Journal of Solar Energy Engineering

Published by American Society of Mechanical Engineers
Print ISSN: 0199-6231
Publications
The heating process of melting margarine requires a vast amount of thermal energy due to its high melting point and the size of the reservoir it is contained in. Existing methods to heat margarine have a high hourly cost of production and use fossil fuels which have been shown to have a negative impact on the environment. Thus, we perform an analytical feasibility study of using solar thermal power as an alternative energy source for the margarine melting process. In this study, the efficiency and cost effectiveness of a parabolic trough collector (PTC) solar field are compared with that of a steam boiler. Different working fluids (water vapor and Therminol-VP1 heat transfer oil (HTO)) through the solar field are also investigated. The results reveal the total hourly cost ($/h) by the conventional configuration is much greater than the solar applications regardless of the type of working fluid. Moreover, the conventional configuration causes a negative impact to the environment by increasing the amount of CO2, CO, and NO2 by 117.4 kg/day, 184 kg/day, and 74.7 kg/day, respectively. Optimized period of melt and tank volume parameters at temperature differences not exceeding 25 °C are found to be 8-10 h and 100 m(3), respectively. The solar PTC operated with water and steam as the working fluid is recommended as a vital alternative for the margarine melting heating process.
 
Horizontal axis wind turbine (HAWT) performance is usually predicted by using wind tunnel airfoil performance data in a blade element momentum theory analysis. This analysis assumes that the rotating blade airfoils will perform as they do in the wind tunnel. However, when stall-regulated HAWT performance is measured in full-scale operation, it is common to find that peak power levels are significantly greater than those predicted. Pitch-controlled rotors experience predictable peak power levels because they do not rely on stall to regulate peak power. This has led to empirical corrections to the stall predictions. Viterna and Corrigan (1981) proposed the most popular version of this correction. But very little insight has been gained into the basic cause of this discrepancy. The National Renewable Energy Laboratory (NREL), funded by the DOE, has conducted the first phase of an experiment which is focused on understanding the basic fluid mechanics of HAWT aerodynamics. Results to date have shown that unsteady aerodynamics exist during all operating conditions and dynamic stall can exist for high yaw angle operation. Stall hysteresis occurs for even small yaw angles and delayed stall is a very persistent reality in all operating conditions. Delayed stall is indicated by a leading edge suction peak which remains attached through angles of attack (AOA) up to 30 degrees. Wind tunnel results show this peak separating from the leading edge at 18 deg AOA. The effect of this anomaly is to raise normal force coefficients and tangent force coefficients for high AOA. Increased tangent forces will directly affect HAWT performance in high wind speed operation. This report describes pressure distribution data resulting from both wind tunnel and HAWT tests. A method of bins is used to average the HAWT data which is compared to the wind tunnel data. The analysis technique and the test set-up for each test are described.
 
Two models for the temperature distribution within a flat-plate solar collector are investigated. The simpler of the two (the one-temperature model) is a distributed-parameter model that determines the temperature of the collector fluid without reference to the temperature of the collector plate. The more accurate model of the two (the two-temperature model) takes into consideration the temperatures of both the collector plate and the fluid. In previous work the authors showed how the solutions of the two-temperature model tend to the solutions of the one-temperature model as the thermal coupling between the fluid and the collector plate increases. In the present work we include an artificial diffusion term in the one-temperature model. We show that this diffusion term causes the solutions of the one-temperature model to better approximate those of the more accurate two-temperature model. In essence, the artificial diffusion term produces effects similar to those caused by the thermal coupling between the fluid and the collector plate, which is not accurately modeled in the original one-temperature model. In realistic collectors the value of the diffusivity associated with this artificial diffusion term is much smaller than the diffusivity of the collector fluid, which we neglect in all models under consideration. Graphical results further demonstrate the good agreement between the revised one-temperature model and the two-temperature model.
 
The Solar Dynamic Power Module being developed for Space Station Freedom uses a eutectic mixture of LiF-CaF2 phase-change salt contained in toroidal canisters for thermal energy storage. This paper presents results from heat transfer analyses of the phase-change salt containment canister. A two-dimensional, axisymmetric finite difference computer program which models the canister walls, salt, void, and heat engine working fluid coolant was developed. Analyses included effects of conduction in canister walls and solid salt, conduction and free convection in liquid salt, conduction and radiation across salt vapor-filled void regions, and forced convection in the heat engine working fluid. Void shape and location were prescribed based on engineering judgment. The salt phase-change process was modeled using the enthalpy method. Discussion of results focuses on the role of free convection in the liquid salt on canister heat transfer performance. This role is shown to be important for interpreting the relationship between ground-based canister performance (in 1-g) and expected on-orbit performance (in micro-g). Attention is also focused on the influence of void heat transfer on canister wall temperature distributions. The large thermal resistance of void regions is shown to accentuate canister hot spots and temperature gradients.
 
Recent advances in the efficiency and manufacturing technology of CulnSe2 (CIS) thin films demonstrate the opportunity for low-cost large-scale production of photovoltaics for utility applications. Large area (0.4 m2) submodules with 9.7 percent aperture efficiencies yielding 37.8 watts have been fabricated. Thin film fabrication techniques used in the production of modules enable reduced production costs compared with those for single crystal silicon. The performance of 0.4 m2 modules is projected to exceed 50 watts, based on performance achieved to date on 0.1 m2 modules and small area test devices. Preliminary tests packaged (encapsulated and framed) modules show no significant losses after 15 1/2 months of continuous outdoor exposure. Fabrication of 0.4 m2 modules to demonstrate the feasibility of large-scale commercialization of CIS thin film photovoltaics for utility applications is currently under way.
 
Space power technologies have undergone significant advances over the past few years, and great emphasis is being placed on the development of dynamic power systems at this time. A design study has been conducted to evaluate the applicability of a combined cycle concept-closed Brayton cycle and organic Rankine cycle coupling-for solar dynamic space power generation systems. In the concept presented here (solar dynamic combined cycle), the waste heat rejected by the closed Brayton cycle working fluid is utilized to heat the organic working fluid of an organic Rankine cycle system. This allows the solar dynamic combined cycle efficiency to be increased compared to the efficiencies of two subsystems (closed Brayton cycle and organic fluid cycle). Also, for small-size space power systems (up to 50 kW), the efficiency of the solar dynamic combined cycle can be comparable with Stirling engine performance. The closed Brayton cycle and organic Rankine cycle designs are based on a great deal of maturity assessed in much previous work on terrestrial and solar dynamic power systems. This is not yet true for the Stirling cycles. The purpose of this paper is to analyze the performance of the new space power generation system (solar dynamic combined cycle). The significant benefits of the solar dynamic combined cycle concept such as efficiency increase, mass reduction, specific area-collector and radiator-reduction, are presented and discussed for a low earth orbit space station application.
 
We developed a maskless plasma texturing technique for multicrystalline Si (mc-Si) cells using reactive ion etching (RIE) that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to low levels. Internal quantum efficiencies higher than those on planar and wet-textured cells have been obtained, boosting cell currents and efficiencies by up to 6% on tricrystalline Si cells.
 
At the DLR a second generation sodium heat pipe receiver for the Schlaich Bergermann und Partner (SBP) 9-kWel Dish/Stirling system has been developed and constructed. Long-term operation occurred from Oct. 1992 until Aug. 1993 at the Plataforma Solar de Almeria (PSA) in Spain, accumulating 950 operating hours. The performance of the SBP 9-kWel System with a soldium heat pipe receiver is evaluated according to the guidelines for dish/Stirling performance evaluation by Stine and Powel, as proposed to the International Energy Agency (IEA). Tests were stopped due to a leak in the receiver absorber surface. The analysis of this damage are reported.
 
Measurement set-up with reflected and observed rays  
Reflected images of the absorber tube with the reflector facing directly the camera (center) and tilted slightly up (left) and down (right). The evaluated facet row is marked with a rectangle  
Picture for photogrammetric evaluation with targets on four facets, on absorber tube and module axis.  
Transversal slope errors (in x-direction) of the facets in milliradians, from photogrammetric data  
Root mean square values of the slope deviations from the ideal slope computed by the absorber reflection method and the photogrammetric method.  
A new fast method for optically measuring the reflector slope of parabolic troughs with high accuracy has been developed. It uses the reflection of the absorber tube in the concentrator as seen from some distance and is therefore called “absorber reflection method”. A digital camera is placed at a distant observation point perpendicular to the trough axis with the concentrator orientated towards it. Then, a set of pictures from the absorber tube reflection is taken with the concentrator in slightly different tilt angles. A specially developed image analysis algorithm detects the edges of the absorber tube in the reflected images. This information, along with the geometric relationship between the components of the set-up and the known approximately parabolic shape of the concentrator, is used to calculate the slopes perpendicular to the trough axis. Measurement results of a EuroTrough segment of four facets are presented and verified with results from a reference measurement using high-resolution close-range photogrammetry. The results show good agreement as well in statistical values as in local values of the reflector slope. In contrast to the photogrammetric data acquisition method, the new technique allows for drastically reduced measurement time.
 
The solar field is the major cost component of a solar thermal power plant and the optical quality of the concentrators has a significant impact on the field efficiency and thus on the performance of the power plant. Measuring slope deviations in the parabolic shape of the mirror panels in the accuracy and resolution required for these applications is a challenge as it is not required with the same characteristics in other industries. Photogrammetry and deflectometry are two optical measurement methods that are typically used to measure this shape accuracy of mirror panels used in CSP applications. They have been compared and validated by measuring a typical mirror panel under optimal conditions. Additionally, a flat water surface has been measured as an absolute reference object using deflectometry. The remaining deviations between the results of both methods and to the reference object are discussed and possible sources of errors during the measurement are identified. A detailed error analysis is conducted for both methods and compared to the experimental findings. The results show that both methods allow for surface slope measurement with the necessary accuracy for present CSP applications and that among the two, deflectometry exhibits advantages in speed, measurement accuracy and spatial resolution. However, for obtaining correct results several sources of errors have to be addressed appropriately during measurement and post-processing
 
The availability of storage capacity plays an important role for the economic success of solar thermal power plants. For today’s parabolic trough power plants, sensible heat storage systems with operation temperatures between 300°C and 390°C can be used. A solid media sensible heat storage system is developed and will be tested in a parabolic trough test loop at PSA, Spain. A simulation tool for the analysis of the transient performance of solid media sensible heat storage systems has been implemented. The computed results show the influence of various parameters describing the storage system. While the effects of the storage material properties are limited, the selected geometry of the storage system is important. The evaluation of a storage system demands the analysis of the complete power plant and not only of the storage unit. Then the capacity of the system is defined by the electric work produced by the power plant, during a discharge process of the storage unit. The choice of the operation strategy for the storage system proves to be essential for the economic optimization.
 
The performance and cost of four 10 MWe advanced solar thermal electric power plants sited in various regions of the continental United States was studied. Each region has different insolation characteristics which result in varying collector field areas, plant performance, capital costs and energy costs. The regional variation in solar plant performance was assessed in relation to the expected rise in the future cost of residential and commercial electricity supplied by conventional utility power systems in the same regions. A discussion of the regional insolation data base is presented along with a description of the solar systems performance and costs. A range for the forecast cost of conventional electricity by region and nationally over the next several decades is given.
 
High air outlet temperatures increase the solar share of pressurized solar receivers for gas turbines, operated in solar-fossil hybrid mode. However, an increase in outlet temperature over 800°C leads to excessive heating of the receiver window, unless it is actively cooled. This paper describes modeling, testing and evaluation of a high-temperature receiver with external multiple air-jet window cooling. An asymmetric window-cooling design with pulsating air mass flow rates achieves suitable cooling of the concave fused-silica window. A thermodynamic receiver model, comprising non-gray radiative heat transfer, convection and conduction is the basis of the external window cooling design. In addition to high-temperature testing with window cooling in operation, solar tests at lower temperatures with no window cooling were conducted to verify the thermodynamic receiver model. Temperature distributions on the quartz window and the absorber were determined by an infrared (IR) scanner which was specially developed for temperature measurement on the high-temperature module. Comparisons of simulations and measurements show good agreement. With multiple air-jet window cooling, receiver air outlet temperatures over 1000°C could be reached, while window temperatures are kept below 800°C.
 
In view of rising energy prices and an increasing share of power generated by renewable energy sources, the importance of energy storage is growing. In the framework of this project a thermal energy storage concept for solar power towers is being developed, in which quartz sand serves as storage medium. Sand is suited due to its properties as high thermal stability, specific heat capacity and low-cost availability. Compared to storages based on ceramic bodies the usage of sand promises to reduce costs of energy storage and thus to reduce the costs of electricity generation. In addition the storage concept could be applicable in steel industry. The central element of the storage concept is an air-sand heat exchanger, which is presently under development. This paper describes simulation results and measurements of the heat exchanger prototype. It includes sand flow behaviour and experience with different porous walls as well as up-scaling options.
 
In view of rising energy prices and an increasing share of power generated by renewable energy sources, the importance of energy storage is growing. In the framework of this project, a thermal energy storage concept for solar power towers is being developed, in which quartz sand serves as a storage medium. Sand is suitable due to its properties such as high thermal stability, specific heat capacity, and low-cost availability. Compared with storages based on ceramic bodies, the use of sand promises to reduce costs of energy storage and thus to reduce the costs of electricity generation. In addition, the storage concept could be applicable in the steel industry. The central element of the storage concept is an air-sand heat exchanger, which is presently under development. This paper describes simulation results and measurements of the heat exchanger prototype. It includes sand flow behavior and experience with different porous walls as well as up-scaling options.
 
A pseudoheat-pipe heat transfer mechanism has been demonstrated effective in terms of both total heat removal efficiency and rate, on the one hand, and system isothermal characteristics, on the other, for solar thermal energy storage systems of the kind being contemplated for spacecraft. The selection of appropriate salt and alkali metal substances for the system renders it applicable to a wide temperature range. The rapid heat transfer rate obtainable makes possible the placing of the thermal energy storage system around the solar receiver canister, and the immersing of heat transfer fluid tubes in the phase change salt to obtain an isothermal heat source.
 
To check the feasibility of solar thermal remelting of aluminum scrap a directly absorbing rotary kiln receiver-reactor was constructed for experimentation in a mini-plant scale in the DLR high flux solar furnace. Conventionally the high energy demand for heating rotary kilns is met by the combustion of fossil fuels. This procedure generates a big exhaust gas volume which is contaminated by volatiles if the the technology is applied to treat waste materials. Application of concentrated solar radiation to provide the high temperature heat enables to substitute the fossil fuel. Thus smaller off-gas streams ai e generated and lower investment and O&M cost are expected for the off-gas purification. in this paper market and environmental issues are discussed and pre-designs both for solar pilot and industrial scale applications ai e presented.
 
An existing software tool for annual performance calculation of concentrating solar power and other renewable energy plants has been extended to enable the simulation of solar tower power plants. The methodology used is shown and a demonstrative example of a 50 MW(e) tower plant in southern Spain is given. The influence of design power and latitude on solar field layout is discussed. Furthermore, a comparison of the tower plant with a 50 MW(e) parabolic trough and a Linear Fresnel plant at the same site is given. [DOI: 10.1115/1.4004355]
 
This paper synthesizes past events in an attempt to define the general magnitude, duration, and location of large surface solar anomalies over the globe. Surface solar energy values are mostly a function of solar zenith angle, cloud conditions, column atmospheric water vapor, aerosols, and surface albedo. For this study, solar and meteorological parameters for the 10-yr period July 1983 through June 1993 are used. These data were generated as part of the Release 3 Surface meteorology and Solar Energy (SSE) activity under the NASA Earth Science Enterprise (ESE) effort. Release 3 SSE uses upgraded input data and methods relative to previous releases. Cloud conditions are based on recent NASA Version-D International Satellite Cloud Climatology Project (ISCCP) global satellite radiation and cloud data. Meteorological inputs are from Version-I Goddard Earth Observing System (GEOS) reanalysis data that uses both weather station and satellite information. Aerosol transmission for different regions and seasons are for an 'average' year based on historic solar energy data from over 1000 ground sites courtesy of Natural Resources Canada (NRCan). These data are input to a new Langley Parameterized Shortwave Algorithm (LPSA) that calculates surface albedo and surface solar energy. That algorithm is an upgraded version of the 'Staylor' algorithm. Calculations are performed for a 280X280 km equal-area grid system over the globe based on 3-hourly input data. A bi-linear interpolation process is used to estimate data output values on a 1 X 1 degree grid system over the globe. Maximum anomalies are examined relative to El Nino and La Nina events in the tropical Pacific Ocean. Maximum year-to-year anomalies over the globe are provided for a 10-year period. The data may assist in the design of systems with increased reliability. It may also allow for better planning for emergency assistance during some atypical events.
 
If a malfunction occurs in a solar thermal point-focus distributed receiver power plant while a concentrator is pointed at the sun, motion of the concentrator may stop. As the sun moves relative to the earth, the spot of concentrated sunlight then slowly walks off the receiver aperture, across the receiver face plate, and perhaps across adjacent portions of the concentrator. Intense local heating by the concentrated sunlight may damage or destroy these parts. The behavior of various materials under conditions simulating walk-off of a parabolic dish solar collector were evaluated. Each test consisted of exposure to concentrated sunlight at a peak flux density of about 7000 kW/square meter for 15 minutes. Types of materials tested included graphite, silicon carbide, silica, various silicates, alumina, zirconia, aluminum, copper, steel, and polytetrafluoroethylene. The only material that neither cracked nor melted was grade G-90 graphite. Grade CS graphite, a lower cost commercial grade, cracked half-way across, but did not fall apart. Both of these grades are medium-grain extruded graphites. A graphite cloth (graphitized polyacrylonitrile) showed fair performance when tested as a single thin ply; it might be useful as a multi-ply assembly. High purity slipcast silica showed some promise also.
 
In commercial power plant technology, the market introduction of ultra supercritical (USC) steam cycle power plants with steam parameters around 350bar and 720°C is the next development step. USC steam cycles are also proposed to decrease the levelized electricity costs of future solar power towers due to their highly efficient energy conversion. A 55% thermal efficiency with decreased specific investment costs is within the potential of USC steam cycles. The required process parameters can be achieved using nickel based alloys in the solar receiver, the tubing and other plant components. For solar tower applications, appropriate high temperature heat transfer media (HTM), high temperature heat exchangers and storage options are additionally required. Using the current development for molten salt power towers (Solar Tres) as a reference, several tower concepts with USC power plants were compared. The ECOSTAR methodology provided by [1] was applied for predicting the cost reduction potential and the annual performance of these power tower concepts applying tubular receivers with various HTM. The considered HTM include alkali nitrate salts, alkali chloride salts and liquid metals such as a Bi-Pb eutectic, tin or sodium. For the assessment, an analytical model of the heat transfer in a parametric 360° cylindrical, tubular central receiver was developed to examine the receiver characteristics for different geometries. The sensitivity of the specific cost assumptions for the levelized electricity costs (LEC) was evaluated for each concept variation. No detailed evaluation was done for the thermal storage, but comparable costs were assumed for all cases. The results indicate a significant cost reduction potential for the liquid metal HTM processes.
 
In photovoltaic systems, the encapsulant material that protects the solar cells should be highly transparent and very durable. Glass satisfies these two criteria and is considered a primary candidate for low-cost, photovoltaic encapsulation systems. In this paper, various aspects of glass encapsulation are treated that are important for the designer of photovoltaic systems. Candidate glasses and available information defining the state of the art of glass encapsulation materials and processes for automated, high volume production of terrestrial photovoltaic devices and related applications are presented. The desired characteristics of glass encapsulation are (1) low degradation rates, (2) high transmittance, (3) high reliability, (4) low-cost, and (5) high annual production capacity. The glass design areas treated herein include the types of glass, sources, prices, physical properties and glass modifications, such as antireflection coatings.
 
The cost of solar tower power plants is dominated by the heliostat field making up roughly 50% of investment costs. Classical heliostat design is dominated by mirrors brought into position by steel structures and drives that guarantee high accuracies under wind loads and thermal stress situations. A large fraction of costs is caused by the stiffness requirements of the steel structure, typically resulting in 20 kg/m2 steel per mirror area. The typical cost figure of heliostats is currently in the area of 150 € /m2 caused by the increasing price of the necessary raw materials. An interesting option to reduce costs lies in a heliostat design where all moving parts are protected from wind loads. In this way, drives and mechanical layout may be kept less robust, thereby reducing material input and costs. In order to keep the heliostat at an appropriate size, small mirrors (around 10x10 cm2) have to be used, which are placed in a box with a transparent cover. Innovative drive systems are developed in order to obtain a cost-effective design. A 0.5x0.5 m2 demonstration unit will be constructed. Tests of the unit are carried out with a high-precision artificial sun unit that imitates the sun’s path with an accuracy of less than 0.5 mrad and creates a beam of parallel light with a divergence of less than 4 mrad.
 
The cost of solar tower power plants is dominated by the heliostat field making up roughly 50 % of investment costs. Classical heliostat design is dominated by mirrors brought into position by steel structures and drives that guarantee high accuracies under wind loads and thermal stress situations. A large fraction of costs is caused by the stiffness requirements of the steel structure, typically resulting in ~20 kg/m² steel per mirror area. The typical cost figure of heliostats1 is currently in the area of 150 €/m² caused by the increasing price of the necessary raw materials. An interesting option to reduce costs lies in a heliostat design where all moving parts are protected from wind loads. In this way, drives and mechanical layout may be kept less robust thereby reducing material input and costs. In order to keep the heliostat at an appropriate size, small mirrors (around 10 cm x 10 cm) have to be used which are placed in a box with transparent cover. Innovative drive systems are developed in order to obtain a cost-effective design. A 0.5 m x 0.5 m demonstration unit will be constructed. Tests of the unit are carried out with a high-precision artificial sun unit that imitates the sun’s path with an accuracy of less than 0.5 mrad and creates a beam of parallel light with divergence less than 4 mrad.
 
For parabolic trough power plants using synthetic oil as the heat transfer medium, the application of solid media sensible heat storage is an attractive option in terms of investment and maintenance costs. One important aspect in storage development is the storage integration into the power plant. A modular operation concept for thermal storage systems was previously suggested by DLR, showing an increase in storage capacity of more than 100 %. However, in these investigations, the additional costs needed to implement this storage concept into the power plant, like for extra piping, valves, pumps and control had not been considered. These aspects are discussed in this paper, showing a decrease of levelized energy costs with modular storage integration of 2 to 3 %. In a Life Cycle Assessment (LCA) a comparison of an AndaSol-I type solar thermal power plant [1] with the original two-tank molten salt storage and with a “hypothetical” concrete storage shows an advantage of the concrete storage technology concerning environmental impacts. The environmental impacts of the “hypothetical” concrete based AndaSol-I decrease by 7 %, considering 1 kWh of solar electricity delivered to the grid. Regarding only the production of the power plant, the emissions decrease by 9.5 %.
 
Solar-hybrid gas turbine power systems offer a high potential for cost reduction of solar power. Such systems were already demonstrated as test systems. For the market introduction of this technology, microturbines in combination with small solar tower plants are a promising option. The combination of a solarized microturbine with an absorption chiller was studied; the results are presented in this paper. The solar-hybrid trigeneration system consists of a small heliostat field, a receiver unit installed on a tower, a modified microturbine, and an absorption chiller. The components are described, as well as the required modifications for integration to the complete system. Several absorption chiller models were reviewed. System configurations were assessed for technical performance and cost. For a representative site, a system layout was made, using selected industrial components. The annual energy yield in power, cooling, and heat was determined. A cost assessment was made to obtain the cost of electricity and cooling power, and eventually additional heat. Various load situations for electric and cooling power were analyzed. The results indicate promising niche applications for the solar-assisted trigeneration of power, heat, and cooling. The potential for improvements in the system configuration and the components is discussed, also the next steps toward market introduction for such systems.
 
Solar-hybrid gas turbine power systems were already demonstrated as test systems. For the market introduction of this technology, microturbines in combination with small solar tower plants are a promising option. The combination of a solarized microturbine with an absorption chiller was studied, the results are presented in this paper. The solar system consists of a small heliostat field, a receiver unit installed on a tower, a modified microturbine and an absorption chiller for use of the exhaust heat. Each component is described, as well as the required modifications for integration to the complete system. Several system configurations were assessed for technical performance and cost. For a representative site a system layout was made, using selected industrial components. For several system configurations the annual energy yield in power, cooling and heat was determined. A cost assessment was made to obtain the cost of electricity and cooling power, and eventually additional heat. Various load situations for electric and cooling power were analyzed. The results indicate promising niche applications for the solar-assisted trigeneration of power, heat and cooling. The potential for improvements in the system configuration and the components is discussed, also the next steps towards market introduction of such systems.
 
Theoretical models to predict the radiation of low frequency and impulsive sound from horizontal axis wind turbines due to three sources: (1) steady blade loads; (2) unsteady blade loads due to operation in a ground shear; (3) unsteady loads felt by the blades as they cross the tower wake. These models are then used to predict the acoustic output of MOD-1, the large wind turbine operated near Boone, N.C. Predicted acoustic time signals are compared to those actually measured near MOD-1 and good agreement is obtained.
 
A flux mapping system able to measure the flux distribution of dish/Stirling systems in planes perpendicular to the optical axis was built and operated at the Plataforma Solar de Almería (PSA). It uses the indirect measuring method with a water-cooled Lambertian target placed in the beam path and a CCD-camera mounted on the concentrator taking images of the brightness distribution of the focal spot. The calibration is made by calculating the total power coming from the dish and relating it to the integrated gray value over the whole measurement area. The system was successfully operated in a DISTAL II stretched membrane dish and in the new EURODISH in order to characterize their beams and improve the flux distribution on their receivers.
 
For large off-shore wind turbines, blades with relative low blade mass are becoming more important. The economic use of large-tow carbon fibers can help achieve lower blade masses. Basic material design data have been established for two promising material combinations, including the fatigue properties for Panex33/epoxy. Blade root joints have been developed in a carbon/glass combination, resulting in a better price performance ratio. The initial cost assessment on a blade dominated by severe fatigue loads shows that application of carbon fibers in the spar leads to cost reductions.
 
For large off-shore wind turbines, blades with relative low blade mass are becoming more important. The economic use of large-tow carbon fibers can help achieve lower blade masses. Basic material design data have been established for two promising material combinations, including the fatigue properties for Panex33/epoxy. Blade root joints have been developed in a carbon/glass combination, resulting in a better price performance ratio. The initial cost assessment on a blade dominated by severe fatigue loads shows that application of carbon fibers in the spar leads to cost reductions.
 
This paper presents results of development tests of various solar thermal parabolic dish modules and assemblies that used dish-mounted Brayton or Stirling cycle engines for production of electric power. These tests indicate that early modules achieve net efficiencies up to 29 percent in converting sunlight to electricity, as delivered to the grid. Various equipment deficiencies were observed and a number of malfunctions occurred. The performance measurements, as well as the malfunctions and other test experience, provided information that should be of value in developing systems with improved performance and reduced maintenance.
 
A study has been completed on the application of latent-heat buffer thermal energy storage to a point-focusing solar receiver equipped with an air Brayton engine. To aid in the study, a computer program was written for complete transient/stead-state Brayton cycle performance. The results indicated that thermal storage can afford a significant decrease in the number of engine shutdowns as compared to operating without thermal storage. However, the number of shutdowns does not continuously decrease as the storage material weight increases. In fact, there appears to be an optimum weight for minimizing the number of shutdowns.
 
Background. Storage technology based on solid media heated in direct contact – so called regenerators – are well suited to promote the market introduction of solar central receiver plants with air receivers. However, starting from existing technologies, several design issues need to be addressed. Method of Approach. A test campaign was performed at the Solar Power Tower Jülich, an experimental central receiver plant, to experimentally verify the functionality and to quantify the performance of the plant’s storage subsystem. To this end, a gas burner used during commissioning of the plant, was reactivated and used to run a series of operation sequences. Computer simulations have been set up and applied to retrace the storage behaviour to confirm the validity of the underlying models and to gain further insight into the relevant phenomena. Results. The test results confirm the full functionality of the storage subsystem, the ability to perform cycling at high discharge heat rates and relatively low heat losses, supporting the view that the technology represents a promising basis for up-scaled implementations. Conclusions. Measurement data and simulation results are in good agreement, confirming the maturity of existing design tools.
 
Solar sailing is a unique form of propulsion where a spacecraft gains momentum from incident photons. Since sails are not limited by reaction mass, they provide continual acceleration, reduced only by the lifetime of the lightweight film in the space environment and the distance to the Sun. Practical solar sails can expand the number of possible missions that are difficult by conventional means. The National Aeronautics and Space Administration's Marshall Space Flight Center (MSFC) is concentrating research into the utilization of ultra lightweight materials for spacecraft propulsion. Solar sails are generally composed of a highly reflective metallic front layer, a thin polymeric substrate, and occasionally a highly emissive back surface. The Space Environmental Effects Team at MSFC is actively characterizing candidate sails to evaluate the thermo-optical and mechanical properties after exposure to electrons. This paper will discuss the preliminary results of this research.
 
The Solar Dynamic Power Module being developed for Space Station Freedom uses a eutectic mixture of LiF-CaF2 phase change salt contained in toroidal canisters for thermal energy storage. Results are presented from heat transfer analyses of the phase-change salt containment canister. A 2-D, axisymmetric finite-difference computer program which models the canister walls, salt, void, and heat engine working fluid coolant was developed. Analyses included effects of conduction in canister walls and solid salt, conduction and free convection in liquid salt, conduction and radiation across salt vapor filled void regions, and forced convection in the heat engine working fluid. Void shape, location, and growth or shrinkage (due to density difference between the solid and liquid salt phases) were prescribed based on engineering judgement. The salt phase change process was modeled using the enthalpy method. Discussion of results focuses on the role of free-convection in the liquid salt on canister heat transfer performance. This role is shown to be important for interpreting the relationship between groundbased canister performance (in 1-g) and expected on-orbit performance (in micro-g). Attention is also focused on the influence of void heat transfer on canister wall temperature distributions. The large thermal resistance of void regions is shown to accentuate canister hot spots and temperature gradients.
 
Two sets of experimental data are examined in this paper, ground and space experiments, for cylindrical canisters with thermal energy storage applications. A 2-D computational model was developed for unsteady heat transfer (conduction and radiation) with phase-change. The radiation heat transfer employed a finite volume method. The following was found in this study: (1) Ground Experiments: the convection heat transfer is equally important to that of the radiation heat transfer; radiation heat transfer in the liquid is found to be more significant than that in the void; including the radiation heat transfer in the liquid resulted in lower temperatures (about 15 K) and increased the melting time (about 10 min.); generally, most of the heat flow takes place in the radial direction. (2) Space Experiments: radiation heat transfer in the void is found to be more significant than that in the liquid (exactly the opposite to the Ground Experiments); accordingly, the location and size of the void affects the performance considerably; including the radiation heat transfer in the void resulted in lower temperatures (about 40 K).
 
Heliostat canting (alignment of mirror facets) is known to have a major influence on the optical efficiency of heliostat fields and therefore on the power output of solar tower plants. In recent years several canting concepts were used, mainly on- and off-axis canting. Several new canting concepts, like stretched-parabolic or target aligned canting, were proposed in order to improve the performance of heliostats. As solar power plants become economically more attractive, knowledge about the influence of canting becomes more important. In this contribution, the influence of several factors on the canting method is discussed and optimal canting strategies are described. The considered factors comprise plant power level, heliostat position in the field, heliostat area, receiver dimension and site latitude. It is concluded that the target aligned tracking method is superior to all other variants in the majority of cases. As for the standard azimuth-elevation tracking methods, no one of these exhibits a clear advantage. It is only the on-axis method that performs worst in all cases.
 
Properly quantified performance of a solar-thermal cavity receiver must not only account for the energy gains and losses as dictated by the First Law of thermodynamics, but it must also account for the quality of the energy. Energy quality can only be determined from the Second Law. In this paper, an equation developed for the Second-Law efficiency of a cavity receiver is presented as an evolution from the definition of available energy or availability (occasionally called exergy). The variables required are all either known or readily determined. The importance of considering the Second-Law is emphasized by a comparison of the First- and Second-Law efficiencies around an example of data collected from two receivers that were designed for different purposes, where the attempt was made to demonstrate that a Second-Law approach to quantifying the performance of a solar-thermal cavity receiver lends more complete insight than does the conventional solely applied First-Law approach.
 
Two tank storage systems using molten salt represent today's state of the art in energy storage for concentrating solar power (CSP) plants. This concept shows a limited potential for further cost reductions, since the capital costs are dominated by the expenses for the salt inventory. The application of solid storage materials represents a promising approach to reduce capital costs. While this approach avoids also the risk of freezing and lessens corrosion problems, the efficiency of the heat transfer between the heat transfer fluid (HTF) and the solid storage medium is crucial. This paper introduces the CellFlux concept, which uses an intermediate closed air loop to transfer energy between the HTF and the solid storage material. A modular concept is chosen to optimize the size of the air flow channels. An initial project will provide the fundamentals needed to design a CellFlux storage unit. The feasibility will be proven by a 100 kW/500 kWh pilot storage module.
 
The high-temperature capability, resistance to corrosive environments and non-strategic nature of ceramics have prompted applications in the solar thermal field whose advantages over metallic devices of comparable performance may begin to be assessed. It is shown by a survey of point-focusing receiver designs employing a variety of ceramic compositions and fabrication methods that the state-of-the-art in structural ceramics is not sufficiently advanced to fully realize the promised benefits of higher temperature capabilities at lower cost than metallic alternatives. The ceramics considered include alumina, berylia, magnesia, stabilized zirconia, fused silica, silicon nitride, silicon carbide, mullite and cordierite, processed by such methods as isostatic pressing, dry pressing, slip casting, extrusion, calendaring and injection molding.
 
Basic prices
Share capital comparison of photochemical production of rose oxide
Annual production cost comparison
Net present values of solar and lamp-operated manufacture of rose oxide depending on global insulation and different costs of electricity, heating and cooling
An economic evaluation regarding the industrial photosynthesis of fine chemical rose oxide using solar light is given and compared to conventional lamps as light source. A plant reaching a capacity of 100 t/a was designed containing app. 1,900 m2 of parabolic troughs, 1,500 m2 of flatbed reactors or 32 high-pressure mercury lamps doped with Thallium iodide (TlI) as a photounit. The investment and production costs were compared and graded. We reached the conclusion that solar application is more profitable than the lamp driven one.
 
In concentrating solar power, high-temperature solar receivers can provide heat to highly efficient cycles for electricity or chemical production. Excessive heating of the fused-silica window and the resulting recrystallization are major problems of high-temperature receivers using windows. Excessive window temperatures can be avoided by applying an infrared-reflective solar-transparent coating on the fused-silica window inside. Both glass temperatures and receiver losses can be reduced. An ideal coating reflects part of the thermal spectrum (lambda>2.5 µm) of the hot absorber (1100°C) back onto it without reducing solar transmittance. Extensive radiation simulations were done to screen different filter types. The examined transparent conductive oxides (TCO) involve a high solar absorptance, inhibiting their use in high-concentration solar systems. Although conventional dielectric interference filters have a low solar absorption, the reflection of solar radiation which comes from various directions is too high. It was found that only rugate filters fulfill the requirements for operation under high-flux solar radiation with different incident angles. A thermodynamic qualification simulation of the rugate coating on a window of a flat-plate receiver showed a reduction of almost 175 K in mean window temperature and 11% in receiver losses compared to an uncoated window. For the configuration of a pressurized receiver (REFOS type), the temperature could be reduced by 65 K with slightly reduced receiver losses. Finally, a first 25-µm thick rugate filter was manufactured and optically characterized. The measured spectra fitted approximately the design spectra, except for two absorption peaks which can be avoided in future depositions by changing the deposition geometry and using in-situ monitoring. The issue of this paper is to share the work done on the choice of filter type, filter design, thermodynamic evaluation, and deposition experiments.
 
A hybrid sodium heat pipe receiver has been developed within the project HYHPIRE, funded 50% by the European Commission. The hybrid receiver was designed for the SBP/LCS 10-kWel dish/Stirling system with the SOLO-161 Stirling engine. Design of the heat pipe reciever and combustion system are described in this paper. The system has been tested successfully in all operation modes. Results and experience from the lab tests in combustion-only mode, as well as results from demonstration testing in the dish in solar-only, gas-only, and hybrid mode on the Plataforma Solar de Almeria (PSA) in Spain, are reported.
 
A new fast and highly accurate method for measuring the slope errors of parabolic dish concentrators has been developed. This method uses a flat target with colored stripes placed close to the focal plane of the concentrator and a digital camera located at an observation point on the optical axis at some distance from it. A specially developed image analysis algorithm detects the different colors of the images of the reflection of the target in the concentrator and assigns them their known position on the color target. This information, along with the geometry of the measurement setup and the theoretical parabolic shape of the concentrator, is used to calculate the normal vectors of the concentrator surface. From these normal vectors the radial and tangential slopes can be calculated and compared to the design values of the concentrator. The resulting slope errors not only give the total concentrator error for general characterization of the dish, but also indicate systematic errors from fabrication and installation with a high spatial resolution. In order to verify the quality of the results obtained, a ray-tracing code was developed that calculates the flux distribution on planes perpendicular to the optical axis. Measured slope errors of a 8.5m dish concentrator are presented and the calculated flux distributions are compared to measured flux distributions. The comparison shows excellent agreement in the flux distribution on the absorber plane. This verifies the promising potential of this fast and highly precise new method for measuring imperfections in dish concentrator shape.
 
The direct steam generation (DSG) is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi megawatt range. According to Price, H., Lupfert, E., Kearney, D., Zarza, E., Cohen, G., Gee, R. Mahoney, R., 2002, "Advances in Parabolic Trough Solar Power Technology," J. Sol. Energy Eng., 124 and Zarza, E., 2002, DISS Phase H-Final Project Report, EU Project No. JOR3-CT 980277 a 10% reduction of the LEC is expected compared to conventional SEGS like parabolic trough power plants. The European DISS project has proven the feasibility of the DSG process tinder real solar conditions at pressures up to 100 bar and temperatures up to 400 degrees C in more than 4000 operation hours (Eck, M., Zarza, E., Eickhoff, M., Rheinlander J., Valenzuela, L., 2003, "Applied Research Concerning the Direct Steam Generation in Parabolic Troughs," Solar Energy 74, pp. 341 351). In a next step the detailed engineering for a precommercial DSG solar thermal power plant will be performed. This detailed engineering of the collector field requires the consideration of the occurring thermohydraulic phenomena and their influence on the stability of the absorber tubes.
 
The direct steam generation in parabolic troughs is a promising option to further reduce levelised electricity costs of solar thermal power plants. The operation of the field in recirculation mode offers advantages concerning the control and stability of the system. Due to the large dimensions of the field compact phase separators installed in each collector row are discussed as an alternative to a central drum-type separation vessel. This paper deals with the design of a distributed phase separation system composed of the separation devices themselves and a common drainage line to the central buffer tank. As a reference configuration for this analysis the collector field developed within the INDITEP project is chosen. In a first step transient numerical simulation of the collector loop is used to define the loads on the separation system especially under transient conditions caused by cloud coverage. It is found that peak liquid flow rates can exceed the steady-state values by a factor of 10. Based on these results a model for the required separation efficiency is suggested and the drainage system is dimensioned. In order to guarantee sufficient draining even under transient conditions the separators have to be equipped with additional buffer volumes. Finally the distributed separation system is compared to the configuration with a central buffer tank considering the criteria materials consumption, thermal inertia, parasitic losses and controllability. The results of the presented analysis are of vital importance for the layout of future DSG-collector fields.
 
A discrete-gradient optimization algorithm is used to identify the parameters in a one-node and a two-node capacitance model of a flat-plate collector. Collector parameters are first obtained by a linear-least-squares fit to steady state data. These parameters, together with the collector heat capacitances, are then determined from unsteady data by use of the discrete-gradient optimization algorithm with less than 10 percent deviation from the steady state determination. All data were obtained in the indoor solar simulator at the NASA Lewis Research Center.
 
Parabolic trough concentrating collectors play a major role in the energy efficiency and economics of concentrating solar power plants. Therefore, existing collector systems are constantly enhanced and new types developed. Thermal performance testing is one step generally required in the course of their testing and qualification. For outdoor tests of prototypes a heat transfer fluid loop (single collector or entire loop) needs to be equipped with measurement sensors for inlet, outlet and ambient temperature as well as irradiance, wind speed and mass or volumetric flow rate to evaluate the heat balance. Assessing the individual measurement uncertainties and their impact on the combined uncertainty of the desired measurement quantity one obtains the significance of the testing results. The method has been applied to a set of EuroTrough collector tests performed at Plataforma Solar de Almería (PSA). Test results include the uncertainty range of the resulting modeling function and exemplify the effects of sensors and their specifications on the parameters leading to an uncertainty of ±1.7%-points for the optical collector efficiency. Measurement uncertainties of direct normal irradiance and mass flow rate are identified as determining uncertainty contributions and indicate room for improvement. Extended multiple sensor deployment and improved calibration procedures are the key to further reducing measurement uncertainty and hence increasing testing significance.
 
Top-cited authors
W.A. Beckman
  • University of Wisconsin–Madison
Reiner Buck
  • German Aerospace Center (DLR)
Delilah Kearney
  • Fordham University
James E. Pacheco
  • Sandia National Laboratories
David Claridge
  • Texas A&M University