High-frequency Magnetocaloric Modules with Heat Gates Operating

with Péltier EffectPeter W. Egolf

University of Applied Sciences of Western SwitzerlandInstitute of Thermal Sciences and Engineering

HEIG-VD / Hes-so

In collaboration with:

Institute of Micro and Nanotechniques

Laurent GravierThibault Francfort

Anne-Gabrielle PawlowskiGilles Courret

Mirco Croci

Table of content

1) The operation principle of magnetic

refrigeration

2) The frequency problem

3) The thermal switch technology

4) Overall device: An engineer’s estimate

5) Conclusions and outlook

1) The operation principle

2) The frequency problem

A provocative personal statement:

We are not competitive enough with present developments (feasibility was demonstrated, good efficiency is possible, but still high cost)! Approaching luxury refrigerator sector!

Demanded:

3-5 x higher magnetocaloric effect of materials

or

3-5 x higher frequency of the machines(seems to me more realistic!)

2) The frequency problem

Heat diffusion in and out of solid structure!

Density: =7900 kg m-3,

Thermal conductivity: k=10.5 W m-1 K-1,

Heat capacity: cH= 886 J kg-1K-1,

Thermal diffusivity: a= 1.5 10-6 m2 s-1.

Example:

s=0.25 mmf =100 Hzmax

2) The frequency problem

L=25 mmL=50 mmL=100 mmL=200 mm

Safety factor: 10

Carry-over leakage

Example:

L=25 mmv=0.25 m/s

Result:

f ≤ 1 Hz

3) The thermal switch technology

Advantage: Constant fluid flows with alternating cold/heat inputs (no carry over leckage, no fluid switches)

Magneticfield

Nofield

Gate closed

A. Kitanovski and P.W. Egolf, Int. J. Refr. 33 (3), 449-464:

3) The thermal switch technology

Upper microchannels for the heat transfer fluid

Lower microchannels

for the heat transfer fluid

Top and bottom contact surface of

the Peltier (thermoelectric)

module

SemiconductorsMagnetocaloric

material I.Magnetocaloric

material II.Magnetocaloric

material III.

Kitanovski and Egolf, Int. J. Refr. 33 (3), 449-464:

The basic plates:

3) The thermal switch technology

Three main elements:

1) Magnetocaloric layered bed

2)Thermoelectric switches

3) Micro channel heat exchangers

Overall system and its performance

1

32

3) The thermal switch technology

Characteristic diameter of theNi wires is 200 nm

SME: Dr. Anne-Gabrielle Paw-lowski, MNT

Nanowires:

3) The thermal switch technology Thermal switches and experimental device:

0

20

40

60

80

100

0 10 20

CO

P

2

1

1

1

1

1

1

2

1

1

2

CarnotCOPCOP

Entire theory outlinedin Thermag V paper.Main result:

chT

R

TT

RI

P

P

2

thT

I

IRP

Q

1

1

ch TT

AZ

21

1

Z: Figure of merit

Coefficient of performance of a thermal switch:

3) The thermal switch technology

4) Overall device: An engineer’s estimate

  Thermal switches (3 layers):      

20 Surface area of thermal switches A --- 800 cm2

21 Temperature difference Th-Tc--- 2 x 2.5 K

22 Thermal resistance, Eq. (7d), Table 2 Rth--- ∞ +++

23 Carnot coefficient of perf. (13a,b) COPCarnot--- 216…254

24 Power thermoelectric effect, Eq. (3a) PT--- 10 mW

25 Power electric loss, Eq. (3b) PR--- 122 mW

26 Total power of source, Eq. (3c) PS--- 132 mW

27 Heat flux, Eq. (10c) --- 0 +++

28 First non-dim. variable, Eq. (11a,b) Q1--- 0 +++

29 Sec. non-dim. variable, Eq. (11c,d) Q2--- 12

30 Coeff. of perf.  (Eq.(13c)) COP --- 17-20 +++

IQ

4) Overall device: An engineer’s estimateNo. Quantity Symbol ULMR

(usual type)TSMR

(thermal switch)

    Overall machine: 1 Nominal cooling power Pn 50 W* 50 W2 Max. cooling power Pmax 96 W 96 W3 Magnetic field strength (ind.) H 2 T* 2 T4 Frequency  f 2 Hz* 10 Hz5 Heat source temperature Tc -5 °C* -5 °C6 Heat sink temperature Th 45 °C* 45 °C7 Adiabatic temp. diff. Tad 5 K* 5 K8 Temp. Diff. Mat.-Fluid T 1 K* 2.5 K9 Coeff. of perf. Carnot COPCarnot 5.4* 5.410 Coefficient of performance COP 2.8* 2.811 Exergy efficiency 52 %* 52 %12 Number of layers (layered bed) N 10 1013 Specific cooling power pc 2.5 kW kg-1** 15 kW kg-1**14 Mass of magnetoc. mat.  mmagneto 384 g 64 g15 Volume of magnetoc. material V 49 cm3 8 cm3

16 Thickness of plates (FIG. 1) s ≈ 0.2 mm* 0.2 mm17 Surface area heat exchange A 4900 cm2 400 cm2

18 Demanded heat transf. coeff. h 392 W m2 K 1920 W m2 K19 Estimate of magnets mass mmag 18 kg* 4-6 kg

(*) Kitanovski et al. (2008a), (**) Kitanovski and Egolf (2008b) based on pure gadolinium.

5) Conclusions and outlook1) First nanowire thermal switches have been success-

fully produced following secret recipes (patent)

2) A substiantial Péltier effect was experimentally deter-mined

3) The electric resistance can be further decreased

4) The thermal resistance has not yet been successfully determined5) Thermal switches are high-frequency devices

6) A physical model for the thermal switches has been developed (see Thermag V proceedings)

7) The energy demand of the switches is of small influence

5) Conclusions and outlook 1)If this technology works, it will be possible to beat conventional refrigeration in numerous important refrigeration markets

2)This technology is so promising that even automobile refrigeration, where a high mass is very critical, could become feasible!

Final slide

Thank you for your attention!

FINISH