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@PHDTHESIS{Tiddens:689803,
      author       = {Tiddens, Arne},
      othercontributors = {Hoffschmidt, Bernhard and Kemna, Andreas},
      title        = {{M}easurement methods for investigating the air return
                      ratio of open volumetric receivers at solar power towers},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Aachen},
      reportid     = {RWTH-2017-04595},
      pages        = {1 Online-Ressource (VI, 130 Seiten) : Illustrationen,
                      Diagrammme},
      year         = {2017},
      note         = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
                      University; Dissertation, RWTH Aachen University, 2017},
      abstract     = {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.},
      cin          = {421010},
      ddc          = {620},
      cid          = {$I:(DE-82)421010_20140620$},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2017-04595},
      url          = {https://publications.rwth-aachen.de/record/689803},
}