One of the most dazzling applications of thermodynamics is the study and design of devices to convert energy to obtain some useful effect. Modern societies are based on the extensive use of electricity, heating, and refrigeration for a broad variety of goals. These services are supplied by a system in which some sort of working substance, usually in the gas or liquid phase, flows through several devices at various temperatures. The same holds for engines, which provide the stunning mobility which is also a pillar of modern times.
Here we therefore describe and analyze systems such as stationary power plants, propulsion engines, refrigerators, heat pumps, and the like. Stationary power plants convert one kind of energy, like the chemical energy of a fuel, the thermal energy of a geothermal reservoir, or solar radiation into electricity. Propulsion engines do the same, but usually the primary energy source is a liquid fuel and the final objective is to obtain mechanical power or thrust to put something into motion. Refrigeration systems and heat pumps make use of electrical, mechanical, or thermal power in order to keep the temperature of a certain space at a value different from that of the natural environment.
Analysis of Thermodynamic Systems
Most energy systems involve a working fluid, like water or air, which is circulated through one or more components, often arranged in a cycle. The working fluid undergoes therefore a sequence of processes, which together form a thermodynamic cycle. In steam power plants (Figure 7.1a) the cycle is usually closed, but thermodynamic cycles can be open (closed by the atmosphere), as is the case for a turboprop engine (Figure 7.1b).
The basic methodology for the analysis of thermodynamic processes and systems has been laid down in Chapter 4, while in this chapter we will use that procedure, together with knowledge on the estimation of fluid properties (Chapter 6), in order to study some notable examples of energy conversion systems. In addition, we can make use of the many consequences of the second law of thermodynamics to discuss their performance and limitations.
In Section 3.6 we have already stressed the importance of thermodynamic charts and of sketching relevant processes on such charts. Graphical representation of concepts is a very powerful tool for the understanding of thermodynamics.