Based on thermodynamic considerations, the use of electrical energy to produce hot water is not recommended, since a prized kind of energy is used and also because the global efficiency of the water heating process is lower than the produc­tion of many other direct water heating processes. In fact, heat is a kind of energy which we inevitably find in every real process as consequence of the irreversibility of this process. So it does not make sense to degrade completely a noble form of energy to obtain heat, without getting the mechanical work which can be obtained from that energy.

An alternative way to produce hot water involves the exploitation of solar energy, which represents a form of clean and inexhaustible energy, by low — temperature thermal solar technologies [16].

These technologies include systems using a solar collector to heat a fluid or the air. The aim of these low-temperature thermal solar systems is to intercept and transfer solar energy to produce hot water or heat buildings. By low temperature we mean the heating of fluids at a temperature of less than 100°C (it rarely reaches 120°C) [1, 2, 5, 13, 17].

The detailed description of low-temperature thermal solar systems can be found in par. 2.2.2.