Recent Developments in Room Temperature Active Magnetic ...20Magnetic%20cooling.pdf · Magnetic Refrigeration Background • Alternative to vapor compression • Magnetic refrigeration

(9.6.08)Temadag om Kølemetoder

Recent Developments in Room Temperature Active Magnetic Regenerative

Refrigeration

Kurt Engelbrecht, Nini Pryds, Christian Bahl

Risø DTU


Magnetic Refrigeration Background

• Alternative to vapor compression

• Magnetic refrigeration used to first break 1K temperature barrier near 1935, but used a one shot technique with a very low temperature span– Giauque awarded the Nobel Prize in chemistry in 1949

• First regenerative magnetic refrigerator proof-of-concept device built by Brown in 1976

• Possible applications include residential/commercial space cooling, refrigeration, hydrogen liquefaction/storage


Advantages of Magnetic Refrigeration

• Environmentally friendly– no ozone depleting or direct global warming associated with

the refrigerant

• Possibly more efficient than current technologies– no superheat or throttling loss– no compressor losses– nominally equivalent heat exchanger loss– relatively easy to achieve efficient part load operation

• Quiet operation


Room Temperature Magnetic Materials

The magnetocaloric effect is greatest at the Curie point of the magnetic material, which can be adjusted by alloying

200 225 250 275 300 325 3500

2

4

6

8

10

12

Temperature (K)

Adi

abat

ic T

empe

ratu

re C

hang

e (K

)0-5 Tesla

0-2 Tesla

Gadolinium (Gd) and Erbium (Er) alloy


Magnetic Refrigeration Concept


MagnetizationHot-to-Cold FlowCold-to-Hot FlowDemagnetization

Cold

ReservoirHot

Reservoir

Heat Rejection

Refrigeration

Active Magnetic Regenerative Refrigerator Cycle (AMRR)


Magnetic Refrigerator Design Considerations

• System configuration – rotary, reciprocating, etc.

• Magnetocaloric material selection

• Regenerator design

• Magnetic field

• Heat transfer fluid selection


AMRR System Configurations

• Stationary bed with an electromagnet– magnetic field is controlled by varying the current

to the electromagnet

QH QC

magnet

regenerator

displacer



• Reciprocating systemmagnet

regenerator

displacer



• Rotary system magnet

regenerator

pump


Permanent

Magnet

bed rotation

Magnetization in a Rotary Bed

Magnetic field “sweeps”through bed.

magnetized region

zero magnetic field


Choosing Magnetocaloric Materials

• Adiabatic temperature change

• Isothermal entropy change

• Thermal conductivity

• Magnetic hysteresis

• Practical issues – cost, corrosion, toxicity, manufacturability


Magnetocaloric Material Properties

• Magnetic materials generally experience a phase change from a ferromagnet to a paramagnet at the Curie Point

• Second-order magnetic phase transitions (SOMT)– transition associated with the alignment of magnetic

moments in the material– effect is nearly instantaneous (order of nanoseconds)– generally reversible magnetization

• First order-magnetic phase transitions (FOMT)– latent heat and large magnetocaloric effect associated with

transition– higher irreversibilities than second order– longer time associated with transition


Summary of Magnetocaloric Material Properties

13 (0-1.45

Tesla)30FOMT287x=0.1

6532FOMT318x=0

MnAs1-xSbx

17.128FOMT287LaFe11.7Si1.3H1.1

2727FOMT269Gd5Si3.5Ge0.5

~05.55.8SOMT294Gd

(K)(K)(J/kg-K)(K)HysteresisΔTad-ΔsMTypeTCurieMaterial


AMRR Loss Mechanisms

• Heat transfer / regeneration losses

• Pumping / viscous dissipation losses

• Axial conduction

• Eddy current heating

• Heat exchanger and motor losses– nominally equivalent to vapor compression


Regenerator Design

• Packed regenerator– packed sphere, packed powder, etc.– relatively easy to achieve high surface area– fluid flow profile is generally well-distributed– high pressure drop

• Flat plate regenerator– low pressure drop– requires small dimensions to achieve high heat

transfer performance– theoretically best regenerator performance– performance highly dependent on geometry


Prototype AMRR Performance Summary

540GdSiGeGa

100Gd5Si2Ge2

spheres (0.2mm)230Gd~200 1.4 (P)Nanjing University

spheres (0.25-0.355mm)250Gd & GdEr


spheres (0.43-0.5mm)1415Gd331.5 (P)

0600

spheres (0.15-0.3mm)38100Gd~6005 (S)Astronautics

24402 (S)

spheres (0.3 mm)26100Gd4844 (S)Chubu Electri / Tokyo University

47Gd252 (S)(2004)

spheres (0.2mm)200Gd492 (S)

crushed part. (0.4mm)500Gd, GdTb & GdEr742 (S)University of Victoria (2006)

KWcm3Tesla

Regen typedTQCRegen materialVregenμoHMaxSystem


Promising AMRR Innovations

• High performance magnetocaloric materials with tunable Curie Temperatures

• Layered regenerators

• Rotary systems


Efficiency Dependence on Regenerator Volume –Packed Sphere Regenerator8.76 kW Space Conditioning Application

5 10 15 20 25 30 352.75

3

3.25

3.5

3.75

4

4.25

4.5

4.75

Regenerator volume, L

Coe

ffici

ent o

f per

form

ance

layered bed

non-layered bed

vapor compression

B

A


Practical AMRR Concerns

• High fluid mass flow– pressure lower than vapor compression systems

• System size

• Materials processing

• Material oxidation

• Cost


AMRR Design Consideration

,~ v refAMRR

vc f mc

hmm c T

ΔΔ

&

&

• The mass flow rate of fluid in an AMRR is much higher than an equivalent vapor compression system

• no phase change in the fluid

• For a practical system, the fluid flow rate is ~20 times the refrigerant flow rate for vapor comp

• Requires larger connecting piping to reduce pressure drop

for equal refrigeration capacity


Magnetic Refrigeration Research at Risø DTU

Partnership between:

Duration: 4 yearsEnding date: 31.12.2010

Funding: 20 Mkr5 Ph.D. students, 3 Postdocs

ChallengesDemonstrate cost-effective systems at commercially relevant temperature spans with high efficiency and environmentally friendly materials


Test Machine at Risø DTU

• Parallel plate regenerator of Gd– plate thickness 0,9mm

• Maximum magnetic field ~1.1 Tesla

• Uses a permanent magnet

• Temperature span of 9 K

• Operational since Nov 2007


Future Work at Risø

• Currently designing next generation prototype AMRR

• Project goal: 100W cooling at 40K temperature span

• System should be practical and cost effective

• Prototype will be built in 2009


Conclusions

• Improvements have been made to prototype AMRR systems recently, but the technology is still not mature

• Systems using a layered regenerator beds have been shown higher performance over a relatively large temperature span

• New magnetocaloric materials have potential to significantly improve AMRR performance

• Gd remains


Modeling Efforts

• Developed a 1D AMRR model– used model to determine limits of AMRR technology

and investigate control techniquesK. L. Engelbrecht, G. F. Nellis, and S. A. Klein, 2006, Predicting the Performance

of an Active Magnetic Regenerator Refrigerator used for Space Cooling and Refrigeration, HVAC&R Journal 12(4):1077-1095

K. L. Engelbrecht, G. F. Nellis, and S. A. Klein, 2006, Modeling the Transient Behavior of an Active Magnetic Regenerative Refrigerator, Proceedings of the 11th International Refrigeration and Air Conditioning Conference at Purdue,

West Lafayette, IN

• Used the model to predict the performance of a rotary AMRR prototype


Model Verification – Rotary Input Data

Modeling parameters - geometry

Parameter Value Parameter Valueregenerator type packed sphere maximum applied field 1.5 Teslaregenerator material Gd sphere size for packing 0.425-0.5 mmregenerator beds 6 porosity 0.362regenerator height 6.6 mm motor efficiency N/Sregenerator width 15.9 mm dwell ratio 1/3regenerator length 2.025 in magnet arc 120°

regenerator volume 33 cm3 heat transfer fluid90% water/10% ethylene glycol


Uncertainty in Model Inputs

x

( )m t&

heat transfer fluid properties:

cold end, fluid enters at TC

magnetic regenerator material properties:

( ) ( ) ( ) ( ), , , , , , , ,rr o r o r

o T

sk T x T x H c T x H xH

μ μ ρμ

⎛ ⎞∂⎜ ⎟∂⎝ ⎠

( ) ( ) ( ), , ,f f f fc T k T Tρ μ

( ),oH x tμ

hot end, fluid enters at TH

Tf (x,t)

Tr (x,t)

regenerator bed configurationAc, L, as, dh, ε, Nu(Re, Pr), f(Re), keff(Re, Pr,..)


Model Results for New Nusselt Number and Reduced Magnetic Field


Modeling Error vs. Temperature Span


Important AMRR Design/Modeling Considerations

• Effective magnetic field in the regenerator material is different than the field in the gap of the magnet

– suggests that the permeability of prospective magnetocaloric materials should be considered along with other properties

• Heat transfer in the regenerator – model agreement improved using a Nu correlation developed for a

liquid rather than primarily for a gas


FLUID PUMP

COLD HX

MAGNETOCALORICWHEEL

PERMANENTMAGNET

DRIVE MOTOR

Example of a Prototype Rotary AMRR


Regenerator Flow Unbalance

• for some flow configurations, it is possible that flow through each regenerator may be dependent on flow direction

fluid flow direction

regenerator bed


Model Sensitivity to Flow Unbalance

5.3633.7739.130.67417.48

1.4320.9322.360.34411.87

6.0538.0444.090.67111.56

3.1221.4924.610.35113.15

0.1467.6667.800.8140.24

0.6534.7335.380.3540.83

1.9451.2653.200.6710.32

1.0634.6635.720.3410.21

WWWLiter/minHzK

Cooling Power Loss

5% Unbalance

No Unbalance

Fluid Flow Rate

Cycle FrequencyΔTCase


Thermal Properties of the Regenerator Housing

• The regenerator housing is made of G-10, k ~ 0.8 W/m-K, α ~ 4e-7

6.6mm

15.9mm


Reduced Magnetic Field in the Regenerator

• in a magnetically permeable object, the magnetic field will be less than the magnetic field in free space

– actual magnetic field depends on the shape of the material and the permeability as a function of magnetic field

• For a sphere of constant permeability in a uniform magnetic field the magnetic field in the material is

00 0

0

3

2effH Hμ μ μ

μ

=+

permeability of regenerator material

• It is important to determine the actual effective magnetic fieldin the regenerator


Reduction in Cooling Power at Reduced Heat Transfer

25.4%39.10.674.017.48

17.1%22.40.344.011.87

19.6%44.10.671.011.56

10.8%24.60.351.013.15

2.2%67.80.814.00.248.3%35.40.354.00.830.3%53.20.671.00.321.0%35.70.341.00.21

WLiter/minHzK

Cooling Power Loss

Predicted Cooling Power

Fluid Flow Rate

Cycle FrequencyΔTCase


Determining Actual Magnetic Field


Governing Equations

( ) ( ) ( ) ( ) ( ) ( )

2

2

f f f ff f f s c f r f f f c f

h

rdisp c

Nu Re ,Pr k Tm t c T T a A T T c T A T

x d tenthalpy flow capacity ofheat transfer to

entrained fluidregenerator material

Tk Ax

axialdispersion

ρ ε∂ ∂⎡ ⎤ ⎡ ⎤+ − +⎣ ⎦ ⎣ ⎦∂ ∂

∂−

∂

&144424443 144424443144444424444443

1 2

( )f

m tpx

viscousdissipation

ρ∂

=∂

&

4 43 14243

( ) ( ) ( ) ( ) ( )2

21 1f f f f r rs c f r c o eff c r c

h

Nu Re ,Pr k T M T ua A T T A H k A Ad t x t

axialmagnetic work energy storedheat transfer from fluid conductionin matrix

ε μ ρ ε∂ ∂ ∂− + − + = −

∂ ∂ ∂144424443 14243 1442443144444424444443

Fluid:

Regenerator:


Modeling Efforts

• Developed a 1D AMRR model flexible design tool capable of predicting the performance of an AMRR system

• Verify the model experimentally

• Make the model publicly available


Representative Experimental Cases

xxx8

xxx7

xxx6

xxx5

xxx4

xxx3

xxx2

xxx1

highlow highlowhighlow

Fluid Flow Rate Cycle FrequencyTemperature SpanCase


Model Sensitivity to Heat Transfer Coefficient in the Regenerator


Modeling Experimental Data – Cycle Configuration


bed rotation

mass flow inmass flow out

Cold to Hot Flow in a Rotary Bed

flow activated as bed moves overcold check-valve

flow activated as bed moves overhot check-valve


Prototype AMRR Performance

540GdSiGeGa

100Gd5Si2Ge2

spheres (0.2mm)230Gd~200 1.4 (P)Nanjing University



spheres (0.43-0.5mm)1415Gd331.5 (P)Astronautics

0600

spheres (0.15-0.3mm)38100Gd~6005 (S)Astronautics

24402 (S)Toshiba

spheres (0.3 mm)26100Gd4844 (S)Chubu Electric/

47Gd252 (S)

spheres (0.2mm)200Gd492 (S)Victoria

crushed part. (0.4mm)500Gd, GdTb & GdEr742 (S)University of

KWcm3Tesla

Regen typedTQCRegen materialVregenμoHMaxSystem


1 1 1

Efficiency Dependence on Aspect Ratio

Volume of magnetic material is held constant while the length and diameter of the regenerator bed are varied

Axial conduction losses begin to dominate as L/d

decreases

regenerator

bed

Pump losses begin to dominate as L/d increases

flow

flow


Numerical Verification: Schumann Solution

• Solid and fluid in a regenerator are initially at a uniform temperature of 0 K and fluid at temperature TH enters at t=0

• Analytical solution for transient temperature profiles of the fluid and solid exists

• Shitzer, A. and M. Levy, 1983, Transient Behavior of a Rock-Bed Thermal Storage SytemSubjected to Varibable Inlet Air Temperatures: Analysis and Experimentation, Journal of Solar Engineering, vol 105: p. 200-206

• Solution assumes constant material properties, no axial conduction, and uniform heat transfer between fluid and solid

• The temperature profiles predicted by the numerical model can beverified against this analytical solution


fluid temp

Model Verification – Single Blow

NTU=50

regenerator temp

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Recent Developments in Room Temperature Active Magnetic ...20Magnetic%20cooling.pdf · Magnetic Refrigeration Background • Alternative to vapor compression • Magnetic refrigeration