Optimization of Brayton cycles for low-to-moderate grade thermal energy sources

• Published in: Energy (Oxford) 2013-06, Vol.55, p.403-416
• Main Authors: Rovira, Antonio, Muñoz-Antón, Javier, Montes, María José, Martínez-Val, José María
• Format: Article
• Language: English
• Subjects: ambient temperature; Applied sciences; Brayton cycle; electricity; Energy; Exact sciences and technology; gases; heat; Moderate temperature heat source; Regenerative cycle; Supercritical fluid
• ISSN: 0360-5442
• DOI: 10.1016/j.energy.2013.03.094

Future electricity generation will involve low or moderate temperature technologies. In such a scenario, optimisation of thermodynamic cycles will be a key task. This work presents a systematic analysis to find the operating regime where Brayton cycles reach the highest efficiency, using real substances and given heat source and sink temperatures. Several configurations using fluids close to its critical point at the compressor inlet are considered. Irreversibility sources are carefully analysed, as well as the type of working fluid. The analysis is performed by means of a theoretical approach to obtain some trends, which are afterwards validated with real gases. Results show that the efficiency and the specific work improve if the compressor inlet is close to the critical point. Furthermore, these cycles are less sensitive to pressure drops and politropic efficiencies than those working with ideal gases. The above features are more evident when the ratio of heat source and heat sink temperatures is low. The selection of the gas becomes a fundamental issue in this quest. Critical temperature should be close to ambient temperature, low critical pressure is advisable and the R/cp factor measured at the ideal gas condition should be low to further enhance the efficiency. •Performance analysis of Brayton cycles with the compressor inlet close to the critical point.•Cycles are not very sensitive to pressure drops and isentropic efficiencies of the compressor.•Gas selection becomes important, regarding the critical pressure and temperature as well as the kind of fluid.•R/cp factor measured at the ideal gas condition should be as low as possible.