Satyanarayanan S

The Transcritical Regenerative Series Two-stage Organic Rankine Cycle (TR-STORC) has improved performance over STORC and single-stage ORC. However, the TR design requires vapor extraction between turbine stages, which prevents the use of compact two-stage ORC turboexpanders. An alternative Transcritical-Recuperative (TREC) design is proposed which eliminates vapor extraction allowing direct integration of compact two-stage turbines. The novel TREC-STORC operates with a recuperator acting as an additional evaporator at the low-pressure turbine exit (LP mode). Another TREC design requiring vapor extraction from the high-pressure turbine stage (HP mode), similar to the TR design, is also presented for comparison purposes. TREC-STORC in LP mode presents a marginally improved performance of 1–3% over HP mode for various IC engine heat source conditions. With cyclopentane as working fluid, at engine design point conditions, LP mode delivers 4% higher power outputs over HP mode and TR-STORC. Compared to STORC and single-stage ORC, the LP mode of TREC-STORC delivers 21% and 28% higher power outputs, respectively. For other working fluids, TREC-STORC (LP mode) delivers between 2–5% additional power output over TR-STORC, with 18–38% and 20–56% increased power outputs over STORC and single-stage ORC, respectively. TREC-STORC (LP mode) presents a superior and robust ORC architecture for dual/multi-source heat recovery applications.

The Transcritical Regenerative Series Two-stage Organic Rankine Cycle (TR-STORC) was shown to deliver improved performance than Series Two-stage ORC (STORC) and single-stage ORC for dual-source heat recovery applications. However, TR-STORC utilizes partial evaporation for regeneration which requires precise control of two-phase flows. In real-time operations involving dual/multiple heat sources, this is difficult to achieve due to fluctuating heat inputs, leading to liquid carryover and subsequent corrosion of turbine blades. This study explores a novel Transcritical Ejector Regenerative STORC (TER-STORC), which replaces the need for two phase flows with a avapor-vapor regeneration via an ejector operating entirely with fully evaporated vapor (FE mode). Partial evaporation (PE) mode of TER-STORC requiring two-phase flows is also analyzed for comparison. Results indicate that the FE mode of TER-STORC can achieve performance comparable to PE mode and TR-STORC. FE mode of ejector operation is less sensitive to variations in ejector pressure drop than PE mode while delivering 0.2–4% lower power outputs with lower heat exchanger requirements than TR-STORC by up to 18%.At the engine design point, only a 2% drop in power output is seen for FE mode compared to TR-STORC. TER-STORC presents a robust system with reduced complexity for multisource heat recovery.

A Transcritical Regenerative Series Two stage Organic Rankine Cycle (TR-STORC) is proposed to improve the efficiency of existing Series Two stage ORC (STORC) architecture by combining supercritical heating in the high pressure (HP) stage and partial evaporation with regeneration in the low pressure (LP) stage. Exhaust gas and jacket water from a stationary IC engine is used as the primary and secondary heat source respectively. Using cyclopentane as working fluid, system exergy performance is analysed for a range of heat source temperatures and for different ratios of heat available between the heat sources. At lower HP evaporator pressures, lower values of vapour outlet temperatures lead to maximum power output. For a wide range of heat ratios and temperatures, TR-STORC delivers improved exergetic performance over STORC and pre-heated ORC. It is the recommended choice for all scenarios of dual source heat recovery. For the engine design point, TR-STORC delivers increased power output by up to16% and 23% than STORC and pre-heated ORC respectively. TR-STORC maintains exergetic superiority for all the working fluids investigated with maximum net power outputs exceeding STORC by15–34% and preheated ORC by 15–52%.

Improvement in performance of Organic Rankine Cycle (ORC) systems, particularly in the context of dual heat sources such as IC engines, leads to better return on investments. However, the choice of architecture to achieve the best performance is not evident from available literature. When two separate heat sources are present concurrently at different temperature levels with heat contents such as in IC engines, single stage pre-heated ORC and dual loop ORC are the two commonly deployed ORC architectures. In this study, two stage architectures: Series two stage ORC (STORC) and Parallel two stage ORC (PTORC) are analysed and their performance is compared against a single stage pre-heated ORC at sub-critical conditions in the utilization of high temperature (primary) exhaust gases (573–773 K) and low temperature (secondary) jacket water (353–393 K) representing IC engine waste heat conditions. Results show that STORC and PTORC are able to achieve the maximum net power output for an intermediate utilization of secondary heat source. The power output gains from two stage layouts improves significantly with a reduction in heat source temperature difference and for lower ratios of the heat available between the primary to secondary heat source. For a 2.9 MW natural gas IC engine operating at its design point, STORC delivers 8.5% more power output whereas PTORC delivers 0.3% less power output than pre-heated ORC. Compared to a dual–loop ORC, STORC presents a less complex and improved cycle architecture with a 13.1% increased power output and a 27.9% reduced heat exchanger requirements.