Assessment of organic Rankine cycle performance with binary working fluid mixtures

Abstract

Over the past several years organic Rankine cycle (ORC) processes have become a promising technology for power production from low grade heat sources, such as solar, biomass, geothermal and waste heat. A key challenge in design is the selection of an appropriate working fluid. ORC systems that use single components as working fluids have two major shortcomings. First, in the majority of applications, the temperatures of the heat sink and source fluid vary during the heat transfer process, whereas working fluid evaporation and condensing is isothermal. As a consequence a pinch point is encountered in the evaporator and condenser giving rise to large temperature differences at one end of 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 its lack of flexibility. For given operating conditions, a certain working fluid may be the optimum choice; however, as the operating conditions change another working fluid would become a more appropriate choice. The 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 the inherent properties of the mixture itself, and through cycle variations which become available with mixtures. The most obvious positive effect is decrease in exergy destruction, because occurrence of the temperature glide at phase change provides a good match of temperature profiles in condenser and evaporator. The paper presents detailed simulations analysis of organic Rankine cycle processes for energy conversion of low heat sources for various binary zeotropic mixtures. The rigorous and the most suitable thermodynamic models are applied in each mixture simulation. 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. In addition, several new criteria are developed in order to provide a new perspective on how ORC performance should be assessed from thermodynamic point of view.