Smallholder farmers — and the communities they serve — suffer from a lack of effective postharvest storage

In hot and dry climates, freshly harvested fruits and vegetables need to be quickly cooled to avoid spoilage. When fruit and vegetables spoil, communities suffer from a lack of nutritious foods and farmers make less money. Without much margin for error, smallholder farmers and the communities that rely on them are particularly vulnerable when it comes to postharvest waste.

Now, researchers from MIT have introduced a forced-air evaporative cooling chamber design and released detailed documentation online — open-source and free to use by all. The chamber can be run from built-in solar panels or grid electricity and can accommodate 3,000 kilograms of produce. Importantly, where access to capital to invest in cold storage is scarce, the chamber can be built at half the cost of refrigerated cold rooms and uses only one-quarter of the energy to operate. Full-scale chambers have been constructed in Kenya and India to demonstrate performance with support from the Abdul Latif Jameel Water and Food Systems Lab.

How it works

Evaporative cooling is the key. Not only is it an affordable solution, but it critically provides a cool and humid environment that many fruits and vegetables thrive in. These systems are capable of providing rapid precooling of fruits and vegetables shortly after harvest in hot, dry regions. The chamber combines the energy-efficient process of evaporative cooling with a carefully designed airflow pathway to maximize the cooling rate.

In this system, ambient hot, dry air is pulled into the chamber and forced through a wetted pad, producing cool, humid air. The cool, humid air is then directed through stacks of vegetable crates inside the container – removing heat from the produce — and then vented out of the chamber. The pump to circulate water and fans for moving air can be powered by a solar photovoltaic (PV) and battery power system designed for off-grid applications.

Piloting the open-source design

Our team chose to make the designs publicly available to reach the widest audience and achieve the greatest impact. After the design was piloted in Kenya and India, we published the detailed design documentation on the website, How to Build a Fruit & Vegetable Cooling Chamber. This documentation includes dimensional design schematics; diagrams for the airflow, plumbing and electrical systems, along with a bill of materials; guidance for sourcing; and a recommended order of construction.

Evaporative cooling: An important addition to the cold storage sector

In the past five years, there have been significant advances in the development and deployment of refrigerated cold rooms for off-grid settings. Recent advancements, including the use of water or ice batteries for thermal storage, are particularly encouraging for improving efficiency and reducing the need for chemical batteries. Evaporation cooling has an important role to play as well. The key areas where evaporative cooling-based systems differ from refrigerated cold rooms include:

Cost

Relying on fans, a water pump and evaporative cooling pads — instead of expensive and energy-intensive compressors for mechanical refrigeration — the forced-air evaporative cooling chamber can be built at half the cost of a similarly sized refrigerated cold room. Furthermore, evaporative cooling systems require neither compressors nor refrigerants, thereby reducing the complexity and cost of the equipment and supplies, and requiring less technical expertise for maintenance and repair.

Cooling rate

One of the most critical and unaddressed stages in the postharvest supply chain for fruits and vegetables is the time immediately after harvest, when what is commonly referred to as “precooling” makes a significant difference in the shelf life of produce. An hour delay in leaving produce at field conditions, often 35 degrees Celsius, can lead to a loss in shelf life of about one day, even with optimal storage conditions later in the supply chain. The rapid cooling rates achievable with forced-air evaporative cooling have significant potential at the precooling stage, especially because this technology can be deployed near the farm gate, reaching produce shortly after harvest. Refrigerated cold rooms typically rely on room cooling through conduction and natural convection, resulting in significantly slower cooling rates. This process is less beneficial to freshly picked fruits and vegetables and, even where available and affordable, cold rooms are rarely situated close enough to farms to provide precooling.

Minimum temperature

The minimum temperature that can be achieved with evaporative cooling is highly dependent on the relative humidity. Lower relative humidity allows for more effective cooling, while higher humidity limits cooling potential. In hot and dry regions, temperature drops of greater than 10 degrees Celsius can be expected and are well-suited to keeping many fruits and vegetables fresh. Refrigerated cold rooms, on the other hand, can achieve temperatures sufficiently low to safely store dairy products, meat and certain medicines; refrigeration equipment and power supply must be available and affordable.

Humidity

Systems based on evaporative cooling generate cool and humid air, whereas refrigeration systems remove moisture from the air, creating a low-humidity environment. Most fruits and vegetables — including leafy greens, tomatoes, eggplants, okra, mangoes and melons — prefer high-humidity environments to avoid dehydration. In contrast, foods such as onions, garlic and cereal grains require low-humidity environments to avoid microbial and fungal growth. The ideal humidity of produce being stored should be considered when selecting a storage method. For most fruits and vegetables, the higher humidity environment of an evaporative cooling-based system is beneficial.

Scaling the technology

With 54 million tons of fruits and vegetables lost or wasted per year across sub-Saharan Africa, accounting for 52% of the total production, there is a huge opportunity to deploy cost-effective storage solutions. This type of large storage chamber is typically operated by a business, gathering revenue by charging customers a daily fee for storing crates. However, access to capital is currently a major challenge for many entrepreneurs working to scale technologies to improve cold chains across Africa. With lower upfront costs than a refrigerated cold room, the forced-air evaporative cooling chamber has the potential to be deployed more widely with less investment.

We are looking to raise awareness of this technology and build capacity for local businesses and technicians to build and operate these chambers. The website, How to Build a Fruit & Vegetable Cooling Chamber, provides extensive documentation for this chamber design and the CoolVeg team is available to partner with organizations interested in deploying this technology in suitable regions.

If you are interested in learning more or exploring partnerships to disseminate forced-air evaporative cooling chambers, please contact Eric Verploegen at [email protected] and Leon Glicksman at [email protected].