International Journal of Engineering Trends and Technology (IJETT) – Volume 23 Number 6- May 2015
An Investigative Review on Recent Developments in
Refrigeration by Evaporative Cooling
Prabodh Sai Dutt R#1, Thamme Gowda C.S.*2
#1
PG Student, Department of Mechanical Engineering, Vidya Vardhaka College of Engineering (VTU), Mysuru, India
* Asst. Professor, Department of Mechanical Engineering, Vidya Vardhaka College of Engineering (VTU), Mysuru, India
2
Abstract— Conventional Refrigerators has its own share of
problems pertaining to environmental safety, usage and
maintenance in rural parts of developing countries. Refrigeration
by evaporative cooling has proved to be a successful alternative
in providing inexpensive and environment friendly way to
preserve food and medicines. Though the concept of evaporative
cooling has been in existence from a long time, application of the
concept towards preservation of food, agricultural produce and
medicines has seen a surge only from past few years. This paper
makes an investigative review on a few of the recent
developmental designs towards refrigeration, both for
agricultural produce and domestic usage by evaporative cooling.
Keywords— Refrigeration, Evaporative Cooling, food, medicines,
agricultural produce, environment
I. INTRODUCTION
Refrigeration can be defined as the process of extraction of
heat from a specified space or object to be cooled and
maintained at a temperature lesser than the temperature of the
surrounding atmosphere/air. The most common and arguably
the important use of refrigeration is preservation of food.
After the discovery of bacteria, it became apparent that food
stored at lower temperatures last longer without losing its
nutrients value and hence more prominence was given to
developing good refrigerators. The other most visible use of
refrigeration is in the world of medicine, where preservation
of drugs, medicines etc are conveniently managed with the use
of refrigeration process.
Along with bringing significant benefits towards food
preservation, conventional refrigerators come with a number
of disadvantages, with reference to the environment.
Chlorofluorocarbon (CFC) which was earlier used as a
refrigerant was found to harm the ozone layer. It was replaced
by Perfluorocarbons(PFCs) and Hydro-fluorocarbons (HFCs).
But although these new refrigerants do not deplete the ozone
layer, the major complaint against their use is in assisting
global warming.
Conventional Refrigerators require electricity to run. Most
of the under developed countries and rural areas does not have
sufficient and in some cases even minimal access to electricity.
But preservation of food being an integral part of human life
sustenance, exploring alternatives to commercial refrigerators
has become necessary. These refrigerators need to be
inexpensive, easily available and should be environmentally
friendly. Refrigeration by evaporative cooling is found to be
successful in this regard. The working principle behind
ISSN: 2231-5381
evaporative cooling is described in section II, followed by
different factors affecting a evaporative cooling refrigerators
in section III. Description of construction and working of
different innovative designs in the field of Refrigeration by
evaporative cooling in recent times, mainly concentrated on
food and medicine preservation is discussed in section IV. The
conclusion is reported in section V.
II. PRINCIPLE OF EVAPORATIVE COOLING REFRIGERATION
The principle of refrigeration by evaporative cooling
involves the storage space covered by a single layer of porous
medium or a double layer of porous medium with evaporating
liquid in between the layer. The opening of the refrigerator is
either covered with a wet cloth or a porous lid. Fig. 1 shows
the working principle of an evaporative cooling refrigerator.
Fig. 1 Principle of Evaporative Cooling Refrigerator
When the refrigerator is subjected to the surrounding dry
air, the evaporating liquid (usually water) diffuses through the
porous medium, approaches the surface exposed and gets
evaporated into the surrounding air. The thermal energy
required for the evaporation is drawn from the storage space
thereby cooling the space intended for refrigeration.
The effectiveness of these sorts of refrigerators is best seen
when relative humidity of surrounding air is low and
temperature high. The porosity of medium surrounding the
storage space in these sorts of refrigerators should be such that
the rate of seepage of evaporating liquid through them should
be slightly higher than the rate of evaporation from the surface
http://www.ijettjournal.org
Page 289
International Journal of Engineering Trends and Technology (IJETT) – Volume 23 Number 6- May 2015
when the relative humidity is lowest. Increasing the porosity
will only result in wastage of evaporating liquid [1].
III. PARAMETERS AFFECTING EVAPORATIVE REFRIGERATION
The
different
parameters
affecting
evaporative
refrigeration are
A. Ambient Temperature
The ambient temperature plays an important role in
evaporative cooling. The higher the temperature higher will be
the rate of evaporation, ensuring better cooling of the storage
space.
B. Ambient Relative Humidity
Relative humidity is the measure of water content already
present in air. When the relative humidity of air surrounding
the refrigerator is less, more water can be evaporated and
hence cooling will be better. Theoretically it is found that with
decrease in ambient relative humidity for a given temperature,
the temperature of the storage space decreases [2].
Many of the designs have been built using clay as the
porous medium. In all these designs water due to its easy
availability, is used as the evaporating liquid. The different
designs that will be looked into are as follows:
A. Zeer Pot or Pot-in-Pot Technique
B. Tin-in-Pot Technique
C. Eco-Fridge
D. Mitti Cool Refrigerator
E. Evaptainer
A. Zeer Pot or Pot-in-Pot Technique
Mohammed Bah Abba invented this technique in 1995
mainly for the poor farmers to preserve their produce. It was
sold freely initially and full commercial production was only
started in the year 2000. The cost of these pots varies between
2 USD to 4 USD. [6]
C. Geometry and Surface Area of Refrigerator
The amount of evaporation taking place is directly
proportional to area available for evaporation. Hence increase
in the area for evaporation around the storage space enhances
the cooling process.
D. Thermal conductivity of Material used to hold water
The material used to hold water between the pots should
have as low a thermal conductivity as possible to obtain lesser
temperature in the storage space for a given ambient
temperature and relative humidity. Theoretical as well as
experimental studies have also confirmed that the material
used should be of low thermal conductivity [2]-[4].
E. Thermal conductivity of material exposed to evaporation
The material in contact with the ambient air should have
lower thermal conductivity as well. But theoretical results
have shown that though the temperature of storage space
decreases with decrease in thermal conductivity, the effect is
minimal [2].
F. Hydraulic Conductivity of material
Hydraulic Conductivity of porous material in contact with
ambient air should be higher, but not too high to allow
seepage. Theoretical analysis shows that the effect of
hydraulic conductivity is also minimal on the cold temperature
achieved at storage space [2].
G. Movement of air
The movement of air enhances the evaporation process
and hence the cooling. Experimental results show that the
temperature of storage space decreases with increase in wind
speed for a given temperature and humidity [5].
IV. RECENT INNOVATIVE DESIGNS
ISSN: 2231-5381
Fig. 2: Zeer Pot or the Pot-in-Pot Technique
This process involves usage of two porous earthenware
pots of different sizes. The smaller pot is placed inside the
larger pot and the space between the pots is filled with sand.
The sand is kept constantly wet. Sand acts both as a medium
to hold water required for evaporation and also to provide
insulation to the inner pot from the outside temperature. The
opening is covered with a wet cloth. The water diffuses
through the sand and permeates through the outer pot. Once it
reaches the pot surface it gets evaporated, thereby cooling the
inner pot.
Studies have shown that vegetables and fruits with shelf
lives ranging between 1-4 days can be effectively stored for
nearly 20 days by this technique [7]. The advantages of this
technique is that the all the materials required for fabrication
are easily available and are inexpensive to the rural masses.
The pot-in-pot has limitation in size. This affects quantity of
food products that can be stored. Fruits and vegetables used
for domestic purposes can be effectively stored but large
amount of agricultural produce cannot be stored. Also there
are no separate compartments to store different products
separately. All food products need to be dumped together. The
assembly is heavy and susceptible to breakages.
http://www.ijettjournal.org
Page 290
International Journal of Engineering Trends and Technology (IJETT) – Volume 23 Number 6- May 2015
B. Tin-in-Pot Technique
This technique is essentially the same as pot-in-pot
technique. But in this method the smaller clay pot used in the
pot-in-pot technique is replaced with a tin pot.
The experimental comparison between the pot-in-pot and
tin-in-pot technique using mangoes has suggested that the tinin-pot technique retains the freshness of mangoes better than
pot-in-pot for initial few days. Later on the fruits were
observed to react with the tin and this led to the deterioration.
It was also observed that the relative humidity of the storage
space was more in tin-in-pot [8].
Again the advantage of this type of refrigeration technique
is that the material required as well as the fabrication process
is easy and inexpensive. This method can be effectively used
when storage of food products is not required for more than 24 days. But again when the agriculture produce is large,
storage becomes difficult. It is best suited for small
agricultural produce and domestic usage. Care should be taken
by painting or any other means to prevent deterioration of tin
due to its reaction with water present in the sand poured
between tin box and outer pot. The outer clay pot is
susceptible to breakage.
C. Eco-Fridge
Emily Cummins, an inventor from UK came up with this
innovative idea. Fig 5.3 shows construction a portable EcoFridge
that this eco-fridge was able to provide constant 6ºC of the
storage space for several days and it can be used to preserve
milk, meat and other food products [9].
This design is best suitable for refrigeration of medicines
since the design is portable. Medicines can be effectively
transferred from one place to another. The lid (not shown in
Fig. 3] can made by welding circular disc to a metal rod
having hook on one end to facilitate easy carriage. By
providing more circular discs below the lid, separate
compartments can be created. The storage space is lesser than
pot-in-pot and tin-in-pot. The material required holding water
for evaporation though easily available, the material and tools
required to create outer and inner cylinder are expensive and
need expertise compared to pot-in-pot technique.
D. Mitti Cool Refrigerator
Mitti Cool Refrigerator is the invention of Mansukhbhai
Prajapati. It is a relatively expensive refrigerator costing about
40 USD. But the advantage of not requiring any power
outweighs the benefits of commercial refrigerators. Its design
is completely different from the designs discussed till now.
It resembles the modern electrical refrigerator at first glance.
But it is smaller in size. It has separate provisions for storing
water and food. The upper chamber is used to store drinking
water and the bottom chamber is provided to store fruits and
vegetables. Usually a top is provided to draw drinking water.
The food storage chamber is covered with a glass pane. The
refrigerator is available in different options such as without
and with paint finish. Fig 4 shows construction of a mitti cool
refrigerator.
Fig. 3: Eco-Fridge
It can be seen that its construction is similar to Zeer Pot
technique of refrigeration. But the material here is not of
earthenware clay. The outer cylinder material can be of any
solid material like wood, plastic etc. The outer cylinder has
many holes in it to facilitate evaporation of water. The inner
cylinder is made of metal and holes are absent. The top of the
inner cylinder is closed with a cap. The space in between the
cylinder can be filled with any locally available material like
sand, wool or soil which can hold water for evaporation. The
wetness of the material is to be ensured. It has been claimed
ISSN: 2231-5381
Fig. 4: Construction of a Mitti Cool Refrigerator
The main constituent of a Mitti Cool refrigerator is clay
[10]. There is no recurring cost in this refrigerator as there is
no usage of electricity or any other power source. The water
from the top chamber permeates through the walls
surrounding food storage chamber and when these water
particles evaporate it brings about cooling of the storage
chambers. It is claimed that this type of refrigerators can store
fruits and vegetables for 6-7 days without causing any
deterioration [11].
http://www.ijettjournal.org
Page 291
International Journal of Engineering Trends and Technology (IJETT) – Volume 23 Number 6- May 2015
The clear advantage with this design is that in the same
refrigerator both drinking water and food can be stored. Also,
usage of sand and other materials to hold water for
evaporation is not required. Unlike Zeer Pot, the food storage
chamber has two compartments to store fruits and vegetables
separately. It is aesthetically good. The fabrication though is
not as easy as Zeer pot. Also preservation is only for 6-7 days
which is much less than the Zeer pot technique of food
preservation. Also it is relatively expensive.
E. Evaptainer
This design invented by Quang Truong and Spencer Taylor,
is in the final phases of rolling out as a commercial
refrigerator. The design is very innovative. The Fig. 5 shows
an evaptainer. (Image source from Evaptainer website)
The material required and fabrication process is complex
compared to clay refrigerators.
V. CONCLUSIONS
The review suggests that there is considerable scope for
research, improvement and innovation in the area of
refrigeration by evaporative cooling. The Zeer Pot design is
found to be the basic design on which most of the
improvements and innovations are being made. The „Tin-inPot‟, „Eco-Fridge‟ and „Evaptainer‟ concepts suggest that clay
pots, susceptible to breakages can be conveniently replaced
with other materials to meet the demand of working
conditions.
The „Mitti Cool‟ concept provides idea towards developing
models that are as aesthetically good looking as the
commercial refrigerators. Improvisation of the design to
accommodate more food and water might prove helpful to the
farmers. Increasing the efficiency by testing different
materials instead of clay might provide an opportunity to
replace most of the electrically driven domestic refrigerators.
If this is achieved, then health and standard of living of
developing and underdeveloped countries will increase
substantially and also result in a positive step towards greener
environment.
REFERENCES
[1]
[2]
[3]
Fig. 5: Evaptainer
[4]
The construction of an evaptainer consists of two separate
chambers. One is used for storing water required for
evaporation and the other for storing food [12]. The highly
conducive aluminium plates are run along the walls of the
storage space. These plates are connected to special fabric
which is always kept wet. The aluminium plates draw heat
from the inside and the evaporation of water from fabric keeps
the storage space cooler [13].
This cooler is primarily designed to assist farmers in Africa
to store their produce effectively before releasing them into
the market. The unit is capable of holding 60 litres of produce
which is equivalent to 150 large tomatoes. It requires six litres
of water and keeps the food cold for 12 hours. The design is
rugged and capable of withstanding small impacts. It is
estimated to cost around 10 – 20 USD [13].
This refrigerator is clearly designed more towards
preservation of agricultural produce than refrigeration for
domestic purpose. The rugged design to resist breakage unlike
clay refrigerators is an added advantage and makes it portable.
ISSN: 2231-5381
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
J.A. Olorunmaiye, “Evaporative Cooling of Earthern Pots in Quiscent
Air”, NSE Technical Transactions, vol. 31, pp. 80-91, 1996
A.W.Date, “Heat and Mass transfer analysis of a clay-pot refrigerator”,
International Journal of Heat and Mass Transfer, vol. 55, pp. 39773983, 2012.
Prabodh Sai Dutt R, Thamme Gowda C.S. “Experimental Study of
Alternatives to Sand in Zeer Pot Refrigeration Technique”,
International Journal of Modern Engineering Research, vol 5, pp. 1-7,
May 2015.
M.M. Kulkarni, K.N. Vijaykumar, N.A. Jadhav, M.J. Bhor, S.S. Shinde,
“Experimental Performance Evaluation of new cooling pad material for
direct evaporating cooling for Pune summer conditions”, International
Journal of Engineering Trends and Technology (IJETT), vol 22, pp.
281-287 April 2015
Harish H.G., Y.T. Krishne Gowda, “Thermal Analysis of Clay Pot in
Pot Refrigerator”, International Journal of Modern Engineering
Research, vol 4, pp. 50-55, Nov 2014.
Oluwasola, Oluwemimo, “Pot-in-pot Enterprise: Fridge for the Poor,
GIM Case Study No. B080.” United Nations Development Programme,
New York, 2011.
Longmone, A.P. 2003. “Evaporative Cooling of food Products by
Vacuum”.,Food Trade Review, (Pennwalt Ltd), pp. 47.
12. Kamaldeen O. S, Anugwom Uzoma, Olyemi F.F and Awagu E.F;
“Effect of NSPRI tin-in-pot compared with pot-in-pot evaporative
cooler on the stored fruits”. International Journal of Engineering and
Technology, vol 2 (1), pp. 63-69, 2013.
Matt Dillon, “Emily‟s Cool Idea”, Ecolibrium, pp.24-25, Dec. 2009
(2015)
The
Mitticool
website.
[Online].
Available:
http://www.mitticool.in
(2015) Case study home page on GIAN West. [Online]. Available:
http://west.gian.org/case_study.php
(2015)
The
Evaptainer
website.
[Online].
Available:
http://www.evaptainers.com
(2015) Evaptainer – Cooler page on CNNMoney. [Online]. Available:
http://money.cnn.com/2015/04/07/smallbusiness/evaptainercooler/index.html?iid=HP_LN
http://www.ijettjournal.org
Page 292