|CERES No. 121 (FAO Ceres, 1988, 50 p.)|
To make cold using heat: that is the challenge that a French company has accepted. It has perfected and manufactured a refrigerator with no moving parts, which needs no maintenance, and which makes ice using the heat of the sun as its only source of energy.
This simple-looking "product" is actually the result of several years of work on the phenomenon of adsorption, the principle, in physics, of the taking up of a gas or a liquid at the surface of a solid. The refrigerator has been nicknamed "Gaspard", which stands for (in French) Solar Adsorption Generator for the Independent Production of Soft Refrigeration.
Seen from outside, Gaspard resembles an ordinary refrigerator with a solar heat collector on top. But there is nothing ordinary about its innards. There is no electric motor or compressor to produce the cycle of alternating liquid and gas phases of a fluid that produces cold by evaporation. Instead the sun does all the work. The alternation of evaporation and condensation follows the rhythm of day and night between a condenser placed under the solar collector and an evaporator in the body of the refrigerator
The condenser contains activated carbon, that is, grains of carbon whose microscopic pores have been created by thermal shock, by heating them to 500-600 degrees centigrade, so that the grains become riddled with microscopic pores. A gram of carbon thus acquires a contact surface of about 1 000 m2 and exercises a strong surface attraction to methanol, something like capillary action. During the night, although the compartment containing this activated carbon is not heated, the activated carbon attracts and adsorbs the molecules of methanol contained in the evaporator. The liquid methanol is evaporated, producing cold. The water around the evaporator turns to ice.
During the day, the active carbon saturated with methanol is reheated by the sun's rays. The force of adsorption diminishes in proportion as the heat increases, and the molecules of methanol are expelled by the pores of active carbon by desorption, as by boiling. The methanol condenses and runs into the evaporator. No chemical reaction is produced between the methanol and the carbon, adsorption being a physical, not chemical, phenomenon. Condenser and evaporator are joined by a tube, forming a tight assembly which keeps the surrounding air out and the methanol in so that its quantity does not change.
The system requires no moving parts and the cycles alternate in absolute silence. It is not necessary to change the position of the solar panel, which always points toward the sun at its zenith.
In a sunny climate, a solar collector with a surface of one square metre produces in 24 hours some 5 kg of ice, whose temperature is between -4 and -7 degrees centigrade. This ice is not even as cold as that which the ice compartment of an ordinary refrigerator can produce (-10 to -15°C), and a freezer can make even colder ice, but at least it allows food and, even more important, medicines and vaccines to be stored. According to WHO, these must be kept between 4 and 8 degrees. Gaspard prototypes, utilized in Senegal, French Polynesia, Guyana, and India have allowed the maintenance inside a compartment of 200 litres of a temperature of 4 degrees.
Despite its apparent simplicity, the system has demanded several years of research at the National Centre for Scientific Research (CNRS) in the laboratory of thermodynamics of fluids directed at Orsay (France) by Professor Francis Meunier. The CNRS patent has been developed by the engineers of Brissoneau and Lotz Marine (BLM), a subsidiary of Jeumont-Schneider.
Grd Paeye, service chief of research and development of the company, stresses that the apparent simplicity of Gaspard conceals a series of sophisticated and advanced techniques, notably for the manufacture of non-deteriorating compartments and vacuum assemblies, and the degassing to obtain the purest carbon possible, without gaseous residues which reduce the yield of the adsorption-desorption cycle.
Thermal shock permits nearly cylindrical carbon particles to be obtained from 2 to 3 micrometres (millionths of metres) long and 2 micrometres in diameter. On this cylinder, the pores form irregular orifices of 2.5 A diameter and a few in depth. These particles, with their numerous pores, constitute a "thermal sponge" which is saturated with methanol during the night.
The standard model manufactured till now looks like an isothermal box about a cubic metre in volume with a solar collector of about one square metre. The refrigeration compartment has a volume of 200 litres. The tests made in tropical countries have established that the adsorbent material is very stable and does not decompose and that the installation does not require either adjustment or maintenance. The refrigerator is guaranteed for ten years.
If the cold obtained is largely sufficient for keeping vaccines and small quantities of food, the ice is not cold enough to be commercially usable for preservation and transport of large quantities of meat and fish. The company envisages the manufacture of cold rooms of several dozen cubic metres with a view to adapting the same system for refrigerated transport vehicles and fishing boats. The gas exhaust of internal combustion engines could serve as a supplementary source of heat, which would make the transport of refrigeration machines or tons of ice to preserve shipped food unnecessary.