Taking the chill off the sun
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.