Anyone who has ever sweated on a hot summer day understands the principles and critical value of evaporative cooling. When we overheat, our bodies produce sweat droplets, which evaporate due to a dry breeze or a nearby fan, absorbing heat in the process and creating a pleasant cooling sensation.
The same scientific principle, evaporative cooling, could transform the way fruits and vegetables grown on smallholder farms are preserved, where the wilting dry heat can quickly spoil freshly harvested produce. If those freshly picked red peppers and leafy greens are not consumed within a short period of time, or quickly transferred to refrigeration (or at least cold storage), much of it will likely go to waste.
Now, Professor Leon Glicksman of the Architectural Technology Program in MIT’s Department of Architecture and Eric Verploegen, a research engineer at MIT’s D-Lab, have released their open-source design of a forced-air evaporative cooling chamber that can be built into used vessels. and powered by grid electricity or built-in solar panels. The chamber, which can accommodate 168 produce crates, holds great promise for small farmers in hot, dry climates who need an affordable way to quickly lower the temperature of freshly harvested fruit and vegetables to ensure their freshness.
“Delicate fruits and vegetables are most susceptible to spoilage if picked during the day,” said Verploegen, a long-time proponent of using evaporative cooling to reduce postharvest waste. “If cold storage is not feasible or affordable,” he continued, “evaporative cooling can make a huge difference to farmers and the communities they feed.”
Verploegen has made evaporative cooling the focus of his work since 2016, initially focusing on small evaporative cooling “Zeer” tanks, typically with a capacity of between 10 and 100 liters, ideal for domestic use, as well as the larger double brick walls known as The cooling chamber, which is a Zero Energy Cooling Chamber or ZECC, can store 6 to 16 vegetable boxes at a time. These designs rely on passive airflow. The newly released forced air evaporative cooling chamber design differs from these two more modest designs by an active airflow system and a significantly larger capacity.
In 2019, Verploegen turned its attention to the idea of building larger evaporative cooling chambers and teamed up with Glicksman to explore using forced airflow instead of passive airflow to cool fruits and vegetables. After studying existing cold storage options and conducting user research with Kenyan farmers, they came up with the idea of using active evaporative cooling and used shipping containers as cold storage structures. As the Covid-19 pandemic intensified in 2020, they procured a second-hand 10-foot shipping container, installed it in a courtyard area outside D-Lab near Village Street, and began working on a forced-air evaporatively cooled prototype chamber.
Here’s how it works: An industrial fan draws hot, dry air into the room, and the air passes through a porous wet pad. The resulting cool, moist air is then forced through bins of fruit and vegetables stored indoors. The air is then directed through the raised floor and into a channel between the insulation and the outer wall of the container, where it flows to vents near the top of the side walls.
Leon Glicksman, a professor of building technology and mechanical engineering, used previous research on natural ventilation and airflow in buildings to propose a vertical forced-air design model for rooms. “The key to the design is tight control of the intensity of the airflow and its direction,” he said. “The intensity of the airflow directly across the fruit and vegetable bins and the airflow path itself make the system work well. The design promotes rapid cooling of harvests picked directly from the field.”
In addition to its novel and efficient airflow system, the forced air evaporative cooling chamber is representative of D-Lab’s reputation for working in resource-poor and off-grid communities: developing low-cost and low-carbon-footprint technologies with partners. Evaporative cooling is no exception. Whether connected to the grid or running through solar panels, forced air rooms consume only a quarter of the power of refrigerated rooms. And, because the chamber is designed to be built out of second-hand shipping containers that are ubiquitous around the world, the project is a great example of upcycling.
pilot design
As with earlier surveys, Welplogen, Glicksman and their colleagues worked closely with farmers and community members. For the forced air system, the team worked with community partners who needed better cooling and storage conditions for their products in climates where evaporative cooling is most effective. Two partners, one in Kenya and one in India, each built a laboratory to test and inform the process in parallel with ongoing work at MIT.
In Kenya, where smallholder farms produce 63% of food consumed and more than 50% of smallholder agricultural products are lost after harvest, they partnered with Solar Freeze, a cold storage company based in Kibwezi, Kenya. Solar Freeze founder Dysmus Kisilu, a 2019 MIT D-Lab Scale-Ups Fellow, built an off-grid forced-air evaporative cooling chamber at a produce market between Nairobi and Mombasa at a cost of $15,000, powered by solar energy Powered by photovoltaic panels. “The chamber is providing a safety net to prevent the huge post-harvest losses previously suffered by local smallholder farmers,” commented Peter Mumo, an entrepreneur and local politician overseeing the construction of solar-powered freezers in Makueni County, Kenya.
As much as 30% of the fruits and vegetables produced in India are wasted every year due to insufficient cold storage capacity, lack of cold storage near farms, poor transportation infrastructure and other shortcomings in the cold chain. Although climates vary across the subcontinent, the hot desert climates there (such as in Bhuj, where the Hunnarshala Foundation is headquartered) are ideal for evaporative cooling. Hunnarshala was contracted to build a grid-tied system at an organic farm near Bhuj for $8,100. “We have achieved very encouraging results,” said Mahavir Acharya, executive director of the Hunnarshala Foundation. “In midsummer, the temperature is as high as 42 degrees [Celsius] We can reach 26 degrees [Celsius] The internal humidity is as high as 95%, which is indeed a good condition for keeping vegetables fresh for three, four, five or six days.In winter we tested [and saw temperatures reduced from] 35 degrees to 24 degrees [Celsius], and the quality has been very good for 7 consecutive days. “
speak out
With the proof of concept and pilot refined, the next step is to spread the word.
“We are continuing to test and optimize the system in Kenya and India and in our test chambers at MIT,” Verploegen said. “We will continue to conduct pilots with users and deploy with farmers and suppliers to collect data on thermal performance, shelf life of fruits and vegetables indoors and how using the technology affects users. And we also hope to work with people who may want to work on Collaborate with suppliers across the horticulture value chain to establish this or other cold storage, such as farmers’ cooperatives, individual farmers and local governments.”
To reach the widest possible audience, Verploegen and team chose not to file for a patent and instead set up a website to disseminate the open-source design and provide detailed instructions on how to build a forced-air evaporative cooling chamber. In addition to extensive printed documentation, complete with detailed CAD drawings and videos, the team also produced instructional videos.
As co-principal investigator in the early stages of the project, MIT mechanical engineering professor Dan Frey contributed to the market research phase of the project and the initial conception of the chamber design. “These forced air evaporative cooling chambers have huge potential, and the open source approach was an excellent choice for this project,” Frey said. “The release of this design is an important milestone on the road to achieving positive impact.”
Research and design of the forced air evaporative cooling chamber was supported by the Abdul Latif Jameel Water and Food Systems Laboratory through an India Grant, Seed Grant, and Solutions Grant.