Waste-Heat Recovery from Combustion Engines based on the Rankine Cycle

Licentiatavhandling, 2013

The majority of the energy in the fuel burned by the combustion engines used in modern vehicles is lost in the form of waste heat and does not contribute to the propulsion of the vehicle. Three different technologies have been proposed for recovering some of this lost heat and thereby increasing the overall efficiency of combustion engines: the turbocompound, thermoelectric converters, and heat engines based on the Rankine cycle. This thesis is about systems based on the Rankine cycle and the challenges associated with their incorporation into vehicles that are not encountered in more conventional applications. One such challenge relates to the selection of a suitable working fluid. To address this issue, a range of candidate fluids were evaluated, including organic fluids, ammonia, and water. In simulations, the best results were achieved using water-alcohol mixtures. Mixtures with a water content of 80 % by mass were found to be particularly useful since they are non-flammable and do not suffer from the freezing problems encountered when using pure water. Pure organic fluids were found to present numerous problems, including their low thermal stability, safety issues and in case of most organic refrigerants the potential to increase global warming. Another key challenge in the development of Rankine cycle systems for vehicles relates to the design of suitable expansion devices. Two expander types are considered suitable for vehicular systems: turbo expanders and displacement expanders. In order to establish a method for determining which type will offer the greatest efficiency in any given case, an analysis based on dimensionless numbers was performed. Displacement expanders were found to have favorable performance characteristics in situations involving high expansion pressure ratios and low flow rates; such conditions tend to increase the thermal efficiency of the Rankine cycle. On the other hand, turbo expanders can be made more compact than displacement expanders and may therefore be more suitable in cases where space is at a premium. Moreover, by using a pure organic working fluid instead of the suggested water-alcohol mixture and decreasing the expansion pressure ratio, the cycle parameters can be adjusted to permit the efficient operation of turbo expanders. Based on the above analyses of the system’s components, a model heat recovery system was created using the GT-Suite 1-D flow simulation program. This model can be used in conjunction with a previously established model of a heavy-duty diesel engine created using the same software, which was in this work converted to mean-value model in order to permit faster computation.