Search

Author(s): Kolsch, B; Walker, IS; Schiricke, B; Delp, WW; Hoffschmidt, B | Abstract: Today, the fan pressurization method is the most frequently used method to evaluate a building's airtightness. However, the localization and quantification of leaks remain difficult. In this paper, an acoustic method is introduced to estimate the leakage size of single leaks. Acoustic and airflow measurements were conducted and compared in laboratory tests within the same boundary conditions. This work aims to investigate if various leak sizes can be predicted using acoustic measurement methods. The test apparatus consists of two chambers, separated by a test wall. This wall represents a single characteristic air leakage path. Various types of wall structures with different slit geometries, wall thicknesses, and insulation materials were investigated. The acoustic measurements were performed with a sound source placed in one chamber and ultrasonic microphones located in both chambers. These results were compared to measured airflows through the test wall to provide estimates of uncertainty in the acoustic approach, which indicate a linear trend. Finally, these laboratory measurements were compared to the same measurements at a real office building. Although the acoustic measurement uncertainty is still significant (greater than +/- 50%), the acoustic method has the potential to give an order of magnitude of single leak sizes.

Various approaches are used to determine direct, sky diffuse, and reflected contributions to the solar irradiance on building surfaces. Among these are ray tracing, pixel counting, and projection and clipping. Projection and clipping is closest to an exact solution but it is often considered as too slow. This paper presents an improved projection and clipping method to determine the direct and the sky diffuse contribution to the irradiance. In a second step a projection and clipping approach is applied to calculate the view factors between building surfaces, so that the reflected contribution can be obtained. The presented approach delivers results which are comparable to those obtained by EnergyPlus in acceptable computation time. This shows that modern building simulation tools can benefit from an approach that applies fewer approximations than common raytracing or pixel counting techniques.

Heating energy represents a significant proportion of the total energy consumption of theFederal Republic of Germany. Thus, a reduction of heating energy demand in buildings is one of the central points of German energy saving and climate goals. Considering that the current rate of new building construction is approximately 1 %, the most significant energy saving potential lies in existing buildings. The analysis of existing buildings will thus become more important in the years to come. As a result, the development of new measurement systems to assess the need for renovation is required. In this context, detailed Information about heat insulation and airtightness is essential. The current approaches to detect air leakage in building envelopes are mostly time consuming and expensive. Therefore, the development of an advanced and more inexpensive method is a vital step. In this paper, we2 present a study about the feasibility and suitability of ultrasound technology to evaluate the size of air leakages in building envelopes. The advantage of ultrasound technology is that the corresponding wavelengths are small enough to penetrate small openings and that disturbing background noise is usually low. Moreover, the frequencies are above the human threshold of hearing; hence, working and living inside the building would be affected less by the measurement than by a blower door test or leakage detection with audible sound. To examine the suitability of this technology, we use an experimental setup of ultrasound microphones and a speaker in a test chamber. Within this setup, we measured different hole sizes and assessed whether they could be detected using ultrasonic frequency swept signals.

In order to achieve high optical efficiency of solar parabolic trough collectors and high performance of the solar field, the concentrator mirrors in concentrating solar power plants are expected to maintain accurate parabolic shape over the daily operation cycles. In addition to shape imperfections introduced by the manufacturing process, deformation due to gravity load and mounting forces is an inevitable factor affecting shape accuracy in all types of parabolic trough collectors. In this paper the effect of gravity load on mirror shape and resulting slope and focus deviation values is characterized and quantified in finite element analyses referenced to specific lab tests. Inner and outer ideally shaped parabolic mirror of RP3 geometry are evaluated for various discrete collector angles relevant for operation on different collector support structures. Three finite-element-models are included in the study: two with idealized support structures (ideal and elastic case) and one including the cantilever arms as relevant parts of the EuroTrough type collector support structure (cantilever case). Constructional design and stiffness of the support structure significantly determine characteristic and magnitude of deformation. Resulting rms values of the sagged mirror panels are as high as SDx = 1.7 mrad and FDx = 6.3 mm (inner mirror, elastic case) and SDx = 1.1 mrad and FDx = 5.6 mm (outer mirror, cantilever case). Depending on the type of support structure, minimum and maximum values occur at different collector angles. Rms slope and focus deviation values are closer to the 0° (zenith) collector angle case than to the non-deformed (ideal) mirror shape. This leads to optimizing the mirror shape for 0° (zenith) collector angle. Different support structures in design and stiffness for shape accuracy assessment in laboratory and those used in the collector make it difficult to find one optimum shape for all types of mirror and collector.

Heat loss testing of parabolic trough receivers with heating cartridge is well-established. In that technique temperature is measured by pressing thermocouples against the inside surface of the absorber tube. As the temperature of the thermocouple is influenced by radiation temperature and air temperature in the annulus, the temperature of the thermocouple is higher than the absorber temperature. The paper presents a measurement of absorber temperature over- prediction as function of heating power between 89 °C and 577 °C for three different insulation configuration yielding over-prediction between 4.4 K and 15.3 K. This shows, that absorber temperature over-prediction is at a relevant order of magnitude for heat loss measurements of parabolic trough receivers. Interpolation using 2nd- and 3rd-order 2d polynomials with the additional condition of disappearing over-prediction at HL=0 for correction of heat loss measurements is demonstrated.

The QUARZ scope assessment label for independent product tests of CSP key components, like parabolic trough receiver or mirrors, is introduced to support the end customer in assessing the product quality and test reports. Two dimensions of quality criteria are defined for assessing product qualification in a standardized manner in QUARZ reports. Aspects of test significance, e.g. number of samples or prototypes vs. independent selection from series production, are covered by the dimension significance category. The completeness of the characterization is represented in the scope level. The scope level compares the tests performed to the tests that could be performed to describe all aspects relevant for a characterization. Goal of the label is to support the end customer of CSP components in assessing the product quality based on the test report

This paper presents results from a characterization of DLR’s second generation sun simulator test facility (OptiRec) for determining the optical performance of parabolic trough receivers. The configuration of the test bench is an elliptical mirror trough with flat end mirrors. Solar simulator lamps are situated in one focal line, the receiver in the other focal line. Water at ambient temperature flows through the receiver and allows for a calorimetric measurement of the absorbed power. Compared to the first generation test bench (ElliRec) the mirror material is changed from aluminum to glass, the ellipse size is changed and the handling is improved. Based on 34 measurements of the absorbed power the repeatability for OptiRec measurements is measured to 0.13% (1σ), compared to 0.2% in the ElliRec. Seven receivers from different manufacturers are measured in both test benches yielding consistent relative optical efficiency with

This paper describes a new2 nd generation linear focus solar simulator test bench. The test bench is used for the non-destructive calorimetric measurement of relative optical efficiency of parabolic trough receivers. Compared to the predecessor, the test bench has been optimized for faster measurement, better long term stability and easier operation. The test bench optics is an elliptical shaped glass mirror trough with flat end mirrors. The receiver and six solar simulator lamps, metal halide lamps with 2.5 kWeeach are located in their respective focal lines. Absorbed power is measured via temperature increase of the water flowing through the receiver and the mass flow rate. By comparison to the absorbed power of a reference receiver, the optical efficiency of a sample can be determined relative to the optical efficiency of the reference receiver.

Shape accuracy of mirror panels for parabolic trough solar collectors has a significant impact on the optical performance of the collectors in a solar power plant and is therefore carefully assessed by test laboratories and manufacturers. Relevant deformation is induced by gravity or mounting forces, so that shape accuracy data measured in different setups cannot be compared. This paper presents a method for conversion of shape measured in a vertical laboratory setup into data for a horizontal laboratory setup. Characteristic deformation matrices for parabolic trough mirror panels of RP3 geometry are determined by deflectometric shape measurements on various mirror panels and by validated finite element analyses (FEA). The resulting root mean square (rms) of measured slope deviation difference (i.e. the gravity induced deformation) between vertical and horizontal setup is on average 2.4 mrad for inner mirrors and 1.25 mrad for outer mirrors loosely positioned on a frame. Measured data from vertical setup, transformed by such characteristic deformation matrices into horizontal shape results, differ by less than 0.2 mrad in rms slope deviation value from data measured in horizontal setup. Whereas the presented approach to convert shape accuracy measurement results is suitable for the calculation of rms values, some of the analyzed mirror samples show differences in local slope deviation values larger than the deflectometric measurement uncertainty. The amount of deviation depends on details of the accuracy of the positioning of the mirrors on the measurement frame and is affected by the fixation and associated mounting forces at the pads.

At DLR’s QUARZ Center a test bench has been established to measure, using steady state calorimetric method, the total hemispherical emittance of cylindrical solar thermal absorber samples at temperatures up to 450 °C. Emittance measurement of solar absorber surfaces is commonly performed by direct-hemispherical reflectance measurements with spectrophotometers. However, the measurement of cylindrical samples with spectrophotometers can be considered still a challenge as integrating spheres, reference samples and calibration services by national metrology institutions are optimized for flat sample measurement. Additionally samples are typically measured at room temperature. The steady state calorimetric method does not rely on reference samples and the measurement is performed at operating temperature. In the steady state calorimetric method electrical power input used to heat the sample is equated to the radiative heat loss from a heated sample to the environment. The total emittance can be calculated using the Stefan-Boltzmann equation from radiative heat loss power, the defined sample surface area and measured surface temperature. The expanded uncertainty (k=2) of the total hemispherical emittance has been determined to ±13 % for a typical parabolic trough absorber sample at a temperature of 300 °C and a heating power of 100 W. The test bench was validated by the measurement of three samples with the spectrophotometer and the steady state calorimetric method.

A round robin for parabolic trough receiver heat loss and optical efficiency in the laboratory was performed between five institutions using five receivers in 2015/2016. Heat loss testing was performed at three cartridge heater test benches and one Joule heating test bench in the temperature range between 100 °C and 550 °C. Optical efficiency testing was performed with two spectrometric test bench and one calorimetric test bench. Heat loss testing results showed standard deviations at the order of 6% to 12 % for most temperatures and receivers and a standard deviation of 17 % for one receiver at 100 °C. Optical efficiency is presented normalized for laboratories showing standard deviations of 0.3 % to 1.3 % depending on the receiver. © 2017 Author(s).

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.

Deviations from the ideal shape of reflector panels for parabolic trough solar power plants have relevant impact on field efficiency and thus on the performance of the whole power plant. Analyzing the gravity-induced deformation of mirror shape for different mirror angles is relevant for performance calculation of solar parabolic trough collectors and identifying optimization potential of the mirror panels. Two mirror model versions (stiff and elastic supports) are evaluated in four angles: in horizontal laboratory angle (mirrors facing upward with mounting points horizontally aligned), and in 0°, 45° and 90° collector angle. The resulting slope maps are calculated in a separate post-processing. In order to evaluate the effect of gravity load on mirror shape, the deformed mirror in each evaluated angle is compared to the non-deformed mirror shape, and to the shapes in 0° (zenith) collector angle, respectively. The resulting slope deviation maps show the mirror deformation in different mirror angles. Stiffness of the mounting to the support structure has a relevant impact. Mirror deformation on elastic brackets (SDx up to 1.6 mrad) is much more pronounced than on an ideal stiff support structure (SDx up to 1.0 mrad).

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 Almería. 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.

In order to achieve high optical collector and therefore high solar field efficiencies parabolic trough concentrators in concentrating solar power plants need to maintain their parabolic shape during operation. Additional to shape deviation already induced by the manufacturing process, deformation due to gravity load is an inevitable factor influencing shape accuracy in all types of parabolic trough collectors. This paper characterizes and quantifies the effect of gravity load on mirror shape and resulting slope and focus deviation values. One inner and one outer ideally parabolic shaped mirror of RP3 geometry mounted onto different support structures are evaluated in finite element analyses for all collector angles relevant for operation. The different support structures include two idealized support structures (ideal and elastic case) and one structure including relevant parts of EuroTrough type collectors (cantilever case). Constructional design and stiffness of the support structure significantly determine characteristic and magnitude of deformation. If compared to non-deformed shape, resulting rms values are as high as SDx = 1.7 mrad and FDx = 6.3 mm (inner mirror, elastic case) and SDx = 1.1 mrad and FDx = 5.6 mm (outer mirror, cantilever case). Depending on the type of support structure, minimum and maximum values occur at different collector angles. If compared to 0° (zenith) collector angle, resulting rms slope and focus deviation values are on average smaller than if compared to non-deformed mirror shape. This implies optimizing mirror shape for 0° (zenith) collector angle. However, it has to be considered that support structures for shape accuracy assessment in laboratory as well as support structures used in collector might differ significantly in design and stiffness, thus making it difficult to find one optimum shape for all types of mirror and collector.

Mirror shape accuracy as a key optical performance parameter for parabolic trough collectors can be assessed accurately by common measurement systems proving the high quality of state-of-the-art mirror panels. However, measurement results cannot always be compared directly because critical boundary conditions are not yet standardized. This paper quantifies the differences in shape accuracy results between the most common measurement setups for parabolic trough mirror panels and identifies measurement position, mounting mode and support frame employed for the measurement as relevant boundary conditions. Deflectometric measurements of mirror panels of RP3 geometry were performed at DLR's QUARZ Center Cologne in vertical (mounting points vertically and curved direction horizontally aligned) and horizontal measurement position (mirrors facing upward with mounting points horizontally aligned), both with and without tightening the mirrors to a support frame with screws. Finite element models were applied to calculate gravity-induced deformation and resulting slope and focus deviation on three different types of support frame: an ideally rigid support frame, a laboratory support frame, and an ideal support frame with elastic brackets. The measurement results demonstrate that the difference in position and mounting mode can lead to relevant deviations of the shape accuracy results higher than the uncertainty of the employed deflectometric measurement system. For RP3 inner mirror panels a difference of up to 0.7 mrad in root mean square slope deviation ( SDx ) and 3.3 mm in root mean square focus deviation ( FDx ) from vertical to horizontal position was measured. Mirror shape specifications may thus not be applicable in all positions. Concerning the mounting onto different types of modeled support frame (in horizontal position) a variation of 0.5 mrad ( SDx ) and 1.8 mm ( FDx ) was calculated for perfectly shaped RP3 inner mirrors mounted onto an ideally rigid support frame compared to the case when mounted to a support frame with elastic brackets.

This paper describes accelerated aging tests on parabolic trough receivers. Overheating of parabolic trough receivers about 80-100 K over the nominal operating temperature is applied for accelerated aging of the absorber coating.A lifetime prediction based on the Arrhenius parameters requires extensive testing; A reduced test with heating receivers to 478 °C for 1000 hours is applied on receivers for oil fluid in order to compare heat loss and optical efficiency before and after such accelerated aging. In a further test set, such receivers are thermally cycled between 200 °C and 478 °C for 100 times. A bellow fatigue test has been developed for testing of the bellows stability for the compression and expansion. Receivers are subjected to 20,000 cycles and rested for a 24 hour waiting period. The receiver heat loss during the bellow fatigue test shall not increase more than 30%. Durability of the antireflex coating on the glass is tested with the Taber linear abrasor using the CS-10F rubber (¾”) and MIL 12397 Eraser (¼”). Both abrasive materials proved to be suited to obtain reproducible results, although the discussion is still ongoing on which material should be used. The larger diameter of the CS-10F rubber offers the advantage that transmittance measurement with the Lambda 950/1050 spectrophotometer can be performed without the necessity for multiple abrading stripes.