At our latitudes, low-temperature solar thermal systems are more common than the medium and high temperature ones (especially among private users, who want to save money on their energy bills). They are usually used [1-5, 17]:

• to heat sanitary water for domestic, hotel and hospital use;

• to heat water for showers (bathing establishments, camping, etc.);

• to heat rooms;

• to heat water used in processes at a low temperature;

• to dry foodstuffs;

• to refresh rooms (although it is still too expensive).

A solar thermal system always works in the same way, although there may be slight changes according to its application and use. First, there are a few solar collectors which absorb large quantities of sunlight and then convert it into heat.

The collectors are crossed by a fluid (thermal vector fluid) which removes the absorbed heat; this fluid, crossing the so-called solar circuit, arrives at an accumu­lator which stores the large quantities of thermal energy to be used in the future when there will be a real need [17].

We will now analyse the low-temperature solar thermal system starting with its most important element: the collector.