Marc Röger

Face-Down Solid Particle Receiver Using Recirculation

Solar thermal energy generation needs receiver technologies which can drive highly efficient turbines and decouple the collection of energy from its use by an economic storage technology. High-temperature solid particle receivers for solar tower systems with particle storage are one option. Important issues regarding high-temperature particle receivers are minimization of convective losses, no particle loss due to susceptibility to wind and high efficiency also in part load operation. A design approach facing these challenges is the face-down receiver using recirculation of particles. A screening performance analysis studying different re-circulation patterns is presented. Using smart recirculation schemes, high receiver efficiencies can be main-tained also at part load operation (100%-load ~90%; 50%-load ~86%; 20%-load ~67%). In terms of total annual solar-to-electric efficiencies the face-down geometry yields excellent 24%, although implicating a surround heliostat field. From the analyses, it can be concluded that solid particle receivers using smart recirculation patterns are a viable receiver option for storage and high-temperature high-efficient turbine processes.

Numerical and Experimental Investigation of a Multiple Air Jet Cooling System for Application in a Solar Thermal Receiver

The transparent quartz glass window of a high temperature solar receiver (1000°C air outlet temperature, 15bars) has to be protected from overheating. The window is an axially symmetric part that can be approximated by a hemisphere with a cylindrical extension (diameter 0.31m height 0.42m 0.42m). The cooling is accomplished by impinging several air jets onto the concave window surface. Due to concentrated solar radiation, the air supply nozzles can only be installed at the circumference of the cylindrical extension. Symmetric configurations with six or nine nozzles, equally distributed around the window circumference, are examined. A second configuration generates a swirl in the window cavity by inclining the nozzles. In a third, asymmetric configuration, only nozzles on one side are simultaneously charged with mass flow, while a spatial homogenization of heat transfer is reached by periodically modulating the air flows with time. Computational fluid dynamics (CFD) calculations and laboratory measurements of the heat transfer have been carried out. In the performed 3-D simulations, the realizable k-e model, the k-ω model, and the SST-k-ω model are compared. For measuring the heat transfer coefficient, a periodic-transient measurement technique with high spatial resolution is used. For the application of cooling of the solar receiver window, the jet cooling system with periodically modulated air flows is identified as the best solution.

Multiple Air-Jet Window Cooling for High-Temperature Pressurized Volumetric Receivers: Testing, Evaluation, and Modeling

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 nongray 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 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.

Measurement of Solar Radiance Profiles With the Sun and Aureole Measurement System

Due to forward scattering of direct sunlight in the atmosphere, the circumsolar region closely surrounding the solar disk looks very bright. The radiation coming from this region, the circumsolar radiation, is in large part included in common direct normal irradiance (DNI) measurements, but only partially intercepted by the receivers of focusing collectors. This effect has to be considered in the performance analysis of concentrating collectors in order to avoid overestimation of the intercepted irradiance. At times, the overestimation reaches more than 10% for highly concentrating systems even for sky conditions with relevant DNI above 200 W/m2. The amount of circumsolar radiation varies strongly with time, location and sky conditions. However, no representative sunshape measurements exist for locations that are now of particular interest for concentrating solar power (CSP) or concentrating photovoltaics (CPV). A new sunshape measurement system is developed and analyzed in this study. The system consists of the sun and aureole measurement instrument (SAM), an AERONET sun photometer and postprocessing software. A measurement network is being created with the presented system. The uncertainty of this system is significantly lower than what was obtained with previous devices. In addition, the spectral optical properties of circumsolar radiation are determined. As a result, the necessary information for CSP and CPV systems, and a basis for the development of modeling methods for circumsolar radiation, can now be achieved.

CFD Simulation and Performance Analysis of Alternative Designs for High-Temperature Solid Particle Receivers

Direct-absorption solid particle receivers are theoretically capable of yielding temperatures in excess of 1000°C, which enables higher efficiency power cycles and lower thermal storage costs. This paper presents rigorous CFD simulations of alternative solid particle receiver designs with recirculation to help identify optimal configurations that maximize the receiver thermal efficiency. The alternative receiver designs considered are a north-facing cavity receiver and a face-down surround-field cavity receiver. The CFD simulations model incident solar radiation from a heliostat field as a boundary condition on the model domain. The CFD simulations also couple convective flow with the thermal and discrete-phase (particle) solutions, which in turn affects absorption of incident solar radiation and thermal re-radiation within the receiver. The receivers are optimized to yield comparable particle temperatures at the outlets of 750–850°C, heated from an injection temperature of 300°C, and are compared on the basis of thermal efficiency. The CFD simulations yielded thermal efficiencies of the north-facing receiver at 72.3% (losses were 6.5% radiative and 20.9% convective) and the face-down receiver at 78.9% (losses were 11.4% radiative and 9.6% convective) at solar noon on March 22. Ongoing efforts are focused on reducing convective and radiative losses from both receiver configurations.Copyright

Automatic Noncontact Quality Inspection System for Industrial Parabolic Trough Assembly

The construction of solar thermal power plants with several thousand m 2 of collector area requires quality control measures for components, subsystems, and the entire collector rows. While quality control has a significant potential to increase the solar field efficiency, the main objective is to assure high-quality standards for the whole solar field. Quality control, assembly documentation, and performance measurements are required by the investors. Based on previous R&D work in collector development and prototype qualification, measurement systems have been developed for use in solar field construction and operation supervision. In particular, close-range photogrammetry can be used to measure the geometry of collector steel structures. The measurement system consists of a digital camera, which moves around the structure automatically while shooting photos of the concentrator structure from various positions. The photos are evaluated with photogrammetry software to check the assembly quality. The whole measurement and evaluation procedure is computer controlled and is fast enough to be integrated in a solar collector production line. This paper deals with the required measurement accuracy and shows ways to reach, maintain, and control this accuracy in the rough environment of an on-site production line.

Infrared-Reflective Coating on Fused Silica for a Solar High-Temperature Receiver

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.

Techniques to Measure Solar Flux Density Distribution on Large-Scale Receivers

Flux density measurement applied to central receiver ystems delivers the spatial distribution of the concentrated solar radiation on the receiver aperture, measures receiver input power, and monitors and might control heliostat aimpoints. Commercial solar tower plants have much larger aperture surfaces than the receiver prototypes tested in earlier research and development (R&D) projects. Existing methods to measure the solar flux density in the receiver aperture face new challenges regarding the receiver size. Also, the requirements regarding costs, accuracy, spatial resolution, and measuring speed are different. This paper summarizes existent concepts, presents recent research results for techniques that can be applied to large-scale receivers and assesses them against a catalog of requirements. Direct and indirect moving bar techniques offer high measurement accuracy, but also have the disadvantage of large moving parts on a solar tower. In the case of external receivers, measuring directly on receiver surfaces avoids moving parts and allows continuous measurement but may be not as precise. This promising technique requires proper scientific evaluation due to specific reflectance properties of current receiver materials. Measurement-supported simulation techniques can also be applied to cavity receivers without installing moving parts. They have reasonable uncertainties under ideal conditions and require comparatively low effort.

A Transient Thermography Method to Separate Heat Loss Mechanisms in Parabolic Trough Receivers

This paper describes a transient thermography method to measure the heat loss of parabolic trough receivers and separate their heat loss mechanisms. This method is complementary to existing stationary techniques, which use either energy balances or glass envelope temperature measurements to derive overall heat losses. It is shown that the receiver heat loss can be calculated by applying a thermal excitation on the absorber tube and measuring both absorber tube and glass envelope temperature signals. Additionally, the emittance of the absorber selective coating and the vacuum quality of the annulus can be derived. The benefits and the limits of the transient method are presented and compared to the established stationary method based on glass envelope temperature measurements. Simulation studies and first validation experiments are described. A simulation based uncertainty analysis indicates that an uncertainty level of approximately 5% could be achieved on heat loss measurements for the transient method introduced in this paper, whereas for a conventional stationary field measurement technique, the uncertainty is estimated to 17–19%.

Techno-Economic Analysis of Receiver Replacement Scenarios in a Parabolic Trough Field

The heat loss of an evacuated parabolic trough receiver of solar thermal power plants ranges typically between values below 150 and 200 W/m at 350°C. Defects, such as glass breakage by wind events and coating degradation, anti-reflection coating degradation or hydrogen accumulation in the annulus, decrease the annual electricity production. This study examines the effect of different receiver performance loss scenarios on the energetic and economic output of a modern 150-MWel–parabolic trough plant with 7.5-hours molten-salt storage, located in Ma’an, Jordan over the whole lifetime by modeling it in an extended version of the software greenius. Compared to the reference scenario, a wind event in year 5 (10, 15) causing glass envelope breakage and consequential degradation of the selective coating of 5.6% of the receivers reduces the electricity output by 5.1% (3.8%, 2.5%), the net present value is reduced by 36.5% (23.1%, 13.1%). The payback time of receiver replacement is only 0.7 years and hence this measure is recommended. The highest negative impact on performance and net present value of a project has the hydrogen accumulation scenario (50% of field affected) in event year 5 (10,15) reducing net electric output by 10.7% (8.1%, 5.4%) and the net present value by 77.0% (48.7%, 27.6%). Replacement of the receivers or even better an inexpensive repair solution is an energetically and economically sensible solution. The option of investing in premium receivers with Xe-capsule during the construction phase is a viable option if the surplus cost for premium receivers is lower than 10 to 20 percent.