Abstract
A numerical model of subcritical and trans-critical power cycles using a fixed-flowrate low-temperature heat source has been validated and used to calculate the combinations of the maximum cycle pressure (Pev) and the difference between the source temperature and the maximum working fluid temperature (DT) which maximize the thermal efficiency (ηth) or minimize the non-dimensional exergy losses (β), the total thermal conductance of the heat exchangers (UAt) and the turbine size (SP). Optimum combinations of Pev and DT were calculated for each one of these four objective functions for two working fluids (R134a, R141b), three source temperatures and three values of the non-dimensional power output. The ratio of UAt over the net power output (which is a first approximation of the initial cost per kW) shows that R141b is the better working fluid for the conditions under study.
Highlights
IntroductionConsiderable quantities of low-temperature thermal energy are available from natural sources (solar, geothermal, biomass) and industrial waste heat (power plants, chemical plants, etc.)
Considerable quantities of low-temperature thermal energy are available from natural sources and industrial waste heat
On the other hand the non-dimensional exergy destruction, the total thermal conductance, the turbine size parameter and the working fluid mass flowrate increase monotonically with the non-dimensional increases linearly with α while the rate of increase of β and SP decreases as power output
Summary
Considerable quantities of low-temperature thermal energy are available from natural sources (solar, geothermal, biomass) and industrial waste heat (power plants, chemical plants, etc.). In an analogous study [7] the present authors considered a subcritical Rankine cycle with superheating, operating between a constant flowrate fixed-temperature (100 °C) heat source and a fixed-temperature (10 °C) sink, for five working fluids (R134a, R123, R141b, ammonia and water) These results show the existence of two optimum evaporation temperatures: one minimizes the total thermal conductance of the two heat exchangers whereas the other maximizes the net power output. By considering that the performance of the system is a continuous function of these two independent variables we determine the combinations of their values which maximize the thermal efficiency or minimize the total exergy losses, the total thermal conductance of the two heat exchangers and the turbine size For each of these conditions the corresponding values of several operating variables (temperature pinch in the evaporator, working fluid mass flowrate, etc.) are determined and analyzed for two working fluids, three heat source temperatures and three non-dimensional power outputs
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
