bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Three Microturbine Primary Surface Recuperator Alloys
Authors: Matthews Wendy J. , More Karren L. , Walker Larry R.
Published: November 2009

Extensive work performed by Capstone Turbine Corporation, Oak Ridge National Laboratory, and various others has shown that the traditional primary surface recuperator alloy, type 347 stainless steel, is unsuitable for applications above 650°C(∼1200°F). Numerous studies have shown that the presence of water vapor greatly accelerates the oxidation rate of type 347 stainless steel at temperatures above 650°C(∼1200°F). Water vapor is present as a product of combustion in the microturbine exhaust, making it necessary to find replacement alloys for type 347 stainless steel that will meet the long life requirements of microturbine primary surface recuperators. It has been well established over the past few years that alloys with higher chromium and nickel contents than type 347 stainless steel have much greater oxidation resistance in the microturbine environment. One such alloy that has replaced type 347 stainless steel in primary surface recuperators is Haynes Alloy HR-120 (Haynes and HR-120 are trademarks of Haynes International, Inc.), a solid-solution-strengthened alloy with nominally 33 wt % Fe, 37 wt % Ni and 25 wt % Cr. Unfortunately, while HR-120 is significantly more oxidation resistant in the microturbine environment, it is also a much more expensive alloy. In the interest of cost reduction, other candidate primary surface recuperator alloys are being investigated as possible alternatives to type 347 stainless steel. An initial rainbow recuperator test has been performed at Capstone to compare the oxidation resistance of type 347 stainless steel, HR-120, and the Allegheny Ludlum austenitic alloy AL 20–25+Nb (AL 20–25+Nb is a trademark of ATI Properties, Inc. and is licensed to Allegheny Ludlum Corporation). Evaluation of surface oxide scale formation and associated alloy depletion and other compositional changes has been carried out at Oak Ridge National Laboratory. The results of this initial rainbow test will be presented and discussed in this paper.
A Micro Gas Turbine Based Test Rig for Educational Purposes
Authors: Ferrari Mario L. , Pascenti Matteo , Magistri Loredana , Massardo Aristide F.
Published: November 2009

The Thermochemical Power Group of the University of Genoa, Italy, has developed a new “Gas Turbine” laboratory to introduce undergraduate students to the Gas Turbines and Innovative Cycles course, and Ph.D.s to advanced experimental activities in the same field. In the laboratory a general-purpose experimental rig, based on a modified commercial 100 kW recuperated micro gas turbine, was installed and fully instrumented. One of the main objectives of the laboratory is to provide both students and researchers with several experimental possibilities to obtain data related to the gas turbine steady-state, transient, and dynamic performance, including the effect of interaction between the turbomachines (especially the compressor), and more complex innovative gas turbine cycle configurations, such as recuperated, humid air, and hybrid (with high temperature fuel cells). The facility was partially funded by two Integrated Projects of the EU VI Framework Program (Felicitas and LARGE-SOFC) and the Italian Government (PRIN project), and it was designed with a high flexibility approach including: flow control management, cogenerative and trigenerative applications, downstream compressor volume variation, grid-connected or stand-alone operations, recuperated or simple cycles, and room temperature control. The paper also shows, as an example of the possibilities offered by the rig, experimental data obtained by both Master and Ph.D. students. The tests presented here are essential for understanding commercial gas turbines and microturbine performance, control strategy development, and theoretical model validation.
Flame Transfer Functions for Liquid-Fueled Swirl-Stabilized Turbulent Lean Direct Fuel Injection Combustion
Authors: Yi Tongxun , Santavicca Domenic A.
Published: November 2009

Heat release rate responses to inlet fuel modulations, i.e., the flame transfer function (FTF), are measured for a turbulent, liquid-fueled, swirl-stabilized lean direct fuel injection combustor. Fuel modulations are achieved using a motor-driven rotary fuel valve designed specially for this purpose, which is capable of fuel modulations of up to 1 kHz. Small-amplitude fuel modulations, typically below 2.0% of the mean fuel, are applied in this study. There is almost no change in FTFs at different fuel-modulation amplitudes, implying that the derived FTFs are linear and that the induced heat release rate oscillations mainly respond to variations in the instantaneous fuel flow rate rather than in the droplet size and distribution. The gain and phases of the FTFs at different air flow rates and preheat temperatures are examined. The instantaneous fuel flow rate is determined from pressure measurements upstream of a fuel nozzle. Applications of the FTF to modeling and control of combustion instability and lean blowout are discussed.
An Analysis of a Direct Radiation Solar Dehumidification System
Authors: Schultz K. , Barlow R. , Pesaran A. , Kreith F.
Published: November 2009

A desiccant dehumidifier regenerated by direct absorption of solar radiation was investigated using a simplified numerical model (DESSIM) of the adsorpotion and desorption processes. This paper presents estimates of the performance of an air conditioning system in the ventilation mode using direct solar radiation regeneration. The effects of dehumidifier NTUs, heat exchanger performance, and insolation levels were also analyzed. The direct radiation regeneration system was found to have a COP less than that of other types of regeneration schemes.
Method for Determining Overall Performances of Solar Kilns
Authors: Guzman J. A. , Lauterbach A. , Jordan R.
Published: November 2009

A theoretical model based on heat and mass transfer equations was developed for determining a reliable comparison between different solar kiln designs. It involves dividing the drying process into two sequential steps, one in which only heat transfer occurs and a second one in which the actual drying process takes place. The model gives an equation for determining the evaporated water in function of the normal drying variables, together with a specially defined parameter which gives account for the air recirculation inside the kiln. The model was validated experimentally for a box-type collector solar kiln operating under constant and falling rate drying periods. It was found that although the model could be applied satisfactorily, the tested design proved to be a very inefficient one, giving low values for the recirculating parameter.
Performance of the Coolidge Solar Thermal Electric Power Plant
Authors: Larson D. L.
Published: November 2009

Energy performance and equipment evaluation results are presented for the grid-connected Coolidge solar thermal-electric power plant. Performance was determined for each of the major subsystems—line-focus collector array, thermal energy storage and 200 kW, organic Rankine cycle engine and generator. Day-long collector array efficiency was about 32, 26, and 9 percent in June, September, and December, respectively. Energy conversion efficiency was about 20 percent; electrical parasitics reduced this by 12 percent. Operation and maintenance required about 90 h/mo, only 20 percent requiring special skills or training. Operating supplies and repair services cost about $6300 per year. Major equipment problem catagories were fluid leakage and electric motor and electronic component failures. The presented operating data provide a basis for improved design and analysis.
Economic Optimization of Stationary Nonevacuated CPC Solar Collectors
Authors: Carvalho M. J. , Collares-Pereira M. , Gordon J. M.
Published: November 2009

Stationary, nonevacuated CPC solar collectors are a promising alternative to corresponding flat plate collectors in that they offer superior yearly energy delivery at comparable cost for low-temperature thermal applications. For realistic cost scenarios, we determine optimal concentrator configurations and concentration ratios, and calculate their sensitivity to variations in relative component costs, climate, orientation and collector operating temperature. For CPC’s that are to have the flexibility of either east-west or north-south orientation, optimized collectors are shown to have low concentrations of around 1.2, achieved by truncation from acceptance half angles of about 45 deg.
Unification of Three Different Theories Concerning the Ideal Conversion of Enclosed Radiation
Authors: Bejan A.
Published: November 2009

The discussion of the ideal conversion of enclosed thermal radiation revolves around three different theories, whose earliest proponents were Petela, Spanner, and Jeter, respectively. Each of these theories has been accused to be incorrect. The objective of this paper is to provide a bird’s-eye-view of the relationship between these three theories. It is shown that the theories are individually correct, and that they complement (rather than contradict) one another. The present “unification” consists of answering two important questions: i) What is the origin of the assumed blackbody radiation system? ii) What is the ultimate fate of the blackbody radiation system? It is shown that the supply of high-temperature isotropic radiation postulated by all three theories is the result of a heat input from a high-temperature heat reservoir. When referenced to the same high-temperature heat input, Spanner’s ideal work output must always be smaller than Petela’s, and, in turn, Petela’s work output must be smaller than Jeter’s.
Monthly-Averaged Utilizability for Multiple Aperture Solar Systems
Authors: Bida M. , Kreider J. F.
Published: November 2009

New solar utilizability correlations for solar collectors with apertures at various orientations are developed. These correlations use the functional form introduced by Klein/Theilacker and an Aperture Configuration Ratio descriptive of the collector’s aperture arrangement. Various glazing areas and aperture orientations were considered in several climates. Good accuracy is found for the new correlations which can be used for thermal collector performance prediction as well as sensible cooling load calculation.
Thermodynamic Properties of Refrigerant-Absorbent Pairs
Authors: Bhaduri S. C. , Varma H. K.
Published: November 2009

An analytical method has been proposed to calculate thermodynamic properties of refrigerant-absorbent mixtures of polar molecules. The critical pressure, critical temperature, critical volume and experimental dipole moment of pure components are required in the proposed method. The calculated properties have been compared with the available experimental data and results reported in the literature. The comparison shows very good agreement. Thus, this method is recommended for use with refrigerant-absorbent pairs having polar molecules.
A Clear Sky Model of Diffuse Sky Radiance
Authors: Hooper F. C. , Brunger A. P. , Chan C. S.
Published: November 2009

A model, previously proposed, describing the sky radiance as a continuous function, has been calibrated from 11,000 individual measurements made in scans taken across springtime skies in Toronto using a narrow field of view radiometer. The model reproduces the measured sky radiance with a mean bias error under five percent and a root mean square error only slightly larger than the standard deviation of the measurements. The model is applied to the calculation of the ratio of the clear sky diffuse irradiance on a slope to that on a horizontal surface. Charts are presented for the direct determination of the expected values of these ratios for surfaces at three tilts and at any azimuth.
A Review of Convective Loss Data From Solar Central Receivers
Authors: Boehm R. F.
Published: November 2009

A summary of previous experimental evaluations to determine convective losses from solar central receivers is given. In this review studies performed directly on actual central receivers or simulations that closely approximate central receiver geometries are considered. Because of the diversity of the previous studies, there are a number of classifications of the studies that can be made. From one perspective, two types of evaluations have been carried out: either those using performance data (often called efficiency tests), or those configuring the receiver so that losses can be more directly inferred (loss tests). In addition to classifying the experimental configurations according to geometry type (either external or cavity forms), it is also beneficial to distinguish between evaluations that take place when normal solar flux levels are incident upon the receiver (flux-on tests) and those made with no flux present (flux-off evaluations). Previous tests are categorized according to the method of testing used. Special attention is given to the range of the physical parameters represented by the experiments, particularly the Grashof number. Too often sufficient information to characterize forced convection contributions is not available, but it is shown that the total convective loss sometimes is lower than the predictions for natural convection alone. It is shown that performance-based data does not correspond well with loss-based data. The effects of directional effects of forced convection and general aspects of mixed (natural and forced) convection are not well established in the existing data. There have been some flux-on techniques recommended recently that may overcome some of the previously established concerns about the accuracy of flux-on data.
Wind-Related Heat Transfer Coefficient for Flat-Plate Solar Collectors
Authors: Shakerin S.
Published: November 2009

Experiments were performed to evaluate the convective heat transfer coefficient for a flat plate mounted in a wooden model of a roof of a building. The experiments were carried out in a closed-circuit wind tunnel and included parametric adjustments of the roof tilt and Reynolds number, based on the length of the plate. The roof tilt was set at 0, 30, 45, 60, and 90 degrees and the Reynolds number ranged from 58,000 to 250,000. A transient, one lump, thermal approach was used for heat transfer calculations. Due to a separation bubble at the leading edge of the model, i.e., the roof, at angles of attack of less than 40 degrees, the flow became turbulent after reattachment. This resulted in a higher heat transfer than previously reported in the literature. At higher angles of attack, the flow was not separated at the leading edge and remained laminar. The heat transfer coefficient for higher angles of attack, i.e., α > 40 deg, was found to be approximately independent of the angle of attack and in good agreement with the previously published results.
Moderate Axisymmetric Deformations of Optical Membrane Surfaces
Authors: Murphy L. M.
Published: November 2009

Large deformable membranes are currently used as optical reflector surfaces for solar concentrators. In this paper, we study moderate axisymmetric deformations of optical membrane surfaces and their impact on the macroscopic optical quality of the deformed surface. A uniform pressure loading applied to an initially flat membrane surface is assumed in attaining the final concentrator surface shape. The problem is important in the design of focusing stretched-membrane heliostat modules. Insights into the design of stretched-membrane parabolic dish reflectors also result from this work. This paper presents several simple design and sizing relationships, based on a nonlinear variational principle, in terms of common engineering parameters. Good approximate predictions of the nonlinear load-deformation response and the stress state in the membrane are derived; simple but accurate predictions for membrane surface slope and nominal focal length also result. The phenomena that lead to deviation of the surface shape from the desired parabolic reflector shape are identified and design procedures to minimize those deviations are deduced. Results show that the macroscopic optical surface quality, for a given nominal f/D (concentrator focal length/diameter), is highly dependent on the non-dimensionalized membrane stiffness parameter, Et/To (membrane modulus × thickness/initial tension). Further, for the heliostat cases of interest, the focal-length variations relative to the desired parabolic shape were shown to be about ±5 percent about the nominal value.
Solid-Fluid Response of Packed-Bed Regenerators by Crump’s Numerical Inversion of Laplace Transforms
Authors: Lu P.-C.
Published: November 2009

The temperature histories in the fluid and the solid phases of a fixed-matrix (packed-bed) thermal storage unit in the regenerative mode of operation, with complete reversals of the flow direction, are traced. The heat conduction and energy storage are considered in the solid, but neglected in the fluid. The problem is solved numerically by the technique of Laplace transforms, with the inversion effected by a fast and robust scheme due to Crump [19]. Sample results are plotted and discussed in physical terms.
Unsteady Natural Convection in Enclosures With Stratified Medium
Authors: Shyu R. J. , Hsieh C. K.
Published: November 2009

A numerical solution is given to study the unsteady natural convection in enclosures with stratified medium. Three designs of the enclosure wall are investigated that encompass a wall without insulation on it, insulation placed over the exterior of the wall, and insulation placed over the interior. The enclosed medium is initially stratified in the middle region and the wall temperature is uniform at the high temperature level of the medium. Computation results indicate the superiority of placing the insulation inside to maintain the thermal stratification in the medium. Flow and temperature fields are given for a quantitative substantiation of this observation.
Gas-Particle Flow Within a High Temperature Solar Cavity Receiver Including Radiation Heat Transfer
Authors: Evans G. , Houf W. , Greif R. , Crowe C.
Published: November 2009

A study has been made of the flow of air and particles and the heat transfer inside a solar heated, open cavity containing a falling cloud of 100-1000 micron solid particles. Two-way momentum and thermal coupling between the particles and the air are included in the analysis along with the effects of radiative transport within the particle cloud, among the cavity surfaces, and between the cloud and the surfaces. The flow field is assumed to be two-dimensional with steady mean quantities. The PSI-Cell (particle source in cell) computer code is used to describe the gas-particle interaction. The method of discrete ordinates is used to obtain the radiative transfer within the cloud. The results include the velocity and temperature profiles of the particles and the air. In addition, the thermal performance of the solid particle solar receiver has been determined as a function of particle size, mass flow rate, and infrared scattering albedo. A forced flow, applied across the cavity aperture, has also been investigated as a means of decreasing convective heat loss from the cavity.
Rapid Heating of Gas/Small Particle Mixture
Authors: Wang K. Y. , Yuen W. W.
Published: November 2009

The concept of using a mixture of particles and air as a medium to absorb radiative energy has been proposed for various applications. In this paper, carbon particles mixed with gas form a medium that absorbs radiation from sources such as concentrated solar energy. A single-particle, two-temperature model is used to study the transient temperature of the particle/gas mixture as it undergoes a constant pressure expansion process. The results indicate that for particles smaller than 1 μm in diameter, the surrounding air can be heated as quickly as the particles, while for particles larger than 1 mm in diameter, the air temperature stays relatively unchanged and the particles are heated to a very high temperature. The scattering albedos from the particles are also calculated, revealing that their contribution from scattering to the heating process is insignificant for particles with diameter less than 1 μm.
A Design Method for Thermosyphon Solar Domestic Hot Water Systems
Authors: Malkin M. P. , Klein S. A. , Duffie J. A. , Copsey A. B.
Published: November 2009

A modification to the f-Chart method has been developed to predict monthly and annual performance of thermosyphon solar domestic hot water systems. Stratification in the storage tank is accounted for through use of a modified collector loss coefficient. The varying flow rate throughout the day and year in a thermosyphon system is accounted for through use of a fixed monthly “equivalent average” flow rate. The “equivalent average” flow rate is that which balances the thermosyphon buoyancy driving force with the frictional losses in the flow circuit on a monthly average basis. Comparison between the annual solar fraction predited by the modified design method and TRNSYS simulations for a wide range of thermosyphon systems shows an RMS error of 2.6 percent.
Simultaneous Production of Desalinated Water and Power Using a Hybrid-Cycle OTEC Plant
Authors: Panchal C. B. , Bell K. J.
Published: November 2009

A systems study for simultaneous production of desalinated water and electric power using the hybrid-cycle OTEC system was carried out. The hybrid cycle is a combination of open and closed-cycle OTEC systems. A 10 MWe shore-based hybrid-cycle OTEC plant was discussed and corresponding operating parameters were presented. Design and plant operating criteria for adjusting the ratio of water production to power generation was described and their effects on the total system were evaluated. The systems study showed technical advantages of the hybrid-cycle power system as compared to other leading OTEC systems for simultaneous production of desalinated water and electric power generation.
Thermal Design of Compound Parabolic Concentrating Solar-Energy Collectors
Authors: Prapas D. E. , Norton B. , Probert S. D.
Published: November 2009

A theoretical analysis of the heat exchanges in a Compound Parabolic Concentrator solar energy collector is presented. The absorber configuration considered is that of a tube (with or without a spectrally-selective surface) either directly exposed or enclosed within one or two glass envelopes. The annular cavity formed between the tube and the surrounding envelope can be either air-filled or evacuated. The optimal annular gap, which leads to the best overall collector efficiency, has been predicted for the nonevacuated arrangement. It was found to be approximately 5 mm for the considered geometry. This is about half that recommended by Rabl and Ratzel and gives a 3 percent better overall collector efficiency than obtained with their design. The evacuation of the annular cavity or the application of a selective surface, separately employed, are demonstrated to yield improvements of the same order. It was necessary, for the particular solar radiation data used, both to evacuate the cavity and apply a selective surface if receiver temperatures exceeding 140°C are required. The comparative performances of different CPC designs have also been considered. The theoretical predictions were compared with experimental results and adequate corroboration was obtained.
System-Theoretic Models for Building Thermal Analysis
Authors: Haghighat F. , Chandrashekar M.
Published: November 2009

A method for computer simulation of thermal analysis and energy consumption in buildings is described. This method is based on the system-theoretic approach which has been used to formulate and solve the set of equations for dynamic thermal analysis of buildings. The technique is well suited for computer formulation and the solution of repetitive structures. The resulting set of equations is sparse and can be solved very efficiently on a computer. The algebraic-differential equations are derived for an example using the system-theoretic approach. A large scale example is also presented.
Analysis of Heat and Mass Transfer Between a Desiccant-Air System in a Packed Tower
Authors: Gandhidasan P. , Ullah M. Rifat , Kettleborough C. F.
Published: November 2009

Heat and mass transfer analysis between a desiccant-air contact system in a packed tower has been studied in application to air dehumidification employing liquid desiccant, namely calcium chloride. Ceramic 2 in. Raschig rings are used as the packing material. To predict the tower performance, a steady-state model which considers the heat and mass transfer resistances of the gas phase and the mass transfer resistance of the liquid phase is developed. The governing equations are solved on a digital computer to simulate the performance of the tower. The various parameters such as the effect of liquid concentration and temperature, air temperature and humidity and the rates of flow of air and liquid affecting the tower performance have been discussed.
Thermal Performance in Multi-Pass Solar Systems With Well-Mixed Storage
Authors: Gordon J. M. , Zarmi Y.
Published: November 2009

An approximate analysis of multi-pass, closed-loop solar systems with well-mixed storage tanks is presented. The thermal coupling of the storage tank both to the collector-storage loop (“charging” cycle) and the load-storage loop (“discharging” cycle) is shown to reduce system performance by a penalty factor, called the partial depletion factor, when compared to one-pass, open-loop systems in which the storage fluid is completely consumed by the load by the end of each 24-hour day (total depletion). This penalty factor is typically around 20 percent for systems with a daytime-only load and 30 percent for systems with a nighttime-only load. Our analysis provides, for the first time, the explanation for the findings of various experiments and numerical simulations. We establish the approximate validity of a “quasi-steady state” approach, wherein actual system performance is approximated by a calculation based on the repetition of one representative day. The approach is general in that it is applicable to all solar collector types.
Optimization of a Packed Bed Thermal Energy Storage Unit
Authors: Torab H. , Beasley D. E.
Published: November 2009

The optimization of the design of a packed-bed thermal energy storage unit is presented. A one-dimensional, transient, two-phase model is chosen for the packed bed which assumes uniformity at each cross section within the packing. The governing equations for the time dependent temperature distributions in both the solid and fluid phases are solved using a fully implicit formulation. The accuracy of the numerical method is demonstrated by comparison with experimental measurements of temperature distribution in a randomly packed bed of uniform spheres. The goal of the optimization is to achieve maximum utilization of the thermal energy storage and recovery capabilities of the storage medium for a given set of operating conditions. The optimum combination of bed length, size of the packing particles, and relative size of the bed cross section to the particle diameter is determined, subject to constraints on the maximum allowable pressure drop across the packing, the maximum outlet fluid temperature, and the total amount of supplied energy. The thermodynamic availability is examined as the measure of storage utilization. The monotinicity method is utilized for the optimization process. This method identifies a global optimum without any special computations, and prevents acceptance of false optimum solutions, as could be generated by numerical techniques. The results of the study provide guidelines for choosing the size of the packing and the packing particle subject to the constraints for a practical operating system.
Low Temperature Latent Heat Storage With Quasi-Eutectic Mixtures Containing Ca(NO3)2•4H2O
Authors: Vaccarino C. , Cali V. , Parmaliana A. , Frusteri F.
Published: November 2009

Two mixtures containing Ca(NO3)2•4H2O and KNO3 or Mg(NO3)2•6H2O of composition close to their eutectics are described as promising thermal heat storage (TES) materials suitable for passive solar heating applications. Supercooling is hindered by a system which apparently may be applied to other salt mixtures of this type. A comparison is made with other salt mixtures containing magnesium salts previously studied. Tests are in progress to confirm that no efficiency reduction after many heating/cooling cycles occurs.
Performance Comparison of Storage Control Strategies in CSHPSS Systems
Authors: Lund P. D.
Published: November 2009

Comprehensive numerical computer simulations have been performed to investigate the effects of various storage control strategies on the thermal performance of a non-heat pump Central Solar Heating Plant with Seasonal Storage (CSHPSS) employing a constructed water volume. Different distribution temperatures, load, collector, and storage sizes were also considered in the analyses. The study indicates that the difference in the yearly solar fraction between a system with a simplified storage control strategy and an optimal, but technically more sophisticated one, would be of the order of 0.05–0.15 units (solar fraction, or, equivalently nonauxiliary fraction units). The worst control showed a performance reduction of 0.1–0.35 units compared to the optimal case, the lower value representing system configurations with adequate storage capacity.
The Dual Medium Storage Tank at the IEA/SSPS Project in Almeria (Spain); Part I: Experimental Validation of the Thermodynamic Design Model
Authors: Geyer M. , Bitterlich W. , Werner K.
Published: November 2009

Efficient and reliable thermal storage is an important requirement for substituting conventional power systems with solar thermal facilities. Storage will synchronize the intermittent supply of solar radiation with the usually constant demand of technical thermal processes. The distributed collector system of the IEA/SSPS Project in Almeri´a (Spain) uses two different storage systems for the 100 to 295°C temperature range: a single thermocline vessel and a dual medium storage tank (DMST). In the first tank, thermal oil is used as the energy carrier as well as for energy storage; in the dual medium tank, the storage medium is cast iron, and the oil acts primarily as a heat transfer fluid. At the SSPS, this concept’s potential for future process heat applications has been assessed. Performance and operational restrictions of the DMST were systematically studied over a wide range of temperatures, and an existing simulation model was verified and adapted at the same time. The thermodynamic model of the DMST is presented and compared with the first results of the 1985 test program.
Receiver/Reactor Concepts for Thermochemical Transport of Solar Energy
Authors: Diver R. B.
Published: November 2009

Thermochemical transport of solar energy based on reversible chemical reactions may be a way to take advantage of the high-temperature capabilities of parabolic dishes, while minimizing pipe network heat loss, since energy is transported at ambient temperatures in chemical form. Receiver/Reactor design is a key to making thermochemical transport a reality. In this paper the important parameters for solar receiver and chemical reactor design and how they relate to each other are presented. Three basic receiver/reactor types, applicable to thermochemical receiver design, are identified: (1) Tube Receiver/Reactors have tubular reactor elements which are directly heated by solar energy in the receiver. (2) Indirect Receiver/Reactors use an intermediate heat transfer fluid between the receiver and reactor. (3) Direct Absorption Receiver/Reactors absorb sunlight directly on the reactor catalyst. Advantages, limitations, and risks associated with each design are discussed and examples of those that have been built are given. Each type offers its own set of advantages and risks, and warrant further investigation.
Energy and Availability Transport Losses in a Point-Focus Solar Concentrator Field
Authors: Stine W. B. , Heckes A. A.
Published: November 2009

This paper presents the results of an experimental study of the losses in transporting thermal energy from a field of 114 point-focus solar collectors to a central thermal energy conversion system at the Solar Total Energy Project (STEP), Shenandoah, Georgia. Conduction and convection heat losses from the collector field piping and solar collector receivers and radiant energy losses from the solar collector receivers were measured. At normal operating conditions the steady state heat losses per unit of collector aperture area are 130 W/m2 (41 Btu/hr-ft2). The thermal mass of the collector field was found to be 3.92 kWh/°C (7,440 Btu/°F), which implies that 17 percent of the energy collected on a typical day is used to warm the field piping to its operating temperature. The loss of availability from the collectors and the field piping shows that only 21 percent of the available solar energy falling on the collector field is delivered to the power cycle for conversion into electricity.
Energy Storage and Transportation Based on Sulfuric Acid Decomposition and Synthesis Processes
Authors: Bilgen E.
Published: November 2009

A chemical energy storage and transportation system is conceived based on sulfuric acid decomposition and synthesis processes using hot oxygen as a vector. A thermodynamic assessment is carried out to determine the thermal performance of the process; it is found that the energy storage density is about 0.58 GJ/m3 for 365 cycles per year operation, the overall thermal energy storage efficiency is about 62 percent, the energy transportation efficiency is about 29 percent, and the thermal to mechanical energy conversion efficiency is about 25 percent.
Closed Loop Thermochemical Energy Transport Based on CO2 Reforming of Methane: Balancing the Reaction Systems
Authors: Fish J. D. , Hawn D. C.
Published: November 2009

The objective of the Closed Loop Efficiency Analysis (CLEA) Project at Sandia National Laboratories is to develop the data base, the calculational tools, and the operational experience necessary for the design of cost-effective energy transport systems based on reversible chemical reactions. The primary application of interest is the transport of solar energy either from the collectors of a distributed system to a central site or from a solar energy plant of any type located in an area of high solar insolation to a population or industrial center a few to several hundred kilometers away. A series of experiments and analysis have been carried out to explore the effect of catalyst selection, starting composition, reactor temperatures, system pressure, and water recycle on the operation and the efficiency of an energy transport system based on the reversible carbon dioxide reforming of methane. Both the experiments in the CLEA laboratory facility and the analyses indicate that the carbon dioxide/methane thermochemical energy transport system can be operated in a stable, closed-loop mode and that the system can be started up and shut down, as it must be daily in a solar application, without triggering either catastrophic instabilities or carbon deposition. Based on the success of the CLEA project, two new hardware projects have been initiated to address scale-up and direct solar interfacing.
Energy Transport With an Integrated Direct Solar Reactor
Authors: Rozenman T.
Published: November 2009

A novel design concept is presented of a high-temperature solar central receiver in which solar energy is directly absorbed by fluids flowing in tubes and undergoing a chemical reaction. The utilization of this concept in a design of a solar Methane-Reforming reactor is discussed, especially as it applies to chemical energy transport systems. An analysis of heat balance and energy efficiency of both Steam-Methane and CO2-Methane reforming presents the technological barriers and potential in applying these systems for the transport of solar energy.
Performance/Economics Comparison of Sensible and Thermochemical Energy Transport Systems for Solar Thermal Dish Applications
Authors: Muir J. F.
Published: November 2009

A major challenge facing the development of distributed receiver solar systems is the efficient transport of high temperature thermal energy from the collectors to the point of use. As receiver temperatures increase, conventional sensible (SEN) energy transport methods become less attractive because of increased heat losses and insulation costs. A promising alternative that is particularly attractive for the high temperatures characteristic of paraboloidal dishes and the extensive piping associated with large collector fields is the concept of themochemical (TC) energy transport. Estimates of the performance and economics of 4 SEN and 2 TC transport systems for a dish collector field are compared at 4 delivery temperatures ranging from 400 to 815° C. On the basis of levelized energy cost (LEC), there is no clear choice between SEN and TC energy transport at 400°C. At higher output temperatures, TC transport is more cost-effective and is the only viable choice at temperatures above ∼700° C. The TC system based on the carbon-dioxide reforming of methane has the best performance and lowest costs at temperatures >400°C and appears closest to meeting the DOE Solar Thermal Technology (STT) Program long-term IPH goal of 3¢/kWhth (9$/MBtuth) LEC.
Alkali Metal/Halide Thermal Energy Storage Systems Performance Evaluation
Authors: Phillips W. M. , Stearns J. W.
Published: November 2009

Materials compatibility and durability of advanced salt/alkali metal slurry thermal energy storage systems has been demonstrated [1]. Applications are being evaluated for both space and terrestrial solar thermal power conversion [2]. High energy density of these thermal storage systems is achieved by colocation of heat input and extraction within the slurry mixture which is overwhelmingly phase-change salt. This paper addresses performance testing of these systems. Understanding of mechanisms of both micro and macro stratification of the slurry is necessary to fully predict system performance as a function of gravity and system geometry. If it can be shown the gravity stratification effects are secondary to a combination of: (1) liquid metal film adhesion (wetting) to the heat exchange surfaces and solidified salt particles, (2) solubility of alkali metal in the salt-rich phase, and (3) stirring produced by liquid to vapor conversion of the alkali metal, then system geometry limitations are greatly relaxed for space application. Performance testing was accomplished using a sodium heat pipe to transfer heat from the slurry canister to a gas gap calorimeter. Testing was accomplished with the heat pipe installed only in the vapor space above the alkali metal/salt slurry and with an increase heat pipe and minimum vapor space. This testing conclusively demonstrated the effectiveness of the pseudo-heat-pipe type heat transfer mechanism operating in the slurry system under terrestrial conditions.
Development of Sputter-Deposited Pt/Al2O3 Selective Absorber Coatings
Authors: Thornton J. A. , Lamb J. L.
Published: November 2009

The Pt/Al2O3 cermet system is extremely stable against oxidation. Therefore, effective high temperature selective absorber coatings can be formed by depositing Pt/Al2O3 cermets onto Pt low emittance base layers. Three Pt/Al2O3 cermet configurations have been investigated for photothermal receiver applications: (1) cermets with graded Pt composition and Al2O3 antireflection layers; (2) cermets with uniform Pt compositions and Al2O3 antireflection layers; and (3) Al2O3-M-Al2O3 multilayer coatings where the M-layer is a Pt/Al2O3 mixture containing about 20 volume percent Al2O3. The coatings were deposited by cosputtering from Pt and Al2O3 sources. Both direct rf sputtering from an alumina target and Ar-O2 dc reactive sputtering from an aluminum target were investigated as methods for depositing the Al2O3 components of the cermet and the antireflection layers. Cr, Mo, Ta, W, and ZrB2 were investigated as alternative low emittance base layer materials. Most of the coatings were deposited onto flat plates of glass, although type 316 stainless steel, type 1020 low carbon steel, and Incoloy 800 substrates were also examined. Hemispherical absorptances as high as 0.97, with room temperature emittances in the 0.06 to 0.08 range, were achieved. The optical properties of the coatings with rf-sputtered Al2O3, and Pt low emittance base layers were stable at 600° C in air for as long as 2000 hr, consistent with data for evaporated Pt/Al2O3 coatings reported by Cornell University. The coatings with reactive sputtered aluminum oxide, or alternative base layers such as Mo, exhibited somewhat reduced thermal stability but should be adequate for intermediate temperature applications (up to 400° C in air).
Near-Optimal Control of a Bilinear, Solar-Assisted Heat Pump System
Authors: Le H. , Zaheeruddin M. , Gourishankar V. , Rink R.
Published: November 2009

A system consisting of a flat-plate solar collector, two heat storage tanks, a heat pump, and all necessary minor components is considered. For each major component a bilinear mathematical model is developed. The system control problem is divided into steady-state and dynamic cases, and the optimal steady-state solution is found. Based on this result a suboptimal dynamic control algorithm is given and is shown to give locally stable and near optimal performance. Results of a simulation study, comparing seasonal performance of the suboptimal system with that of the conventionally-controlled system are given.
Measurement of Air Velocity Components of Natural Convective Interzonal Airflow
Authors: Mahajan Bal M.
Published: November 2009

Recent flow visualization tests performed at the National Bureau of Standards Passive Solar Test Facility indicated that the natural convective interzonal flow through a doorway is three-dimensional with the velocity components perpendicular to the plane of the opening and the plane of the floor appearing dominant. In order to further investigate the velocity components of the interzonal airflow through a doorway, an experimental study was undertaken. A simple one-dimensional model is modified to apply to two-dimensional airflow. Empirical expressions for the X velocity component and interzonal mass flow rate are developed. The measured velocity and mass flow rate data and the resulting empirical expression are compared with the values predicted by the simple one-dimensional model. It is found that the natural convective interzonal airflow may be adequately represented by two one-dimensional equations, one for the outflow and a different one for the inflow, provided that proper values of the outflow and inflow discharge coefficients are known.
Comparison of Short-Term Testing and Long-Term Monitoring of Solar Domestic Hot Water Systems
Authors: Beale S. B.
Published: November 2009

This paper reports on the results of a comparison between short-term indoor testing and long-term outdoor monitoring of solar domestic hot water systems. Five solar-preheat systems were monitored under side-by-side conditions of irradiance and load, for a period of two years. The systems were then tested according to a standard day test, using a solar simulator, and a load schedule identical to that imposed on each system during the monitoring. The systems were found to deliver 19.7 MJ–25.8 MJ daily in the test, compared to a two-year average of 19.1 MJ–26.0 MJ (1.5 to 2.0 GJ/m2 annually) outdoors. System rank was reasonably well preserved. Comparison of results on the basis of efficiency and solar fraction suggests that good correspondence exists between long-term outdoor results and those of indoor testing, at least for systems with stable controllers. Selected systems were also tested at different load schedules and radiation levels. Methods of predicting the performance of a solar-preheat system from the results of a standard day test are discussed, and the possibility of reducing testing time to a single day is explored.
Solar-Load-Ratio Statistics for the United States and Their Use for Analytical Solar Performance Prediction
Authors: Cowing T. , Kreider J. F.
Published: November 2009

This paper presents a new analytical method for estimating the annual auxiliary space heating energy requirements of passively heated solar buildings. It features closed-form, analytical expressions for solar system performance that are amenable for use with an inexpensive, programmable calculator. The method is applicable to direct gain, water storage wall, and masonry storage (Trombe) wall passive solar space heating systems and does not restrict the user to a particular “reference” system. It permits the rapid assessment of the performance effects of variations in such system features as south glazing, thermal storage mass, and fenestration insulation. Site specific data for its implementation are included for twenty-six cities in the United States. Also, the method is illustrated with examples. Auxiliary energy estimates from this model are compared with measured auxiliary energy use data from owner-occupied, passively heated solar houses. Statistically significant agreement is demonstrated between predicted and measured results in the 1981–1982 space heating season. While this does not constitute a complete validation of the method, it does demonstrate its viability when applied to actual residences.
A Heat Transfer Analysis for Passively Cooled “Trumpet” Secondary Concentrators
Authors: Suresh D. , O’Gallagher J. , Winston R.
Published: November 2009

Some practical questions associated with the use of hyperboloidal “trumpet” shaped terminal concentrators for use in solar thermal applications are addressed. Computer ray-trace calculations show that the flux distribution is strongly peaked over a small neck area at the exit of the trumpet, which will be subjected to a substantial thermal load. A quasi-transient heat transfer model has been developed to analyze the thermal behavior of passively cooled trumpets. The thermal analysis shows that simple techniques exist such that one can design passive secondary trumpets which will remain below safe temperature limits under normal operation for many applications. The wall thickness and its variation along the body of the bell-shaped shell from the exit are found to play an important role in controlling the temperature at all flux levels. As a check on the validity of the model, a set of electrical simulation experiments was conducted and excellent agreement was found.
Optical Cascade Heat-Collection for Effective Solar Energy Gain
Authors: Oshida Isao , Suzuki Akio
Published: November 2009

Optical cascade heat collection of solar energy has proved effective for gaining thermal energy in the medium temperature region (80∼150° C). The cascade system consists of two separate absorbers (one hot and the other warm) and an optical system including CPC’s and Fresnel lenses which concentrate the direct solar radiation on one of the two absorbers. The temperature of the heat transfer fluid can be increased effectively if the fluid flows into the warm absorber first and then flows into the hot absorber. This type of solar collector, having no mechanical tracking device, can theoretically collect heat at a higher temperature than usual flat-plate collectors.