In the paper presented are definitions of specific indicators of power which characterize the operation of the organic Rankine cycle (ORC) plant. These quantities have been presented as function of evaporation temperature for selected working fluids of ORC installation. In the paper presented also is the procedure for selection of working fluid with the view of obtaining maximum power. In the procedure of selection of working fluid the mentioned above indicators are of primary importance. In order to obtain maximum power there ought to be selected such working fluids which evaporate close to critical conditions. The value of this indicator increases when evaporation enthalpy decreases and it is known that the latent heat of evaporation decreases with temperature and reaches a value of zero at the critical point.

A review of the available literature shows that analyses of organic Rankine cycle systems with a zeotropic mixture working medium practically concern single-circuit systems. In these works, it has been shown that the standing of zeotropic mix-tures in organic Rankine cycle systems makes it possible to achieve higher power and efficiency compared to organic Rankine cycle systems with pure fluids. In this article, the authors present an analysis of the efficiency of a two-circuit organic Rankine cycle (binary) power plant with a zeotropic mixture in the upper cycle of this power plant. The proposed binary power plant system uses a zeotropic mixture circulating medium in the upper organic Rankine cycle circuit, while the lower circuit uses a homogeneous low-boiling medium. The results of this analysis showed that with properly selected parameters of the binary power plant system, i.e. with appropriate selection of the pressure during the evaporation trans-formation in the upper and lower circuits, the power obtained in it is higher than for a single-circuit power plant in the same temperature range (for the same heat source and the same condensing temperature). The increase in the power of the binary power plant system was achieved by using the heat contained in the water stream to preheat the medium in the bottom circuit. For example, for the binary organic Rankine cycle power plant with R413A refrigerant in the upper circuit, the generated power is 17.8 kWe, which is 20% higher than for a single-circuit power plant (for the reference power plant, the power is 14.8 kWe).