Eckhard Lüpfert

Experimental Analysis of Overall Thermal Properties of Parabolic Trough Receivers

The heat loss of a receiver in a parabolic trough collector plays an important role in collector performance. A number of methods have been used to measure the thermal loss of a receiver tube depending on its operating temperature. This paper presents methods for measuring receiver heat losses including field measurements and laboratory set-ups both based on energy balances from the hot inside of the receiver tube to the ambient. Further approaches are presented to measure and analyze the temperature of the glass envelope of evacuated receivers and to model overall heat losses and emissivity coefficients of the receiver. Good agreement can be found between very different approaches and independent installations. For solar parabolic trough plants operating in the usual 390°C temperature range, the thermal loss is around 300W/m receiver length.

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.

Calibration corrections of solar tower flux density measurements

The PSA flux density measuring system PROHERMES measures the concentrated solar radiation in the entrance aperture of solar tower receivers with a white rotating bar as target and a CCD-camera taking images. The calibration is done with commercial flux gauges placed in the measurement plane. To im prove the calibration of the system and to reveal systematic errors, measurements are performed with two different types of commercial flux gauges (Thermogage sensors with and without quartz window) and a large custom-made calorimeter used as reference. The comparison shows that the sensors without quartz window measure about 5–8% higher and the sensors with quartz window about 100% higher. This error is explained with the differences in the spectral composition of the radiation and different angles of incidence between the manufacturer calibration and the solar measurements and corrections are proposed. Spectral changes of the sunlight during the day and year can affect the measurements by more than 10%. By selecting a correction filter adapted to the camera sensitivity, this influence can be reduced to less than 2.5%. Due to the reflective properties of the target coating, changes in angle of incidence can affect the measurements. In standard solar field conditions, this error is less than 0.5%, but for special conditions a correction of the systematic error of up to 8% is proposed.

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.

Application of concentrated solar radiation to high temperature detoxification and recycling processes of hazardous wastes

Abstract In many cases, hazardous wastes are subject to thermal treatment at elevated temperatures. Some types of wastes do not have a sufficient calorific value to cover the heat demand of the high temperature process. For thermal treatment of e.g. filter residues, dusts, sulfuric acid, aluminium dross, foundry sand, or waste water, supplementary energy supply is needed. The specific energy demand ranges from 0.5 to 2.5 kWh/kg (2–10 MJ/kg). An important aim of process optimisation is the reduction of (fossil) energy consumption and exhaust gas flow. Concentrated solar energy promises advantages when applied to high energy consuming waste treatment processes with regard to substitute fossil or electric energy consumption, to reduce CO2 emissions, and exhaust gas flow. In parallel to conceptional studies, a solar-heated rotary kiln mini-plant has been designed and constructed for tests in the DLR solar furnace. The tests will give indications of boundary conditions for solar thermal treatment or conversion of selected hazardous materials.

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.

Pyrometric Temperature Measurements on Solar Thermal High Temperature Receivers

The knowledge of the absorber surface temperature distribution is essential for efficient operation and further development of solar thermal high temperature receivers. However, the concentrated solar radiation makes it difficult to determine the temperature on irradiated surfaces. Contact thermometry is not appropriate and pyrometric measurements are distorted by the reflected solar radiation. The measurement in solar-blind spectral ranges offers a possible solution by eliminating the reflected solar radiation from the measurement signal. The paper shows that besides the incoming solar radiation and the absorber emittance, the bi-directional reflection properties and the temperature of the object are determining for the required selectivity of the spectral filter. Atmospheric absorption affects the solar blind pyrometric measurements in absorption bands of CO2 and water vapor. The deviation of temperature measurement due to atmospheric absorption is quantified and the possibilities and limitations of accounting for the atmospheric absorption with models based on radiation transfer calculations are discussed.

OXIDATION RATES OF CARBON BLACK PARTICLES EXPOSED TO CONCENTRATED SUNLIGHT

Abstract Small carbon particles have excellent absorption properties for concentrated solar radiation. Oxidation rates of micron and submicron sized carbon black particles have been investigated under high flux solar conditions. These oxidation rates are required for a thermal model of direct absorbing particle receivers as well as for the development of solar chemical technologies to treat toxic wastes. An experimental set-up has been developed for kinetic measurements in the solar furnace of the DLR. In the temperature range 600–900°C no significant acceleration of the oxidation could be observed due to insolation up to irradiances I ≤700 kW m −2 as compared to thermal oxidation of carbon particles (Derussol C©).

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.