The application of process integration to the optimisation of cruise ship energy systems: a case study

In recent years, the shipping industry has faced an increasing number of challenges in terms of fluctuating fuel prices, stricter environmental regulations, and concerns about global warming. In this situation, passenger volumes on cruise ships have increased from around 4 million to 13 million from 1990 to 2008 and keep growing today. A small cruise ship can emit about 85 tons of CO2 per day, and require around 27 tons of fuel per day. To keep up with market demand, while reducing their impact on the environment, cruise ships will need to improve their energy efficiency. Most previous research in marine technology relates to energy efficiency focused on propulsion, which for most ship types constitutes the largest energy demand. On cruise ships, however, auxiliary heat and electric power also have a significant importance. For this reason, we focus in this paper on the heat demand and its integration with available sources of waste heat on board. In this study, the principles of process integration are applied to the energy system of a cruise ship operating in the Baltic Sea. The heat sources (waste heat from the main and auxiliary engines in form of exhaust gas, cylinder cooling, charge air cooling, and lubricating oil cooling) and sinks (HVAC, hot water, fuel heating) are evaluated based on one year of operational data and used to generate four operating conditions that best represent ship operations. Applying the pinch analysis to the system revealed that the theoretical potential for heat integration on board could potentially allow the reduction of the external heat demand by between 35% and 85% depending on the investigated case. A technoeconomic optimisation allowed the identification of the most economically viable heat exchanger network designs: two in the “retrofit” scenario and one in the “design” scenario, with a reduction of 13-33%, 15-27% and 46-56% of the external heat demand, respectively. Given the high amount of heat being available after the process integration, we also analysed the potential for the installation of a steam turbine for the recovery of the energy available in the exhaust gas, which resulted in up to 900 kW of power being available for on board electric power demand.