© Fraunhofer IFAM
Integration of thermally active materials during the formation of open porous metal structures
Jörg Weise1, Joachim Baumeister1, Marc Möllers2, Hannes Seifarth2, Sebastian-Johannes Ernst2
1Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Bremen
2Fraunhofer Institute for Solar Energy Systems ISE, Freiburg
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System: Adsorption and desorption effects
allow to ”pump“ heat between different temperature levels
E.g. water adsorption by zeolite Essential components are the adsorber
and the evaporator:
Are basically heat exchangers, combined with thermally active material (zeolite)
Made from metal (Al, Cu, steel)
Crucial components, determine system efficiency and power density
Introduction
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Introduction
Combining heat exchangers with thermally active material, SotA:
Loose bulk of zeolite granules between the lamellae of the metal heat exchanger
Bad thermal contact zeolite/metal
Coating of the lamellae with mixture of zeolite powder/binder
Limited specific surface
Difficult coating of fine structures
How can adorber performance be improved?
[ISE]
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Introduction
Requirements influencing the performance of an adsorber:
High thermal conductivity
Heat transfer from active material to fluid system
Low (thermal) mass
”Dead weight“ will lower coefficient of performance
Large surface
Thinner coating improves system kinetics
Accessibility
For coating and for reactants during system operation
Good adhesion and thermal contact of active material
Long time stability
Costs
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Conventional approach (SotA)
Introduction
Metal Active material
Production of metal semi-finished parts (lamellae, fibres, pipes,…)
Combination with active material (filling, coating)
Assembly to metal adsorber structure (soldering,…)
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Alternative approach:
New technological approach
E.g.
Fabrication of the porous metal structure can be done by casting or sintering
Direct integration of the active material during the fabrication of the metal structure
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Production of composite structure - casting
Basis: casting of metal sponges
Organic place-holder structure
Pressure assisted metal melt infiltration
Pyrolysis of place holder
Adapted approach:
Integration of granules of the active material into the preform
“Granules transplantation” to solidifying metal during casting
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New approach (casting):
coating of the place holder net with zeolite granules
Pla
ce h
old
er
Production of composite structure - casting
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Act
ive m
ate
rial
Production of composite structure - casting
New approach (casting):
coating of the place holder net with zeolite granules
Pla
ce h
old
er
© Fraunhofer
Meta
l Production of composite structure - casting
New approach (casting):
coating of the place holder net with zeolite granules
infiltration with metal melt, “transplantation” of the zeolite granules to solidifying metal during casting
Act
ive m
ate
rial
Pla
ce h
old
er
© Fraunhofer
Production of composite structure - casting
New approach (casting):
coating of the place holder net with zeolite granules
infiltration with metal melt, “transplantation” of the zeolite granules to solidifying metal during casting
removal of the placeholder structure (e.g. by pyrolysis)
Meta
l
Act
ive m
ate
rial
© Fraunhofer
Production of composite structure - casting
Example: open cell aluminium sponges, zeolite granules integrated into the surface of every strut
Very good thermal contact between zeolite and heat exchanger
Ratio of zeolite/metal and porosity can be adjusted
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Production of composite structure - casting
Placeholder EVA polymer net
Coating with zeolite granules
Wrapping around fluid pipe
Final shaping of preform
Infiltration, machining, polymer pyrolysis,
assembly
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Production of composite structure - sintering
New approach (sintering):
Coating of the active material granules with binder
Mixing with metal powder/flakes/fibres, adherence to granules surface
Transfer to mould, shaping
Sintering
Finished porous composite
Cu-Fibres Cu-Powder Cu-Flakes
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Combination of metal foam with
+ expanded glas granules (boiling stones)
+ zeolite (adsorbens)
evaporator adsorber
Production of composite structures
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Expanded glass integration - evaporator
New composite evaporator structure:
expanded glass granules act as nucleation sites (bubble formation)
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Expanded glass integration - evaporator
Impact of expanded glass granules
High wall superheat: Sponge sample without granules performs better
Low wall superheat: Sponge sample with granules performs better relevant for adsorption heat pump application!
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combination metal foam +
+ expanded glas granules(boiling stones)
+ zeolite (adsorbens)
evaporator adsorber
Production of composite structures
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Zeolite integration - adsorber
Variable compositions possible
Typical: 50vol% zeolite, 10vol% binder, 15vol% metal, open porosity for vapour transport 20vol%
Size of zeolite granules: >60µm
Thermal conductivity (Cu, Al): 2-5 W/m/K
Active materials like zeolites are often not stable at high temperatures.
Casting/sintering: >600°C
Investigation of zeolite degradation
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Water adsorption behaviour of zeolite is retained
Structure is affected by chosen process slight degradation of sample IFAMB-18
XRD investigation shows a slight shift of peaks and some amorphous background only on sample IFAMB-18
No signs of degradation for IFAMB-19 and IFAMB-20
Zeolite integration – adsorber zeolite – stability
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
300
350
wat
er u
pta
ke [
mg
/ g]
rel. pressure [p / p0]
KOE-14 / Pure zeolite
IFAMB-18 / glass granules + zeolite
IFAMB-19 / EVA-precurser + zeolite
IFAMB-20 / zink + zeolite
water adsorption @ 25°C
5 10 15 20 25 30 35 40 45 50
2q / °
IFAMB-20
IFAMB-19
IFAMB-18
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Successful manufacturing of an adsorber module made of zeolite-Al-composite
Directly integrated fluid pipes
Successful measurements on manufactured adsorber give promising results
Zeolite integration – adsorber zeolite – potential
0 10 20 30 40 50
0
200
400
600
800
1000
po
we
r [W
]
time [min]
Ads:35°C - Evap:10°C
Ads:49°C - Evap:4°C
0 10 20 30 40 50
0
20
40
60
wat
er u
pta
ke [
g]
time [min]
Ads:35°C - Evap:10°C
Ads:49°C - Evap:4°C
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Summary and conclusion
New approach for production of porous metal structures combined with active materials
Integration of the active material in-situ during the formation of the metal porous structure
Simple process
Ratio of active material/metal and porosity can be adjusted
Application: evaporator and adsorber for adsorption heat pumps / chillers
Evaporator: significantly improved boiling behaviour
Adsorber: using suitable processing parameters the degradation of zeolite can be avoided
The work was done in the framework of the project „Harvest“ funded by the Fraunhofer-Zukunftsstiftung.