Design optimization of ORC systems for waste heat recovery on board 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 generation plant. This option is investigated here to recover the heat available from three of the four engines of a real electrically driven Liquefied Natural Gas (LNG) carrier. A detailed analysis of the engines operation is first performed to evaluate all thermal streams released by the engines. Heat associated with the jacket water, lubricating oil and charge air cooling of the engines is found to be available for the ORC, while the heat from the exhaust gases is already used to generate low pressure steam for ship internal use. Simple, regenerative and two-stage ORC configurations are compared using six different organic fluids that are selected as the most suitable for this application. The thermal matching that maximizes the net power output of the total system composed by engine cooling circuits and ORC cycle is then found by searching for the optimum heat transfer between thermal streams independently of the structure/number of the heat exchangers. Three layouts of the engine cooling systems are compared. Results show that the maximum net power output (820 kW) achieved by the two-stage ORC configuration almost doubles the simple cycle and regenerative ones (430-580 kW), but structure complexity and reliability issues may give different indications in terms of economic feasibility.