Modelling and simulation of waste heat recovery systems for marine applications

Waste heat recovery systems have assumed an important role in the last decade as an effective way to improve fuel utilization in thermal engines, since they provide an opportunity to produce eco-friendly electrical power from an otherwise wasted energy source, leading to a reduction of the pollution and an increase of the overall system efficiency. In this scenario, Rankine cycle technology, based on simple Rankine cycle or Organic Rankine cycle (ORC), earned a good market position, thanks to its ability of producing additional electric power from relatively low- Temperature heat sources (80 to 350°C); this feature makes these cycles a very suitable solution as bottomers to Internal Combustion Engines (ICE), geothermal sources, solar thermal modules and micro-gas turbines. This paper presents a waste heat recovery system specifically designed as a bottomer to ICE of different power range for marine applications, using a simple Rankine cycle or a Rankine cycle with ORC further bottoming. The particular application considered shows several advantages for the installation of a waste heat recovery system; in particular, the basically infinite availability of the cooling medium represented by the water sea, avoids any issues in the condenser design. A steady state model of the system is developed via the process simulator CAMEL-Pro™, in order to identify the most convenient configuration, from a thermodynamic point of view. A most interesting feature of this study with respect to previous works, is the comparison of the organic fluids R245fa and R600, widely recognized as two of the best candidate ORC fluids, within a configuration analysis that critically and analytically compares the improvements regeneration can possibly bring in the low grade regime. To this scope, different cycle configurations have been modeled, simulated and comparatively assessed, in order to figure out whether the more expensive Rankine bottoming ORC brings substantial improvements over the simpler Rankine cycle configuration and to investigate system behaviors in bottoming applications to atmospheric intake or turbo-charged marine Diesel engines. The paper shows how adding a bottoming ORC to the Rankine cycle improves the WHR system performance both in terms of recovered electric power (up to 8.11% and 2.67% respectively in small and large application size) and heat source utilization rate, since the heat source temperature could reach values as low as 70 °C when considering a Sulphur free fuel. In addition, R 245fa is to be preferred over the R 600 since it allows for the production of the same power considering lower values for the ORC top pressures.