An improvement to waste heat recovery in internal combustion engines via combined technologies

Waste heat recovery (WHR) in internal combustion engines (ICEs) is very interesting opportunity for reducing fuel consumption and CO2 emissions. Among the different heat sources within an ICE, exhaust gases are certainly the most suitable for potential recovery. The most promising technology is represented by power units based on organic Rankine cycles (ORCs). Unfortunately, their actual efficiency is far from that obtainable using only thermodynamic evaluations: low efficiencies of small-scale machines, strong off-design conditions, and backpressure effect are the main reasons. To improve the conversion efficiency, this paper presents a combined solution, coupling two thermodynamic cycles: Joule-Brayton and Rankine-Hirn ones. The first (top cycle) considers supercritical CO2 as the working fluid and the second considers an organic fluid (R1233zDe, bottom cycle). The combined recovery unit inherently introduces further complexity, but realizes an overall net efficiency 3-4% higher than that of a single ORC-based recovery unit. The hot source is represented by the exhaust gas of an IVECO F1C reciprocating engine, considering twelve experimental operating conditions that fully represent its overall behaviour. The combined unit was modelled via a software platform in which the main components of the ORC unit were experimentally validated. The best thermodynamic choices of the top and bottom cycles, as well as their mutual interference, were identified under the condition of maximum power recoverable; this implies an overall optimization of the combined unit considered as an integrated system. Moreover, the components must be handled when off-design conditions (produced by the unavoidable variations of the hot source) occur: insufficient heat transferred to the two working fluids, produced by an over- or under-designed heat exchanger, would prevent the proper operation of the two units, thereby reducing the final mechanical power recovered. To address this critical issue, the three main heat exchangers were designed and sized for a suitable ICE working point, and their behaviour verified to guarantee a suitable maximum temperature of the supercritical CO2 and full vaporisation of the organic fluid. These two conditions assure the operability of the combined recovery system in a wide range of engine working points and a maximum recovered mechanical power up to 9% with respect to the engine brake one.