Waste heat recovery with Organic Rankine Cycle (ORC) in marine and commercial vehicles Diesel engine applications

Heavy and medium duty Diesel engines, for marine and commercial vehicles applications, reject more than 50-60% of the fuel energy in the form of heat, which does not contribute in terms of useful propulsion effect. Moreover, the increased attention towards the reduction of polluting emissions and fuel consumption is pushing engine manufacturers and fleet owners in the direction of increasing the overall powertrain efficiency, still considering acceptable investment and operational costs. For these reasons, waste heat recovery systems, such as Organic Rankine Cycles (ORC), are undergoing a period of intense research and development. However, in most of engine waste heat recovery studies in literature, the engine side analysis is not considered in a detailed way, even though the engine architecture and the operational behaviour strongly influence the availability of heat sources, and their characteristics, to be recovered using heat recovery systems. As an example, the use of emission reduction strategies, such as Exhaust Gas Recirculation (EGR), can introduce an additional heat source and modify the temperatures in the engine gas lines, thus leading to new possible scenarios for the exploitation of the engine wasted heat. The scope of this work is to introduce a combined engine-waste heat recovery system analysis and design methodology, which could go beyond the traditional development approach, considering both the engine and the ORC system as a synergic and integrated powertrain. For this reason, industry-standard engine gas dynamics simulation software and thermodynamic process simulation techniques have been used and developed in order to study the combined effects and performance of engine-ORC systems in the commercial vehicle and marine sectors, addressing at the same time several development issues, such as: working fluid and layout choice, powertrain thermal management, energy utilization, turbocharging and emission reduction strategies, in the direction of a co-simulation approach, which is one of the industry’s main interests, to reduce development time and costs. After a detailed literature review and modelling approach explanation, four different case studies have been proposed, to show an increasing level of integration between engine and ORC system analysis, addressing also applications which are not commonly considered in literature, such as off-highway vehicles and two-stroke ships propulsion units. The combination of energy, exergy and economic analysis, allows the developer to deeply understand the thermodynamics of the combined engine-ORC systems, addressing all the energy and exergy streams available for heat recovery, highlighting the main sources of inefficiencies in the powertrain, and proposing improvements to increase the overall system efficiency at acceptable investment and operational costs. The methodology can be, in principle and with further developments, applied to any kind of engine-waste heat recovery system powertrain. Moreover, the combined use of emission reduction strategies and new technologies, such as EGR and ORC, can allow to develop clean, but at the same time efficient, propulsion units. However, while for commercial vehicles the recovery of high temperature exhaust gas and EGR heat is more beneficial in terms of compromise between performance, system costs and packaging issues, in the case of large ship propulsion units, the recovery of lower temperature heat sources, such as coolant and scavenge air, could become very interesting for future developments, because of the high amount of heat available, even if at lower temperature levels, suitable for the use of an ORC technology. The results of the proposed case studies show a fuel consumption reduction up to around 5-10% when adopting ORC systems, depending on the application, type of engine, overall system architecture and design point chosen, showing the potential of the technology in the considered sectors.