suggests the advantage of the random peak model, which appears to capture the rotating blade behavior sufficiently well to accurately predict extremes.

Journal ArticleDOI
TL;DR: In this article, a study of simultaneous heat and mass transfer was conducted on a vertical falling film absorber to better understand the mechanisms driving the heat-and mass transfer processes, and the results showed that the mass flux and therefore the absorber load can be solved for using a constant flux approximation.
Abstract: A study of simultaneous heat and mass transfer was conducted on a vertical falling film absorber to better understand the mechanisms driving the heat and mass transfer processes. Thermographic phosphors were successfully used to measure the temperature profile along the length of the absorber test tube. These measures of the local variations in temperature enabled calculation of the bulk concentration along the length of the absorber. The bulk concentration varied linearly, which infers that the concentration gradient in the direction of flow is approximately constant. The implication is that the mass flux and therefore the absorber load can be solved for using a constant flux approximation. Design data and correlations are sparse in the open literature. Some experimental data are available; however, all literature data to date have been derived at mass fractions of lithium bromide ranging from 0.30 to 0.60. Experiments were therefore conducted with no heat and mass transfer additive on an internally cooled smooth tube of 0.01905-m outside diameter and of 1.53-m length. The data, for testing at 0.62 and 0.64 mass fraction, were scaled and correlated into both Nu and Sh formulations. The average absolute error in the Nu correlation is about ±3.5% of the Nu number reduced from the experimental data. The Sh correlation is about ±5% of the reduced Sh data. Data from the open literature were reduced to the authors Nu and Sh formulations, and were within 5% of the correlations developed in the present study. The study therefore provides test data with no heat and mass transfer additive and correlations for the coupled heat- and mass-transfer process that are validated against the extensive experimental data.

Journal ArticleDOI
TL;DR: In this paper, a test bed for building integrated photovoltaic test bed has been constructed at the National Institute of Standards and Technology to provide the performance data needed for model validation.
Abstract: The photovoltaic industry is experiencing rapid growth. Industry analysts project that photovoltaic sales will increase from their current $1.5 billion level to over $27 billion by 2020, representing an average growth rate of 25 %. (Cook et al. 2000) [1]. To date, the vast majority of sales have been for navigational signals, call boxes, telecommunication centers, consumer products, off-grid electrification projects, and small grid-interactive residential rooftop applications. Building integrated photovoltaics, the integration of photovoltaic cells into one or more of the exterior surfaces of the building envelope, represents a small but growing photovoltaic application. In order for building owners, designers, and architects to make informed economic decisions regarding the use of building integrated photovoltaics, accurate predictive tools and performance data are needed. A building integrated photovoltaic test bed has been constructed at the National Institute of Standards and Technology to provide the performance data needed for model validation. The facility incorporates four identical pairs of building integrated photovoltaic panels constructed using single-crystalline, polycrystalline, silicon film, and amorphous silicon photovoltaic cells, One panel of each identical pair is installed with thermal insulation attached to its rear surface. The second paired panel is installed without thermal insulation. This experimental configuration yields results that quantify the effect of elevated cell temperature on the panels' performance for different cell technologies. This paper presents the first set of experimental results from this facility. Comparisons are made between the electrical performance of the insulated and non-insulated panels for each of the e four cell technologies. The monthly and overall conversion efficiencies for each cell technology are presented and the seasonal performance variations discussed. Daily efficiencies are presented for a selected month. Finally, plots of the power output and panel temperatures are presented and discussed for the single-crystalline and amorphous silicon panels.

Journal ArticleDOI
TL;DR: The National Institute of Standards and Technology (NIST) has undertaken a multiple-year project to collect high quality experimental performance data for building integrated photovoltaics as mentioned in this paper, where the data will be used to validate computer models and, where necessary, to develop algorithms that may be incorporated within these models.
Abstract: The widespread use of building integrated photovoltaics appears likely as a result of the continuing decline in photovoltaic manufacturing costs, the relative ease in which photovoltaics can be incorporated within the building envelope, and the fact that buildings account for over 40 percent of the U.S. energy consumption. However, designers, architects, installers, and consumers need more information and analysis tools in order to judge the merits of building -integrated solar photovoltaic products. In an effort to add to the knowledge base, the National Institute of Standards and Technology (NIST) has undertaken a multiple -year project to collect high quality experimental performance data. The data will be used to validate computer models for building integrated photovoltaics and, where necessary, to develop algorithms that may be incorporated within these models. This paper describes the facilities that have been constructed to assist in this effort. The facilities include a mobile tracking photovoltaic test facility, a building integrated photovoltaic “test bed”, an outdoor aging rack, and a meteorological station.

Journal ArticleDOI
TL;DR: In this paper, the performance of an air breathing proton exchange membrane (PEM) fuel cell stack has been experimentally measured to investigate the steady-state and transient effects of temperature, humidity and air flowrate.
Abstract: The performance of an air breathing proton exchange membrane (PEM) fuel cell stack has been experimentally measured to investigate the steady-state and transient effects of temperature, humidity and air flowrate. The results show that hydrogen leaks to the cathode through the membrane causing internal heating of the fuel cell. The leakage rate is found to be linearly dependent on the pressure difference between the hydrogen side and air side which is at atmospheric pressure. Temperature was found to not have a significant effect on the PEM performance, except through its indirect effect on humidity. The humidity of the membrane is found to be the most significant variable in determining the fuel cell performance. The airflow also influences the performance of the fuel cell directly by supplying oxygen and indirectly by influencing the humidity of the membrane. Experiments show that an optimum air flowrate exists that is much larger than required for stoichiometric oxidation of the fuel.

Journal ArticleDOI
TL;DR: In this paper, a hot-salt pump was developed for moltensalt solar power tower applications that will reduce the capital cost of the plant, eliminate many of the piping, valve and sump problems associated with the handling of molten salt and improve the reliability of a critical part of the operating system.
Abstract: A new hot-salt pump has been developed for moltensalt solar power tower applications that will reduce the capital cost of the plant, eliminate many of the piping, valve and sump problems associated with the handling of molten salt and improve the reliability of a critical part of the operating system of the plant. Previous systems required that the pumps in these plants be housed in shallow sumps that were gravity fed by the storage tanks. This new pump arrangement will eliminate the sumplevel control valves and the potential for overflowing the pump sump vessels. Until now only cantilever pumps were qualified for hot molten-salt service because no suitable bearing materials had been tested. This paper describes the successful qualification of a long-shafted pump with salt-lubricated bearings tested for over 5000 hours with nitrate salt at 565 °C. BACKGROUND


Journal ArticleDOI
TL;DR: In this paper, the first three statistical moments (mean, coefficient of variation, and skewness) of a wind turbine were mapped to wind conditions with a two-dimensional regression over ten-minute average wind speed and turbulence intensity, and the longterm distribution of ranges was determined by integrating over the annual distribution of input conditions.
Abstract: International standards for wind turbine certification depend on finding long-term fatigue load distributions that are conservative with respect to the state of knowledge for a given system Statistical models of loads for fatigue application are described and demonstrated using flap and edge blade-bending data from a commercial turbine in complex terrain Distributions of rainflow-counted range data for each ten-minute segment are characterized by parameters related to their first three statistical moments (mean, coefficient of variation, and skewness) Quadratic Weibull distribution functions based on these three moments are shown to match the measured load distributions if the non-damaging low-amplitude ranges are first eliminated The moments are mapped to the wind conditions with a two-dimensional regression over ten-minute average wind speed and turbulence intensity With this mapping, the short-term distribution of ranges is known for any combination of average wind speed and turbulence intensity The longterm distribution of ranges is determined by integrating over the annual distribution of input conditions First, we study long-term loads derived by integration over wind speed distribution alone, using standard-specified turbulence levels Next, we perform this integration over both wind speed and turbulence distribution for the example site Results are compared between standarddriven and site-driven load estimates Finally, using statistics based on the regression of the statistical moments over the input conditions, the uncertainty (due to the limited data set) in the long-term load distribution is represented by 95% confidence bounds on predicted loads

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TL;DR: In this paper, the authors used a numerical model to determine the heat loss to the air stream over a perforated, sinusoidal plate with suction to air flowing over the plate, perpendicular to the corrugations.
Abstract: Heat transfer from a perforated, sinusoidal plate with suction to air flowing over the plate, perpendicular to the corrugations, has been studied numerically and experimentally. This study used a numerical model, validated by wind tunnel tests and hot wire anemometer/resistance thermometer measurements, to determine the heat loss to the air stream over the plate as a function of wind speed, suction velocity, and plate geometry. Both attached and separated flow regimes were observed, and the criterion for flow attachment was determined to be Re V0,P ≥6.93 Re 0.5 U ∞ ,A . Correlations were developed for heat transfer to the air stream for each flow regime. For attached flow, the heat transfer can be represented as Nu att = Nu falat {1 +0.81(A/P) 0.5 }. For separated flow, the following correlation applies: Nu sep = 2.05(A/P) 1.40 Re 1.63 .

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TL;DR: In this paper, Muneer's models for the luminous efficacy of global and diffuse radiation, useful for illumination engineers, are statistically assessed using data obtained at Madrid during a seven-year period.
Abstract: From luminous efficacy models, illuminance can be estimated if irradiance is known or can be estimated. In the present paper, Muneer's models for the luminous efficacy of global and diffuse radiation, useful for illumination engineers, are statistically assessed using data obtained at Madrid during a seven-year period. Several other models inspired on the simple approach proposed by Muneer are developed and statistically assessed. Some of those model the luminous efficacy of global or diffuse solar radiation, while others directly model the global or the diffuse illuminance. It is shown that for diffuse illuminance estimation the ratio of diffuse to extraterrestrial irradiance is to be preferred as independent variable to the ratio of global to extraterrestrial irradiance proposed by Muneer. Some models that estimate global illuminance from global irradiance and solar elevation, or diffuse illuminance from diffuse irradiance and solar elevation, perform practically on a par with the corresponding luminous efficacy models. It is also concluded that the striking difference between global luminous efficacy between the United Kingdom and Japan observed by Muneer is not due to a latitude effect.