Material requirements for receiver designs with secondary reflectors …raiselife.eu/dissemination/docs/2nd_Workshop/22 RAISELIFE... · 2019-11-29 · Material requirements for receiver

Material requirements for receiver designs with secondary reflectors

RAISELIFE DISSEMINATION WORKSHOPThursday, November 28th 2019, Düsseldorf, GermanyRalf Uhlig

Deutsches Zentrum für Luft und Raumfahrt (DLR)Institut für Solarforschung

Overview

• Introduction – why there is a need for secondary reflectors?• Boundaries - principle• Active cooled systems – history, design, example• Secondary reflectors for molten salt receivers

• Thermal analysis• Conceptual design

• Summary

DLR.de • Chart 2 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

Why using secondary reflectors?• Reducing spillage, reducing costly ceramic heat shields

• Homogenize flux distribution

• Clustering of cylindrical receivers

• High flux, high temperature receivers

DLR.de • Chart 3 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

Useful radiation

Heat shield

Source: Fraunhofer Institute for Solar Energy Systems ISE

Receiver

Sekundär-konzentratoren

Solarstrahlung

Secondary Concentrator

Solar flux

Receiver

Source: SYNHELION

Boundaries• Thermal gradients:

• Absorbed solar radiation• Heat sink by:

• Natural convection• Forced convection (active cooling / wind)

• Transients (start up / shut down / clouds)• “Unforeseen situations” (operation failure in aiming of heliostats, bird droppings…)

• Environment:• Gravity• Forces by wind load (static, fatique)• Humidity and contamination (dust, salts, acids…) corrosion• Freezing• Abrasion (sand storm)

DLR.de • Chart 4 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

Active cooled glass mirrors• Glass silver mirror glued on active cooled substrate• DLR development since 20 years• Max. incident flux: 1.5 MW/m2

• Optical efficiency: up to 91%• Complex system:

• Different materials• Active cooling (water)• Difficult manufacturing process• Thermal strain compensation

• Always problems:• Mirror cracking• Corrosion• Manufacturing

• “Cold” device (100°C) Heat sink!

DLR.de • Chart 5 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

* referred to the used heliostat field

• 1998• hexagon to hexagon (6

corners)• for receiver clusters• 5 segments in length

with extension• planar mirrors segments• α=20°• η=75% *• connections welded

• 1999 – 2002• hexagon to octadecagon (18 corners)• for receiver clusters• 5 segments in length with extension

(SecNT2+3, SecNT1 without extension)• bended mirrors segments• α=25° for trapezoid parts• α=17° for triangle parts• η=86% *• connections welded

• 2005• Dodecagon to

dodecagon (12 corners)• Not for clusters• 4 segments in length

with extension• bended mirrors

segments• α=30°• η=91% *• connections glued

SOLTREC

• 2015• For receiver cluster• Scaled dimension (600mm 800mm

outlet aperture• Connections welded• Massive problems during

manufacturing

Glass mirrorGlue

Water cooled structure (alumina)

Example of active cooled silver mirror secondary reflector• Developed for use with volumetric pressurized receivers (air 1000°C / 16 Barabs)• Inlet aperture diameter: 1.4 m• Outlet aperture diameter: 0.8 m• Thermal power receiver: 1.5 MWth

DLR.de • Chart 6 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

Receiver

Sekundär-konzentratoren

Solarstrahlung

Secondary Concentrator

Solar flux

Receiver

Design of active cooled silver mirror secondary reflector

Optical

DLR.de • Chart 7 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

Thermal / hydraulic Mechanical

High temperature secondary reflectorDLR.de • Chart 8 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

• High temperature reflector (300°- 400°C)• Silver coated stainless steel instead of glued glass mirrors• Reduced heat sink (radiative exchange with absorber)• Less complex:

• No active water cooling• Nor sandwich (glass , glue, metallic structure)• Simple manufacturing

• Development started in RAISELIFE

Useful radiation

Heat shield

Thermal analysis of a high temperature secondary reflectorDLR.de • Chart 9 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

• Parametric thermal FEM Model simulating thermal field of secondary reflector• Varying optical properties of reflector and heat transfer coefficient for convective losses• Different load cases (operation modes)• More than 70 variations

• Model:• Reflector and absorber modelled as shell, no support structure• Absorbed heat fluxes from solar radiation on reflector• Temperature field absorber• Radiative heat exchange• Forced convection

• Results:• Reflectivity ≥ 95%• Convective cooling ≥ 10W/m2K

Conceptual design of a high temperature secondary reflectorDLR.de • Chart 10 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

• Profiles enhancing chimney effect• Profiles made of sheet metal by laser cutting, bending• Reflective coating after bending

• Several profiles are arranged to form the reflector segment• Connected to T-beam by thread bolts / screws• T-beams are connected to an truss support structure of hollow profiles

CFD analysisDLR.de • Chart 11 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

Lower sec Upper Sec

Max T (°C) Max T (°C)

250 380

Profile taken from worst case reflectivity of 92%

• Aim: Check if natural convection is sufficient for cooling the reflector• One closed channel, inlets are able to suck ambient air (20°C)• Absorber: 500°C• Net heat flux from thermal simulation (profile)• Results: Max temperature of 380°C in worst case reflectivity:

• Outlook: Ongoing development in German national project: SOLTREC• Further development of coating• Optimizing cooling• Design build and test of a prototype in DLR SYNLIGHT facility

Summary – material requirements for secondary reflectors?DLR.de • Chart 12 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig

As Christmas is near, here is the wish list for materials useable for secondary reflectors:

• High reflectivity: • As high as possible reduces cooling effort and thermal gradients• ≥ 95% for natural cooled reflector in molten salt systems• Low degeneration over time

• Temperature stable: Aim is 400°C for molten salt systems• Resistant against oxidation and corrosion• Tolerant for thermal strain (no brittle behavior)• Easy to clean• Resistant against abrasion (sand storm, cleaning)• Low costs

Thank you for your attention!DLR.de • Chart 13 / Material requirements for receiver designs with secondary reflectors / RAISELIFE DISSEMINATION WORKSHOP / November 28th 2019, Düsseldorf, Germany / DLR / R.Uhlig