Thermag VII, Turin, Italy 11-14 SEPTEMBER 2016

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

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

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

Typical HEX Design in Domestic Refrigerators Warm side natural convection (UA ≈ 5 WK-1 @ ∆T = 20 K)

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

Typical HEX Design in Domestic Refrigerators Cold side natural convection; Evaporator integrated in the back wall (UA ≈ 3 WK-1 @ 20 K )

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

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

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

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

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

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

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

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

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

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

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

Measured Performance

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

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

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

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

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.

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.

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