From sun to ice

Publish date 06-11-2021

by Mauro Palombo

The specificity of the Technological Restitution activities is to study and test technological solutions as needed in areas of underdevelopment, as simple and accessible as possible both in terms of cost and of autonomous management. Over the years, the areas of work have been many: water - with various techniques to make it safe for consumption even if with bacteriological and / or chemical-physical contamination, portable photovoltaic lamps, hygiene - self-production of sodium hypochlorite, hydroponic agriculture for areas lacking farmland and water, and so on. Important elements of projects developed with the beneficiary communities.

Today, making use of the extensive skills of a group of friends, we are working to put a great deal of availability - the sun - at the service of a much less widespread possibility: refrigeration, and its use for the preservation and marketing of food. Therefore, the realization, at the lowest possible cost, of a refrigerator powered by photovoltaic energy; both vertical and horizontal. The photovoltaic power supply responds to the lack of access to the electricity grid in many areas; but also, where it exists, due to the discontinuity of its supply, a serious problem for a traditional fridge. On the other hand, a constant power supply from the generator is too expensive.

The solution adopted makes use of a set of significant technical innovations.
Refrigeration takes place with a refrigeration unit operated by a variable speed dc (direct current) compressor powered exclusively by a photovoltaic panel. The thermal accumulation of the cold is obtained with a new eutectic solution developed specifically: the heat exchange process will take place thanks to ice walls that completely cover the internal surface of the refrigerator. Each wall is made up of a plurality of rectangular pipes inside which water and a copper coil of eight millimeters in diameter are inserted; the coil is crossed by the refrigerant gas which causes the water to freeze. The thermostat immersed in the eutectic solution, specially developed to be calibrated with great precision, will control the operation of the refrigerator.
The ice walls thus accumulate the cold which will be returned uniformly to the cell. This system gives autonomy in the absence of available energy at night and on days when there will be a lack of sun, equal to about forty hours, even with external temperatures of 40 degrees.

The cooling of the internal compartment and of what it will contain will therefore take place at a temperature that will oscillate from -4 degrees to about +4 degrees.
The refrigerator will be insulated by a polyurethane structure (or derivatives) from 20 to 60 mm thick and a metal supporting structure.

To the innovative solutions are added the interface between photovoltaic panels and compressor which allows in case of low insolation to capture the little solar energy by charging, through a booster circuit, a pack of electric supercapacitors to a voltage level compatible with the power supply voltage of the compressor - to facilitate its starting -, before connecting to it.

Support awarded under the Cariplo Foundation's "Technologies for Sustainable Development" call for proposals accelerates the development of the project.
A first field test has already been carried out in Burkina Faso, at the Fada-N'Gourma Professional School of the Brothers of the Holy Family, actively involved in the development of the fridge; a first prototype made by adapting commercial refrigerators to the various technical solutions envisaged, gave good results even at intense local temperatures.
The on-site activity will continue with the sending and testing of other prototypes, made completely from scratch; with the aim of experimenting with local production as well, giving work to trained students, in order to propose this innovation to the market on a small scale.
Intense months await us ...

Mauro Palombo
NP June / July 2021

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