Organic Rankine Cycles are the preferred choice for low to medium temperature heat recovery. Many waste heat recovery sites such as large IC engines, refineries, and process plants have multiple heat sources existing concurrently. However, the existing multi-source ORC systems suffer from design complexity and underutilization of heat sources resulting in low power outputs and high costs. The primary aim of this thesis is to design and develop advanced next-generation dual/multi-source ORCs capable of delivering best-in-class performance without entailing intricate system designs.

As a first step, an initial study explored the dual-source heat recovery capability of existing two-stage ORCs architectures: Series Two-stage ORC (STORC) and Parallel Two-stage ORC (PTORC). A 2.9MW natural gas IC engine is selected to supply high temperature (430°C) exhaust gas and low temperature (90° C) jacket water as the primary and secondary heat source to the ORC systems. Cycle simulations indicate that both STORC and PTORC have improved secondary heat source utilization than the basic single-stage ORC. STORC shows improved performance than PTORC and delivers higher power outputs over basic single-stage ORC and dual-loop ORC by 8.5% and 13.1%, respectively.

In the second study, to further improve the STORC, a combined transcritical-regenerative (TR) design is developed. The proposed TR-STORC adopts transcritical evaporation in the high pressure (HP) stage and partial evaporation in the low pressure (LP) stage. Superheated vapor resulting from the transcritical expansion in the HP turbine is utilized in a direct mixing vapor regenerator, wherein vapor-liquid mixing takes place. This regeneration leads to complete evaporation of the two-phase fluid, thereby generating additional vapor for expansion in the LP turbine. System optimization using Genetic Algorithm indicates that TR-STORC delivers 15-52% and 15-34% higher power outputs than single-stage ORC and STORC for various organic working fluids and heat ratios.

The TR-STORC proposed in the second study utilizes partial evaporation for regeneration, requiring 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. The third study proposes a modified Transcritical Ejector Regenerative STORC (TER-STORC), which eliminates partial evaporation, and achieves regeneration via an ejector operating in full evaporation (FE) mode. The partial evaporation (PE) mode of TER-STORC is also analyzed. System optimization using Genetic Algorithm indicates that the FE mode of TER-STORC can achieve performance comparable to PE mode and TR-STORC. FE mode is less sensitive to ejector pressure drop variations than PE mode while delivering 0.2-4% lower power outputs. TER-STORC in FE mode is also associated with lower heat exchanger requirements by up to 18% and 6-40% lower turbine sizes than TR-STORC.
The TR and TER designs proposed above still require vapor extraction between turbine stages. This feature requires two separate turbine units in the HP and LP stages. The fourth and final study proposes an advanced Transcritical-Recuperative (TREC) design, eliminating the need for vapor extraction, thereby allowing the use of a low-cost, single compact two-stage ORC turbine unit. The TREC-STORC operates with a recuperator acting as an additional evaporator at the low-pressure turbine's exit (LP mode). Another TREC design requiring vapor extraction from the high-pressure turbine (HP mode), similar to the TR design, is also presented for comparison. System optimization results show that the LP mode outperforms the HP mode for all scenarios. TREC-STORC in LP mode also generates 2-8% additional power output over TR and TER designs proposed in the previous studies. Compared to existing STORC and single-stage ORC, TREC-STORC (LP mode) also leads to 18-38% and 20-56% increased power outputs, respectively. TREC-STORC (LP mode) presents a robust and superior ORC architecture for multi-source heat recovery applications