how to reduce energy consumption of built-in...

Institut für Landtechnik

Sektion Haushaltstechnik

How to reduce energy consumption of

built-in refrigerators?

Jasmin Geppert, Rainer Stamminger University of Bonn

Institute of Agricultural Engineering

Household and Appliances Technology Section

5th Cold Chain Management Workshop in Bonn,

10th – 11th of June, 2013



11 June, 2013 CCM Workshop - J. Geppert 2

Background and motivation

29 % of electrical energy in private households used to operate

refrigerators and freezers

Great efforts to reduce this consumption

Problematic case: built-in refrigerators and freezers

Standardised niche measures

Insulation limited to a certain extent

Additional insulation material decreases refrigerator’s net volume

Sensitive to deviations in air vents

Seldom graded into the highest energy efficiency classes




State of the art

Installation instructions have remained unchanged for many years

Over this period, efficiency was improved and heat dissipation reduced

Do modern refrigerators and installation conditions still fit together?

How do installation conditions influence energy

consumption?

Is it possible to reduce energy consumption

by modifying installation conditions?

Air outlet

min. 200cm2

Air inlet in the

socket

min. 200cm2

Measures in

mm

Source: http://www.bosch-home.com




State of research

Melo et al. (2004): Study on performance of wire-and-tube condensers

(special experimental apparatus) dependent on:

Geometry

Distance to adjacent walls

Gap: refrigerator – rear

wall

Position of condenser Gap: refrigerator – side

wall

Figures: Melo et al. (2004)




Objectives

Is it possible to reduce energy consumption of built-in refrigerators and freezers

by modifying installation conditions or by using additional features without

impairing safety and quality of stored food?

Accelerating heat removal from the condenser

Reducing the condenser’s temperature, which is the most crucial factor for

refrigerators’ energy consumption




Material and methods – Numerical simulation (CFD)

Geometry of condenser simplified (nine equal parts)

Each part was assigned a constant temperature (deduced from

IR thermography)

Figures: Transsolar Energietechnik GmbH




Material and methods – Numerical simulation (CFD)

Assumptions/ simplifications to reduce computational efforts:

Walls are adiabatic

Radiation neglected

Heat transfer and airflow were examined at a discrete and representative

point of time

Reference Variant 1

(V1)

Variant 2

(V2)

Variant 3

(V3)

Power 90 W 90 W 90 W 90 W

Distance

refrigerator back

wall-condenser

10 mm 25 mm 10 mm 10 mm

Air vents (inlet/

outlet)

200 cm2 200 cm

2 400 cm

2 200 cm

2

Gap refrigerator-

rear wall

50 mm 50 mm 71 mm 71 mm




Material and methods – Experiments

Test enclosure made of 20 mm dull

black-painted plywood

Positions of side walls and rear wall

variable

Air vents variable

Measured parameters:

Internal fridge temperature (3 positions)

Condenser temperature (9 positions)

Air velocity (near inlet and outlet)

Ambient temperature and humidity

Energy consumption

Refrigerator

Air vent (inlet)

Air vent (outlet)

Side wall




Material and methods – Experiments

Climatically controlled test conditions

Ambient temperature 21 °C

Test period: at least 24 h under steady state conditions (following

EN ISO 15502:2005)

Reference point: according to installation instructions

Gap between condenser and rear wall: 50 mm

Area of air vents: 200 cm2




Results




Changes in convective heat dissipation compared to

reference (numerical simulation)

-10%

-5%

0%

5%

10%

15%

20%

V1 V2 V3

Re

lati

ve

ch

an

ge

in p

erc

en

t

V1: condenser centrally

arranged

V2: gap between

refrigerator and rear

wall and air vents

increased

V3: gap between

refrigerator and rear

wall increased




Air temperature/ velocities inside the gap between

refrigerator and rear wall (numerical simulation)

V1 V2Reference V3V1 V2Reference V3

V1 V2Reference V3V1 V2Reference V3




Changes in energy consumption by modifying gap

between refrigerator and rear wall compared to reference

(experiments)

-10%

0%

10%

20%

30%

40%

50%

Gap 10 mm Gap 20 mm Gap 30 mm Gap 80 mm Gap 110 mm

Rela

tive c

han

ges i

n p

erc

en

t




Changes in energy consumption by modifying air vents

(air inlet/ outlet) compared to reference (experiments)

-1,00%

0,00%

1,00%

2,00%

3,00%

4,00%

5,00%

Air vent 400 cm2 Air vent 100 cm2 Air vent 50 cm2

Rela

tive c

han

ges i

n p

erc

en

t




Conclusion

Gap between refrigerator and rear wall has highest impact on energy

consumption

Energy consumption decreases with increasing gap, reaching a

minimum at 30 mm

Gap can be reduced by 20 mm compared to actual instructions

(50 mm) without negative effects for energy consumption

Additional space might be used to extend insulation layer

Optimal position of the condenser is half the distance between

refrigerator and rear wall

Effect of air vents is vanishingly low




Outlook

Further experiments and simulations needed:

Gap between refrigerator and side walls

Forced ventilation

Material of rear wall (increased thermal conductivity)

Additional features (e.g. drawers inside the refrigerator)

…

Results applicable to other refrigerators/ freezers?




Thank you for the attention!

Contact:

Dr. Jasmin Geppert, Prof. Dr. Rainer Stamminger

University of Bonn

Institute of Agricultural Engineering

Nussallee 5

53115 Bonn/ Germany

phone: +49 228 732384

email: [email protected]

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