Ralf Uhlig

THERMAL MODELLING AND SIMULATION OF PARABOLIC TROUGH RECEIVER TUBES

Receiver tubes (or heat collecting elements — HCE) are a key component of parabolic trough solar thermal power plants. They are mounted in the focal line of the collectors, absorb the concentrated solar irradiance and transfer the absorbed energy to the heat transfer fluid flowing through them. During the design phase of the receiver tubes and for the performance prediction of solar thermal power plants it is helpful to derive their technical properties, like the thermal losses or the temperature field in the receiver tubes, from their physical and geometrical properties. For this purpose, several models have been developed in the past [1–3]. In this paper, the different existing models are presented, compared and assessed. It is found that a simple analytical model is a helpful tool for the fast prediction of the temperature distribution in the receiver tube. Furthermore, a 2-dimensional and a 3-dimensioanl model are compared regarding the heat losses of a HCE at different operation conditions. Both tools show a good agreement with available measurements. Finally with these tools the efficiency factor F′ is calculated that considers the heat losses of an irradiated receiver compared to that of an un-irradiated receiver. According to the performed calculations, the efficiency factor of parabolic trough receivers is higher than expected.Copyright

Thermal Load of Direct Steam-Generating Absorber Tubes with Large Diameter in Horizontal Linear Fresnel Collectors

Solar thermal power plants are presently the cheapest technology for solar electricity production. Today, level electricity costs of 15 ct/kWh are achievable at good sites with high levels of direct normal irradiation. Nevertheless, further cost reductions are necessary to make solar thermal power plants economically feasible. One possibility for a further cost reduction might be the use of so-called Linear Fresnel Collectors in solar thermal power plants. Preliminary cost estimates of this option show that this is a promising potential for cost reduction. The technical feasibility of the linear Fresnel collector has to be checked in theoretical studies as well as during the operation of a life-sized prototype under real solar conditions. This paper presents results of preliminary theoretical studies regarding the thermal load of the absorber tube used in a linear Fresnel collector with a secondary concentrator.

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.

Numerical Simulation of a Centrifugal Particle Receiver for High-Temperature Concentrating Solar Applications

A three-dimensional, steady-state finite-element model (FEM) model of a centrifugal particle receiver for high-temperature concentrating solar applications has been developed. The model is built according to an experimentally examined 15 kW prototype in order to investigate the thermal receiver performance and the governing heat transfer mechanisms of the proposed receiver concept. Comparison to the experimental data reveals good agreement in terms of particle outlet temperature and absorbed heat. For a target particle outlet temperature of 900°C and a design input flux of 1 MW/m², a receiver efficiency of >85% is predicted, indicating promising perspectives for the proposed receiver concept.

Optical Performance and Weight Estimation of a Heliostat With Ganged Facets

In standard heliostat design the usual strategy of cost reduction is to increase the mirror area of the heliostats. This leads to a reduction of specific drive cost, but also to an increase of the torques caused by the wind loads, resulting in higher specific weight and higher specific drive power. The opposite strategy focuses on the reduction of the specific weight and driving power. Therefore the mirror area is decreased. In total the drive power is much lower then, but it has to be divided into more units which means a drawback for cost reduction. A combination of both strategies is to couple small heliostats so that they can be tracked by the same drive. At the Torque Tube Heliostat (TTH) the mirrors are mounted on torque tubes to simplify the coupling mechanism for the elevation angle. The optimal tracking speed of heliostats depends on the position in the field. In a heliostat with ganged facets all facets are tracked with the same speed. This leads to higher astigmatism losses compared to independently tracked mirrors. To reduce shading a certain space is foreseen between the facets of the TTH. To achieve a high mirror density and to avoid too long and therefore too flexible torque tubes this distance is limited. Thus the shading is higher than usually. Ray tracing calculations were done to obtain optimized heliostat configurations and to estimate the energy yield of a heliostat field built of 288 m² TTHs. The results are compared to a conventional heliostat field. To get an idea of the weight reduction potential of the TTH the dimensions of the torque tube, the secondary axis tube and the mirror support structure were weight optimized via FEM. The layout criterion was a mean mirror error of 5 mrad under a wind load and gravity, which was calculated from the deviations of the FE model.

16 – Central tower systems using the Brayton cycle

Solar-hybrid gas-turbine (SHGT) systems are a promising alternative to conventional solar thermal power plants, as gas turbine systems are cost effective and can reach higher efficiencies than steam cycles. Therefore, SHGT systems offer the potential for future reduction of solar electricity cost, while providing full dispatchability with the integrated hybrid option. Additional advantages are the reduced water consumption, the integrated hybrid option, fast system response, and simple plant control. Past developments in SHGT systems are summarized, and the actual status of the technology is discussed. The requirements for adaptation of commercial gas turbines and for the associated high-temperature receivers are outlined. Several configurations for the gas turbine cycle are described. Results of thermodynamic and techno-economic studies are discussed, supporting the economic potential of SHGT systems.

Reducing the convective losses of cavity receivers

Convective losses reduce the efficiency of cavity receivers used in solar power towers especially under windy conditions. Therefore, measures should be taken to reduce these losses. In this paper two different measures are analyzed: an air curtain and a partial window which covers one third of the aperture opening. The cavity without modifications and the usage of a partial window were analyzed in a cryogenic wind tunnel at −173°C. The cryogenic environment allows transforming the results from the small model cavity to a large scale receiver with Gr≈3.9·1010. The cavity with the two modifications in the wind tunnel environment was analyzed with a CFD model as well. By comparing the numerical and experimental results the model was validated. Both modifications are capable of reducing the convection losses. In the best case a reduction of about 50 % was achieved.

Effects of vertically ribbed surface roughness on the forced convective heat losses in central receiver systems

External receiver configurations are directly exposed to ambient wind. Therefore, a precise determination of the convective losses is a key factor in the prediction and evaluation of the efficiency of the solar absorbers. Based on several studies, the forced convective losses of external receivers are modeled using correlations for a roughened cylinder in a cross-flow of air. However at high wind velocities, the thermal efficiency measured during the Solar Two experiment was considerably lower than the efficiency predicted by these correlations. A detailed review of the available literature on the convective losses of external receivers has been made. Three CFD models of different level of detail have been developed to analyze the influence of the actual shape of the receiver and tower configuration, of the receiver shape and of the absorber panels on the forced convective heat transfer coefficients. The heat transfer coefficients deduced from the correlations have been compared to the results of the CFD simulations. In a final step the influence of both modeling approaches on the thermal efficiency of an external tubular receiver has been studied in a thermal FE model of the Solar Two receiver.

Dynamic modeling of molten salt power towers

A detailed understanding of the transient behavior of a receiver using molten salt as heat transfer fluid is of great importance for an efficient and safe operation. To analyze the transient operation a dynamic model for the flow in the receiver is currently under development, which will be capable to analyze the one-phase flow during normal operation and the two-phase flow during filling and draining. The model can be coupled to raytracing simulation in order to use a realistic flux density distribution as input for the model. In the paper the modelling approach for the receiver model is described shortly and validation results are discussed. This includes a detailed discussion of the heat transfer during the filling procedure, where an interesting phenomenon was discovered. Finally, the results for a parameter variation of the filling procedure and the simulation results for the impact of certain cloud events on the operation of the receiver are presented.

Fused silica windows for solar receiver applications

A comprehensive study of optical and mechanical properties of quartz glass (fused silica) with regard to application in high temperature solar receivers is presented. The dependence of rupture strength on different surface conditions as well as high temperature is analyzed, focussing particularly on damage by devitrification and sandblasting. The influence of typical types of contamination in combination with thermal cycling on the optical properties of fused silica is determined. Cleaning methods are compared regarding effectiveness on contamination-induced degradation for samples with and without antireflective coating. The FEM-aided design of different types of receiver windows and their support structure is presented. A large-scale production process has been developed for producing fused silica dome shaped windows (pressurized window) up to a diameter of 816 mm. Prototypes were successfully pressure-tested in a test bench and certified according to the European Pressure Vessel Directive.