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Evaporative cooling is the process by which the temperature of a substance is reduced due to the cooling effect from the evaporation of water. The conversion of sensible heat to latent heat causes a decrease in the ambient temperature as water is evaporated providing useful cooling. This cooling effect has been used on various scales from small space cooling to large industrial applications.
- 1 Introduction
- 2 History
- 3 Engineering Theory
- 4 Operation
- 5 Evaporative Cooler Design
- 6 Dissemination
- 7 Usage and Performance
- 8 Improvements
- 9 References
Evaporative cooling differs from common air conditioning and refrigeration technologies in that it can provide effective cooling without the need for an external energy source. Effective cooling can be accomplished by simply wetting a surface and allowing the water to evaporative. Humans can most notably feel this effect as the body is cooled by evaporating sweat from the skin during physical exertion. This simple form of evaporative cooling is the basis for more mechanized and complex evaporative cooling systems.
The concept of using water for air cooling has been around for millennia. While complex evaporative cooling processes and machines have been invented, the basic concept has not changed. Egyptian frescoes portraying large, porous jars of water being fanned to force evaporation and subsequent cooling have shown the prevalence of evaporative cooling since ancient times.
In evaporative cooling, the surrounding air is used as a heat sink where sensible heat is exchanged for latent heat of water .
Latent and Sensible Heat
The fundamental governing process of evaporative cooling is heat and mass transfer due to the evaporation of water. This process is based on the conversion of sensible heat into latent heat. Sensible heat is heat associated with a change in temperature. While changes in sensible heat affect temperature, it does not change the physical state of water. Conversely, latent heat transfer only changes the physical state of a substance by evaporation or condensation .
As water evaporates, it changes from liquid to vapor. This change of phase requires latent heat to be absorbed from the surrounding air and the remaining liquid water . As a result, the air temperature decreases and the relative humidity of the air increases. The maximum cooling that can be achieved is a reduction in air temperature to the wet-bulb temperature (WBT) at which point the air would be completely saturated .
Conditions Affecting Cooling
The cooling effect of an evaporative cooler depends on the rate of evaporation and on conditions within the cooler. Environmentally, temperature and humidity have the most significant impact on evaporation. The temperature must be high enough to allow for evaporation and the relative humidity must be low enough to allow for more water vapor to enter the air. Water quality also has an important role in both evaporation and keeping the system well maintained . Evaporative coolers can only provide cooling down to 10C under optimal conditions. Lower temperatures require different cooling technologies .
Evaporation vs. Boiling
Evaporation differs from boiling and can be accomplished at a temperature lower than the boiling temperature of water because it occurs at the liquid-vapor interface. Evaporation occurs when the vapor pressure of water is lower than the saturation pressure of water. Conversely, boiling occurs when a liquid contacts a surface at a temperature at or above the saturation temperature. This solid-liquid interaction causes vapor bubbles to form and rise to the free service of the water .
The basic operation of an evaporative cooler is to allow air to flow across a wet surface, thus causing evaporation and cooling. To be effective, evaporative coolers need to be placed in areas of adequate air flow.
Evaporative Cooler Design
Direct vs. Indirect
Evaporative cooling technologies fall into one of two major categories: direct or indirect. Direct evaporative cooling occurs when air has direct contact with water. Indirect evaporative coolers utilize a heat exchanger, therefore the air never comes into direct contact with the cooling water . For developing countries, direct evaporative coolers have the highest applicability due to less complicated design and construction than indirect coolers.
Types of Evaporative Coolers
Evaporative cooler designs vary based on absorbent medium, storage chamber construction, method of evaporation. Many different evaporative cooler designs exist in both unpowered and externally powered forms. Evaporative coolers that need no external power are designed to allow for natural airflow to provide the convection needed for adequate cooling. Various designs within this group allow for heat transfer in slightly different ways. While all coolers rely on converting sensible heat to latent heat, the mechanism for this conversion can be different.
Some evaporative cooler designs are created using porous materials to allow water to seep from an inside container to an outer surface where evaporation occurs. For these designs, evaporation is aided by drawing heat from the inner cooling chamber. As warmer water molecules evaporate and leave the outer surface, the surface cools and a heat gradient causes heat to diffuse from the inner chamber out to the surface . As a result, the temperature within the inner chamber decreases causing a cooling effect on whatever is stored in the cooler.
A second type of evaporative cooler operation relies on airflow through a wetted pad to provide cooling. In this design, an absorbent pad forms part of the wall of a cooling chamber and air, either through natural airflow or forced flow from a powered fan, passes through the wetted pad. Water is evaporated from the pad into the air and causes a drop in the air temperature. This air then flows through the cooling chamber to provide cooling.
One of the most notable evaporative coolers is the “pot-in-pot” cooler due to its simplicity and effectiveness. The pot-in-pot evaporative cooler design is a simple passive cooler designed by Mohammed Bah Abba who received a Rolex Award for his innovation . The cooler consists of two concentric clay pots, one placed inside the other with a buffer layer of sand between them. The sand acts as a wetted medium that holds water needed for cooling. A wet cloth is placed over the top of the pots to aid cooling. Also called a “Zeer pot” due to the name of the pots used in some parts of Africa, the pot-in-pot system has been tested for food preservation by the organization Practical Action and has shown to significantly increase produce storage life, in some cases up to five times as long as for certain vegetables. The pot-in-pot cooler has an approximate capacity of 12kg of vegetables and is manufactured for US$2. While very effective, the pot-in-pot cooler is limited in storage volume by the size of the pots used.
Many other passive cooler designs exist in addition to the pot-in-pot design. Developed by the Food and Nutrition Board of India, the Janata cooler is similar to the pot-in-pot cooler in that it uses ceramic pots. The Janata uses a storage pot placed in a large bowl of water with a cloth placed over the pot and dipped into the bowl to soak up water which is then evaporated to provide cooling. A charcoal cooler consists of a wooden frame holding two layers of wire mesh with a gap between them that encloses charcoal pieces which act as the absorbent. An Almirah cooler is made from a wooden frame with cloth enclosing the frame to create a storage chamber. The cloth is dipped into a tray of water to absorb water to provide a cooling surface for evaporation.
Unpowered static cooling systems, often called Evaporative Cooling Chambers (or ECCs), provide a larger chamber for increased storage. These systems are generally designed with a double wall chamber made of bricks. The gap between the walls varies with multiple designs using a gap between 3-5 inches. The gap is filled with sand like the pot-in-pot cooler. The sand is either kept moist by manually wetting it or through the use of a drip hose connected to a water reservoir. In the cooling chamber, food is stored, usually separated into different trays .
Evaporative coolers can be designed using an externally powered fan to provide airflow and a pump to provide continuous wetting. These systems are constructed differently than unpowered systems in that they use a cooling pad in place of an absorbent medium and do not rely on water flowing through a porous structure for evaporation. Instead, a fan forces air through a wetted absorbent pad from which water is evaporated thus cooling the air. The cooled air then travels through the cooling chamber to reduce the storage temperature.
In these systems, the absorbent pad plays a major role in the cooling process. Many factors affect pad performance including pad material, pad thickness, and size of perforations . Current commercial pads are often made of cellulose, plastic, or fiberglass which can be expensive and are not made of locally available materials  . Investigation has been done into using local materials to make effective cooling pads for evaporative cooling systems. One criteria for cooling pad evaluation is cooling efficiency calculated by:
where is the change in actual temperature, and are the dry bulb and wet bulb temperatures respectively. Experiments with palash and coconut fibers show comparable performance with aspen and khus pads with improved effectiveness in some situations . Luffa fibers were determined to be a suitable material for evaporative pads based on performance and slower degradation than other materials . Volcanic tuff and pumic were compared to commercial cellulose pads with volcanic tuff pads found to be a suitable alternative to the commercial pad at a specific air velocity . Straw and sliced wood pads have also been investigated for greenhouse use with sliced wood providing the best performance .
Other alternative evaporative cooling methods include fog or misting systems, which spray small diameter water droplets into the air for evaporation, and roof top evaporation systems which involve a thin water layer on the top of a building resulting in evaporation and cooling 
Evaporative cooling is not a modern concept and since it has been a known way of cooling since ancient times, it has naturally been adopted in many different ways. Even though it is not a proprietary concept, it is still not as widespread today as it could be to provide useful cooling.
Different organizations have been promoting evaporative cooling for the storage of produce, with Practical Action, formerly the Intermediate Technology Development Group, being an active promoter. Practical Action has published multiple technical briefs and case studies outlining the details of evaporative cooling and its usefulness. The have been involved in field testing pot-in-pot coolers in Sudan to demonstrate their performance and generate interest in their use..
Mohammed Bah Abba has also been active in promoting his pot-in-pot innovation. He personally has supplied thousands of coolers at his own expense to villages in Nigeria to increase their visibility. He has also created an education campaign targeted to uneducated populations to showcase the potential benefits of using a pot-in-pot cooler to improve food storage life .
Like any development project, education is an important component of disseminating evaporative cooling technologies and ensuring it is used properly. In one project in Gambia, the pot-in-pot cooler was successfully used to store vegetables, but was not working at peak performance due to villagers keeping the cooler in enclosed spaces. Since this limits the air flow and thus evaporation, the cooling effect was limited . Another project in India resulted in users using improper amounts of water in a pot-in-pot cooler first resulting in too little cooling which was overcompensated for by adding too much water which caused water to pool in the storage chamber.
Usage and Performance
Vegetable and fresh produce storage has proven to be a good application for evaporative cooling. High temperature is an important factor in produce preservation that can be combated by evaporative cooling . To lengthen storage life, fresh produce needs to be stored in conditions of high humidity to reduce water loss which evaporative cooling can also achieve .
One study of an evaporative cooling system in Ethiopia used an evaporative cooler to successfully increase the storage life of mangoes. The storage cooler was able to reduce the ambient temperature throughout the day from a range of 23-43C to 14.3-19.2C with an increase in relative humidity from 16-79% to 70-82.4%. On average, the temperature and relative humidity differences were 10.7C and 36.7% respectively. This caused the storage life of mangoes to double from 14 to 28 days with a 55% increase in the number of marketable mangoes. Another study in Ethiopia showed that a multi-pad evaporative cooler resulted in a 5C temperature drop and 18% relative humidity increase compared to a single pad cooler .
Multiple studies have been conducted regarding vegetable storage in Nigeria. One study used a cooler with coconut fiber as the absorbent in a cabinet shaped cooler. During a no-load test, the cooler was able to achieve cooling of 0.1C-8.3C. When the relative humidity was 80%, it was found that no cooling occurred. Using the cooler, pumpkins were stored 60 hours instead of 12 hours without cooling and tomatoes lasted for 93 hours compared to 32 hours . A different study in Nigeria constructed an evaporative cooler out of clay with one cooling pad and a reflective surface on the roof. The cooler reduced ambient temperatures from 32-40C to 24-29C throughout the day. This cooler was found to have a cooling capacity from 870-1207 W and was able to store tomatoes for 19 days . Similar studies have shown increases in storage life of 14 days over ambient storage conditions .
The study of a pot-in-pot cooler also had successful cooling results. During the hottest part of the day, the cooler was 15C cooler than the ambient temperature. The cooler also had smaller temperature fluctuations throughout the day compared to the ambient temperature. In this study, carrots were preserved for 40 days and bell peppers were kept for 25 days compared to 18 days without cooling .
The financial impact of evaporative cooling in produce preservation can be substantial. For example, in India where post-harvest losses can reach 30% annually, the use of an evaporative cooler can reduce food loss resulting in increased income for farmers and decrease the amount of money spent on food as it can be preserved longer . Certain economic evaluations have estimated a payback period of 1.2 years for an evaporative cooling system depending on the size and quantity of produce sold .
Other Agriculture Uses
Evaporative cooling has been used for other agricultural purposes besides food storage. In Thailand, an evaporative cooling system was created to reduce the temperature of a silkworm rearing house. Heat stress is believed to be a cause of low silkworm production, therefore cooling can be an important addition to the silkworm environment. Using a fan to force air through a wetted pad, a 6-13C drop in temperature was observed along with an increase of 30-40% relative humidity .
Potential improvements for evaporative cooling devices include improvements in materials used, mainly in absorbent pads and ceramic pots. Experimentation needs to be performed to identify more locally available materials that can successfully be used as absorbents.
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