With the exception of nuclear energy, the Sun is the source of all mechanical energy used by man. The total proportion at which the energy from the Sun reaches Earth is impressive, but the proportion at which it falls on a square meter of surface is small. The difficulty lies in concentrating the energy collected over a large area so that its use is practical in the production of some work; Research in this field continues. But not It is widely used for large-scale power generation, radiation solar finds application for direct water heating, for generation of elevated temperatures in metallurgical applications (solar ovens) and for the evaporation of water in the production of salt.
The kinetic energy associated with the movement of air masses has been used to a certain degree for the production of work (windmills), especially in rural areas. Variations and uncertainties are problems within this field in wind speed and the need for large equipment to produce significant amounts of work. It is conceivable that the potential energy of the tides can be exploited. Attempts have been made in this direction on a large scale in parts of the world in where the tides are especially high. However, the total production of energy from this source does not appear to be significant compared to the world demand.
By far the most important sources of energy are chemical energy (molecular) of fuels and nuclear energy. A significant generation of energy also results from the conversion of the potential energy of water into work, a process that in principle can be carried out with an efficiency of 100 percent. On the other hand, all the methods used until today in the production and large-scale use of molecular or nuclear energy are based on detachment of heat and in the subsequent conversion of part of the heat into usable work. Despite the improvements in the equipment design, the efficiency of the conversion is relatively low (values above 35 percent are rare), a consequence of the Second law. When it is possible to convert fuel energy in work without intermediate generation of heat, it is considerably improved the conversion efficiency. The usual device for direct conversion of chemical energy in electrical energy is the electrolytic cell. Progress has been made in the development of cells that operate with hydrogen and carbon fuels, for example, natural gas or bituminous coal. % fuel cells are already in use for a modest supply of energy requirements with special purposes. The efficiency of these cells ranges from 55 to 85 percent, approximately double the value obtained through the conventional process of converting first the chemical energy in heat.
In a conventional power plant, the molecular energy of the fuel It is released by a combustion process. The function of the production device of work consists of converting part of the heat of combustion into mechanical energy. In a nuclear power plant, the fission or fusion process releases the energy of the nucleus of the atom as heat, and then this heat is partially converted at work. Thus, the thermodynamic analysis of thermal machines, As presented in this chapter, it applies equally well to power plants conventional (fossil fuels) and nuclear. The steam power plant is a large-scale thermal machine in the which the working fluid (H20) is flowing continuously, in a state uniformly, successively through a pump, a boiler, a turbine and a capacitor in a cyclic process. The working fluid is separated from the source of heat and heat is transferred through physical limits. In a plant combustion of coal, the combustion gases are separated from the steam by the walls of the boiler pipe. The internal combustion machine is another form of thermal machine, in where elevated temperatures are achieved by direct conversion of the chemical energy of a fuel in internal energy within the producing device of work. Examples of this type are the Otto engine and the gas turbine. To illustrate the calculation of thermal efficiencies, they will be analyzed in this chapter several cycles of common thermal machines.
