Pot-in-pot refrigerator

Task

The ‘pot-in-pot refrigerator’ is a device that keeps food cool using the principle of evaporative cooling. It consists of a pot placed inside a bigger pot with the space between them filled with a wet porous material, e.g. sand. How might one achieve the best cooling effect?

Scheme

Principle

Water from the wet sand soaks into the outer pot. Then it evaporates from its surface, along with heat. Outer pot gets cooled. To keep balance inside the pot-in-pot system, heat is transfered from the inner pot to the outer, and the inner pot takes heat from its inside part. As a result, temperature inside the inner pot decreases.

Conditions and constraints

The pots themselves should consist of a porous material (clay). They need to be moisture-permeable.

The inner pot can be from a non-permeable material. In fact, it would be better, because we don't want the moisture to contaminate our food.

The more quality evaporation, the better effect is achieved. Therefore, if the refrigerator is put on a dry place where air is streaming, we are able to achieve lower temperature.

Theory

Efficiency of the refrigerator should follow this equation:

f – „freezing factor“ = efficiencyQ - heat transferred into the outer environmentW – heat transferred from the inner pot

f =Q 0/W

0

Experiment

We made a provisional aparature out of two flowerpots, wet soil and a piece of cloth.

(illustrational image)

Experiment

The experiment was performed in a room with constant temperature, no direct sun light and no air currents.

We were measuring temperature inside the inner flowerpot.

Measurements

Final temperature: 15,5 °C

Initial inside temperature: 21 °C

0 5 20 30 60 90 120 300

0

5

10

15

20

25

t (m in )

t (°C

)

Time (in hours) Temperature (in °C)

0 : 00 21

0 : 05 19

0 : 20 17,5

0 : 30 17

1 : 00 16,75

1 : 30 16,5

2 : 00 16

5 : 00 15,5

Measurements

Measurement #2: Initial inside temperature: 18 °C

Final temperature: 15,5 °C

Time (in hours) Temperature (in °C)

0 : 00 18

0 : 10 17

0 : 30 16,5

1 : 20 16

5 : 00 15,5

0 10 30 80 300

14

14,5

15

15,5

16

16,5

17

17,5

18

18,5

t (m in )

t (°C

)

Conclusion

Even after repeatedly performing the experiment with flowerpots of different scales, we haven't been able to reach lower temperature than 15,5 °C in our conditions.

(However, it has been proven that in conditions of African desert, this type of refrigerator can reach temperature under 6 °C.)

End of presentation

All of the photos were illustrational, they were not taken by us. Sources:http://rebuildingcivilization.com/content/busting-myths-about-zeer-pothttp://deconstructingproductdesign.com/pot-in-pot-cooler/