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The Zeer pot is a refrigeration device invented in Africawhich operates by the principle of evaporative cooling.According to Science in Africa, each device can store12kg of vegetables, keeping them fresh for up to 20 dayswhile costing less than 2USD to produce[1]. This articlewill showcase how to build a pot-in-pot refrigerator,discuss various aspects of the technology and present theresults of a parametric analysis.

1 Overview of Technology1.1 Operating Principle1.2 How to make a Pot-in-Pot Refrigerator

2 Analysis of Technology2.1 Relative Humidity2.2 Permeability2.3 Flow Characteristics

2.3.1 Flow Velocity2.3.2 Laminar vs Turbulent Flow2.3.3 Boundary Layer Considerations

2.4 Area available for Evaporation3 Discussion

3.1 Interpretation of Reported Values3.2 Convection3.3 Radiation3.4 Rough Approximation for Net Energy Balance3.5 Limitations of Device

4 Conclusion5 References6 Appendix A: Calculation Methodology

In harsh dry climates, food preservation plays a vital role in maximizing both economic andnutritional yield from the rare opportunity of a good harvest. The dry heat significantly

Zeer pot refrigeration (design) - Appropedia: The sustainability wiki http://www.appropedia.org/Zeer_pot_refrigeration_(design)

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Figure 1: Conceptual Flow of Energy and Water in Zeer Refrigerator

reduces the life ofproduce and as aresult there is ahigh level ofwasted crop[2].

The followingshelf-lifecomparison hasbeen adapted froma case studyperformed by thePractical ActionOrganization andcan be directlyaccessed here

(http://practicalaction.org/?id=zeerpots) .

Table 1: Shelf of common produce with Zeer pot refrigerator[3]

Produce Shelf-Life without Zeer Shelf-Life with ZeerTomatoes 2 days 20 daysGuavas 2 days 20 daysOkra 4 days 17 daysCarrots 4 days 20 daysArugula 1 day 5 days

The impact of the pot-in-pot refrigerator is immediately realized as the shelf life for mostproduce is extended by 5 to 10 times. This means that farmers are not only able sell theirproduce beyond the initial days following a harvest, but will also be able to safely consumethe unsold goods because of their extended shelf life.

When evaporation occurs from a surface, there is an energy associated with the phasechange known as the latent heat of vaporization. In a given system, as a gas species flowsover the wet surface, evaporation and condensation continuously occur to maintainsteady-state conditions.

In order to sustain evaporation, there must be a draw of internal energy in the liquid,which would result in a temperature reduction. This cooling effect is known as evaporativecooling and is most effective in dry climates due to the lack of moisture content (relativehumidity) in the air.[4]

In the case of the Zeer refrigerator, water evaporates out of the sand through the surfaceof the outer clay pot and from the whole top surface of the moist sand exposed to the solar


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Figure 2: A stone is used toshape the inside of the

mould into a bowl shape

Figure 3: Dimensions forMould

radiation, removing energy from the system. Figure 1 is a graphical representation of thewater and energy flow within a Zeer refrigerator.

Additional information on evaporative cooling technologies can be accessed here.

The document File:ClayTech.pdf has been made available by the Practical ActionOrganization. Within the document, pages 15 to 19 outline in detail how to make apot-in-pot refrigerator. The remainder of the document discusses other clay basedtechnologies that may be of interest. The steps for making a pot-in-pot refrigerator areoutlined below in case the reader is unable to open PDF files[5].

Making the Moulds

Make a small hole in the ground and cover with amat. Use a small amount of wood

chippings on the mat to prevent sticking

Mix and knead an even mixture of mud, dung andwater into a ballRepeatedly press a stone into the mixture toeventually form a bowl shape. Continue to

do this, adding more material if necessary, until themould reaches the dimensions given in Figure 2

Once the mould has been made it must be left to dryin the sun for approximately 30 minutes

Making the Clay Pots

Make a small hole in the ground and cover with a amat. Use a small amount of wood

chippings on the mat to prevent sticking

Knead the clay into a dough like mixtureRoll the clay flat and place on top of an upside downmouldSpread the mixture over the mould, maintaining athickness of approximately 10mmUse a flat stone and some water to smooth thesurfaceOnce the round shape has been formed the mouldcan be removedThe pot walls can be extended to the desired heightAdd an extra thick layer to the rim (approximately20mm)


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Figure 4: Using water and astone the surface of themixture is smoothened

Figure 5: The mould isturned over and the clay is

pressed on top of it

Figure 6: The pots areburned under a pile of rocks

and cow dung

This completes the first potForm the larger second pot in the same manner,using the appropriate mouldExtend the height of this pot as neededForm the rim for the larger pot, using a thickness ofapproximately 30mm

The final part of this process is to add decorationaround the outside of the pot. This

is done by rolling the mixture into a long sausage shapeand sticking it around

the outside of the pot about two thirds of the way up. Thepattern is made by pressing

fingers into it. The large pot is now complete.

Both pots are now left in the sun to dry. Thisnormally takes between two to four days

depending on the ambient temperature and sunlight

Preparing the Clay Pots

Identify an area that is cleared and make a circlewith rocksCover the ground within the circle with cow dungPlace as many clay pots as possible inside the circleCompletely cover the pots with wooden sticks andmore cow dungLight the fire and let it burn for 24 hoursTo make this process more efficient it is better toburn as many pots as possible at once

Assembly and Operation

Sand should be placed in the bottom of the large potforming a layer of approximately 5cm in depthPlace the smaller pot on top of the sand and centre itin the large pot (the pots should now be level)


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Figure 7: Assembledpot-in-pot refrigerator

Fill the remaining space between the pots with sandIf possible place the assembled device on a stand tomaximize air flowThe small pot should be covered with a lid (clay orcloth) to prevent hot air from

entering the storage chamber

The pot-in-pot refrigerator operates passively aslong as the sand remains moistCheck the sand twice a day and add water asneeded

The success of the pot-in-pot refrigerator is heavily dependent on the surroundingconditions. Due to the device's reliance on natural evaporative cooling it can only beviewed as an appropriate technology for regions that demonstrate a suitably low relativehumidity and a sufficient level of air flow. In order to maximize the effectiveness of thepot-in-pot refrigeration device, the rate of evaporation must be increased. To properlyinvestigate the technology it is important to quantifying the effects of:

Relative HumidityPermeabilityFlow Characteristics

Flow VelocityLaminar vs Turbulent FlowBoundary Layer Considerations

Area available for Evaporation

It should be noted that the values presented in the following sections indicate themaximum cooling effect for a given set of parameters. The actual cooling will likely belower due to irregularities in parameters, such as wind speed and relative humidity.

Relative humidity is a measure of the content of water than can be held in the air at aspecific temperature. An environment with a low relative humidity in comparison to onewith a high relative humidity will evaporate moisture more readily and to a greater extent.For this reason, the pot-in-pot refrigeration device is only effective in environments with alow relative humidity (Figure 8). Northern central regions of Mexico (such as Chihuahua)and Africa (such as Sudan) are the most appropriate locations for use of this technology.

In some cases, the technology may prove to be suitable for specific times of the year whenthe humidity level is low. For a set of given conditions, information regarding humidity canbe measured directly off of a psychrometric chart. Additionally, local weather centerstypically keep record of humidity levels.


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Figure 8: Cooling Effect vs. Wind Speed for varying Relative Humidity Levels(Device Radius = 0.25m, Permeability Correction Factor = 0.3, Ambient

Temperature = 35 degrees Celsius, Turbulent Flow)

A relative humidity of 0.3 will has been used for all calculations where this value is heldconstant. At this relative humidity and a typical wind speed of 2.5 m/s, a cooling effect of4.46kW is observed under turbulent flow conditions. The relationship established above, inwhich the device is expected to perform better in low humidity environments, is clearlyindicated by this graph.

While evaporation undoubtedly occurs through the outer clay pot, the permeability of thislayer plays a significant role in determining the actual rate at which water is evaporated.Earthenware is the type of clay that is used to construct this device. This type of clay isrelatively porous and permeable in relation to other forms of clay, such as porcelain andstoneware[6]. In addition to permeating through the outer clay pot, water must travelthrough the sand to continuously replace moisture that has passed through the clay andevaporated. The cooling effect will be limited both by this diffusion rate and thepermeability of the clay.

With these considerations, a permeability correction factor of 0.3 is incorporated into theperformance calculations (as detailed in Appendix A). Additionally, a graph has beenproduced (Figure 12) varying only this correction factor in order to provide a generalinsight as to how this number effects the findings presented.


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Figure 12: Cooling Effect vs. Permeability Correction Factor for varying WindSpeeds (Device Radius = 0.25m, Ambient Temperature = 35 degrees Celsius,

Relative Humidity = 0.3, Turbulent Flow)

It is acknowledged that the validity of setting this correction factor to 0.3 may not berepresentative of the actual limitations created by the mechanisms involved with moisturediffusion and clay permeability within the device. This provides a platform for a futureproject to investigated this in greater detail, including the identification of methods toincrease the permeability of the clay.

Flow Velocity

As the water is evaporated into the surrounding air, the local relative humidity increasesand thereby reduces the likelihood of further evaporation. Air flow is necessary to replacethis moist air with dry air. This is provided by natural winds in the area. A high flow velocitywill result in the body of air immediately surrounding the device to continuously remain in adry state, and therefore will induce a higher rate of evaporation.

An average wind speed of 2.5m/s or 5.6mph has been selected based on a review ofweather data for regions of Northern and Central Africa.

Laminar vs Turbulent Flow

In a given system the existence of turbulence within a fluid flow increases the level ofmixing, heat transfer and mass transfer. In this particular case, for a typical setting theflow can be assumed to be turbulent. If we consider air flow over a flat plate (where the airflow is wind and the flat plate is the ground), the distance at which the flow transitionsfrom laminar to turbulent flow can be calculated. This transition occurs for a Reynoldsnumber of approximately 5 x 105.

Reynolds Number is dimensionless parameter that is defined as a measure of the ratio ofinertial forces to viscous forces and can be calculated as follows:


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[7]

For an ambient air temperature at 35 degrees Celsius and a wind speed of 2.5m/s, it isfound that for the flow transitions into turbulence at a distance of approximately 2.6m.

Herein lies the justification for our turbulent assumption. If we rely on natural wind, it ishighly unlikely that the pot-in-pot refrigerator will be located within 2.6m of where the airflow initially interacts with the ground surface. For completeness, a graph has beenproduced to highlight how performance varies between laminar and turbulent flow (Figure9).

Figure 9: Cooling Effect vs. Wind Speed for Laminar and Turbulent Flow (DeviceRadius = 0.25m, Permeability Correction Factor = 0.3, Ambient Temperature =

35 degrees Celsius, Relative Humidity = 0.3)

Boundary Layer Considerations

Continuing with the flat plate analysis, the existence of a boundary layer must beacknowledged. Within this boundary layer a velocity gradient exists in which there is anincrease in velocity as we move away from the ground. The actual wind velocity (freestream velocity) exists only beyond this boundary layer[8]. Due to this velocity gradient,the boundary layer height becomes an important factor that effects the performance of thepot-in-pot refrigerator. This height can be calculated as follows:

[9]

[10]


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Figure 8 provides a graphical representation of how this value increases in the direction offlow. It becomes clear that by increasing the height of the pot-in-pot refrigerator, thedevice can be exposed to a greater wind velocity. The wind velocity has a significantimpact on the performance of the device. Observation of the various graphs presented inthis article indicates that their is an increase in the cooling effect as the wind speedincreases.

Figure 10: Boundary Layer Development and Velocity Profile for Flow Over a Flate Plate (FrVelocity of 2.5m/s and Air Temperature of 35 degrees Celsius). Image is not to sca

The existence of a velocity gradient within the boundary layer, and the expectedperformance gains due to higher velocities suggest that the device should be placed ashigh as possible and without obstruction of air flow. This can be accomplished by using aframe structure for the device to sit on top off. If possible, the frame itself should be placedon high ground or on top of existing solid structures.

The surface area available for evaporation tooccur on/through can be approximated for thepot-in-pot system as:

Total Area = Surface Area of Spherical Portionof Outer Pot

+ Surface Area of CylindricalPortion of Outer Pot

+ Surface Area of Exposed Sand inbetween Pots


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Figure 11: Dimensions for Pot-in-PotRefrigerator Recommended by The

Practical Action Organization

For example, if we use the recommendeddimensions given by the Practical ActionOrganization (Figure 11), the area is found tobe:

This value has been used for all calculations in which area is held constant.

Figure 12 highlights how performance of the device varies with area. An overview of thesection detailing How to make a Pot-in-Pot Refrigerator, will show that the radius is thesimplest dimension to vary in the process. As such, the radius of the outer pot has beenselected to vary the area available for evaporation.

Figure 12: Cooling Effect vs. Wind Speed for varying Device Radii (PermeabilityCorrection Factor = 0.3, Relative Humidity = 0.3, Ambient Temperature = 35

degrees Celsius, Turbulent Flow)

As expected, there is a noticeable increase in the cooling effect as the surface area isincreased. For the chosen wind speed of 2.5m/s, there is a cooling effect of 4.46kW,6.58kW and 8.85kW for radii of 0.25m, 0.35m and 0.45m respectively.

An increase in the area, however, implies the need to use more clay and mould material foreach device, which will drive up the average price of 2USD. Additionally, the device willrequire more sand and more water to operate. The option to increase the area available forevaporation should only be implemented after consideration of these factors. It may proveto be a viable approach for families wishing to pool resources in order to create a largerand more effective pot-in-pot device. In this case, the inner pot should also be made largerto increase capacity of goods to be stored inside the device.


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The values for the cooling effect have been reported in kilo-Watts(kW) and not in the formof energy. The actual energy removed from the system depends on the existence of theseconditions and how long they are present for.

Additionally, energy derived from this number is an indication of the total energy that canbe removed from the entire pot-in-pot device. This includes both clay pots, the volume ofsand, water content within the sand, the air within the storage chamber and the contentsof the storage chamber. The value therefore, should not be interpreted as the energywithdrawn exclusively from the storage chamber. A complex conduction and mass diffusionmodel would be required to provide a full analysis of the heat transfer mechanisms withinthe pot-in-pot structure, which falls outside the scope of this article.

The pot-in-pot refrigerator will experience both forms of convection. When the air is idlethe device will experience natural convection and subsequently, when the air is moving at agiven velocity, the device will undergo the effects of forced convection.

In both cases it is likely that the convective process will transfer energy to the system dueto the high ambient temperature and cooler device surface temperature. This energy that istransferred into the system will reduce the overall cooling effect. In comparison to theenergy associated with a change of phase reaction (evaporation) however, the convectivelosses/gains are relatively small.

The Zeer pot refrigerator is expected to operate in open conditions with exposure to directsunlight during the day. This will transfer a significant amount of energy to the system,reliant on the level solar irradiation, surface temperature of the outer pot and materialproperties of the clay.

This section will attempt to quantify the amount of energy withdrawn from the storagechamber by applying an energy balance including evaporative cooling and radiation.

The following conditions describe the base case scenario used throughout the article:


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To include the effects of radiation let us consider only daylight hours, assumed to bebetween the 8am and 6pm (10 hours).

The maximum cooling effect for the parameters listed above was reported to be 4.46kW.Over the course of 10 hours, the overall cooling (considering periods of flow stagnation,environmental irregularities, and periods of reduced of moisture content in the sand) hasbeen approximated to be equal to three hours at this maximum value for the prescribedconditions. This gives us an effective energy loss from the system of 13.4kWh.

The following calculation determines the net energy transfered to the system by radiationfor the described scenario. Additionally let:

Performing an energy balance on the device (only for radiation):

=

We can approximate the area exposed for radiative on heat transfer equal to:

Over an exposure period of 10 hours the total net radiative energy transferred to thesystem is 1.92kWh.

Therefore, the energy flowing out of the device over a period of 10 hours is


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found to be (13.4 - 1.92) = 11.48kWh

As mentioned above, this value represents the energy loss for the entire pot-in-pot device.Due to the nature of conduction, a temperature gradient within the device must be presentto create an out-flux of energy. Each layer of the device therefore must be cooled: theouter pot; volume of sand; water content within the sand; inner pot; air within the storagechamber; and the contents of the storage chamber. Taking this into consideration, it isreasonable to estimate that the only 5% of the total energy out-flux directly impacts thestorage chamber.

Therefore, the actual cooling within the storage chamber for the prescribedconditions is found to be roughly 0.57kWh.

Beyond the limitations of the required climatic conditions for the pot-in-pot refrigerator tobe successful, there is also a need for a continuous supply of water. For many regions,water may be prioritized for other purposes making the it difficult for communities to adoptthe technology. The device also has no proper seal for the storage chamber, reducing it'soverall effectiveness since warm ambient air can seep into this chamber and increase thetemperature of chilled zone. (however warm air will rise and chilled air is heavier anddrops, so the temperature will always be coldest on the bottom)

A parametric analysis concerning the performance of a Zeer pot-in-pot refrigeration devicehas been performed. As expected, the device performs well only in climates possessing alow relative humidity. The velocity of the wind and the area available for evaporation tooccur on/through are two primary factors that can addressed to improve the performanceof the pot-in-pot refrigerator.

It has been shown that increasing the radius of the outer pot from 0.25m to 0.45m, almostdoubles the total cooling effect. The adaptation of this however, is restricted by theincrease in cost associated with using more materials. It is suggested that the strategy tomake larger pot-in-pot refrigerators be employed only if community members are willingand able to pool their resources to share a device with superior performance.

It is unrealistic to assume that electricity is available to ensure that their is a constant andadequate source of air flow. The device is dependent solely on naturally occurring winds.To maximize air flow, it is recommended that Zeer refrigerator be placed as high above theground as possible. This can be accomplished by building a simple frame to support thedevice, and placing them on high ground or on top of buildings.

There remains the potential for future analysis of this device. The development of adetailed conduction model to analyse heat transfer and mass diffusion mechanisms withinthe various layers would aid in identifying factors limiting performance and how they canbe addressed. Additionally, by experimentation, a study could be performed to replace thepermeability correction factor used in this analysis with real diffusion rates of moisturethrough clay.

↑ Elkheir, M., "The Zeer Pot - a Nigerian invention keeps food fresh without electricity",1.


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Science in Africa, 2002↑ Elkheir, M., "The Zeer Pot - a Nigerian invention keeps food fresh without electricity",Science in Africa, 2002

2.

↑ Practical Action Organization, "How a zeer pot fridge makes food last longer", 2009,http://practicalaction.org/home/zeerpots

3.

↑ Committee on Microgravity Research; Commission on Physical Sciences, Mathematics andApplications; "Microgravity Research in Support of Technologies for the Human Explorationand Development of Space and Planetary Bodies", Space Studies Board, National ResearchCouncil, 2000

4.

↑ Practical Action Organization, "Clay Based Technologies", 20075.↑ The Clay Room, Telephone Interview with Representative, 20106.↑ Incropera, F.P., Dewitt, D.P., Bergman, T.L., Lavine, A.S., "Fundamentals of Heat andMass Transfer", John Wiley and Sons, 2007

7.

↑ Incropera, F.P., Dewitt, D.P., Bergman, T.L., Lavine, A.S., "Fundamentals of Heat andMass Transfer", John Wiley and Sons, 2007

8.


9.


10.

All equations have been solved used the software package EES (EngineeringEquation Solver). The equations used are outlined below.

Given,

Outer_Clay_Pot_Radius = 0.25 m

Inner_Clay_Pot_Radius = 0.185 m

Thickness = 0.015 m

Height = 0.45 m

Ambient Temperature = 308 K

Ambient Pressure = 101.3 kPa

Wind Speed = 2.5 m/s

Relative Humidity = 0.3

Permeability Correction Factor = 0.3

Fluid Properties,

Density of Air acquired as a function of T and P


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Viscosity of Air acquired as function of T

Density of Water acquired as a function of T and P

Density of Water Vapour acquired as a function of T, P and RH

Latent Heat of Vaporization for Water = 2270 kJ/kg

Diffusion Coefficient of Water into Air (D) =

2 (Curve Fit by Boltz and Tuve 1976)

Manipulations,

Area: 2

Reynolds_Number:

where,

(Lchar for exposed sand + Lchar for outer claypot)

(Length of exposed sand along diameter of device+ Maximum circumference of device)

Schimdt_Number:

Avg_Sherwood_Number(Laminar):

Avg_Sherwood_Number(Turbulent):

Coeff_MassTransfer: (Employing

Heat and Mass Transfer Analogy)


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