Prospects of Mixtures as Working Fluids in Real-Gas Brayton Cycles

Abstract

This paper discusses the thermodynamic characteristics of the closed Brayton cycles in which the compression is placed near the critical point of the working fluid. Under these conditions, the specific volumes of the fluid during the compression are a fraction of the corresponding values under ideal gas conditions, and the cycle performances improve significantly, mainly at moderate top temperatures. As the heat is discharged at about the critical temperature, the choice of the correct working fluid is strictly correlated with the environmental temperature or with the temperature of potential heat users. To resort to mixtures greatly extend the choice of the right working fluid, allowing a continuous variation of the critical temperature. These cycles have a high power density, and the use of ordinary turbomachinery is accompanied by high capacities (tens of megawatts). In the low power range, microturbines or reciprocating engines are required. One important constraint on the choice of the right working fluid is its thermochemical stability that restricts the operative temperatures. Among the organic compounds, the maximum safe temperatures are limited to about 400 • C and, forecasting high temperature applications, it could be interesting to explore the potentiality of the inorganic compounds as secondary fluids in binary mixtures.

Performance Analyses and Evaluation of Co2 and N2 as Coolants in a Recuperated Brayton Gas Turbine Cycle for a Generation IV Nuclear Reactor Power Plant

“…CO2 undergoes rapid changes in its thermodynamic properties due to variations of the system pressures and temperatures, which negatively influence the cycle efficiency and specific power, especially below its critical conditions. The design for an optimal CO2 performance will mean that it operates above its critical points and the use of recompression within the cycle will be an added advantage for its selection [8,19].…”

“…With regards to a coolant such as helium, it brings several benefits to plant operations such as chemical inertness, single phase cooling and neutronic transparency [5][6][7]. However, adopting other working fluids and mixtures for reactor cooling such as carbon dioxide, nitrogen, argon have been proposed in different studies [8,9]. There are planned and on-going developmental projects for GFR and VHTR which focus on testing the basic concepts and performance phase validation [4,10,11].…”

Pure and Hydrocarbon Binary Mixtures as Possible Alternatives Working Fluids to the Usual Organic Rankine Cycles Biomass Conversion Systems

“…These relatively low maximum temperatures and high condensation temperature combined with the relatively low thermal power available exclude the possibility to consider as alternative working fluid carbon dioxide (another "natural" fluid) or, for example, mixture of carbon dioxide and hydrocarbons. As a matter of fact, carbon dioxide should be reserved to power plants with very high power densities and to greater maximum to minimum temperatures ratio [18][19][20][21].…”

Effect of Working Fluid on Selection of Gas Turbine Cycle Configuration for Gen-Iv Nuclear Power Plant System

“…Alpy et al (Alpy et al 2011) compared gas testing for nitrogen and carbon dioxide in a closed-cycle gas turbine component design prototype. Other researches that have explored several options of working fluid for closed cycle gas turbine technology are documented in references (Invernizzi 2017;Osigwe, Gad-Briggs, Nikolaidis, et al 2018;Yousef and Zaamout 1992;Lee, J. Campbell, and Wright 1981).…”