T-Q̇ diagram analyses and entransy optimization of the organic flash cycle (OFC)

Abstract

The electrical power generation from low temperature heat source attracts more and more attentions but the temperature mismatching between the heat sources and working medium in the organic Rankine cycle (ORC) becomes an issue. The organic flash cycle (OFC) is an effective solution to this issue. In this paper, the OFC is analyzed by the concept of entransy loss and the T-Q (temperature-heat flow rate) diagram for the heat-work conversion. The equations for cycles of the basic OFC and the OFC whose heat source is the exhaust gas of the turbine in a Brayton cycle (the combined cycle) are derived theoretically and the results indicate that larger entransy loss rate leads to larger output power with prescribed inlet parameters of the hot stream in the discussed cases, which is displayed by the T-Q diagram intuitively. Two numerical examples demonstrate that the optimal mass flow rate of the working medium for the maximum entransy loss rate is the same as that for the maximum output power. The T-Q diagram analyses is in accordance with the numerical results. The concept of entransy loss can be used as the criteria for the OFC optimization.