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 system complexity and underutilization of heat sources resulting in lower power outputs. The primary aim of this thesis is to design and develop advanced next generation multi-source ORCs capable of delivering higher power outputs without entailing complex system designs.

As a first step, an initial study extended the application of two stage ORCs to a 2.9MW natural gas IC engine. Exhaust gas and engine jacket water serves as the primary and secondary heat source to the ORC systems. Cycle simulations indicate Series Two stage ORC (STORC) to deliver higher power outputs over existing single stage ORC and dual-loop ORC by 8.5% and 13.1% respectively. 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 followed by regeneration of superheated vapour in the low pressure (LP) stage. TR-STORC delivers 15-52% and 15-34% higher power outputs than single stage ORC and STORC for various organic working fluids. Next, to avoid any two phase flows that could damage the turbine blades, an ejector retrofit is added to TR-STORC. This retrofitted version is seen to generate 0.2-4% lower power outputs over TR-STORC.

The TR designs proposed above still require vapour extraction between turbine stages. Existing two stage ORC turbines do not support this feature. Therefore, an alternative Transcritical-Recuperative (TREC) design which eliminates vapour extraction is developed. The novel TREC-STORC generates between 2-8% additional power output over TR based systems, with 18-38% and 20-56% increased power outputs over basic STORC and single stage ORC respectively. TREC-STORC presents a robust and superior ORC for multisource heat recovery applications.