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Session: Parallel Session: Small-capacity systems
Session starts: Friday 23 September, 14:00
Presentation starts: 14:20
Room: Auditorium

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Wolfgang Lang (Institute for Internal Combustion Engines and Thermodynamics, Graz university of Technology)
Piero Colonna (Process and Energy Department, Delft University of Technology)
Raimund Almbauer (Institute for Internal Combustion Engines and Thermodynamics, Graz university of Technology)

Abstract:
This presentation documents recent work performed within a cooperative project funded by the Austrian funding agency involving TU Graz, TU Delft and two European OEMs targeted at investigating options for converting the thermal power discharged by automotive engines via flue gases and the cooling system into mechanical/electrical power. The efficiency of reciprocating engines of cars and trucks has arguably reached its maximum limit and only marginal gains can be obtained by improving on currently adopted technologies. These engines discharge to the environment approximately 66% of the fuel energy content as thermal energy. The energy is available at different temperature levels depending on the type of engine: in car engines the temperatures range from 300 °C to 900 °C for the exhaust gases, and from 90 °C to 110 °C for the cooling system; in truck engines the two heat sources with the highest potential are the exhaust gases with temperatures ranging from 200 °C to 400 °C but thermal energy is available at even higher temperatures (280 °C to 580 °C), if also the heat from the exhaust gas recirculation (EGR) system is recovered. It is apparent that there is still a large fraction of the primary energy that is still untapped and the potential overall energy efficiency gain offered by effectively recovering wasted thermal power is very large [1]. The principle is already widely exploited in stationary power plants, while application on board of vehicles is very challenging and no commercial application exists. The current energy scenario has resumed strong interest into automotive heat recovery systems, much like it happened in the 70’s as a consequence of the first oil crisis [2]. This work is focused on one of the possible solutions to the technical problem of heat recovery for car and truck engines, namely a compact ORC turbogenerator using a siloxane as the working fluid. Two paradigmatic examples of operating conditions taken from existing automotive propellers are considered, one for a truck engine and one for a car engine. The design envelope is explored in terms of working fluid selection, thermodynamic cycle configuration, preliminary turbine and heat exchangers design, taking into consideration all the stringent requirements imposed by the automotive application. A Rankine power system using water as the working fluid is taken as a benchmark and the challenges related to adopting water as the working fluid are discussed. The results of simulations are analysed in order to provide initial guidelines and the most promising routes to successful implementation are outlined.