Multi-approach evaluations of a cascade-Organic Rankine Cycle (C-ORC) system driven by diesel engine waste heat: Part A – Thermodynamic evaluations

A novel transcritical cascade-Organic Rankine Cycle (C-ORC) system was proposed for recovering multi-grade waste heat from a typical heavy-duty diesel engine. The C-ORC comprises a high temperature ORC loop (HT-Loop) and a low temperature ORC loop (LT-Loop) for recovering waste heat from engine exhaust gas (EG), exhaust gas recirculation (EGR), jacket water (JW) and charge air (CA) in a cascaded pattern. The basic thermodynamic evaluation on energy and exergy aspects were covered in the companion piece-‘Part A-thermodynamic evaluations’, indicating that the proposed C-ORC possesses great heat-recovery capacities and efficiency-promotion potentials. The techno-economic evaluations in Part B of this paper will further explore the performance of the C-ORC system based on costs and benefits in order to reveal its practical benefits. Four techno-economic indexes are mainly focused on: component-to-system cost ratio (CSCR), electricity production cost (EPC), depreciated payback period (DPP) and savings-to-investment ratio (SIR). Under rated engine conditions, working fluids were initially compared to find the most economical fluids for the HT-Loop and the LT-Loop respectively. Results indicated that toluene and R143a still make the perfect match for the C-ORC with the lowest EPC (0.27 Dollar/kW h), DPP (7.8 years) and the highest SIR (1.6). As to component-to-system cost ratio (CSCR), the cost of expanders and heat exchangers together account for more than 3/4 of the total system cost, and the expander of the LT-Loop is the most costly single component. The C-ORC’s techno-economic performance was further evaluated based on engine’s practical operating conditions. Results showed that, under available conditions, the CSCR of expanders is within 25.9–48.0%, and it gradually increases as engine’s speed and torque increase; the CSCRs of heat exchangers and pumps are within 31.1–54.8% and 17.7–22.5% respectively. The EPC, DPP and SIR are within 0.27–0.48 Dollar/kW h, 8.5–20.5 years and 0.85–1.6 respectively, and according to their similar variations, an increase in engine speed will evidently benefit three indexes. In addition, the increasing torque will also improve the indexes to some extent. Summarily, the techno-economic evaluations in Part B of this paper further reveal the C-ORC’s merits and drawbacks, and in order to improve its full performance, provide directions for optimizations for higher system capacities and commercial potentials.