An experimental study on an evaporative cooler for hot rural areas

University essay from KTH/Energiteknik; KTH/Energiteknik

Author: Katarina Gustafsson; Hanna Simson; [2016]

Keywords: ;

Abstract:

In developing countries about 40 % of the food-waste is due to post-harvest losses, such as

improper storage. The Zeer-pot is an evaporative cooler, which cools the inside by convective

heat transfer, and can be used to keep fruits and vegetables fresh longer. This is typically

convenient in hot rural areas without access to electricity and is more efficient in non-humid

areas.

This study will investigate if there is a correlation between the temperature decrease inside the

pot and the wind velocity, how the efficiency of the Zeer-pot is affected by hanging it in the

air to additionally expose the underside to the airflow and also how the efficiency is affected

from glazing of the inner pot, to prevent the food from getting damaged from high humidity.

It will also consider the feasibility of combining the Zeer-pot with a solar dryer, also to

improve its efficiency.

The evaporation is increased by higher wind velocity due to forced convection. A solar dryer

can create an airflow when the heated air rises along the surface of the solar collector and

creates a temperature difference between the upper and the lower part. A design for a solar

dryer that could be appropriately integrated with a Zeer-pot to achieve a greater airflow

around it is modelled in CAD and presented.

The tests on the pots took place in a climate chamber where the ambient temperature was

controllable. In the climate chamber a fan and a dehumidifier was installed in order to create

wanted conditions. One pot was tested only for wind velocities and the other only for the

cases of the pot hanging in the air and being glazed on the inside. A reference case was

designed and tested for the second pot in order to compare the glazed and hanging pot in the

same conditions.

The relative humidity was not controllable in this setup, and thereof a way to compare these

results was to calculate the final temperature the pot achieved relative to the lowest possible

theoretical temperature, the wet bulb temperature.

For the first pot an almost linear correlation between the time it took to reach the final

temperature depending on the wind velocities could be observed, apart from two values. A

rather nice coherent curve, also apart from two values, was found for how close to the wet

bulb temperature the final temperature was depending on the wind velocity. For the second

pot the cooling capacity was enhanced for both the hanging construction and the glazed pot.

For the hanging pot this was expected, but for the glazed one it was not.

If a solar dryer is combined with the Zeer-pot, a wind velocity around 3-3.5 m/s is guaranteed

to improve the Zeer-pots cooling capacity. A lower wind velocity could probably make a

large difference too, but the experiments in this study is insufficient to make any conclusions.

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