1. Thermag VII, Turin, Italy
11-14 SEPTEMBER 2016

2. Content Introduction Heat exchanger, project design requirement
Typical heat exchangers designs in domestic refrigerators
Design options considered
Developed heat exchangers
Measurement results
Conclusions

3. Introduction
Goal: Develop low cost heat exchangers which extract heat from the cabinet and reject this heat to the ambient and that can be integrated into an existing appliance platform (built-in refrigerator of 153 dm3).
Step 1) Performance optimisation:
Maximum heat transfer at minimum temperature difference for minimum energy use.
Step 2) Cost optimisation:
In both the production of the heat exchangers as well as in the integration into an existing appliance platform

4. HEX Design Requirements
Cold Side
Heat absorption 23 W
Temperature difference working fluid and inner air < 5 K
Overall Heat Transfer coefficient UA > 4.6 WK-1
Warm Side
Heat rejection 25 W
Temperature difference working fluid and ambient < 5 K
Overall Heat Transfer coefficient UA > 5 WK-1
General
Minimum energy use of peripherals (e.g. fans)
Fluid pressure drop < 0.1 bar at fluid flow rate of 0.5 dm3min-1
Minimum loss in internal volume
Minimum increase in appliance installation volume
TRoom 25 ºC
Tinternal 5 ºC
Tcold = 0 ºC
Twarm 30 ºC
Q = 23 W
Q = 25 W
Magnetic Cooler

5. Typical HEX Design in Domestic Refrigerators
Warm side natural convection (UA ≈ 5 ∆T = 20 K)

6. Typical HEX Design in Domestic Refrigerators
Warm side forced convection (UA >> 10 WK-1)

7. Typical HEX Design in Domestic Refrigerators
Cold side natural convection; Evaporator integrated in the
back wall (UA ≈ 3 20 K )

8. Typical HEX Design in Domestic Refrigerators
Cold side forced convection; Fin & Tube in air duct (UA > 10 WK-1); Cold wall + small fan UA ≈ 4 to 5 WK-1

9. Natural convection Pro: No additional power consumption of peripherals
Con: Reduction in UA with reducing temperature difference
Relatively large surface area required

10. Forced convection: Pro: Con:
Larger heat transfer values, less surface area required
Approximately constant heat transfer coefficient for constant air flow rate
More uniform temperature distribution throughout the cabinet
Con:
Fan and ducting needed
Additional energy consumption
Reduction in robustness of the design
Reducing internal volume

11. Difference in HEX design aspects
Lower fluid pressure than vapour compression:
Reduction in strength requirements (i.e. reduced wall thickness or using plastic tubing)
Single phase fluid (water glycol):
No major fluid distribution issues with parallel circuits
No safety limitation in fluid quantity (150 g for R-600a)
Marginal heat rejection of magnetic cooler and electric motor
Larger area available for warm side heat exchanger at the back of the appliance

12. Design options considered
WARM SIDE:
Conventional, natural convection based, double plate designs utilising the complete back area of the appliance
Using metal foam to increase the heat transfer area; See the next presentation

13. Design options considered
Cold side:
Thermodynamically ideal:
Integrating the tubing (or cold plate) inside the inner liner
Direct: Issue with condensate formation
Indirect: Less severe condensate formation on side walls, easier to drain
Relatively large mass of conductive material required (aluminium)
Not applicable on an existing appliance platform

14. Designs options considered
Cold side: Forced convection
Using small efficient fan ~ 0.5 W
Minimise volume occupation
Fin and tube
Double plate

15. Developed Heat Exchangers
Cold side:
Compact fin and tube with 0.5 W Fan (33 m3/h)
Volume loss of 5 dm3:
297 mm
105 mm
50 mm
Two parallel fluid circuits
Fin pitch 5 mm

16. Developed Heat Exchangers
Cold side:
Two Polypropylene channel plates (3 * 3 * 0.2 mm)
with 0.5 W Fan (33 m3/h)
Height = 350 mm and Width = 320 mm
PP Tubing with O.D. = 12 mm and t = 1.8 mm
assembled using adhesive.
Volume loss: 8.5 dm3

17. Developed Heat Exchangers
Warm side:
Aluminium roll bond (two plates)
Height = 700 mm; Width = 540 mm
Plate thickness = 1.5 mm

18. Developed Heat Exchangers
Warm side:
Plastic channel plate (two plates):
Height = 600 mm; Width = 530 mm:

19. Measured Performance

20. Measurement results warm side
Similar heat rejection: Both configurations meeting target UA > 5 WK-1 (Q > 25 W at ΔT = 5 K)

21. Measurement results warm side
Larger pressure drop for Aluminium heat exchanger: Both configurations meeting target (∆P < 0.1 at 0.5 dm3min-1)

22. Measurement results cold side
Aluminium: heat transfer well above target
Plastic: heat transfer meeting project target

23. Measurement results cold side
Pressure drop similar for both configurations:
Both meet project target (∆P < 0.1 at 0.5 dm3min-1)

24. Conclusions
Compared to vapour compression much lower mechanical strength is requirement and no limitation in fluid charge exists (max 150 g of R-600a).
Compared to vapour compression based refrigerators the heat needs to be transferred at much lower temperature difference; therefore requiring much larger heat transfer area and / or larger heat transfer coefficients.
Heat exchangers fulfilling the project requirements are developed built and have been evaluated.

25. Conclusions
The heat exchangers developed can be produced using conventional and available manufacturing techniques
The developed heat exchangers can be directly integrated into current appliance platforms as shown by the demonstrator
Improved appliance efficiency (reduction of temperature lift and no loss in volume) can be obtained by integration of the cold side heat exchanger into the complete inner liner of the appliance;
No option for an existing appliance platform.

26. Thank You. Marcel van Beek marcel. van. beek@re-gent. nl www. re-gent
Thank You! Marcel van Beek elicit-project.eu #MagneticCooling