Thermodynamic Cycles for Low- and High-Temperature Waste Heat Recovery from Heavy-Duty Engines

Doktorsavhandling, 2021

To reduce the environmental impact of heavy-duty vehicles, it is critical to reduce their CO2 emissions by improving the engine efficiency. A promising way to do this is by extracting waste heat from the engine during operation and converting it into useful work. This thesis presents a comprehensive evaluation of the performance of thermodynamic cycles for waste heat recovery from heavy-duty engines. First, by identifying the combination(s) of heat source, working fluid, and thermodynamic cycle that maximizes the performance. Then, by evaluating the performance of the most promising solutions using experimental investigations and detailed simulations.

The potential for waste heat recovery was investigated with steady-state simulations considering two low-temperature and two high-temperature heat sources, a wide variety of working fluids, and four thermodynamic cycles: the organic Rankine cycle (ORC), the transcritical Rankine cycle, the trilateral flash cycle, and the organic flash cycle. The best overall performance was obtained with the ORC using acetone, benzene, cyclopentane, ethanol, or methanol as the working fluid, or with R1233zd(E), MM, or Novec649 if a non-flammable and non-toxic fluid was preferred. The engine coolant was the best performing low-temperature heat source, recovering 1.5 % of the engine power, and the exhaust gas was the best performing high-temperature heat source, recovering up to 5 %. By combining multiple heat sources in series, almost 8 % was recovered. Using a dual-loop system with the engine coolant and exhaust gas as the heat source, fuel consumption was reduced by over 5 %, rising to 9 % if the engine coolant temperature was increased to 140 C.

Two test setups were constructed to experimentally investigate the performance of the simulated systems. The high-temperature setup consisted of a Rankine cycle with water using the exhaust gases as the heat source while the low-temperature setup recovered heat from the engine coolant using an ORC with R1233zd(E) as the working fluid. Based on the experimental findings, models of both setups were developed to predict their performance over a driving cycle. The low-temperature system was able to recover 0.73 % of the total energy required by the engine, while the high-temperature system could recover 3.37 %.