Showing papers in "Journal of Solar Energy Engineering-transactions of The Asme in 2009"

Predicted Efficiency of a Low-Temperature Nanofluid-Based Direct Absorption Solar Collector

Abstract: Due to its renewable and nonpolluting nature, solar energy is often used in applications such as electricity generation, thermal heating, and chemical processing. The most cost-effective solar heaters are of the "flat-plate" type, but these suffer from relatively low efficiency and outlet temperatures. The present study theoretically investigates the feasibility of using a nonconcentrating direct absorption solar collector (DAC) and compares its performance with that of a typical flat-plate collector. Here a nanofluid-a mixture of water and aluminum nanoparticles—is used as the absorbing medium. A two-dimensional heat transfer analysis was developed in which direct sunlight was incident on a thin flowing film of nanofluid. The effects of absorption and scattering within the nanofluid were accounted for. In order to evaluate the temperature profile and intensity distribution within the nanofluid, the energy balance equation and heat transport equation were solved numerically. It was observed that the presence of nanoparticles increases the absorption of incident radiation by more than nine times over that of pure water. According to the results obtained from this study, under similar operating conditions, the efficiency of a DA C using nanofluid as the working fluid is found to be up to 10% higher (on an absolute basis) than that of a flat-plate collector. Generally a DAC using nanofluids as the working fluid performs better than a flat-plate collector, however, much better designed flat-plate collectors might be able to match or outperform a nanofluids based DAC under certain conditions.

Characterization of MgSO4 Hydrate for Thermochemical Seasonal Heat Storage

Abstract: Water vapor sorption in salt hydrates is one of the most promising means for compact, low loss, and long-term storage of solar heat in the built environment. One of the most interesting salt hydrates for compact seasonal heat storage is magnesium sulfate heptahydrate MgSO4 ·7H 2O. This paper describes the characterization of MgSO4 ·7H 2 Ot o examine its suitability for application in a seasonal heat storage system for the built environment. Both charging (dehydration) and discharging (hydration) behaviors of the material were studied using thermogravimetric differential scanning calorimetry, X-ray diffraction, particle distribution measurements, and scanning electron microscope. The experimental results show that MgSO4 ·7H 2O can be dehydrated at temperatures below 150° C, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store 2.2 GJ/ m 3 , almost nine times more energy than can be stored in water as sensible heat. On the other hand, the experimental results indicate that the release of the stored heat is more difficult. The amount of water taken up and the energy released by the material turned out to be strongly dependent on the water vapor pressure, temperature, and the total system pressure. The results of this study indicate that the application of MgSO4 ·7H 2O at atmospheric pressure is problematic for a heat storage system where heat is released above 40° C using a water vapor pressure of 1.3 kPa. However, first experiments performed in a closed system at low pressure indicate that a small amount of heat can be released at 50° C and a water vapor pressure of 1.3 kPa. If a heat storage system has to operate at atmospheric pressure, then the application of MgSO4 ·7H 2O for seasonal heat storage is possible for space heating operating at 25° C and a water vapor pressure of 2.1 kPa. DOI: 10.1115/1.4000275

A New Fast Ray Tracing Tool for High-Precision Simulation of Heliostat Fields

Abstract: A completely new ray tracing software has been developed at the German Aerospace Center. The main purpose of this software is the flux density simulation of heliostat fields with a very high accuracy in a small amount of computation time. The software is primarily designed to process real sun shape distributions and real highly resolved heliostat geometry data, which means a data set of normal vectors of the entire reflecting surface of each heliostat in the field. Specific receiver and secondary concentrator models, as well as models of objects that are shadowing the heliostat field, can be implemented by the user and be linked to the simulation software subsequently. The specific architecture of the software enables the provision of other powerful simulation environments with precise flux density simulation data for the purpose of entire plant simulations. The software was validated through a severe comparison with measured flux density distributions. The simulation results show very good accordance with the measured results.

The Actuator Surface Model: A New Navier-Stokes Based Model for Rotor Computations

Abstract: This paper presents a new numerical technique for simulating two-dimensional wind turbine flow. The method, denoted as the 2D actuator surface technique, consists of a two-dimensional Navier-Stokes solver in which the pressure distribution is represented by body forces that are distributed along the chord of the airfoils. The distribution of body force is determined from a set of predefined functions that depend on angle of attack and airfoil shape. The predefined functions are curve fitted using pressure distributions obtained either from viscous-inviscid interactive codes or from full Navier-Stokes simulations. The actuator surface technique is evaluated by computing the two-dimensional flow past a NACA 0015 airfoil at a Reynolds number of 10 6 and an angle of attack of 10 deg and by comparing the computed streamlines with the results from a traditional Reynolds-averaged Navier-Stokes computation. In the last part, the actuator surface technique is applied to compute the flow past a two-bladed vertical axis wind turbine equipped with NACA 0012 airfoils. Comparisons with experimental data show an encouraging performance of the method.

Test Results of Concrete Thermal Energy Storage for Parabolic Trough Power Plants

Abstract: Efficient energy storage is vital to the success of solar thermal power generation and industrial waste heat recovery A sensible heat storage system using concrete as the storage material has been developed by the German building company Ed Zublin AG and the German Aerospace Center (DLR) A major focus was the cost reduction in the heat exchanger and the high temperature concrete storage material For live tests and further improvements, a 20 m 3 solid media storage test module connected to an electrically heated thermal oil loop was built in Stuttgart The design of the test module and the test results are described in this paper By the end of November 2008, the second generation solid media storage test module had accumulated five months of operation in the temperature range between 300°C and 400°C and almost 100 thermal cycles with a temperature difference of 40 K The tests will be continued in 2009

Prediction of Wind Farm Power Ramp Rates: A Data-Mining Approach

Abstract: In this paper, multivariate time series models were built to predict the power ramp rates of a wind farm. The power changes were predicted at 10 min intervals. Multivariate time series models were built with data-mining algorithms. Five different data-mining algorithms were tested using data collected at a wind farm. The support vector machine regression algorithm performed best out of the five algorithms studied in this research. It provided predictions of the power ramp rate for a time horizon of 10-60 min. The boosting tree algorithm selects parameters for enhancement of the prediction accuracy of the power ramp rate. The data used in this research originated at a wind farm of 100 turbines. The test results of multivariate time series models were presented in this paper. Suggestions for future research were provided.

Development and Validation of a CFD Technique for the Aerodynamic Analysis of HAWT

Abstract: This paper demonstrates the potential of a compressible Navier–Stokes CFD method for the analysis of horizontal axis wind turbines. The method was first validated against experimental data of the NREL/NASA-Ames Phase VI (Hand, , 2001, “Unsteady Aerodynamics Experiment Phase, VI: Wind Tunnel Test Configurations and Available Data Campaigns,” NREL, Technical Report No. TP-500-29955) wind-tunnel campaign at 7 m/s, 10 m/s, and 20 m/s freestreams for a nonyawed isolated rotor. Comparisons are shown for the surface pressure distributions at several stations along the blades as well as for the integrated thrust and torque values. In addition, a comparison between measurements and CFD results is shown for the local flow angle at several stations ahead of the wind turbine blades. For attached and moderately stalled flow conditions the thrust and torque predictions are fair, though improvements in the stalled flow regime are necessary to avoid overprediction of torque. Subsequently, the wind-tunnel wall effects on the blade aerodynamics, as well as the blade/tower interaction, were investigated. The selected case corresponded to 7 m/s up-wind wind turbine at 0 deg of yaw angle and a rotational speed of 72 rpm. The obtained results suggest that the present method can cope well with the flows encountered around wind turbines providing useful results for their aerodynamic performance and revealing flow details near and off the blades and tower.

Optimal Design of a Molten Salt Thermal Storage Tank for Parabolic Trough Solar Power Plants

Abstract: This paper presents an optimal design procedure for internally insulated, carbon steel, molten salt thermal storage tanks for parabolic trough solar power plants. The exact size of the vessel and insulation layers and the shape of the roof are optimized by minimizing the total investment cost of the storage system under three technical constraints: remaining within the maximum allowable values of both temperature and stress in the steel structure, and avoiding excessive cooling and consequent solidification of the molten salt during long periods of no solar input. The thermal, mechanical and economic aspects have been integrated into an iterative step-by-step optimization procedure, which is shown to be effective through application to the case study of a 600 MW h thermal storage system. The optimal design turns out to be an internally insulated, carbon steel storage tank characterized by a maximum allowable height of 11 m and a diameter of 22.4 m. The total investment cost is about 20% lower than that of a corresponding AISI 321H stainless steel storage tank without internal protection or insulation.

Analysis Model of Mismatch Power Losses in PV Systems

Abstract: A novel procedure to extract and analyze the power losses, mainly due to mismatch effects, in a photovoltaic (PV) system is presented. The developed model allows the extraction of the main PV module and PV array parameters from I-V characteristics, as well as in dynamic behavior under real conditions of work. The method allows a good estimation of the mismatch effect on the total PV system power losses.

Experimental Study of PCM Inclusion in Different Building Envelopes

Abstract: The main objective of this paper is to demonstrate experimentally that it is possible to improve the thermal comfort and reduce the energy consumption of a building without substantial increase in the weight of the construction materials with the inclusion of phase change materials (PCM). PCM are a suitable and promising technology for this application. This paper presents an experimental setup to test PCM with various typical insulation and construction materials in real conditions in Puigverd de Lleida (Lleida, Spain). Nine small house-sized cubicles were constructed: two with concrete, five with conventional brick, and two with alveolar brick. PCM was added in one cubicle of each typology. For each type of construction specific experiments were done. In all cubicles, free-floating temperature experiments were performed to determine the benefits of using PCM. A Trombe wall was added in both concrete cubicles and its influence was investigated. All brick cubicles were equipped with domestic heat pumps as Heating, Ventilation, and Air Conditioning (HVAC) system; therefore, the energy consumption was registered, providing real information about the energy savings. Results were very good for the concrete cubicles, since temperature oscillation were reduced by up to 4°C through the use of PCM and also peak temperatures in the PCM cubicle were shifted in later hours. In the brick cubicles, the energy consumption of the HVAC system in summer was reduced by using PCM for set points higher than 20°C. During winter an insulation effect of the PCM is observed, keeping the temperatures of the cubicles warmer, especially during the cold hours of the day.

Slope Error Measurements of Parabolic Troughs Using the Reflected Image of the Absorber Tube

Abstract: A new fast method for optically measuring the reflector slope of parabolic troughs with high accuracy has been developed. It uses the reflection of the absorber tube in the concentrator as seen from some distance and is therefore called “absorber reflection method”. A digital camera is placed at a distant observation point perpendicular to the trough axis with the concentrator orientated towards it. Then, a set of pictures from the absorber tube reflection is taken with the concentrator in slightly different tilt angles. A specially developed image analysis algorithm detects the edges of the absorber tube in the reflected images. This information, along with the geometric relationship between the components of the set-up and the known approximately parabolic shape of the concentrator, is used to calculate the slopes perpendicular to the trough axis. Measurement results of a EuroTrough segment of four facets are presented and verified with results from a reference measurement using high-resolution close-range photogrammetry. The results show good agreement as well in statistical values as in local values of the reflector slope. In contrast to the photogrammetric data acquisition method, the new technique allows for drastically reduced measurement time.

Development of PCM Storage for Process Heat and Power Generation

Abstract: The increased interest in solar thermal systems using steam as a working medium either for power generation or process heat applications gave rise to a growing demand for latent heat storage units. Essential for the development of cost-effective latent heat storage systems is the achievement of a sufficient power level in spite of the characteristic low thermal diffusivities of latent heat storage materials. The sandwich concept using fins made either from graphite or aluminum has been identified as the most promising option for latent heat storage systems. The feasibility of this approach has been demonstrated by three prototypes using graphite and one prototype using aluminum fins. The prototype with aluminum fins was filled with sodium nitrate and was operated for more than 4000 h without degradation of power. The volume specific average power density is in the range 10-25 kW/m 3 , so it is proven that the major problem of phase change material (PCM) storage of low heat transfer rates has been overcome and high-temperature PCM storage with large capacity factor is possible.

Comparative Evaluation of Maximum Power Point Tracking Methods for Photovoltaic Systems

Abstract: This paper presents a study of two maximum power point tracking methods for grid connected photovoltaic systems. The best operation conditions for the perturbation and observation and the incremental conductance methods are investigated in order to identify the efficiency performances of these most popular maximum power point tracking methods for photovoltaic systems. Improvements of these methods can be obtained with the best adjustment of the sampling rate and the perturbation size, both in accordance with the converter dynamics. Practical aspects about the incremental conductance method are discussed, and some modifications are proposed to overcome its problems. A procedure to determine the parameters is explained. This procedure helps to identify which method is better suited for grid connected photovoltaic systems with only one conversion stage. The methods’ influences on the quality of the currents injected in the grid are evaluated and compared. The performance improvement achieved with the choice of the best parameters is proved by means of simulation and experimental results performed on a low power test system. The simulation results have been obtained by modeling a photovoltaic system in MATLAB. A simplified model was used that employs only parameters of interest and therefore decreases simulation time. Experimental results corresponding to the operation of a grid connected photovoltaic converter controlled with a digital signal processor have been obtained. DOI: 10.1115/1.3142827

Rapid Reflective Facet Characterization Using Fringe Reflection Techniques

Abstract: Mirror facets for Concentrating Solar Power (CSP) systems have stringent requirements on slope accuracy in order to provide adequate system performance. This paper presents a newly developed tool that can characterize facets quickly enough for 100% inspection on a production line. A facet for a CSP system, specifically a dish concentrator, has a parabolic design shape. This shape will concentrate near-parallel rays from the sun to a point (or a line for trough systems). Deviations of surface slope from the design shape impact the performance of the system, either losing power that misses the target, or increasing peak fluxes to undesirable levels. Three types of facet slope errors can impact performance. The first is a focal length error, typically caused by springback in the facet forming process. In this case, the wavelength of the error exceeds the size of the facet, resulting in a parabola, but with the wrong focal length. The results in a slope error that is largely systematic across the facet when the measured slope is compared to the design slope. A second shape error, in which the period of the error is on the order of the length of the facet, manifests also as a systematic slope error. In this case, the facet deviates from a parabolic shape, but can be modeled with a higher order curve. Finally, the residual errors after a model is proposed are usually lumped through a Root Mean Square (RMS) process and characterized as the 1-sigma variation of a normal distribution. This usually characterizes the small-scale imperfections in the facet, and is usually called “slope error”. However, all of these deviations from design are in facet errors in the slope of the manufactured facet. The reported characterization system, named SOFAST (Sandia Optical Fringe Analysis Slope Tool) has a computer-connected camera that images the reflective surface, which is positioned so that it views the reflection of an active target, such as an LCD screen. A series of fringe patterns are displayed on the screen while images are captured. Using the captured information, the reflected target location of each pixel of mirror viewed can be determined, and thus through a mathematical transformation, the surface normal map can be developed. This is then fitted to the selected model equation, and the errors from design are characterized. The reported system currently characterizes point focus mirrors (for dish systems), but extensions to line focus facets are planned. While similar approaches have been explored, several key developments are presented here. The combination of the display, capture, and data reduction in one system allows rapid capture and data reduction. An “electronic boresight” approach is developed accommodating physical equipment positioning errors, making the system insensitive to setup errors. A very large number of points are determined on each facet, providing significant detail as to the location and character of the errors. The system is developed in MatLab, providing intimate interactions with the data as techniques and applications are developed. Finally, while commercial systems typically resolve the data to shape determination, this system concentrates on slope characterization and reporting, which is tailored to the solar applications. This system can be used for facet analysis during development. However, the real payoff is in production, where complete analysis is performed in about 10 seconds. With optimized coding, this could be further reduced.Copyright © 2009 by ASME

Speed and Direction Shear in the Stable Nocturnal Boundary Layer

Abstract: Numerous previous works have shown that vertical shear in wind speed and wind direction exist in the atmospheric boundary layer. In this work, meteorological forcing mechanisms, such as the Ekman spiral, thermal wind, and inertial oscillation, are discussed as likely drivers of such shears in the statically stable environment. Since the inertial oscillation, the Ekman spiral, and statically stable conditions are independent of geography, potentially significant magnitudes of speed and direction shear are hypothesized to occur to some extent at any inland site in the world. The frequency of occurrence of non-trivial magnitudes of speed and direction shear are analyzed from observation platforms in Lubbock, Texas and Goodland, Indiana. On average, the correlation between speed and direction shear magnitudes and static atmospheric stability are found to be very high. Moreover, large magnitude speed and direction shears are observed in conditions with relatively high hub-height wind speeds. The effects of speed and direction shear on wind turbine power performance are tested by incorporating a simple steady direction shear profile into the fatigue analysis structures and turbulence simulation code from the National Renewable Energy Laboratory. In general, the effect on turbine power production varies with the magnitude of speed and direction shearmore » across the turbine rotor, with the majority of simulated conditions exhibiting power loss relative to a zero shear baseline. When coupled with observational data, the observed power gain is calculated to be as great as 0.5% and depletion as great as 3% relative to a no shear baseline. The average annual power change at Lubbock is estimated to be -0.5%« less

Imbalance Estimation Without Test Masses for Wind Turbines

Abstract: Rotor imbalances of a wind turbine can cause severe damage of the turbine or its components and thus reduce the lifespan and security of the turbine significantly. At present, balancing of the rotor is a time consuming and expensive process due to the necessity of mounting test weights to measure a reference imbalance state. We describe a new method for the detection and reconstruction of imbalances in the rotor of a wind turbine avoiding test weight measurements. The method is based on a wind turbine model, which is derived by the finite element method. In some respect, the model information replaces the information of a reference imbalance state. A mathematical equation linking imbalances and the resulting vibrations is derived using the model. Thus the inverse problem of computing an imbalance from vibration measured at the nacelle is solvable with the usual techniques for ill-posed problems. We show that our model for a wind turbine can be used to predict the vibrations for a given imbalance distribution. In particular, it can be used to reconstruct the imbalance distribution of a wind turbine from noisy measurements in real time, which is verified both for artificial and real data. Also, an optimization strategy is presented in order to adapt the model to the wind turbine at hand. The new method requires a simple model of a wind turbine under consideration but reduces the measuring effort for the computation of balancing weights. It can be implemented into a condition monitoring system (CMS). For the first time, there can be not only an alarm generation but also the actual imbalance and balancing weights and positions can be computed directly from the observed CMS data.

Phase Change Material Sandwich Panels for Managing Solar Gain in Buildings

Abstract: In this study, a phase change material (PCM) sandwich panel was developed and tested to evaluate the resulting decrease in heating and cooling loads of a test cabin in Adana, Turkey, where Mediterranean climate prevails. The panel was formed by a macropackage of microencapsulated PCM layer together with an insulation panel. Two different PCMs, with melting points 26°C and 23°C, were used in the panel. Temperature distribution in the cabin was measured for four different cases. In summer, the maximum average temperature reduction achieved in the cabin was 2.5°C when only the PCM was used. This corresponded to a summer cooling load reduction of 7%. In winter, the maximum average temperature increase achieved in the cabin was 2.2°C with the PCM sandwich panel. The winter heating load was decreased by 17%. Energies conserved in cooling and heating were calculated as 186 kWh/year and 206 kWh/year, respectively.

A Simplified Method for Calculating the Effective Solar Optical Properties of a Venetian Blind Layer for Building Energy Simulation

Abstract: The use of venetian blinds to control solar gain through windows is common in both residential and commercial buildings, and their potential for reduction in peak cooling load and energy consumption is recognized to be large. As such, there is a strong need for models that allow a venetian blind to be included in glazing system analysis. A simplified method of calculating the "effective" solar optical properties of a venetian blind is presented. The solar optical properties of the entire blind are determined based on slat geometry and the optical properties of the slat material, and on the direction and nature (beam or diffuse) of incident radiation. The slat material optical properties are assumed to be independent of the angle of incidence, and are assumed to transmit and reflect beam radiation diffusely. As a first approximation, the slats are assumed to be flat with negligible thickness. A correction is then developed and applied that accounts for the curvature of the slat. The results of the flat and curved slat models are compared with experimental data for commercially available venetian blinds. Both models demonstrate excellent agreement with experiments, except when the profile and slat angles are aligned. In that case, the flat slat model predicts blind transmissions that are too large. The models developed in this study provide useful input to multilayer glazing/shading models used for rating or for building energy simulations.

Experimental Verification of Optical Modeling of Parabolic Trough Collectors by Flux Measurement

Abstract: In order to optimize the solar field output of parabolic trough collectors (PTCs), it is essential to study the influence of collector and absorber geometry on the optical performance. The optical ray-tracing model of PTC conceived for this purpose uses photogrammetrically measured concentrator geometry in commercial Monte Carlo ray-tracing software. The model has been verified with measurements of a scanning flux measurement system, measuring the solar flux density distribution close to the focal line of the PTC. The tool uses fiber optics and a charged coupled device camera to scan the focal area of a PTC module. Since it is able to quantitatively detect spilled light with good spatial resolution, it provides an evaluation of the optical efficiency of the PTC. For comparison of ray-tracing predictions with measurements, both flux maps and collector geometry have been measured under identical conditions on the Eurotrough prototype collector at the Plataforma Solar de Almeria. The verification of the model is provided by three methods: the comparison of measured intercept factors with corresponding simulations, comparison of measured flux density distributions with corresponding ray-tracing predictions, and comparison of thermographically measured temperature distribution on the absorber surface with flux density distribution predicted for this surface. Examples of sensitivity studies performed with the validated model are shown.

Beating Betz: Energy Extraction Limits in a Constrained Flow Field

Abstract: Experiments with diffusers and other flow concentrating devices have shown that the power performance coefficient Cp of an energy extraction device, defined in relation to the area of flow intercepted at the device, may exceed the Betz limit. "Beating Betz, " in that sense, has been long established but no theory has existed to define in a generalized way what ideal limit may apply to C p in such situations. Recent analysis has resolved this. This indicates that, irrespective of the presence of flow concentration systems or other influences that perturb the flow but do not in themselves extract energy, there is a universal ideal limit of energy extraction. This is found to be 8/9 of the upstream kinetic energy in the streamtube associated with the energy extraction. Moreover, the familiar Betz equations for power and thrust coefficients can be generalized in a simple way to express this. Although this work has been developed in the context of wind energy it will be apparent that the results are of general significance for any application of ducted rotors or propellers in a fluid stream.

Optimization of Solar Photovoltaic Fields

Abstract: The design of stationary and single axes tracking collectors in a field consisting of rows of collectors involves relationships between the field and collector parameters and solar radiation data. In addition, shading and masking of adjacent rows affect the collector deployment of the field by decreasing the incident energy on the collector plane. The use of many rows, densely deployed in a given field, increases the field incident energy but also increases the shading. Therefore, there is an optimal deployment of the collectors in the field yielding, for example, maximum energy minimum required field area, or other objectives. For photovoltaic collectors, the output energy depends on the module efficiency, the solar cell operating temperature, and on the scheme of the electrically interconnected modules. Series interconnection between the photovoltaic modules may have a significant effect on the output energy of the solar plant in event of shading. The present article deals with the optimal design of photovoltaic solar fields for stationary and single axes tracking collectors to obtain maximum annual output energy.

Thermodynamic Analysis of Freezing and Melting Processes in a Bed of Spherical PCM Capsules

Abstract: The solidification and melting processes in a spherical geometry are investigated in this study. The capsules considered are filled with de-ionized water, so that a network of spheres can be thought of as being the storage medium for an encapsulated ice storage module. ANSYS GAMBIT and FLUENT 6.0 packages are used to employ the present model,for heat transfer fluid (HTF) past a row of such capsules, while varying the HTF inlet temperature and flow rate, as well as the reference temperatures. The present model agrees well with experimental data taken from literature and was also put through rigorous time and grid independence tests. Sufficient flow parameters are studied so that the resulting solidification and melting times, exergy and energy efficiencies, and exergy destruction could be calculated. All energy efficiencies are found to be over 99%, though viscous dissipation was included. Using exergy analysis, the exergetic efficiencies are determined to be about 75% to over 92%, depending on the HTF scenario. When the HTF flow rate is increased, all efficiencies decrease, due mainly to increasing heat losses and exergy dissipation. The HTF temperatures, which stray farther from the solidification temperature of water, are found to be most optimal exergetically, but least optimal energetically. The main reason for this, as well as the main mode of loss exergetically, is due to entropy generation accompanying heat transfer, which is responsible for over 99.5% of exergy destroyed in all cases. The results indicate that viewing the heat transfer and fluid flow phenomena in a bed of encapsulated spheres, it is of utmost importance to assess the major modes of entropy generation; in this case from heat transfer accompanying phase change.

Modeling of a Multitube High-Temperature Solar Thermochemical Reactor for Hydrogen Production

Abstract: O-splitting thermochemicalcycle using concentrated solar energy. The continuity, momentum,and energy governing equations that couple the rate of heat trans-fer to the Arrhenius-type reaction kinetics are formulated for anabsorbing-emitting-scattering particulate media and numericallysolved using a computational fluid dynamics code. Parametricsimulations were carried out to examine the influence of the solarflux concentration ratio (3000–6000 suns), number of tubes (1–10), ZnO mass flow rate (2–20 g/min per tube), and ZnO particlesize 0.06–1 m on the reactor’s performance. The reaction ex-tent reaches completion within1sresidence time at above 2000K, yielding a solar-to-chemical energy conversion efficiency of upto 29%.

Optical Design of a Novel Two-Stage Solar Trough Concentrator Based on Pneumatic Polymeric Structures

Abstract: An innovative concept for fabricating solar trough concentrators based on pneumatic polymer mirrors supported on precast concrete frames is presented. Optical aberration is corrected by means of a secondary specular reflector in tandem with a primary cylindrical concentrator. The optimal design is formulated for maximum solar flux concentration. The Monte Carlo ray-tracing technique is applied to determine the effect of reflective surface errors and structural beam deformations on the performance of the combined primary and secondary concentrating system. The numerical results are validated with field measurements on a 49.4 m length, 7.9 m width sun-tracking prototype system. Theoretical maximum solar concentration ratio is 151 suns; the measured one with a flat secondary reflector was 55 suns.

Numerical and Experimental Investigation on a Combined Sensible and Latent Heat Storage Unit Integrated With Solar Water Heating System

Abstract: The present work investigates, theoretically and experimentally the thermal performance of a packed bed combined sensible and latent heat storage unit, integrated with the solar water heating system. A one-dimensional porous medium approach with the finite difference technique is used to develop the numerical model to obtain the temperature profiles of both the phase change material (PCM) and heat transfer fluid (HTF), and the molten mass fraction of the PCM at any axial location of the cylindrical storage tank during the charging process. The model also incorporates the effect of the varying fluid inlet temperature to accommodate the actual conditions that prevails in the solar collector. Experimental apparatus utilizing paraffin as PCM, which is filled in high-density polyethylene spherical capsules, is constructed and integrated with a solar ftat plate collector to conduct the experiments. The water used as HTF to transfer heat from the solar collector to the storage tank also acts as a sensible heat storage (SHS) material. The results of the numerical model are compared into the experimental results of the temperature profile for various porosities and HTF flow rates. It is found that the results of the numerical model are in good agreement with the experimental results. The performance parameters, such as instantaneous heat stored, cumulative heat stored, and charging rate are also studied in detail.

Methods for Controlling a Wind Turbine System With a Continuously Variable Transmission in Region 2

Abstract: Variable speed operation enables wind turbine systems to increase their aerodynamic efficiency and reduce fatigue loads. An alternative to the current electrically based variable speed technologies is the continuously variable transmission (CVT). A CVT is a transmission whose gear ratio can be adjusted to take on an infinite number of settings within the range between its upper and lower limits. CVT research in wind turbine applications predicts an improvement in output power and torque loads compared with fixed-speed machines. Also, a reduction in the harmonic content of the currents is anticipated by eliminating the power electronics. This paper develops a model that combines a CVT model with the FAST wind turbine simulator for simulating the system's performance in MATLAB/SIMULINK. This model is useful for control development for a variable-speed wind turbine using a CVT. The wind turbine with CVT is simulated using two controllers: a proportional-integral controller and a nonlinear torque controller of the type commonly used in the wind industry.

Dry Methane Reforming Without a Metal Catalyst in a Directly Irradiated Solar Particle Reactor

Abstract: Dry methane reforming with carbon dioxide in a directly irradiated particle receiver seeded with carbon black is presented in this study. Carbon particles were entrained in the reacting gases and acted as heat transfer and reaction surface. The reactions were not catalyzed by a metal catalyst. The molar ratio between the entrained carbon particles and the working gases (Ar, CO 2 , and CH 4 ) was 4-7 mmol carbon/mol gas. The temperature of the reforming experiments varied from 750°C to 1450°C with CO 2 /CH 4 ratios varying from 1:1 to 1:6. Experimental results show that methane reacts at lower temperatures than expected for its thermal decomposition; this indicates that the decomposing reaction is enhanced by the presence of the carbon black particles. At 1170°C 90% of the methane reacted in the receiver during a residence time of 0.3 s. The reaction between carbon dioxide and carbon black is faster than is documented in the literature, but the reaction rate does not seem to change if only carbon dioxide and carbon black are present in the receiver, compared with experiments where methane is also part of the gas mixture. The experimental results indicate that a high solar flux, i.e., about 2500 kW/m 2 or higher, significantly accelerates the reaction rate of methane decomposition. Total or partial blockage of the solar radiation reduced the yield by about 50%, compared with tests when the receiver was exposed to the full solar radiation flux, at the same operating temperature.

Multiphase Stirling Engines

Abstract: Analysis, design, fabrication, and experimental assessment of a symmetric three-phase free-piston Stirling engine system is discussed in this paper. The system is designed to operate with moderate-temperature heat input that is consistent with solar-thermal collectors. Diaphragm pistons and nylon ftexures are considered for this prototype to eliminate surface friction and to provide appropriate seals. In addition, low loss diaphragm pistons, etched and woven-wire screen heat exchangers, and plastic flexures, as the main components of the system, are outlined. The experimental results are presented and compared with design analysis. Experiments successfully confirm the design models for heat exchanger flow friction losses and gas spring hysteresis dissipation. Furthermore, it is revealed that gas spring hysteresis loss is an important dissipation phenomenon for low-power Stirling engines and should be carefully addressed in design. Analysis shows that the gas hysteresis dissipation is reduced drastically by increasing the number of phases in a multiphase Stirling engine system. It is further shown that for an even number of phases, half of the engine chambers could be eliminated by utilizing a reversing mechanism within the multiphase system. The mathematical formulation and modal analysis of multiphase Stirling engine system are then extended to a system that incorporates a reverser. By introducing a reverser to the fabricated prototype, the system successfully operates in engine mode. The system proves its self-starting capability and validates the computed start-up temperature.

Study of a Quench Device for the Synthesis and Hydrolysis of Zn Nanoparticles: Modeling and Experiments

Abstract: The synthesis and hydrolysis of zinc nanoparticles are carried out in a tubular reactor A key component of the reactor is a coaxial jet quench device. Three coaxial and multi-inlet confined jets mix Zn(g), steam, and argon to produce and hydrolyze zinc nanoparticles. The performance of the quench device is assessed with computational fluid dynamics modeling and measurements of hydrogen conversion and particle size and composition. Numerical data elucidate the impact of varying jet flow rates on temperature and velocity distributions within the reactor. Experiments produce hydrogen conversions of 61-79%. Particle deposition on sections of the reactor surface above 650 K favors hydrolysis. Residence time for in-flight particles is less than 1 s and these particles are partially hydroLyzed.

Performance of Large-Scale Seasonal Thermal Energy Stores

Abstract: More than 30 international research and pilot seasonal thermal energy stores (TESs) were realized within the past 30 years Experiences with operation of these systems show that TES are technically feasible and work well Seasonal storage of solar thermal energy or of waste heat from heat and power cogeneration plants can significantly contribute to substitute fossil fuels in future energy systems However, performance with respect to thermal losses and lifetime has to be enhanced, while construction costs have to be further reduced This paper gives an overview about the state-of the-art of seasonal thermal energy storage with the focus on tank and pit TES construction Aspects of TES modeling are given Based on modeled and measured data, the influence of construction type, system configuration, and boundary conditions on thermal losses of large-scale TES is identified The focus is on large-scale applications with tank and pit thermal energy stores and on recent investigations on suitable materials and constructions Furthermore, experiences with the operation of these systems with respect to storage performance are discussed