Klaus Pottler

Photogrammetry: A Powerful Tool for Geometric Analysis of Solar Concentrators and Their Components

Digital close range photogrammetry has proven to be a precise and efficient measurement technique for the assessment of shape accuracies of solar concentrators and their components. The combination of high quality mega-pixel digital still cameras, appropriate software and calibrated reference scales in general is sufficient to provide coordinate measurements with precisions of 1:50,000 or better. The extreme flexibility of photogrammetry to provide high accuracy 3-D coordinate measurements over almost any scale makes it particularly appropriate for the measurement of solar concentrator systems. It can also provide information for the analysis of curved shapes and surfaces, which can be very difficult to achieve with conventional measurement instruments. The paper gives an overview of quality indicators for photogrammetric networks, which have to be considered during the data evaluation to augment the measurement precision. A selection of measurements done on whole solar concentrators and their components are presented. The potential of photogrammetry is demonstrated by presenting measured effects arising from thermal expansion and gravitational forces on selected components. The measured surface data can be used to calculate slope errors and undertake raytrace studies to compute intercept factors and assess concentrator qualities.

Parabolic Trough Optical Performance Analysis Techniques

Analysis of geometry and optical properties of solar parabolic trough collectors uses a number of specific techniques that have demonstrated to be useful tools in prototype evaluation. These are based on photogrammetry, flux mapping, ray tracing, and advanced thermal testing. They can be used to assure the collector quality during construction and for acceptance tests of the solar field. The methods have been applied on EuroTrough collectors, cross checked, and compared. This paper summarizes results in collector shape measurement, flux measurement, ray tracing, and thermal performance analysis for parabolic troughs. It is shown that the measurement methods and the parameter analysis give consistent results. The interpretation of the results and their annual evaluation give hints on identified relevant improvement potentials for the following generation of solar power plant collectors.

Slope Error Measurements of Parabolic Troughs Using the Reflected Image of the Absorber Tube

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.

Experimental Verification of Optical Modeling of Parabolic Trough Collectors by Flux Measurement

In order to optimize the solar field output of parabolic trough collectors (PTCs), it is essential to study the influence of collector and absorber geometry on the optical performance. The optical ray-tracing model of PTC conceived for this purpose uses photogrammetrically measured concentrator geometry in commercial Monte Carlo ray-tracing software. The model has been verified with measurements of a scanning flux measurement system, measuring the solar flux density distribution close to the focal line of the PTC. The tool uses fiber optics and a charged coupled device camera to scan the focal area of a PTC module. Since it is able to quantitatively detect spilled light with good spatial resolution, it provides an evaluation of the optical efficiency of the PTC. For comparison of ray-tracing predictions with measurements, both flux maps and collector geometry have been measured under identical conditions on the Eurotrough prototype collector at the Plataforma Solar de Almeria. The verification of the model is provided by three methods: the comparison of measured intercept factors with corresponding simulations, comparison of measured flux density distributions with corresponding ray-tracing predictions, and comparison of thermographically measured temperature distribution on the absorber surface with flux density distribution predicted for this surface. Examples of sensitivity studies performed with the validated model are shown.

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.

Influence of Measurement Equipment on the Uncertainty of Performance Data from Test Loops for Concentrating Solar Collectors

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 were 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 Almeria, Spain. 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. The 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.

Measurement Techniques for the Optical Quality Assessment of Parabolic Trough Collector Fields in Commercial Solar Power Plants

The optical quality of the collector field of concentrating solar power plants is a fundamental factor for their profitability. High optical quality can be achieved and guaranteed when the manufacturing process is continuously monitored and adjusted in its essential steps. Therefore, a reliable and automatic method is needed to check the geometric accuracy of the concentrator support structures during their manufacturing in the assembly workshop. To verify the overall optical quality of the solar field after completion, an additional method is needed that allows measuring the slope errors of large mirror surfaces in the field with reasonable effort. This paper presents two measurement techniques that fulfill the required measurement demands: The stationary, automatic photogrammetry system “Q-Foto” has been developed for the shape accuracy control of concentrator structures. The measurement system consists of a digital camera, moved 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, robust and fast enough to be integrated in a solar collector production line. This paper describes how to reach, maintain and control this accuracy in the rough environment of an on-site production line. The new optical method “TARMES” has been developed for on-site measurements of mirror slope errors in parabolic trough collector fields. It uses the reflection of the absorber tube in the concentrator and is therefore called “Trough Absorber Reflection Measurement System (TARMES)”. A set of pictures of the absorber tube reflection is taken with the concentrator in slightly different and known tilt angles. The developed image analysis algorithm detects the edges of the absorber tube in the reflected images and corrects for distortions from perspective. This information is then used to calculate the slope errors of the mirror surface with high spatial resolution and accuracy. The consequences of the measured reflector slope deviations on the collector performance are calculated with raytracing. The results give detailed information about the optical quality of the concentrator, systematic errors in the manufacturing process and their optical performance penalty.

Parabolic Trough Analysis and Enhancement Techniques

Analysis of geometry and optical properties of solar parabolic trough collectors uses a number of specific techniques that have demonstrated to be useful tools in prototype evaluation. These are based on photogrammetry, flux mapping, ray-tracing, and advanced thermal testing. They can be used to assure the collector quality during construction and for acceptance tests of the solar field. The methods have been applied on EuroTrough collectors, cross-checked and compared. This paper summarizes results in collector shape measurement, flux-measurement, ray-tracing, and thermal performance analysis for parabolic troughs. It is shown that the measurement methods and the parameter analysis give consistent results. The interpretation of the results and their annual evaluation give hints on identified relevant improvement potentials for the following generation of solar power plant collectors.Copyright

Validation of Optical Modeling of Parabolic Trough Collectors by Flux Measurement

In order to optimize the solar field output of parabolic trough collectors (PTC) it is essential to study the influence of collector and absorber geometry on the optical performance. The optical ray-tracing model of PTC conceived for this purpose uses photogrammetrically measured concentrator geometry in commercial Monte Carlo ray tracing software. The model has been validated with measurements of a scanning flux measurement system, measuring the solar flux density distribution close to the focal line of the PTC. The tool uses fiber optics and a CCD-camera to scan the focal area of a PTC module. Since it is able to quantitatively detect spilled light with good spatial resolution it provides an evaluation of the optical efficiency of the PTC. For comparison of ray tracing predictions with measurements, both flux maps and collector geometry have been measured under identical conditions on the Eurotrough prototype collector at PSA. The validation of the model is provided by three methods: the comparison of measured intercept factors with corresponding simulations; comparison of measured flux density distributions with corresponding ray tracing predictions; and comparison of thermographically measured temperature distribution on the absorber surface with flux density distribution predicted for this surface. Examples of sensitivity studies performed with the validated model are shown.

Full parabolic trough qualification from prototype to demonstration loop

On the example of the HelioTrough® collector development the full accompanying and supporting qualification program for large-scale parabolic trough collectors for solar thermal power plants is described from prototype to demonstration loop scale. In the evaluation process the actual state and the optimization potential are assessed. This includes the optical and geometrical performance determined by concentrator shape, deformation, assembly quality and local intercept factor values. Furthermore, its mechanical performance in terms of tracking accuracy and torsional stiffness and its thermal system performance on the basis of the overall thermal output and heat loss are evaluated. Demonstration loop tests deliver results of collector modules statistical slope deviation of 1.9 to 2.6 mrad, intercept factor above 98%, peak optical performance of 81.6% and heat loss coefficients from field tests. The benefit of such a closely monitored development lies in prompt feedback on strengths, weaknesses and potentia...