Performance comparison between single and dual pressure Organic Rankine Cycle systems

Organic Rankine Cycle (ORC) systems are a well-known technology for power generation from low-to-medium temperature heat sources such as geothermal resources, solar thermal heat and waste heat. The majority of the existing plants worldwide are single pressure ORC systems taking advantage of their simple configuration. Recently, more complex cycle architectures have been proposed in the literature to improve the system performance. In particular, in the dual pressure layout the organic fluid is evaporated at two different pressure levels to improve the heat recovery from the heat source (similarly to steam in dual pressure natural gas combined cycles). However, only specific applications have been investigated in the literature in which the more efficient heat recovery was binding to get a good performance so that the potential of this layout has not been fully explored till now. The aim of this work is to compare the performance of single and dual pressure ORC systems in the utilization of heat sources between 100 and 200°C, and find possible guidelines for the choice between the two cycle architectures. In both layouts evaporation pressures and degrees of superheating were taken as decision variables to maximize power output without any constraint on the minimum outlet temperature of the heat source. Results show that dual pressure ORC systems generally improve power output (on average by 20%) when the critical temperature is similar or higher than the heat source inlet temperature. However, when the working fluid has a critical temperature approximately 35°C lower than the heat source inlet temperature, single pressure ORC systems do give their best performance which is not overcome by the dual pressure layout. Thus, dual pressure ORC systems are to be preferred only when the best fluid has to be selected within a limited set of working fluids which do not fulfill this optimum temperature difference due to specific constraints on costs, environment, availability, material compatibility, etc.