Over the past several years ORC processes have become very promising for power production from low grade heat sources: solar, biomass, geothermal and waste heat. The key challenge in the design process is the selection of an appropriate working fluid. A large number of authors used pure components as working fluid, and assess ORC performance.

ORC systems that use single working fluid component have two major shortcomings. First, the majority of applications involve temperatures of the heat sink and source fluid varying during the heat transfer process, whereas the temperatures of the working fluid during evaporation and condensing remains constant. As a consequence a pinch point is encountered in the evaporator and condenser, giving rise to large temperature differences at one end of the heat exchanger. This leads to irreversibility that in turn reduces process efficiency. A similar situation is also encountered in the condenser. A second shortcoming of the Rankine cycle is lack of flexibility.

These shortcomings result from a mismatch between thermodynamic properties of pure working fluids, the requirements imposed by the Rankine cycle and the particular application. In contrast, when working fluid mixtures are used instead of single component working fluids, improvements can be obtained in two ways, through inherent properties of the mixture itself, and through cycle variations which, become available with mixtures. The most obvious positive effect is decrease in energy destruction, since the occurrence of a temperature glide during a phase change provides a good match of temperature profiles in the condenser and evaporator.

This paper presents detailed simulations and economic analyses of Organic Rankine Cycle processes for energy conversion of low heat sources. The paper explores the effect of mixture utilization on common ORC performance assessment criteria in order to demonstrate advantages of employing mixtures as working fluid as compared to pure fluids. We illustrate these effects based on of zeotropic mixtures of paraffins, as ORC working fluids.


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