The chamber in pieces
The walls of the chamber comprise two layers of bricks separated by a 7 cm gap filled with moistened sand. During day, the sand in between the bricks is heated leading to evaporation of the water therein and as this water evaporates, there is a cooling effect on the sand. (The escaping water vapour carries away latent heat from the sand). Inside the chamber, the temperature is higher than in the cooled sand due to the respiration of the harvested plant material.
So, when the temperature of the sand is lowered, a temperature gradient is created between the sand and the interior of the cooling chamber resulting in a net heat flow from the interior of the cooling chamber to the sand (heat flow from a region of higher temperature to a region of lower temperature) so that thermal equilibrium is established and as a result, the produce temperature reduces as it loses heat to its cooler surrounding air.
This is the same feeling you get when you have a bout of sweating during an intense physical activity followed by a rest. The water in your sweat vaporises when the body is in contact with hot air and you immediately feel cold because as the water leaves your skin, it carries energy with it (from your body) and this lowers your body temperature.
The chamber is equipped with a fan, air inlet and outlet chimneys and a lid. The fan, which can be solar powered or manually operated aids to keep the inside conditions of the chamber homogenised (air, temperature and humidity) so that all the produce is uniformly cooled.
The chimneys help to replenish the air in the chamber. Harvested plant materials especially fruits continue to respire, producing carbon dioxide gas. In addition, they also produce ethylene gas and the two gases speed up the senescence process. Therefore, the chimneys serve to exhaust and replenish the atmosphere of the chamber. To prevent rise in temperature while replenishing the atmosphere of the chamber, the inlet air is passed through a moistened mass of charcoal which serves to cool the inlet air.
Covering the chamber is a lid, equipped with a wire mesh to keep out rodents and other physical contaminants.