Energy Conversion
In the preceding sections we have evaluated the second law with respect to its ability for the description of basic equilibration processes, e.g., the equilibration of temperature, the direction of heat transfer, the dissipation of kinetic energy, and friction losses in gears. Now we shall apply thermodynamic analysis to conversion processes between work and heat.
The science of thermodynamics emerged from an engineering question:
How much work can be obtained from a given amount of heat? This question arose when the first steam engines were built, which had efficiencies of only a few percent. The question is still of outmost importance, as a sustainable way of living requires the optimal use of resources. Having a good understanding of the possibilities and limitations in energy conversion processes is the first step in building better—more efficient—engines.
Before we discuss more complex energy conversion processes, we consider a relatively simple problem: Energy conversion processes between two thermal reservoirs at different temperatures TH and TL, with TH > TL.
The natural environment, usually assumed to be at T0 = 25 ◦C, is the prototype of a thermal reservoir. Due to its size, the environment has al- most infinite thermal mass mcv , and hence it can provide or accept a large amount of heat without changing its temperature. Ultimately, all systems are in thermal contact with the natural environment, and it serves as heat sink or source for most energy conversion processes.
Many of today’s heat engines rely on the combustion of a fuel (coal, oil, gas). Combustion processes do not create a reservoir of constant high temperature, but rather a flow of hot combustion gases that provides heat at varying temperature. Therefore, the following considerations are not always directly applicable to combustion systems. Nevertheless, the subsequent sections give important and relevant insights, to which we shall come back again and again.
Pure heat transfer between the two reservoirs was discussed already in Sec. 4.8, with the statement that by itself heat goes from warm to cold, but cannot go from cold to warm. We now consider processes that involve heat and work. The systems considered are engines that operate at steady state, that is they do not accumulate or loose energy or entropy over time, dE = dS = 0. The dt dt detailed processes inside the engines will be discussed extensively later. For the present overall evaluation, however, they are of no concern, and thus the set-up considered is as simple as shown in Fig. 5.1: The thermal engine E exchanges heat with both reservoirs, and produces or consumes power. The direction of the arrows in Fig. 5.1 simply indicates the convention for heat and work: heat in and work out are positive. In the following figures, however, we will use absolute values of heat and work, and the direction of the flows will be indicated by the directions of the arrows.
For steady state processes, the first and second law for this set-up read
