Converting a portion of the waste heat into usable power by implementing Rankine and Organic Rankine Cycles (ORC) on long-haul trucks is seen as a potential way to improve the overall system efficiency. To identify techno-economical heat sources across the drive cycle of a Heavy Duty Diesel Engine (HDDE), an energy and exergy analysis was performed on all the available heat streams. As a result, to recover the combined exhaust gases and coolant heat, a reference cascade system was analysed. Owing to the nature of this application, a size vs. performance optimisation was performed for the cascade system utilising water and R245fa fluid combination. Despite a 1.8% Brake Thermal Efficiency (BTE) improvement, the key consideration in the research and development efforts for ORC systems was identified as the investigation of technical paths that may improve the practicality of such a heat-to-power conversion concept. For this, simple holistic solutions were considered vital to meet the impending CO2 regulations. To provide a potential solution, an innovative dual-pressure ORC system is therefore proposed to partially address the shortcomings of the cascade system. This innovative system is a function of new working fluids (i.e. water blends), its associated cycle operating mode and a novel architecture (i.e. direct engine block heat recovery). A screening and evaluation methodology applied to water–organic blends is presented. Simulations conducted in Aspen HYSYS V8 showed that, compared to the reference cascade system, the proposed dual-pressure system has the potential to deliver an average of 20% improvement in the system power, a 50% reduction in the total heat exchanger footprint, and a reduced system complexity. These advantages bode well for an integrated and relatively compact engine cooling and exhaust heat recovery solution for future automotive HDDEs. Implementation of the proposed system at mid-speed high-load engine operating condition increased the overall BTE from 41.4% to a maximum of 43.6%.
Bibliographical note© 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
- Organic Rankine Cycle
- Diesel engines
- Energy and exergy analysis
- Size and performance trade-off
- Water blends
- Dual-pressure system