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Case* Results [1] [2]
A –
B –
Case* Structure
A –
B –
Knowledge for Tomorrow
Wissen für Morgen
Comprehensive numerical tool for performance evaluation and design of porous ceramic microstructures in solar thermal application
Raffaele Capuano 1; Thomas Fend 2; Bernhard Hoffschmidt 2
1 - German Aerospace Center - Karl-Heinz-Beckurts-Str. 13, 52428 Jülich, Germany 2 - German Aerospace Center - Linder Höhe, 51147 Köln, Germany
Contact: [email protected]
Institute of Solar Research
Heliostats
Solar receiver Steam
Condenser
Turbine
Introduction & main aim Porous ceramic structures like extruded monoliths, foams and combined micro-shapes have been tested during the last years as thermal absorbers in open volumetric solar receivers to be used in solar tower power plants.
Incoming radiation
Incoming air flow
Porous structure
Hot air exit flow
In order to have high efficiency, material structure parameters and coefficients (porosity, specific surface area, optical properties) must be optimized.
Numerical optimization has to be carried out for the designing procedure.
Due to the complexity of the structure, numerical simulation can be computationally heavy and slow optimized numerical approach needed
Scientific approach Comprehensive numerical tool: discrete models for effective properties evaluation + continuum-based numerical simulation for absorber performance evaluation
Discrete numerical tool s- Effective properties evaluation
Discrete representation of porous absorber unit element.
Carbon-based Honeycomb Monoliths – Single channel Carbon-based foams – Weaire & Phelan foam cell
0
10
20
30
40
50
60
0 0,5 1 1,5 2
Nu
sse
lt n
um
be
r
Inlet flow velocity [m/s]
Sim. Result - Foam cell 76% por.
Dittus-Boelter correlation
Exp. Result - Foam cell 76% (Wu et al.)
Convective heat transfer analysis
Effective parameter: Nusselt number
Effective parameter: Extinction coefficient
π2
π1
Radiative heat transfer analysis
Continuum-based numerical tool – Absorber performance evaluation
Homogeneous representation of porous absorber by means of effective properties.
2 cm
2 cm
14 cm
14 cm
14 cm
14 cm
Continuum representation Heat transfer
coefficient Specific area
Air inlet volume
Homogeneous volume
Channel depth [m]
0 0,01 0,02 0,03 0,04 0,05
1
0,75
0,5
0,25
0
Discrete Ordinates (DO)
Ray-tracer (STRAL)
Attenuation curve
Channel depth [m]
0 0,002 0,004 0,006 0,008 0,01
400
350
300
350
200
150
100
50
0
ξ
Incoming air flow
Single channel
Foam cell
Cont.Model Hoffsch. Pitz-Paal
Fluid outlet temperature [K] Efficiency
Cont.Model Hoffsch. Pitz-Paal
Cont.Model Experimental Cont.Model Experimental
Fluid outlet temperature [K] Efficiency
Cont.Model Experiment Cont.Model Experiment
1200
800
400
0
1200
800
400
0
1
0,5
0
1
0,5
0
1048 1046 1050 0,777 0,775 0,779
979 976 0,710 0,709
* Different environmental conditions
T volume distribution 1179
1050
Channel depth [m] 0 0,01 0,02 0,03 0,04 0,05
1400
1200
1000
800
600
400
200
0
Temperature linear distribution
Tem
per
ature
[K
]
T volume distribution 1163
979
Channel depth [m] 0 0,01 0,02 0,03 0,04 0,05 0,06
1400
1200
1000
800
600
400
200
0
Temperature linear distribution
Tem
per
ature
[K
] Solid temperature
Fluid temperature
Solid temperature
Fluid temperature
y(STRAL) = 0.9895 e -231.6x y(DO) = 0.9923 e -220.1x
(STRAL) = 231.6 [1/m] (DO) = 220.1 [1/m]
Irradiation directions Extinction coefficient (ξ)
Conclusions In the present work, a comprehensive numerical tool for the design and optimization of porous structures has been developed leading to a faster and computationally lighter complete numerical simulation. With the combined use of continuum and discrete approach, it is now possible to predict with a good approximation the thermodynamic performances of ceramic absorbers and, furthermore, to optimize their shape in order to reduce the losses and to enhance the heat transfer efficiency in high temperature solar applications.
Acknowledgements This work was carried out with financial support from the Ministry of Innovation, Science and Research of the State of North Rhine-Westphalia (MIWF NRW), Germany under contract 323-2010-006 (Start-SF).
References [1] Hoffschmidt, B.: Vergleichende Bewertung verschiedener Konzepte volumetrischer Strahlungsempfänger, Deutsches Zentrum für Luft- und Raumfahrt e.V., RWTH Dissertation,1996. [2] Pitz-Paal, R.: Entwicklung eines selektiven, volumetrischen Receivers für Solarturmkraftwerke- Parameterunterschungen und exergetische Bewertung, Dissertation. DLR-Forschungsbericht,1993.
Gordon Research Conferences - Ceramics, Solid State Studies in Mount Holyoke College, South Hadley, MA, U.S. - July 20-25, 2014
