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The air return ratio is a key factor for the overall efficiency of the open volumetric receiver concept. Although first measurements of the air return ratio exist for smaller setups of the open volumetric receiver concept, so far no measurement of the air return ratio has been presented for the Solar Thermal Test and Demonstration Power Plant Julich ( ≈ 1.5 MW, ≈10 (kg air)/s; ≈700 °C). This paper describes the application of a tracer gas method at the solar tower Julich to determine this substantial ARR. As tracer gas the environmentally friendly helium has been chosen. The helium is injected dynamically into the circular air flow of the system and the helium mole fraction is measured using a mass spectrometer. The dynamic concentration response of the system is used to determine the air return ratio. This dynamic method only requires one location of measurement. First measurements with this dynamic method were conducted at the solar tower Julich. The ARR of STJ was measured with and without irradiation of the main receiver with high accuracy. Under low-wind conditions and without irradiation of the main receiver the air return ratio was measured to be ( 67.7 ± 0.5 ) % for an air mass flow of ( 9.96 ± 0.04 ) kg / s . A slightly higher air return ratio of ( 68.6 ± 0.7 ) % was measured under irradiation with an air mass flow of ( 9.94 ± 0.04 ) kg / s . The air return ratio was sensitive to the air mass flow, showing significantly lower rates when moving further away from the 10 kg / s design air mass flow to 5 kg / s .

Current measurement techniques do not allow the visualization of the return air flow of open volumetric receivers in solar tower power plants. The reason is that the region of interest is irradiated by concentrated solar radiation and is located on top of a tower. Therefore, a novel method of measurement, the Induced Infrared Thermography (IIT) is introduced within this paper. With this method the return air can easily be observed with an infrared camera. As air has a very low emissivity in the infrared region the activity has to be induced by the addition of an infrared-active component, here carbon dioxide. The temperature of the infrared-active component has the greatest influence on the signal strength but the mole fraction of the component and the distance to the infrared camera are also important. Due to temperature restrictions, the measured signal to noise ratio is low and therefore several post-processing steps have to be conducted to visualize the return air. The most important step for the visualization is subtracting a background image. Furthermore, a video filter is employed in order to remove noise. A time series of ITT images can be used to obtain information on the velocity fields of the flow. The process, called Infrared Image Velocimetry (IRIV) here, is similar to Particle Image Velocimetry (PIV) and is applied in this paper to external fluid flows of the open volumetric receiver. As IRIV is in an early stage of development the depicted results are treated as qualitative vector fields.

Kostenreduzierung spielt bei der konzentrierenden Solarthermie eine maßgebliche Rolle. Daher ist es essentiell, alle Faktoren, die den Systemwirkungsgrad einer Solarturmanlage beeinflussen, zu quantifizieren. Die Luftrückführrate ist ein Schlüsselfaktor des offenen volumetrischen Solarreceivers, welche das Verhältnis von zurückgeführter Luft zu eingesaugter Luft beschreibt. Um einen hohen Receiverwirkungsgrad zu erreichen, ist es wichtig, die Luftrückführrate zu erhöhen. Viele Parameter, wie z. B. die Windgeschwindigkeit und -richtung, die Geometrie des Receivers sowie die Betriebsweise, haben einen Einfluss auf die Strömung vor dem Receiver und somit auch auf die Luftrückführrate. Daher ist es essentiell, die Luftrückführrate messtechnisch zu quantifizieren und die Strömung vor dem Receiver zu visualisieren. Bislang wurde dies weder auf einem industriellem Maßstab noch unter dem Einfluss von konzentrierter Solarstrahlung umgesetzt. Die Entwicklung eines Messverfahrens zur Quantifizierung der Luftrückführrate mit möglichst hoher Präzision stellt den Kern dieser Arbeit dar. Weiterhin wurde erstmalig die Rückführluft vor dem Receiver visualisiert. Diese erlaubt ein besseres Verständnis der auftretenden Strömungsphänomene, welche maßgeblich die Luftrückführung bestimmen. Messverfahren, die dies ermöglichen, wurden in dieser Arbeit im Labormaßstab entwickelt, unter Betriebsbedingungen erprobt und erfolgreich am Solarturm Jülich angewandt.Um die Luftrückführrate quantitativ zu bestimmen, wurden drei Varianten eines neuartigen, zirkulären Indikatorgasverfahren entwickelt. Das Indikatorgas wurde hierbei entweder kontinuierlich oder intermittierend dem offenen Luftkreislauf hinzugefügt und über die unvollständige Luftrückführung verdünnt. Der Stoffmengenanteil des zugeführten Edelgases Helium wurde mittels eines Massenspektrometers im Luftkreislauf bestimmt und daraus die Luftrückführrate berechnet. Eine zeitliche Auflösung von 0.5 s wurde erreicht. Eine maximale Luftrückführrate von (68.6 ± 0.7)% mit 95% Konfidenzintervall wurde unter Bestrahlung mit konzentrierter Solarstrahlung am Versuchskraftwerk Jülich gemessen. Dieser Wert ist höher als die bisher angenommene Luftrückführrate von 60%. Dieser Unterschied in der Luftrückführrate entspricht einem 4−5% höherem Gesamtsystemwirkungsgrad. Die Visualisierung der Rückführluft wurde erstmalig mit einer zu diesem Zweck entwickelten Induzierten Infrarot Thermographie erzielt. Hierbei wurde die Rückführluft mit Kohlenstoffdioxid angereichert und somit eine erhöhte Strahlung im infraroten Wellenlängenbereich induziert. Die von der Rückführluft abgegebenen Strahlung konnte mittels einer Infrarotkamera visualisiert werden. Cost reduction plays a significant role in the field of concentrated solar thermal energy. It is therefore essential to quantify all factors that influence the energy conversion efficiency. The air return ratio is a key factor for the overall efficiency of the open volumetric receiver. It is the fraction of the blown out air which is sucked in again through the solar receiver. To achieve a high receiver efficiency it is therefore important to increase the air return ratio. Many variables such as wind speed and direction, geometry of the receiver design and operational mode influence the air flow in front of the receiver. This in turn influences the air return ratio. It is therefore of vital importance to be able to measure the air return ratio and furthermore visualize the air flow in front of the receiver. The air return value was prior to this work unknown on a large scale and under concentrated solar irradiation.The development of a measurement technique for the quantification of the air return ratio with maximum accuracy is the main objective of this thesis. The second objective lies in the visualization of the returned air. This improves the understanding of the occurring flow phenomena which govern the air return ratio. The measurement methods were developed at a lab scale, tested under operating conditions and successfully demonstrated at the solar tower Jülich. In order to measure the air return ratio, three variants of a novel circular tracer gas measurement technique have been developed. The tracer gas is injected either continuously or intermittently into the open air system. The tracer gas is diluted by the imperfect air return ratio. The mole fraction of the injected noble gas helium is measured with a mass spectrometer within the air system, from which the air return ratio is determined. A temporal resolution of 0.5 s has been achieved. A maximal air return ratio of (68.6 ± 0.7)% with 95% confidence interval has been measured during irradiation with concentrated sunlight at the solar tower power plant Jülich. This is higher than thepreviously assumed air return ratio of 60%. This difference corresponds to a 4 − 5% higher overall system efficiency. The return air in front of the receiver was visualized for the first time with the newly developed Induced Infrared Thermography. Hereby, carbon dioxide is added to the return air. This induces a larger amount of radiationbeing given off in the infrared region. This radiation from the return air is visualized using an infrared camera.

RWTH Aachen University, Diss., 2017; 130 pp. (2017). = RWTH Aachen University, Diss., 2017, Cost reduction plays a significant role in the field of concentrated solar thermal energy. It is therefore essential to quantify all factors that influence the energy conversion efficiency. The air return ratio is a key factor for the overall efficiency of the open volumetric receiver. It is the fraction of the blown out air which is sucked in again through the solar receiver. To achieve a high receiver efficiency it is therefore important to increase the air return ratio. Many variables such as wind speed and direction, geometry of the receiver design and operational mode influence the air flow in front of the receiver. This in turn influences the air return ratio. It is therefore of vital importance to be able to measure the air return ratio and furthermore visualize the air flow in front of the receiver. The air return value was prior to this work unknown on a large scale and under concentrated solar irradiation.The development of a measurement technique for the quantification of the air return ratio with maximum accuracy is the main objective of this thesis. The second objective lies in the visualization of the returned air. This improves the understanding of the occurring flow phenomena which govern the air return ratio. The measurement methods were developed at a lab scale, tested under operating conditions and successfully demonstrated at the solar tower Jülich. In order to measure the air return ratio, three variants of a novel circular tracer gas measurement technique have been developed. The tracer gas is injected either continuously or intermittently into the open air system. The tracer gas is diluted by the imperfect air return ratio. The mole fraction of the injected noble gas helium is measured with a mass spectrometer within the air system, from which the air return ratio is determined. A temporal resolution of 0.5 s has been achieved. A maximal air return ratio of (68.6 ± 0.7)% with 95% confidence interval has been measured during irradiation with concentrated sunlight at the solar tower power plant Jülich. This is higher than thepreviously assumed air return ratio of 60%. This difference corresponds to a 4 − 5% higher overall system efficiency. The return air in front of the receiver was visualized for the first time with the newly developed Induced Infrared Thermography. Hereby, carbon dioxide is added to the return air. This induces a larger amount of radiationbeing given off in the infrared region. This radiation from the return air is visualized using an infrared camera., Published by Aachen

A pressurized air-based solar receiver is considered for power generation via gas turbines using concentrated solar energy. The modular solar receiver is designed for heating compressed air to the entrance conditions of a gas turbine in the pressure range 4 – 30 bar and temperature range 800 – 1200 °C. The development work involved the design, fabrication, testing, and modelling of a 3 kWth and a 35 kWth solar receiver prototypes. System integration of an array of modular solar receivers with fossil-fuel hybridization was analysed. ISSN:1876-6102

Qualitative measurements of leaks in air flows are frequently conducted in engineering. However, an environmentally friendly leak rate measurement technique which yields quantitative results and is applicable to hot, large-scale circular air flows (≈10 kg/s; ≈ 700 °C) with high leak rates has not been presented yet. This paper describes the development, test and validation of a stationary and a dynamic, helium based tracer gas method on a lab-scale model of a solar air receiver with partial air recirculation. Helium is chosen as tracer gas since it is environmentally friendly, stable under high temperatures and is also cheap for large-scale air flows. The tracer gas is injected either continuously or intermittently into the model system, its concentration is measured using a mass spectrometer and the static or dynamic concentration response of the system is used to determine the leak rate. The stationary method needs two measurement points upstream and downstream the leak, the dynamic method only one measurement point if applied to a circular air flow system. Since the dynamic method is time dependent the transfer function of the measurement setup was determined and the dynamic measurement error was considered. An extensive uncertainty analysis is presented for both the stationary and the dynamic method. Exemplary measurements were conducted at the model system with very good results. Both dynamic and static measurements yield the same result within their confidence intervals. The leak rate of the solar receiver model with a mass flow rate of m in = ( 0.247 ± 0.008 ) / kg s was measured to be l stat = ( 36.1 ± 2.3 ) % and l dyn = ( 34.5 ± 3.6 ) % with the static and the dynamic method respectively.

This presentation gives an overview on latest research activities in the field of hot gas flow through highly porous materials. The overall objective is to describe the thermal behaviour of these kinds of materials when applied as solar air receivers for solar tower technology. In this application, concentrated solar radiation heats up the material, which continuously transfers the energy to an air circuit. After that, the hot air feeds a steam turbine process. Since equivalent physical processes as well as similar materials are involved a number of spin off activities such as effusion cooling, particle filtration and micro heat exchanger arose from the solar application. The presentation includes a short description of the solar application as well as the further activities and the corresponding material requirements. In all of these applications the temperature and fluid velocity distribution inside the porous material is of special interest. Methods to analytically or numerically calculate these quantities are presented. An appropriate modelling of the physical processes requires a precise determination of homogenized material properties such as effective heat conductivity, volumetric heat transfer coefficient and permeability constants. Methods to determine these quantities experimentally as well as results of selected materials are shortly presented. Finally, an outlook on perspectives and future activities is given.

A pressurized air-based solar receiver is considered for power generation via gas turbines using concentrated solar energy. The modular solar receiver is designed for heating compressed air to the entrance conditions of a gas turbine in the pressure range 4 – 30 bar and temperature range 800 – 1200 °C. The development work involved the design, fabrication, testing, and modelling of a 3 kWth and a 35 kWth solar receiver prototypes. System integration of an array of modular solar receivers with fossil-fuel hybridization was analysed., Energy Procedia, 49, ISSN:1876-6102, Proceedings of the SolarPACES 2013 International Conference