1D and 3D Simulation. C. Hochenauer

Transcription

1 Solar thermal flat-plate l t collectors 1D and 3D Simulation C. Hochenauer

2 Introduction Description of a solar thermal flat-plate collector 1D Simulation - Description of the model - Simulation vs. measurement - Parametric analysis 3D CFD Simulation - What is CFD - CFD results - Common weak points of collectors Summary Seite 2

3 1D Simulation Seite 3

4 Common collector construction absorber with selective coating solar glass flow tubes glazing frame flange insulation 50 mm connector Al frame edge insulation Al frame Seite 4

5 Heat flows G* global irradiance S absorbed solar flux c cover co cover outer ci cover cover inner p absorber plate f fluid b...back a ambient Seite 5

6 Heat flows G* global irradiance S absorbed solar flux cond conduction conv convection rad radiation u useful gain (in fluid) c cover co cover outer ci cover cover inner p absorber plate f fluid b...back a ambient Seite 6

7 Tube-and-sheet absorber k thermal conductivity (W m -1 K -1 ) U L total heat loss coefficient (W m -2 K -1 ) Seite 7

8 Collector efficiency factor F' W U L ( d 0 1/ U 1 ( W d L 0 ) F) d i 1 h fi F Collector efficiency factor F Fin efficiency h fi heat transfer coefficient of fluid h fi (W m -2 K -1 ) Diagrams: Eisenmann (2004) Seite 8

9 Seite 9

10 Instantaneous Efficiency: q u G * = Useful energy gain in fluid Global irradiation incident on aperture Seite 10

11 Characteristic efficiency curve Sim. Meas. Collector: 0.4 Marvel CLS 2510 ( f - a )/G* f mean fluid temperature ( C) a ambient temperature ( C) G* global irradiance (W m -2 ) YONSIS LTD (Turkey) Steinhagen: Tests 2003 ITW Seite 11

12 Comparison: Simulation vs. Measurement Sim. (High-Flow) 0.7 Meas. (High-Flow) 0.6 Sim. (Low-Flow) 0.5 Meas. (Low-Flow) 0.4 Sim. (sel) 0.3 Meas. (sel) 0.2 Sim. (non-sel) 0.1 Meas. (non-sel) ( f - a )/G* f mean fluid temperature ( C) a ambient temperature ( C) G* global irradiance (W m -2 ) Seite 12

13 Heat flows and mean temperatures at 0.05 G* global irradiance S absorbed solar flux cond conduction conv convection rad radiation u useful gain (in fluid) c cover co cover outer ci cover cover inner p absorber plate f fluid b...back a ambient Seite 13

14 Heat flows and mean temperatures at 0.05 Seite 14

15 Parametric analysis ( f - a )/G* original normal glass half insulation non-sel Seite 15

16 Summary A steady 1 D model using average surface temperatures produces accurate results 13 test reports of modern collectors were used to verify the model Properties of solar materials from manufacturers are not reliable: c;s, p;s, p Useful tool for collector design and development Seite 16

17 3D CFD Simulation Seite 17

18 Computational Fluid Dynamics What is CFD? Computational Fluid Dynamics (CFD) is the science of predicting fluid flow, heat and mass transfer, chemical reactions, and related phenomena by solving numerically the set of governing mathematical equations. conservation of mass, momentum, energy, The results of CFD analyses are relevant in: conceptual studies of new designs detailed product development troubleshooting redesign 18 Seite 18

19 How does CFD work? CFD solvers are based on the finite it volume method. - Domain is discretized onto a finite set of control volumes (or cells) - General conservation (transport) equations for mass, momentum, energy etc. are solved on this set of control volumes Unsteady Convection Diffusion Generation - Partial differential equations are discretized into a system of algebraic equations. - All algebraic equations are then solved numerically to render the solution field. Control Volume Fluid region of pipe flow is discretized into a finite set of control volumes (mesh) Equation Variable Continuity 1 X momentum u Y momentum v Z momentum w Energy h Seite 19

20 CFD modeling overview Solver Solid modeler Pre-Processing Mesh Generator Solver Settings Transport Equations -Mass - Momentum - Energy Equations of state Supporting physical models Physical Models - Turbulence - Combustion - Radiation - Multiphase - Phase Change - Moving Zones - Moving Mesh Post Processing Material Properties Boundary Conditions Initial Conditions Seite 20

21 Absorber Temperature ( C) Optimal design not enough insulation on the side Seite 21

22 Fluid Temperature ( C) Optimal design not enough insulation on the side Seite 22

23 Fluid Velocity y( (m/s) Optimal design Manifolds are too small non-uniform flow distibution Seite 23

24 Absorber Temperature ( C) Optimal design Manifolds are too small Seite 24

25 Fluid Temperature ( C) Optimal Design Manifolds are too small Seite 25

26 Summary A steady 3-D CFD-model produces accurate local temperature and velocity results Different collector prototypes can be compared before manufacturing CFD can show local weak points of the collector e.g. small manifolds, thermal losses through side/edges,... Useful tool for collector design and development Seite 26

27 Contact Prof. Dr. Christoph Hochenauer Professor for Fluid Mechanics & Thermal Engineering Head of Department Energy Engineering Upper Austria University of Applied Sciences Stelzhamerstraße 23 A-4600 Wels/Austria Tel.:+43(0)7242/ Fax.:+43(0)7242/ Seite 27