Integrated system approach for increase of engine combined cycle efficiency

Internal combustion engines (ICE) are widely used as independent power producers due to their high electrical efficiency (up to 47 %), their fuel flexibility and their ability to respond fast to load alternations. By applying a power cycle for waste heat recovery (WHR cycle) to the ICE, the electrical efficiency can be increased further up to 50 %. The recoverable engine waste heat is usually available at two different temperature levels: 300 °C to 400 °C (exhaust gas) and 90 °C (cooling water). According to studies found in literature, just a small portion of low temperature heat (ca. 6 %) can be used for the preheating of the working fluid either in water/steam or Organic-Rankine- cycles (ORC).For the integrated optimization of the overall system efficiency in an engine combined cycle, not only the most favorable design of the WHR cycle, but also the optimal configuration of the ICE cooling system, have to be investigated. The aim of this paper is to examine the possible changes in the cooling system, with respect to technical limitations given by the ICE, in order to define the optimal temperature level of the low temperature heat source. For that reason, different types of engines combined with a water/steam WHR cycle are simulated. An energetic, exergetic and economic analysis is conducted in order to determine the impacts of the temperature level to the WHR system and to the design of the engine cooling system. The calculations performed, showed that up to 19 % of the low grade heat can be efficiently recovered.