Design and performance evaluation of an Organic Rankine Cycle system exploiting the low grade waste heat of the main engines in a LNG carrier

Organic Rankine Cycle (ORC) technology may represent an interesting way to exploit the low grade waste heat rejected by the ship power generating plant. This option has been investigated here to recover the heat available from three of the four engines of a real electrically driven Liquefied Natural Gas (LNG) carrier, having an electric power output of 23,375 kW. A detailed analysis of the engines operation was first performed to create a reliable set of thermodynamic parameters that fulfil the energy balance of the engines. Heat associated with the jacket water, lubricating oil and charge air cooling of the engines has been considered to be available for the ORC, while the heat from the exhaust gases is already exploited to generate low pressure steam for ship internal use. Simple, regenerative and two-stage ORC configurations have been compared using six different organic fluids that were selected as the most suitable for this application. The thermal matching that maximizes the net power output of the total system including engine cooling circuits and ORC cycle is found by applying the Heatsep method, which allows the optimum heat transfer between thermal streams to be evaluated independently of the structure/number of the heat exchangers within the system. Three layouts of the cooling systems collecting the heat available from the engines have been compared. Results show that the maximum net power output (820 kW) that is achievable by the two-stage configuration almost doubles the simple cycle and regenerative ones (430-580 kW). Economic feasibility is in any case to be verified.