bluuebunny/crossref_metadata_2025 huggingface dataset:
A 50 kW Fluidized Bed High Temperature Solar Receiver: Heat Transfer Analysis
Authors: Flamant G. , Gauthier D. , Boudhari C. , Flitris Y.
Published: November 2009

A theoretical and experimental investigation of a pilot scale solar fluidized bed receiver is presented. Large diameter alumina particles were used. Experimental data with the bed in the temperature range of 550° C to 915° C (wall 740° C–1035° C) are compared with a simple model based on one parameter: the mean front wall temperature. At 950° C, the predicted efficiency is 73 percent and the measured efficiency is about 65 percent. In addition, unsteady behavior of the receiver is described by a simple heat transfer model.
Fluidized Bed Solar Collector
Authors: Glicksman L. R. , Azzola J. , Modlin J.
Published: November 2009

An air fluidized bed, contained in the wall cavity of an exterior building wall, forms the basis of a new solar collector design which is simpler than a water-cooled collector and has a thermal performance superior to that of an air-cooled collector. The fluidized bed serves as an intermediate heat transfer medium between a solar flux absorbed on the external building surface and a liquid thermal transfer loop. Fluidized beds yield heat-transfer coefficients an order of magnitude higher than single phase air flow. Low density particles are used in the bed to minimize power consumption. When defluidized, the bed acts as a good thermal insulator. Recent experimental results are presented for the heat-transfer coefficients of the immersed tubes, bounding walls, the effective conductivity of the bed, and the overall full-scale thermal design efficiency for various low density materials. Structural and power consumption performance is examined as well. An integrated fluidized bed solar collector design is proposed and compared with representative water and air collector designs.
Lithium Bromide and Water Thermal Storage System
Authors: Rizza J. J.
Published: November 2009

An analysis of a thermal storage system using a lithium bromide and water solution both as a refrigerant and as a storage material is considered. The proposed thermal storage system can be used to shift electric demand from periods of high demand to periods of low demand. The system is considered for both the summer cooling and winter heating season. The system’s evaporator and absorber are similar to that of a conventional heat-operated absorption refrigeration system; however, the generator heat is supplied by a self-contained electrically-driven vapor compression heat pump. The heat pump is operated during the off-peak period to recover the thermal storage by reprocessing the stored solution to a higher lithium bromide concentration. The water vapor liberated from solution in the generator is compressed and then condensed in the generator. The storage volumetric efficiency is determined and compared to storage systems based on water ice for the cooling season only and on a liquid water storage system for both cooling and heating. The storage volumetric efficiency of the proposed system is greater than or comparable to that of a thermal storage system based upon water ice and far exceeds the value for a thermal storage system based upon liquid water. The system can be constructed from standard HVAC components and appears to be a competitive alternative to a water ice storage system for summer cooling only, and it could be a competitive alternative to a liquid water system for combined summer and winter operations.
A Method for Analyzing Wind Turbine Dynamic Response Test Data
Authors: Hansen A. C.
Published: November 2009

Correlation of wind turbine structural response with ambient wind conditions is an essential but expensive and difficult task. The numbers of variables involved in a typical atmospheric test, the poor correlation between measured instantaneous winds and the actual wind across a rotor disc, and the range of input and response time scales involved all make the correlation task formidable. This paper describes a method which has proven effective for analyzing test data and gaining insight into wind turbine behavior. The method basically consists of representing the dynamic response data in terms of its Fourier Series. A time-series of Fourier coefficients is then created to replace the original time-series raw data. The entire data set, consisting of thousands of rotor revolutions is subdivided into hundreds of sets, each consisting of the azimuth average of (typically) two to ten revolutions. One set of Fourier coefficients (magnitudes and phases of response) is calculated for each azimuth average. The resulting reduced data has a greatly compressed volume with virtually no loss of information. The result is greater insight and a manageable data set size. This new technique is demonstrated for two different wind turbines, an ESI-80 and a Hamilton Standard WTS-4.
Nusselt Number and Melt Time Correlations for Melting Inside a Horizontal Cylinder Subjected to an Isothermal Wall Temperature Condition
Authors: Prasad Ajay , Sengupta Subrata
Published: November 2009

The design of latent-heat storage devices requires the knowledge of the cycle time (the total melt-down time of the Phase Change Material (PCM)), and the variation of the Nusselt number along the wall over time, given the properties of the PCM and the wall temperature. In order to answer these questions, we have extended a previous work by the same authors, which studied numerically the process of melting of a PCM within a horizontal tube subjected to a constant wall temperature. This numerical model accounts for the irregular, time-dependent shape of the solid-liquid interface, the downward motion of the solid core due to density differences and the natural convection in the liquid phase. In the present work, we study the effect of a variation in the Rayleigh, Stefan, and Prandtl numbers, and obtain useful correlations for the melt time and the Nusselt number.
On Optimizing Solar Collectors Orientation Under Daily Nonrandom Cloudiness Conditions
Authors: Segal M. , Pielke R. A. , Ookouchi Y.
Published: November 2009

Seasonal daily nonrandom cloudiness is typical in many geographical locations. Optimization of flat-plate solar collectors orientation in such situations requires azimuth and tilt modifications from those when daily cloudiness is random. The present study evaluates the significance of optimizing solar radiation gains, while considerating an illustrative case of nonrandom afternoon/morning cloudiness. Results suggest that for fixed flat-plate collectors the related gain in solar energy is practically insignificant. For nonfixed collectors the solar energy gains can be improved on a monthly basis by up to ∼6 percent.
A Numerical Study of the Natural Convection in CPC Solar Collector Cavities with Tubular Absorbers
Authors: Chew T. C. , Tay A. O. , Wijeysundera N. E.
Published: November 2009

The laminar free convection in a compound parabolic concentrator (CPC) solar collector cavity is numerically simulated using the finite element method. Results are presented for representative CPC collectors with tubular absorbers of concentration ratio 2. The effect of Grashof number, truncation and tilt angle were investigated. Generally, higher rates of heat transfer between the tubular absorber and the flat cover plate of the cavity are associated with larger Grashof numbers and shallower cavities. The maximum heat transfer rates occur when the tilt angle is about 60 deg. Contour plots are obtained for the field variables and these provide an insight into the spatial characteristics of the convective mechanisms within the cavity.
Evaluation of Advanced Sodium Receiver Losses During Operation of the IEA/SSPS Central Receiver System
Authors: Carmona R. , Rosa F. , Jacobs H. , Sa´nchez M.
Published: November 2009

This article presents the measurements and experiments conducted on the external receiver: the so-called Advanced Sodium Receiver (ASR) of the Small Solar Power Systems (SSPS) Project of the International Energy Agency (IEA) in southern Spain. The basis of this experiment was to provide loss measurements for later use in determining receiver performance. The tests to evaluate thermal losses consisted in operating the receiver with the doors open and circulating the sodium in normal and reverse flow without providing any incident power from the heliostat field (flux-off technique). In this way, total thermal losses are calculated as the energy lost by the sodium. Radiative losses have been calculated based on theoretical calculations and some results have been compared with infrared thermography measurements. Conductive losses are small and have been estimated by flux-off experiments with the receiver doors closed. Convective losses were evaluated subtracting radiative and conductive losses from the total thermal losses. Optical losses were assessed using absorptance measurements of the receiver coating. A simplified analytical model has been developed to calculate losses and ASR efficiency during operation. In spite of the method’s simplicity, the results are very similar to those found by other investigators, verified simulation programs, and test results.
Measurement and Analysis of Heliostat Images
Authors: Kiera M. , Schiel W.
Published: November 2009

A method is proposed allowing the physical interpretation of the sun image generated by reflecting systems, e.g., heliostats, on a target plane. The instantaneous flux distributions reflected from the target are measured by a distant video-type camera system in the visible spectrum and analyzed in terms of their constituents (sunshape, astigmatic aberration, mirror glass waviness, canting, and curvature errors). The single contributions to the total image are parmetrized by appropriate physically-based model functions and composed to a theoretical image to be compared with the measured image. Within a fit procedure, the parameter set is determined for every experiment allowing to check the consistency and uniqueness of the approach. A series of measurements on GAST heliostats performed between 1984 and 1987 was analyzed. They demonstrate the good optical performance of these systems, thus justifying their use in large solar thermal central receiver tower plants.
Evaluation of the Optical Perfomance of a Prototype Stretched-Membrane Mirror Module for Solar Central Receivers
Authors: Alpert D. J. , Houser R. M.
Published: November 2009

The United States Department of Energy’s solar thermal program is testing an innovative concept for the heliostats used in a solar central-receiver power plant: replacing the familiar glass mirrors with stretched-membrance reflectors. Because of their simplicity and lighter weight, heliostats using membrane reflectors have the potential to cost significantly less than glass-mirror heliostats. To assess the optical performance of these new heliostats, a 50m2 prototype mirror module, representative of a commercial scale design, was built and installed for testing at the Central Receiver Test Facility. The results of the testing program are presented, including the optical performance of the stretched-membrane mirror, the performance of the mirror’s focus control system in windy conditions, the performance of the defocus system, and the durability of the reflective film.
Thermal Analysis of Stratified Storage Tanks
Authors: Shyu Ruey-Jong , Lin Jian-Yuan , Fang Liang-Jyi
Published: November 2009

This paper presents theoretical and experimental studies on the stratification decay in stratified storage tanks. The effects of the thicknesses of tank wall and thermal insulation were discussed. The experimental results showed that the outside insulation can enhance tank wall axial conduction which tends to degrade the stratification. However, the reduction of heat loss outweighs the enhancement of axial conduction and better stratification is still maintained when compared to a bare wall tank. The axial conduction enhancement is sensitive to the wall thickness for the same thickness of insulation, however, it is not strongly dependent on the insulation thickness for identical thick wall. The addition of inside insulation is helpful in preserving stratification. A two-dimensional laminar flow model was also developed to interpret the temperature and flow fields inside storage tanks and the numerical results were compared with the experimental data.
Input Turbulence Features at a Megawatt-Size Wind Turbine at Medicine Bow, Wyoming
Authors: Connell J. R. , Morris V. R. , Hinchee M. E.
Published: November 2009

In response to recent observations that wind turbulence has a strong effect on wind turbine fatigue life, measurements of turbulent wind velocity profiles have been made at the Medicine Bow, Wyo., WTS-4 wind turbine site. These measurements were taken at seven levels spanning the WTS-4 rotor height on a single meteorological tower 1.75 rotor diameters upstream of the turbine. Concurrent measurements of rotor response were also made. Analysis of the winds at the Medicine Bow site reveals the influence of atmospheric and geographic conditions on what may, at first glance, appear to be a “classic, simple, flat terrain” case. The turbulence data are analyzed to (1) characterize the wind in terms of fixed-point, single-tower properties and (2) model the wind properties as they would be experienced by points on rotating turbine blades. The STRS-2 model, developed by the Pacific Northwest Laboratory, is used to convert single-tower wind characteristics into an estimate of those that would be measured from an array of towers arranged in a crosswind line covering the rotor disk. A response function for the flatwise root bending moment of one of the WTS-4 turbine blades is computed using the turbine data and the STRS-2 data. The STRS-2 model provides a substantially improved correlation with wind turbine blade bending moment over other single-tower estimation methods.
On the Performance of the Savonius Wind Turbine
Authors: Modi V. J. , Fernando M. S. U. K.
Published: November 2009

An extensive wind tunnel test program is described which assesses the relative influence of system parameters on the Savonius rotor performance. The parametric study leads to an optimum configuration with an increase in efficiency by around 100 percent compared to the reported efficiency of ≈12–15 percent. Of particular interest is the blockage correction procedure which is vital for application of the wind tunnel results to a prototype design, and facilitates comparison of data obtained by other investigators. Next, using the concept of a central vortex, substantiated by a flow visualization study, a semiempirical approach to predict the rotor performance using measured stationary blade pressure data is developed. The simple approach promises to be quite effective in predicting the rotor performance, even in the presence of blockage, and should prove useful at least in the preliminary design stages.
Heat Transfer Studies in Matrix-Type Solar Air Heaters
Authors: Hassab M. A. , Sorour M. M.
Published: November 2009

The effectiveness of a matrix-type solar air heater with a thick, porous absorber and a double-glass cover is analyzed theoretically and experimentally. The theory presents an exact formulation of simultaneous radiation, conduction, and convection heat transfer for quasi-steady, one-dimensional, isotropic, homogeneous, absorbing, emitting, scattering gray porous slab along with the energy conservations for the cover and floor plates of the collector. This set of the nonlinear coupled equations governing the transfer of energy in the collector is solved numerically by employing several iterative schemes. The effects of air flow rate, scattering, optical thickness, thickness, and conductivity of porous materials on the collector performance are discussed. Experimental results are found to partially support numerical predictions.
Heat Transfer Analysis in an Automated Venetian Blind Window System
Authors: Rheault S. , Bilgen E.
Published: November 2009

The heat transfer through an automated venetian blind window system is theoretically studied. The assumptions for the solution of the problem were: Heat transfer is unidimensional, heat exchanges are at steady-state for the whole system, thermal exchange by convection over the blind is negligible, materials used are diffuse-gray surfaces, and radiative exchanges with the room walls are considered as if all the surfaces are blackbodies at indoor air temperature. The solution of the governing equations (system of nonlinear equations) is carried out by using an iterative numerical method. Summarized results are presented and compared to the case of a regular window in a typical Canadian climate during winter and summer seasons and with different louver angles. It is found that by using an automated blind system, the predicted auxiliary load can be reduced by up to 36 percent and 47 percent for winter and summer, respectively.
Effectiveness of a Differential Temperature Controller on a Solar Water Heating System: An Experimental Study
Authors: Muralidhar G. K. , Nagaraju J. , Mohan S.
Published: November 2009

A differential temperature controller is incorporated in a solar water heating system to study the influence of its set points on system performance. The effectiveness of the controller set points ΔT ON and ΔT OFF on the pump cycling and energy collection has been studied experimentally and the results are presented in this paper.
Analysis of Composite Stretched-Membrane Heliostats
Authors: Kiang H. J. , Dharan C. K. H.
Published: November 2009

The stretched-membrane concept has the potential for reducing the cost of heliostats used for solar thermal energy generation in central receiver systems. In this concept, a high strength/structural membrane carrying the reflective film is stretched uniformly on a toroidal frame. The resulting structure has high stiffness and is structurally efficient. It is practically suitable for composite materials, since the loading is primarily in-plane. In this paper, the application of composite materials to stretched-membrane heliostat design is investigated. The results of the analysis show that composite materials for both the membrane and the frame exhibit significant advantages over metals. These include high strength-to-weight design, higher resistance to wind-induced deformation, the possibility of independently tailoring bending and torsional stiffness, and better transportability.
Amorphous Silicon Photovoltaic Solar Cells—Inexpensive, High-Yield Optical Designs
Authors: Collares-Pereira M. , Gordon J. M.
Published: November 2009

We propose a new method for manufacturing and deploying amorphous silicon solar cells which is based on creating an effectively “bifacial” photovoltaic device by utilizing part of the glazing of a CPC-type nonimaging concentrator as active absorber. This solar collector could enhance the yearly energy delivery of amorphous silicon solar cells by about 100 percent if the cells are manufactured so as to exploit illumination on both cells sides.
Thermal Modeling of Solar Central Receiver Cavities
Authors: Skocypec R. D. , Romero V.
Published: November 2009

Results are presented from a numerical model of the steady-state energy transfer in molten-salt-in-tube solar cavity receivers that includes convective energy transfer at a local (spatially resolved) level. Molten salt energy absorption and gray radiative transfer between all cavity surfaces are also included. This model is applied to the Molten Salt Subsystem Component Test Experiment (MSS/CTE) cavity receiver. Results for this receiver indicate the global (entire cavity) receiver thermal efficiency is invariant within a few percent to most parameters investigated, although front surface temperatures of the nonabsorbing walls vary considerably, and are particularly sensitive to the type of convective submodel used. Absorption efficiencies indicate the effects of the cavity enclosure environment. For all conditions investigated, tube inner wall temperatures remain under 855 K, ensuring that the salt remains chemically stable. Global results for the receiver indicate thermal conditions in the receiver are temporally constant within an hour of solar noon, and solar panel temperatures are governed by the temperature of the flowing salt (the outlet temperature is maintained at 839 K by varying the salt mass flow rate). The dominant loss mechanism is radiative transfer, although convective loss predictions are of the same order of magnitude. The absorbing panel front surface temperatures and the panel thermal losses are somewhat invariant with incident flux. Losses from the nonabsorbing surfaces (comprising over 60 percent of the cavity surface area), however, do vary with incident flux levels. These results suggest that a correction for nonconstant losses in the Barron flux-on loss method is necessary. Global predictions compare well with data obtained in the MSS/CTE experiment. Predicted salt flow rates and absorbed powers were within one percent of values measured during steady-state tests. Predicted global loss values compare well with current loss estimates from measured data.
Development of Long-Term Performance Correlation for Solar Thermosyphon Water Heater
Authors: Huang B. J.
Published: November 2009

A semi-empirical long-term performance correlation for a solar thermosyphon water heater was derived. The derivation used the simulated results from a well-tested computer program developed previously. It was shown that the mean-day simulation results agree very well with the hour-by-hour simulation for the long-term performance of solar thermosyphon water heaters and hence were adopted for the derivation of the performance correlation. The correlations for the yearly- and monthly-mean efficiency in terms of eleven parameters which characterize the performance of solar thermosyphon water heaters were derived and shown to accurately fit the simulation results. Several design rules for the thermosyphon water heater were deduced from the present study. The total incident global radiation is not present in the correlation function of the yearly efficiency. For the performance evaluation on a monthly basis, the monthly efficiency of thermosyphon water heater was shown to vary linearly with the monthly-mean daily total incident global radiation. This result can be useful in developing a rating method for the thermal performance of thermosyphon water heaters.
Convective Heat Transfer Coefficients From a Full-Scale Test in the REPEAT Facility
Authors: Kahwaji Ghalib , Burns Patrick J. , Winn C. Byron
Published: November 2009

Convective heat transport coefficients are computed from data obtained from a full-scale test in a closely controlled, passive solar sunspace. The subtractive convective heat transport at the surface is calculated from the conductive and radiative fluxes. Methods for calculating these fluxes from measured data, and estimating “best fit” coefficients are presented. The results obtained are found to be in very good agreement with an independent approach used to analyze the same data sets.
Heat Transfer Modeling of the IEA/SSPS Volumetric Receiver
Authors: Skocypec R. D. , Boehm R. F. , Chavez J. M.
Published: November 2009

During the summer and fall of 1987 in Almeria, Spain, a wire-pack receiver was tested by the International Energy Agency/Small Solar Power Systems (IEA/SSPS). The basic operation of the receiver is that: air is drawn through several layers of stainless steel wire screen; concentrated solar flux is directed on the face of the screen pack; the oxidized wires absorb the solar energy; and heat is transferred to the air flowing through the screen. Although the experiment goal was strictly proof-of-concept and was not receiver characterization, modeling efforts were initiated to help understand the experimental results. The steady-state performance of the receiver is modeled using the fact that the net solar and infrared radiative energy absorbed by each screen layer must be transferred to the air by convection. Basic performance trends and typical calculations of receiver efficiency are given. Model predictions and experimentally measured temperatures and flow rates are compared. Model predictions of receiver power and efficiency are generally higher than the test results (operational modifications of the receiver absorber as tested are believed to have produced nonideal conditions), but trends are consistent with experimental data.
The Power Production Operation of Solar One, the 10 MWe Solar Thermal Central Receiver Pilot Plant
Authors: Radosevich L. G. , Skinrood A. C.
Published: November 2009

Solar One, the 10 MWe Solar Thermal Central Receiver Pilot Plant near Barstow, Calif., completed a three-year period of power production testing in July 1987. During this period the plant’s capacity factor, system efficiency, and availability were studied to assess the operational capability of Solar One to reliably supply electrical power. The long-term performance of key plant components, such as the heliostats and the receiver, was also studied. During the three years of power production operation, Solar One achieved increases in capacity factor, system efficiency, and availability. Heliostat operation was reliable, and only small amounts of mirror corrosion were observed. Receiver tube leaks did occur, however, and were the main cause of the plant’s unscheduled outages. Solar One provided valuable lessons which will aid in the design of future solar central receiver plants.
Thermal Performance of a Heat Storage Module Using PCM’s With Different Melting Temperatures: Mathematical Modeling
Authors: Farid Mohammed M. , Kanzawa Atsushi
Published: November 2009

In the present study, the performance of a heat storage unit consisting of number of vertical cylindrical capsules filled with phase change materials, with air flowing across them for heat exchange has been analyzed. Earlier theoretical models did not consider temperature distribution in the radial direction within the capsules, an assumption that limits their applications for small diameter capsules. The mathematical model developed in this work is based on solving the heat conduction equation in both melt and solid phases in cylindrical coordinates, taking into account the radial temperature distribution in both phases. Heat flux was then evaluated at the surface of the first row of the capsules to determine the temperature of the air leaving that row by a simple heat balance. It was found that such computation may be carried out for every few rows rather than for a single row to minimize computer time. The simulation study showed a significant improvement in the rate of heat transfer during heat charge and discharge when phase change materials with different melting temperatures were used. Air must flow in the direction of decreasing melting temperature during heat charge, while it must be reversed during heat discharge.
Mean and Peak Wind Loads on Heliostats
Authors: Peterka J. A. , Tan Z. , Cermak J. E. , Bienkiewicz B.
Published: November 2009

Mean and peak wind loads on flat rectangular or circular heliostats were measured on models in a boundary layer wind tunnel which included an atmospheric surface layer simulation. Horizontal and vertical forces, moments about horizontal axes at the ground level and at the centerline of the heliostat, and the moment about the vertical axis through the heliostat center were measured. Results showed that loads are higher than predicted from results obtained in a uniform, low-turbulence flow due to the presence of turbulence. Reduced wind loads were demonstrated for heliostats within a field of heliostats and upper bound curves were developed to provide preliminary design coefficients.
Solar Heating Systems Design Procedure for Greenhouse With Internal Collection and Sensible Heat Storage
Authors: Lau A. K. , Staley L. M.
Published: November 2009

A simplified design procedure for solar heating systems for greenhouses is presented. Computer modeling and simulations were carried out to analyze the effects of design variations in greenhouse construction and storage characteristics on long-term average system thermal performance. The key performance indices were defined in terms of the total solar contribution and solar heating fraction. The results of many simulations were transformed into correlations between monthly solar load ratio and the performance indices. Knowing monthly average meteorological data, designers and engineers can readily extract the necessary technical information for subsequent economic analysis to select the most cost-effective solution for a given design problem.
High Performance, Solid Desiccant, Open Cooling Cycles
Authors: Kang T. S. , Maclaine-cross I. L.
Published: November 2009

Solid desiccant, open cooling cycles use low temperature heat efficiently making them attractive for solar air conditioning. Advanced cycles using nearly reversible evaporative coolers have previously been proposed and shown to have high ideal performance. This parametric study shows that, with real components comparable to those used in studies of classical cycles, these open cycles can give more than twice the thermal coefficient of performance of a ventilation cycle.
Thermal/Hydraulic Analysis of a Commercial-Size Cavity Receiver
Authors: Carli G.
Published: November 2009

This paper presents the results of the thermal/hydraulic analysis of a commercial-size molten salt solar central cavity receiver. Included are a description of the receiver; performance characteristics; sensitivity of salt and tubewall temperatures to variations in heat flux, flow distribution, and uncertainties in design parameters.
Optimizing the Value-to-Cost Ratio for Central Receiver Electric Power Plants
Authors: Dirks James A. , Chiang Clement J.
Published: November 2009

Typically, solar thermal power plants are designed to minimize the levelized energy cost. However, to maximize the benefit of a solar plant and, hence, maximize the wealth of an investor or a utility, a solar plant should be designed and operated with the objective of maximizing the value-to-cost ratio. This paper describes a value and cost analysis of solar central receiver power plants using molten salt external receiver technology. These plants were assumed to operate within the service area of the Southern California Edison Company. The SOLERGY computer code was used to simulate the performance of the solar plants using 1984 weather data for Barstow, California. A value-maximizing dispatch strategy that uses thermal storage to shift operation of the turbine from nonpeak demand periods to the utility’s peak demand period, is shown to greatly increase the value of a solar central receiver power plant with little increase in the levelized energy cost. Results are presented as functions of storage capacity, type of dispatch strategy, size of the field relative to the turbine, and turbine size.
A Comparison Study of One-Dimensional Models for Stratified Thermal Storage Tanks
Authors: Zurigat Y. H. , Maloney K. J. , Ghajar A. J.
Published: November 2009

A survey of the stratified thermal storage tank one-dimensional models available in the literature has been conducted. Six of these models were tested and compared against the experimental data obtained at our laboratories and from the literature. Although various factors affecting the performance of a stratified tank can be accounted for by the higher order models, i.e. two- and three-dimensional models, the introduction of empirically-based mixing parameters into the one-dimensional models renders them widely applicable and practical in the simulation of energy systems incorporating thermal storage tanks.
Experimental Study of a Self-Pumping Boiling Collector Solar Hot Water System
Authors: Davidson J. H. , Walker H. A. , Lo¨f G. O. G.
Published: November 2009

Results of an experimental study of a full-scale passive solar hot water heating system utilizing a boiling collector are presented. The self-pumping system alternates between two modes of operation. During the run cycle, vapor pressure drives the evaporated refrigerant downward from the collector to the condenser. Once a preset quantity of refrigerant is condensed, vapor pressure is again used to force the return of the condensate to the collector during the pump cycle. In order to assess the thermal penalty of self-pumping operation, the system is also operated with a mechanical pump. Daily operating characteristics and seasonal performance are discussed. Performance of the self-pumping system is strongly influenced by the duration of the pump cycle. The average thermal loss attributed to self-pumping during one heating season in Fort Collins, Colo. is approximately nine percent of the solar energy incident on the collector. Refinement of the system design, including individual components, is required to reduce losses and improve performance.
Thermal Performance of a Heated Plate With Open-Channel Infrared Radiation Shields
Authors: Beard J. T. , Ribando R. J. , Adams R. R. , Arthur J. H.
Published: November 2009

The heat transfer associated with various radiation shield designs has been investigated. Mathematical models were used to characterize the thermal performance of a vertical heated plate, with and without a continuous radiation shield, under various environmental conditions. Predictions were compared with laboratory results taken under simulated solar radiation and wind conditions. Measurements of channel temperature and velocity distributions were made to help characterize the thermal performance. Plate temperature predictions from the model, using measured power input, ambient temperature, and channel air velocities, gave results which were within 5° C of the experimental values. The infrared energy emitted in the 8—14μ band was calculated as a function of the measured plate and/or screen temperatures for each set of experimental conditions. Double layers of incised radiation shields were found to have the lowest effective infrared energy emission. For the two screen incision sizes tested, the size of the incision did not significantly influence the temperature and velocity profiles or the amount of radiant energy emitted from the plate/screen combination.
Consideration on Thermal Decomposition of Calcium Hydroxide Pellets for Energy Storage
Authors: Fujii I. , Tsuchiya K. , Shikakura Y. , Murthy M. S.
Published: November 2009

The reversible chemical reaction of Ca(OH)2/CaO appears to be attractive for storage of solar thermal energy, in view of the nonpolluting and nontoxic nature of the reactants. This paper presents some data on thermal decomposition of calcium hydroxide pellets along with its additives of aluminum, aluminum hydroxide, zinc, and copper. The addition of aluminum and zinc powder enhanced the rate of decomposition considerably at 450° C, but copper had no effect. Considerations on the effect of additives are also discussed in some detail, though their effects are not established with certainty. There is some evidence that heat transfer into the pellet, and the number of potential nucleation sites due to thermal stresses, influence the kinetics and mechanism of decomposition.
Studies of Radiative Cooling Systems for Storing Thermal Energy
Authors: Ito S. , Miura N.
Published: November 2009

The thermal performance of an uncovered radiator and a radiative cooling system was investigated experimentally and theoretically. The net radiative power of a black painted surface at ambient air temperature was measured by heat flux plates at night in order to use the results for predicting temperatures of the radiator surface and the fluid in the cooling system on the same night. The net radiative power obtained by the measurements was 40–60 W/m2 on clear nights in the summer and 60–80 W/m2 in the fall and winter. The average temperature of the energy storage tank on clear nights became 2–5°C below the ambient temperature. The experimental and analytical results agreed well with each other.
Comparison of Measured and Modeled Turbulence Spectra for a Point Rotating as on a Horizontal-Axis Wind Turbine
Authors: Connell James R. , Powell David C.
Published: November 2009

A theoretical model of the spectrum of rotationally sampled wind speed is tested against corresponding measured wind spectra. The ability to generate such a spectrum, independent of measurement, is important in wind turbine design and testing. The measured spectra were selected from a number of cases measured with a vertical plane array of anemometers. The array was set up to correspond to the tip circle of a nearby MOD-OA wind turbine rotor blade. For testing purposes, the turbulence and mean wind shear were treated separately. In each case the test was based on two correspondence criteria, each involving comparison of results from the theoretical model and results from real data analysis. These criteria were rotational spectra and their integration by harmonic band into energy, particularly the latter. The measured spectra were selected to represent different atmospheric stability conditions. The theory is shown to have respectable accuracy except when applied to the stable atmosphere. Recommendations were made for selection of the values of atmospheric parameters to be used for computing the theoretical result. The importance of the time length of a measured turbulence record to be used in estimating several parameters for input to the theoretical model applied to real wind conditions at a selected turbine site is discussed. The VAX 11/780 computer time required to generate a model spectrum is usually less than one minute.
Influence of Aperture Height and Width on Interzonal Natural Convection in a Full-Scale Air-Filled Enclosure
Authors: Boardman Charles R. , Kirkpatrick Allan , Anderson Ren
Published: November 2009

The topic of this paper is the influence of aperture height and width on interzonal high Rayleigh number, natural convection heat transfer. Experiments were conducted in an 8 ft. air-filled cube divided into two zones by a vertical partition which was centered between a constant flux hot wall and an isothermal cold wall. The partition was configured to form doorway-like apertures. The aperture height relative to test cell height range from 1/8 to 1 and the aperture width relative to test cell width ranged from 0.009 to 1. The zone-to-zone temperature difference and the overall Nusselt number were determined experimentally, and correlated with the overall Rayleigh number, aperture, and enclosure geometry, using a series resistance model for the enclosure. A turbulent boundary layer resistance was used to represent the hot and cold boundary layer flow, while an orifice resistance was used to represent the aperture flow. For flux Rayleigh numbers between 5*1011 and 5*1012, the enclosure Nusselt numbers ranged between 15 and 165, with a strong dependence on aperture height.
Comparison of the DESSIM Model With a Finite Difference Solution for Rotary Desiccant Dehumidifiers
Authors: Schultz K. J. , Mitchell J. W.
Published: November 2009

A heuristic “pseudo-steady-state” model of the heat and mass transfer occurring in desiccant dehumidifiers, embodied in the program DESSIM, has been proposed as a conceptually and numerically simple analysis tool (Barlow, 1982). A comparison is made with a finite difference solution to determine the accuracy and limitations of the pseudo-steady-state model. The comparison indicates that the pseudo-steady-state model can produce accurate results of dehumidifier performance relative to the finite difference solution when used carefully, although at greater computational expense. Substitution of the finite difference solution into the overall DESSIM program results in a potentially accurate and useful analysis tool.
Two-Phase Heat Transfer in Thermosyphon Evacuated-Tube Solar Collectors
Authors: Den Braven Karen R.
Published: November 2009

This work analyzes the heat transfer within a tilted thermosyphon and its use in a heat pipe evacuated-tube solar collector. A detailed examination is made of the laminar film condensation process, including the effects of interfacial shear due to the moving vapor. Effects of film surface waves are later included. Including the shear term in the constitutive equations changes the predicted film thickness in the condenser portion of the device by less than one percent, depending on location along the surface. This change causes only a slight increase in the predicted heat transfer. Accounting for surface waves increases the heat transfer rate 10 percent to 50 percent in the Reynolds number range studied. The condenser results are combined with a simple trough model for the evaporator portion of the thermosyphon to give the effective heat-transfer coefficient for the entire tube. Predicted performances of the condenser, the evaporator, and the entire tube compare favorably with available data.
Development of an Optimal Control Strategy for the Themis Solar Plant: Part I—Themis Transient Model
Authors: Ferriere A. , Bonduelle B. , Amouroux M.
Published: November 2009

This first part of a twofold article is devoted to the presentation of a model of the Themis central receiver solar plant. The Themis experiment showed that the energy balance is deteriorated by internal (parasitic) energy consumption during outages and that the operating procedures were not optimized. Our model takes into account all blocks of the main transfer chain that are involved in the global energy balance. Thanks to a set of control variables, we can simulate various operating procedures. This paper describes the state equations and details the case of the solar receiver.
Development of an Optimal Control Strategy for the Themis Solar Plant: Part II—Themis Model Operation: Validation and Simulation
Authors: Ferriere A. , Bonduelle B. , Amouroux M.
Published: November 2009

This second part of a twofold article presents the operating system associated with the physical model. Basic control laws and operating rules are described, and the global model is validated comparing energy balances for a sunny day and the model production results. Then, three examples using the model to improve the plant utilization are presented. First, an improved control law for the salt mass flow rate in the receiver is proposed. Then, we use the model to estimate the secondary-loop conditioning cost with various operating strategies. Finally, we make basic modifications of procedures to increase electricity production on cloudy days.
Temperature Distribution Inside Hot Water Storage Tanks of Solar Collectors
Authors: Issa M. , AL-Nimr M.
Published: November 2009

An analytical experimental investigation into the temperature field inside the hot water storage tank of a solar collector was carried out. A transient two-dimensional semi-infinite cylindrical length model with time-and-space boundary-conditions dependency was selected. Conduction and convection heat-transfer modes in the axial direction together with conduction in the radial direction were neglected, since these were considered to be small in comparison with the axial conduction and convection diffusion terms, which in turn were considered small relative to the energy stored. Good agreement between theoretical and experimental prediction was verified. The radial direction temperature dependency disappeared for axial lengths greater than one quarter of the tank depth for most practical operating conditions especially for low inflow velocities and low inlet to outlet temperature ratios. The axial conduction term in the governing equation can be dropped out for inflow velocities greater than a certain critical value without distorting the theoretical consequences. Temperature profiles in the axial direction of a cylindrical storage tank can be assumed to be linear especially for high inflow velocities as well as low temperature differences at inlet and outlet of the storage tank.
System Performance of Solar Thermal High Temperature Utilization Systems Constructed in Japan
Authors: Tanaka Tadayoshi
Published: November 2009

Two solar thermal electric power plants and a solar total energy experimental facility were constructed for the effective use of solar thermal energy and then operated for about three years. Their development for commercial use has been completed because of low system performance and the poor solar conditions in Japan. Under these circumstances, for future development of these systems, we believe it is most important to improve system performance under poor solar conditions such as those experienced in Japan. Therefore, we consider here a method to improve system availability based on the operating characteristics of the system. It is shown that under poor solar conditions, it is important to collect not only high temperature heat but also low temperature heat, according to the solar condition, and to use the collected heat for either heat energy supply or electricity generation.
Radiative Transfer From a Gray, Absorbing-Emitting, Isothermal Medium in a Conical Enclosure
Authors: Kaminski Deborah A.
Published: November 2009

Radiative heat transfer from an isothermal participating medium in a truncated, conical enclosure is investigated numerically. Two methods of solution are employed: the Monte-Carlo technique and the P1 differential approximation. The solution to the P1 representation is obtained from a control-volume-based discretization of the governing equation. The medium is assumed to be gray and nonscattering, and the absorption coefficient and temperature are uniform throughout the medium. Overall and local heat flux rates at the walls predicted by the P1 method are compared to the quasi-exact results of the Monte-Carlo solution. Results are obtained for a variety of optical thicknesses and cone shapes.
Production and Storage of Ice for Cooling Buildings
Authors: Wubben Eugene A. , Shapiro Howard N. , Nelson Ron M.
Published: November 2009

A strategy that may provide economic benefits in buildings is to use an ice production system to provide cool storage for later use when cooling is needed. Understanding the fundamental dynamics of the storage tank is critical in determining the feasibility of such strategies. For this purpose, a lumped parameter model of ice growth on a heat exchanger submerged in a water storage tank is described. Results of an experimental study of an ice storage system installed in a residential research facility are also presented. The results of a parametric study are also presented that show some of the effects of geometric and operating variables on system performance. Trends exhibited in the results suggest ways to optimize ice production for the particular heat exchanger geometry studied.
VAWT Stochastic Loads Produced by Atmospheric Turbulence
Authors: Homicz Gregory F.
Published: November 2009

Blade fatigue life is an important element in determining the economic viability of the Vertical-Axis Wind Turbine (VAWT). A principal source of blade fatigue is thought to be the stochastic (i.e., random) aerodynamic loads created by atmospheric turbulence. This paper discusses the development of a model for the simulation of these stochastic loads, given the rotor geometry, operating conditions, and assumed turbulence properties. A Double-Multiple-Streamtube analysis is employed, which includes the effects of wind shear, Reynolds number variations, different airfoil sections and chord lengths along the blade span, and an empirical model for dynamic stall effects. Calculations are presented for the VAWT 34-m Test Bed recently erected at Bushland, Texas. Time histories of the loads, as well as their Fourier spectra, are presented and discussed. An unexpected finding is that the average output power is predicted to be more sensitive to turbulence level than had previously been thought.