Organic Flash Cycles: Off-design behavior and control strategies of two different cycle architectures for Waste Heat Recovery applications

Off-design characterization of energy systems has become interesting, especially for waste heat recovery application, where the heat source temperature and mass flow rate can vary over time. Low-grade heat is generally converted into power through ORC modules: the problem of the constant temperature evaporation lead to the definition of alternative architectures, among which organic flash cycles. In this work, the off-design behavior of two different architectures of single-stage Organic Flash Cycles has been analyzed in steady-state condition, for small scale waste heat recovery (WHR) purposes. The main difference between the two architecture is the regeneration: in the first architecture (Single-Stage Organic Flash Cycle SS-OFC), the liquid of the flash evaporator, after lamination is mixed with the vapor from the expander and then sent to the condenser; in the second architecture Single-Stage Organic Flash Regenerative Cycle, SS-OFRC, the liquid from the flash evaporator is mixed with the liquid from the condenser, to regenerate the cycle. The most appropriate fluid for the two cycles was selected from a list of sixteen fluids with the objective of minimizing volume flow rates and maximizing the system efficiency and i-Pentane was chosen. For the off-design behavior, a rotary volumetric expander derived from a Wankel engine was considered, taking into account the performance variation of the device at various rotating speed and pressure ratios. Three different control strategies were considered and compared in off-design analysis for both the cycle architectures: sliding-pressure, in which the expander speed was constant and flash pressure varied with the load; sliding-velocity, in which the load was controlled by the speed variation of the expander and flash pressure was retained constant; combined strategy in which the expander speed was varied to drive the flash pressure according to a function which maximized the system efficiency. Results showed that the efficiency of the two cycles was similar in all the operating field whatever was the control strategy considered: SS-OFRC demonstrated a better behavior at low temperatures of the heat source (<170 °C), while SS-OFC had a better efficiency at higher temperature. The maximum absolute efficiency difference in off-design conditions between the two cycles was lower than 0.3%. SS-OFRC however had a wider field of operation than SS-OFC, due to the better flexibility of this type of cycle. As for the control strategy, with both the architectures, combined strategy maximized the system efficiency and flexibility for every temperature and mass flow rate of the heat source considered.