Evaporative Cooling Technology

Summary

Evaporative cooling devices function without the use of electricity and provide a solution for effective and affordable postharvest storage options in regions where electricity and electric cooling are not available or affordable. The Zero Energy Cooling Chamber (ZECC) and clay pot device can increase the shelf life of horticulture produce, and farmers using ZECCs in Rwanda reported shelf life improvements ranging from 2 to 4 times compared to ambient conditions.

Challenge/Problem

The horticulture sector plays a vital role in supporting human nutrition and income generation. A lack of affordable and effective postharvest fruit and vegetable storage solutions often leads to spoilage, loss of income, reduced access to nutritious foods, and significant amounts of time spent travelling to sell and purchase fresh produce (fruits and vegetables), particularly in rural communities. Postharvest losses for fruits and vegetables are between 30–40 percent in developing countries. Cold storage, or temperature management, is critical to preserve fresh fruits and vegetables for longer periods of time. Longer storage life can help with access to markets, receiving higher prices for products, and increasing access to fruits and vegetables for consumers.

Solution

The ZECC and clay pot cooler devices function on the principle of direct evaporative cooling, where heat is removed as water evaporates from the surface of the storage device. The evaporative cooling effect causes a decrease in temperature and an increase in the relative humidity inside the storage device, conditions that increase the shelf life of many fruits and vegetables. Water must be added at regular intervals to maintain the cooling effect. The watering frequency required can vary from several times a day to only a few times a week, depending on the storage device’s material and design as well as the weather conditions. However, the rate of evaporation of water is highly dependent on the ambient humidity. When the ambient humidity is higher, there is a less significant reduction of the interior temperature.
 

Optimal markets for the Zero Energy Cooling Chamber (ZECC) and the pot-in-pot technologies are in hot, dry locales with access to water. Additionally, farmers and households that already have clay pots in their homes as a conversion to an evaporative cooling unit is easy. Within these contexts, any stakeholder that needs cool storage to preserve the life of perishable produce could benefit from the use of the ZECC and pot-in-pot evaporative cooling units. 
 

This solution was developed under the Feed the Future Horticulture Innovation Lab, implemented by the University of California, Davis and in partnership with MIT D-Lab.

Results

- ZECCs and Pot-in-pot evaporative coolers work optimally in dry, hot conditions where there is access to water.

- In optimal conditions, there is roughly a 10 to 15 degree Celsius decrease in temperature compared to the open-air environment.

- Repurposing of clay pots already within homes to cool perishable items has proven to be a better scaling strategy, and commercial uptake of the ZECC has been minimal. Locations that have proven more successful are market centers where there is reliable access to water.

- Farmers using brick ZECCs in Rwanda reported shelf life improvements ranging from 2 to 4 times for specific vegetables compared to the shelf life in ambient conditions

- These devices provide increased humidity, decreased daily average temperatures, decreased peak daily temperatures, and improved temperature stability, all of which can lead to improved vegetable shelf life.

- Considering the costs of a powered, cold storage unit, evaporative cooling units such as the ZECC and the pot-in-pot can be a viable, lower-cost alternative for short-term storage of fruit and vegetable crops, providing more people access to diverse and nutritious diets.

Lessons Learned/Potential for replication

Hot and dry climates with access to needed materials who also would benefit from cool storage is the potential market size. This could be a very large number of individuals. Postharvest management of fruits and vegetables is an enormous challenge in numerous geographies. The pots are affordable and have a low barrier to entry as they already exist in many households. The ZECCs have had some success in other projects when they are built in market centers and cost is shared among vendors and there is access to consistent water supply. This may build better awareness of the importance of temperature (and humidity) and horticulture perishability. Even simple shade structures can make a difference, or harvesting at the coolest parts of the day. But, without this awareness, the adoption of cooling technologies like pot in pot or ZECCs may be hindered.

Next Steps

Connect potential users with existing users of evaporative cooling devices to share advice on construction, use, and cost of these devices. In addition to households and farmers, specifically, target the dissemination of evaporative cooling devices (ZECCs and clay pot coolers) to fruit and vegetable vendors and markets with permanent structures who suffer financial losses due to spoilage.