Thermodynamic optimisation of an open - air solar thermal Brayton cycle with fixed temperature constraints

Abstract

This project mainly focused on the implementation of the second law of thermodynamics relating to the design of heat-exchanging components in an open-air solar thermal Brayton cycle. These components include one or more regenerators (in the form of cross-flow heat exchangers) and the receiver of the parabolic dish where the system heat was absorbed. The generation of entropy is under close investigation since the generation of entropy goes hand in hand with the destruction of exergy, or available work. This phenomenon is caused by two factors, namely the transfer of heat across a finite temperature difference and also the friction that is caused by the flow of a working fluid in a system consisting of components and ducts. The dimensions of some components were used to optimise the cycles under investigation. Entropy Generation Minimisation (EGM) was employed to optimise the system parameters by considering their influence on the total generation of entropy. Various assumptions and constraints were considered and discussed to aid in the solution process, making it simpler in some cases and more feasible in others. The total entropy generation rate and irreversibilities were determined by considering all of the individual components and ducts of the system, and their respective inlet and outlet conditions such as temperature and pressure. The major system parameters were evaluated as functions of the mass flow rate to allow for proper discussion of the system performance. Ultimately, conclusions and recommendations were made, which state the optimum system to be used in this type of solar application, where the amount of net power output is the main driving factor.