Performance of an Organic Rankine Cycle with two expanders at off-design operation

Highlights: • An ORC system with two expanders in series at off-design operation is simulated. • Thermal efficiency reached with two expanders was higher than the achieved with one. • Intermediate pressure affects more than the volume expansion ratio of the expander. • The performance of the first expander was mainly affected by leaks. • The performance of the second one was more affected by the discharged pressure. -- Abstract: The objective of this work was to simulate the behavior of an Organic Rankine Cycle (ORC) system with two expanders in series at off-design working conditions. The influence of both the intermediate pressure and the volumetric expansion ratio of the expanders on the off-design performance of the ORC was studied and the irreversibilities of the components were analyzed. The performance of the ORC with two expanders for two different designs was also discussed. The thermal efficiency reached using two expanders was higher than the obtained using only one. However, this increase conveyed an increase in the complexity of the design and control of the expanders. As an additional conclusion, it was found that the influence of the intermediate pressure is higher than that of the volume expansion ratio of each expander. The irreversibility of the first expander was mainly due to leaks. However, the performance of the second expander was particularly affected by the difference between the discharged pressure and the condensation pressure. The off-design analysis allowed the definition of a methodology to achieve the desired power with the maximum thermal efficiency, and the identification of the best actuation for the part load operation.