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\u2013580 kW), but structure complexity and reliability issues may give different indications in terms of economic feasibility

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

Conceptual development and optimization of the main absorption systems configurations

Absorption cycle is the leading technology among solar thermally driven cooling/heating systems thanks to its high performance and favourable economics. Most studies deal with specific layouts or compare the performance of different systems. However, a clear comparison of the main configurations at the same boundary conditions and modelling approach is still missing, and the search for improved configurations is still an open field. The goal of this article is twofold: searching for the conceptual development in the evolution of the main existing configurations (single, half, double effect and GAX systems) and identifying new configurations to improve system performance. The first goal is accomplished by analysing in detail the design features and variables of all configurations and creating and running design models at the same cooling duty and ambient conditions. The second one is accomplished by optimizing the design of each configuration using the general methodology “HEATSEP”, which is able to take into account every possible internal thermal interaction in each optimization step. Results include a complete conceptual overview of the development of the existing configurations and the proposal of new ones, which show higher than 3% COP gains in comparison with the best corresponding single effect configurations in the literature

Comparison between the optimum performance of different absorption systems

The various papers appeared in the literature regarding absorption systems often refer to different working conditions and assumptions, making the comparison of results not easy. This work searches for a fair comparison among the performances of different absorption cycle configurations using the same modelling approach and external conditions. A preliminary analysis of type, number and values of the independent design variables of each configuration is first performed. The purpose is identifying proper ranges of variations of these variables for the subsequent design optimization of each configuration. The attention focuses on the main absorption systems proposed in the literature, i.e. single and half effect systems, and GAX one. The goal is finding the optimal COP of each configuration when it operates in the proper range of the heat source temperature, for a fixed value of the evaporation temperature and a minimum allowed condensation temperature. Results confirm the higher coefficient of performance of the GAX cycle compared to the single effect one using ammonia-water mixture. Moreover, the half effect system using water-lithium bromide is suitable to exploit waste heat sources with temperatures too low to activate a single effect. New options of internal heat integration are analysed by applying the HEATSEP approach to foresee possible increases in the performance of the best existing configurations