Integrated system approach for increase of engine combined cycle efficiency

Internal combustion engines (ICEs) are widely used as independent power producers due to their high electrical efficiency (up to 47%), which can be further enhanced by operating them in combined cycle mode with a water/steam cycle as bottoming cycle.This study presents an integrated approach to optimize the combined cycle overall system efficiency. Therefore, not only the most favorable design of the waste heat recovery (WHR) cycle, but also the optimal configuration of the ICE cooling system have to be investigated, in order to integrate both available engine waste heat sources (exhaust gas, 300–400 °C, engine cooling water, 90 °C) into the waste heat recovery cycle. For the definition of the most favourable temperature level of the engine cooling water three variants of engine cooling systems are examined, with respect to technical limitations given by the ICE. In order to determine the types of engines for which this optimization approach is suitable, three types of engines with different characteristics (fuel, exhaust gas parameters) combined with a water/steam cycle are simulated, by using the calculation tools Excel and Ebsilon Professional. An energetic, exergetic and economic analysis is conducted. These reveal 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 engine cooling water heat can be efficiently recovered compared to a portion of 6% in the standard system. The better recovery leads to a 5%-decrease in the costs of electricity generation.

► A new approach for the optimization of engine combined cycle systems is presented.
► The efficiency can be increased by integrating all available waste heat sources.
► The optimization of the ICE cooling system towards higher temperatures is examined.
► Higher engine cooling water temperatures increase the combined cycle efficiency.
► The costs of electricity generation can be decreased by five integrated system.