Application of the second law of thermodynamics to control: A review

The development of recently-designed energy efficient components and complex energy concepts has heightened the need to develop advanced control strategies. Exergy-based control strategies are among the recently introduced control strategies, which apply the second law of thermodynamics to control. Exergy is defined as the maximum available work that can be extracted from a system during a process that brings the system into equilibrium with its environment. An exergy analysis can be applied as a powerful thermodynamic technique for assessing and optimizing the efficiency of complex energy systems. A lot of research has been carried out on the exergy analysis of energy systems, but they are typically based on steady-state assumptions. These studies may be helpful to identify or rank the sources of irreversibilities in an energy system, but usually, the main challenge that is faced in real-life is how to efficiently control the energy systems, which are naturally dynamic. Most of the time, the inefficient components of a system are either known or can readily be found, but due to financial concerns, replacing those components with efficient ones or even changing the current system layout in order to maximize the efficiency is not feasible, while achieving the most efficient operation of an imperfect system is always a desire. Despite being common, the steady-state analyses are not capable of offering practical solutions to the real life problems. Although dynamic exergy analysis has recently attracted the attention of researchers, not many studies have been performed on this topic, among them only a few are really control-oriented. Therefore, there is still a real need to evaluate the practical application of exergy from a control point of view, especially on a building scale. This paper aims at presenting a comprehensive literature review of the studies performed on the application of exergy to control, highlighting the still existing gaps. The paper presents an introduction to exergy. The challenging topic of the reference environment is then discussed followed by a thorough review of the application of exergy to control. A summary of control techniques used in the literature, as well as the applications and research focuses of the literature is also presented.