The botijo is one of the most traditional Spanish symbols. Its operation only needs water, clay and a dry and hot climate, it is based on a principle as old as it is effective: the jug sweats to cool the water inside.
In the 90s, Gabriel Pinto quantified –together with his partner José Ignacio Zubizarreta– the ability of the jug to lower the temperature of the water it contains, using theoretical and experimental models. Both were professors in the department of chemical engineering at the Polytechnic University of Madrid. Now, his student Carla Ortiz has studied its application in a vessel calledpot in pot which is used in Africa to preserve vegetables. It even refrigerates the insulin vials.
"We did not intend to discover or patent anything," Pinto explains to Sinc in his laboratory at the university. “Our study was published in a journal for the teaching of chemical engineering and what we wanted was to offer a didactic tool. The cooling of the water in the jug - by transfer of heat and mass - is a subject of study in chemical engineering ”.
But this system is also known in Nigeria, the inventor Mohammed Abba applied evaporative cooling - the same thermodynamic principle - to create the'Pot-in-pot'. It consists of inserting a clay pot into a larger one, insulated one from the other with wet sand and covering them with a damp cloth. In Arabic, the invention is called‘Zeer’.
As the sand that separates the vessels is wet, the water tends to escape through the pores of the vessel in contact with the dry air –as in the jug– and through its evaporation it cools the interior.
The girls go to school thanks to the jug
Unable to preserve food, Nigerian girls were forced to harvest fresh vegetables every day to contribute to the family economy. Now with the jug they have time to go to school.
"This type of spinach only lasted one day fresh, due to the high temperatures. Inside the 'pot-in-pot' it lasted several days. Ingenuity even makes it possible to keep insulin vials, ”explains Pinto.
For Carla Ortiz, her work was oriented for academic purposes. The study lasted two months and in its experimental phase compared the state of food inside and outside the device for two weeks. Outside, the temperature exceeded 40 degrees, –those of that hot July 2017– but inside the 'pot-in-pot' it was about 20 degrees.
After eight days, the vegetables that were inside the pot-in-pot were in good condition for consumption. Those that were left out of the system like spinach, carrots, tomatoes and eggplants had been spoiled.
The 'pot-in-pot' and the jug share a sustainable refrigeration system, which does not need electricity. All you need is water, clay, sand and a suitable climate: hot and dry.
The complex mechanism
Until 25 years ago there was no scientific study to explain why the jug can lower the temperature of the water in the strong summer heat and to what extent it is capable of doing so. Gabriel Pinto, who is part of the Educational Innovation Group for Chemistry Didactics of his faculty, demonstrated it.
The jug, a clay pot with handles and a spout, is perfect for someone who is studying thermodynamics in college. It is an example of evaporation with which you can learn concepts such as the so-called wet bulb temperature, a parameter that depends on the temperature of the air, its relative humidity and atmospheric pressure. "Their resolution is not easy: they are differential equations that must be solved by numerical methods", explains Pinto.
“In this thermal balance, on the one hand, the water is cooling because it evaporates, but it is also heating up - due to the heat of the air - where the jug is dry. On the one hand the water is heated and on the other it cools ”.
The jug in the world
As it depends on the humidity of the external air, the jug does not work the same in all places on the planet. If the heat is humid, the jug does not cool. For this reason, this instrument is not well known in other countries.
"The Mediterranean climate is an exception in the world because it is characterized by a dry summer." To find out where the botijo mechanism could work, Professor Andrés Martínez de Azagra, from the University of Valladolid, prepared a map forJournal of Maps with countries with potentially suitable conditions for its operation.
“That is why this type of cooling mechanism is known in countries like Tunisia or Morocco. Also, someone who lives in London or Paris doesn't need a jug either because summers aren't that hot. The essence of the jug is that someone who was mowing at 40 degrees, in the middle of summer, could conserve water at around 20 degrees ”.
A jug applied to irrigation
Andrés Martínez de Azagra, Professor of Hydraulics and Hydrology at the University of Valladolid, has also studied the operation of the jug. But his focus has not been on cooling, but rather on the transfer of water through the porous clay for use in irrigation.
In ancient times, the Chinese and the Romans already buried clay pots filled with water near some crops to irrigate little by little. Azagra has been based on this principle to patent theoozing bottle. A buried ‘plastic jug’, used for watering.
This innovation would serve as a backup irrigation for young woody plants affected by drought. "It would be like a kind of bottle for the plant, which very slowly distributes a liter of water in a month," Azagra explains to SINC. This also ensures efficient use of the water resource.
In addition, the oozing bottle releases liquid when the ground is dry, but when it rains it can also collect the water autonomously. And so it is not necessary to fill it.