Exergy efficiency and EEXI analysis of a marine power plant at partial load conditions

Energy efficiency and environmental protection are becoming more important in the world. Almost 90% of the world trade is carried by sea which makes energy efficiency very important for the maritime sector. Exergetic method is a powerful scientific tool for measuring energy efficiency. The ships are not always cruising at maximum load conditions. The situations such as narrow water passages, maneuvering, and, hoteling etc. are partial load conditions and the exergy loss in power systems reaches maximum values at these conditions. Besides the new IMO regulation forces the sector to reduce emissions through Energy Efficiency Existing Ship Index (EEXI) and one of the most popular solutions is Engine Power Limitation (EPL). In the literature there are limited studies investigating exergy efficiencies of partial load conditions of a marine power plant. In this article, the performance analyses of a combined power system in partial load conditions are carried out using exergy method. The considered partial load conditions are maximum-ahead, full-ahead, half-ahead, slow-ahead and dead-slow-ahead loads of the engine. The results show that, the exergy efficiency of the overall system is decreasing at lower load conditions. It is observed that the maximum exergy efficiency is 51.6% which is reached at 96 rpm. The best condition considering both exergy efficiency and EEXI is 80 rpm of main engine. At slow-ahead and dead-slow-ahead conditions the exergy efficiency decreases to about 33%.