Design of organic Rankine cycle power systems for maritime applications accounting for engine backpressure effects

The installation of an organic Rankine cycle unit on the exhaust line of a marine engine imposes an increase in the backpressure on the engine, resulting in a decrease of the engine performance and a variation of the available waste heat. In this paper, a method is presented for the optimal design of organic Rankine cycle power systems for waste heat recovery in marine applications. The method is based on the use of performance maps for the engine and numerical models for the organic Rankine cycle unit and the waste heat recovery boiler, thereby enabling consideration of the effect of the increased backpressure on the performance of both the main engine and the organic Rankine cycle unit. The method is evaluated on a hypothetical containership fuelled by liquefied natural gas. The results of the study indicate that the overall system fuel consumption can be reduced by 0.52 g/kWh to 1.45 g/kWh by allowing higher backpressure levels on the engine. In addition, the results of the study indicate that for a fixed power output of the organic Rankine cycle unit, a reduction of the space requirement for the waste heat recovery boiler by up to 35% can be attained when increasing the maximum allowed engine backpressure from 3 kPa to 6 kPa.