Thermodynamic Efficiency Maximum of Simple Organic Rankine Cycles

Abstract

The increase of the maximal cycle temperature is considered as one of the best tools to increase cycle efficiency for all thermodynamic cycles, including Organic Rankine Cycles (ORC). Technically, this can be done in various ways, but probably the best solution is the use of hybrid systems, i.e., using an added high-temperature heat source to the existing low-temperature heat source. Obviously, this kind of improvement has technical difficulties and added costs; therefore, the increase of efficiency by increasing the maximal temperature sometimes has technical and/or financial limits. In this paper, we would like to show that for an ideal, simple-layout ORC system, a thermodynamic efficiency-maximum can also exist. It means that for several working fluids, the thermodynamic efficiency vs. maximal cycle temperature function has a maximum, located in the sub-critical temperature range. A proof will be given by comparing ORC efficiencies with TFC (Trilateral Flash Cycle) efficiencies; for wet working fluids, further theoretical evidence can be given. The group of working fluids with this kind of maximum will be defined. Generalization for normal (steam) Rankine cycles and CO2 subcritical Rankine cycles will also be shown. Based on these results, one can conclude that the increase of the maximal cycle temperature is not always a useful tool for efficiency-increase; this result can be especially important for hybrid systems.

The efficiency of transcritical CO2 cycle near critical point and with high temperature

“…Being the Carnot cycle the thermodynamic cycle featuring the highest possible to obtain efficiency within the given temperature range, one could assume that ORC is more efficient than TFC. According to some recent studies, this is true only for cycles using wet or moderately dry working fluids, including CO2 [14,15].…”

“…Figure 2 also shows that the pump and the expander are modelled as isentropic. This method is similar to the methods used in studies [14,15] for understanding the low and maximal efficiencies of ORC near the critical point.…”

Past, Present and Near Future: An Overview of Closed, Running and Planned Biomethanation Facilities in Europe

“…In line with the long-term strategy of the European Union to become climate-neutral, the energy storage challenge [1] that is induced by volatile renewable electricity production (e.g., with rapidly growing photovoltaic capacities) must be handled [2][3][4]. Power-to-gas (P2G), and especially power-to-methane (P2M), technologies, however, are capable of providing flexibility [5] and efficient seasonal energy storage [6] with the reuse of CO 2 and the utilization of the existing capacities of the natural gas grid [7].…”

“…Following a novel abductive methodological approach in this area with (1) using quantitative and qualitative data, (2) starting the analysis through the lens of a technology developer company, and (3) iteration with former theories and results to identify trends and gaps which can define the scope of future facilities; 3.…”

“…How can future P2M, and especially biomethanation facility development projects, increase sectoral competitiveness in Europe? Figure 1 summarizes (1) why the research is relevant, (2) what is in the focus of the research, and (3) how the research was conducted. In sum, the study has a more future-oriented approach rather than a retrospective one, and this future orientation requires specificity regarding:…”