MODULE III Air-conditioning and Cooling technology Page 71

Cooling with solar actuation

The sun shines everywhere, and, especially in southern lands, delivers surplus heat. With the help of a refrigerating set and a parabolic solar collector, this heat can be transformed into cold. The prototype of a solar refrigerator will be introduced in the following.


Absorption or Adsorption ?

The solar refrigerator works on the basis of Adsorption of water by zeolite. Zeolite is chemically similar to common sand, but does not form compact molecules. Instead it has a crystal structure with a very large internal surface. In nature there are circa 40 different types of zeolite. These mostly come into existence due to volcanic activity and have major impurities, which has the result that they are not suitable for refrigeration technology applications. The chemical industry manufactures a number of artificial types of zeolite. Synthetic zeolite is mainly used as a substitute for phosphate in laundry detergent or, for example, as a desiccant in double-glazed windows. The use of large quantities of zeolite in laundry detergent led to comprehensive examination of its environmental effects. It was thereby shown that zeolite in no way endangers the environment and has no toxic quality. In addition, because mass production has caused the price of zeolite to fall to between 1 and 7 Euro/kg, it represents a very low priced alternative.


Basics of cooling technology with zeolite

Of basic importance for the application of zeolite is a very large inner surface of between 800 and 1200 m²/g. Strong electrostatic forces are active within this hollow space. Polar molecules such as, for example, water are thereby absorbed, and as a result of heat dissipation they are integrated into the crystal structure (adsorption). If the process takes place in airless containers, the absorption of steam takes place at such a dramatic rate that the remainder of the water cools off dramatically and freezes to ice as a result of the high latent heat. This process continues until the zeolite is saturated with water. Depending upon the type, zeolite can absorb up to 25 % of its own weight in water. To use it further, the zeolite must again be dried, whereby it regains its full functional capability. This regeneration takes place through the supply of high temperature heating. Water is thereby driven out of the zeolite as steam (desorption).

This can take place with electrical energy, fossil fuels or (an especially positive energy choice) solar energy. The next adsorption phase can take place following a cooling-off phase. Because the zeolite is not used up, any number of cycles can be carried out.