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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 investment cost of the ORC unit. Given the high volatility of fuel prices, the way to enhance the economic feasibility is to decrease the specific cost of the WHRS. This can be accomplished by improving the system performance, e.g., adopting mixtures as the working fluid or super- critical cycles, while simultaneously abating the purchase cost of the components. There may be prospects for attaining the latter by em- ploying alternative manufacturing processes and materials. A trade-off often exists between power output and total investment cost. The payback period can thus be minimized by adopting multi-objective optimization methods; see, e.g., Pierobon et al. [103]. Geometrical constraints, such as space and weight availability on ships, can also be added to the set of considerations. At this end, the traditional design work-flow whereby one: (i) selects the fluid, (ii) designs the thermodynamic cycle, and (iii) performs the preliminary design of the heat exchangers and the turbine based on the output of the previous phase, proves to be inadequate. For instance, a thermodynamic cycle with a high expansion ratio may achieve better conversion efficiency, whilst it can result in a turbine with a high number of stages and prohibitive production costs. Similarly, a too bulky heat exchanger may not fit within the area available in the engine room. Hence, the more adequate approach is to carry out the optimi- zation of the cycle and the components simultaneously. Pioneering configurations of the marine energy system that integrate the ORC system with other energy systems aboard, e.g., the refrigeration system for reefer containers, shall also be devised. For this purpose, Prater [179] listed three prerequisites for WHRSs: (i) minimal losses from the heat en route to the conversion unit, (ii) efficient vapor expansion, and (iii) limited system complexity. These aspects point towards the design and optimization of the engine and the WHRS as a whole. The Still engine, combining these three prerequisites, is one example of such approach [180]. Steam, generated from waste heat, is expanded under the piston to reduce compression work in a marine diesel engine. This, in turn, improves the thermal efficiency of the system. Prater [179] proposed an integrated six-stroke piston engine and a WHRS. The modelled efficiency was 56%, a significant increase from a 38% base- line. Conklin and Szybist [181] studied a six-stroke concept where water was injected directly into the cylinder, thus using the exhaust heat to produce additional power. In these examples, no costly turbine is required. More recently, Larsen et al. [182] embraced these pre- requisites in a proposal to integrate the ORC expansion process into a two-stroke marine main engine design by using an engine cylinder as the expander. Thus, the need for the generator, turbine and electrical equipment could be removed and thereby entail a 50% reduction in the ORC capital costs and fuel savings up 8.3%. In a recent study, Larsen at al. [59] compared the performance of five different main engine configurations, two of which included ORC units for exhaust heat recovery. The design and tuning of the main engine and the design of the ORC units were optimized in a multi-ob- jective optimization considering minimization of NOx emissions and specific fuel oil consumption. The results indicate that for the combined cycle (main engine and ORC unit) fuel savings of up to 3.5 g kWh−1 can be obtained if the main engine fuel consumption is increased by 0.5–1 g kWh−1. Further research is required to fulfill this potential, for ex- ample, by investigating to what extent such tuning could be done in practice and what barriers exist. With regards to the use of alternative fuels, in order to maximize the performance of the whole machinery system, its optimization and the engine design and tuning, need to be accomplished simultaneously by considering the whole energy system on board the vessel. By employing such approach, it is possible to adapt the engine design and tuning for the WHR system, and take advantage of the possibilities for integration of the fuel system with the WHR system. For the combined optimization of WHR systems and engines using alternative fuels, for which previous operational experience is scarce, accurate simulation tools need to be developed in order to predict the effects of engine tuning on the per- formance and the formation of NOx emissions for different fuels. In the case of vessels running on highly flammable gases such as natural gas or hydrogen, safety and classification requirements need to be carefully considered with respect to the placement of the ORC unit (possibly using a flammable working fluid) on board the vessel. These precau- tions (e.g., double piping, placement of the ORC unit in a separate gastight compartment) may result in a more expensive installation. In the case of an LNG engine, where the ORC unit could utilize the LNG as the cold source, a number of crucial aspects in addition to the safety and classification requirements would need to be addressed for its practical implementation. These include the design of expanders tailored for a high-pressure ratio, and the derivation of control strategies suitable for ORC units using the varying LNG fuel flow rate as a heat sink. Additionally, ship voyage pattern-based optimizations are relevant. Baldi et al. [60] analyzed the importance of optimizing the system considering the engine operational strategy and ship voyage speed. The optimization of an ORC unit considering only the design-point speed gives fuel savings of 7%. Conversely, the results indicate that this value can be increased to 11% when optimizing the design taking into ac- count the ship voyage. As for the working fluid, the main challenge is to comply with the environmental and safety requirements. For instance, the most suitable compounds to recover the exhaust gas heat are hydrocarbons. These have a high flammability risk. Research on new non-flammable fluids tailored to marine applications should be carried out considering, e.g., HFOs with a higher number of carbon atoms. For the low-temperature heat sources, the new working fluids (HFOs) with low ODP and GWP are viable alternatives. Most of them are non-toxic, and have a low or moderate flammability risk. Currently, refrigerant manufacturers are working on the development of HFO blends suitable for retrofitting HFCs, such as R410A or R134a. These fluids are expected to be com- mercially available in the coming years. The development of equations of state and correlations for the thermophysical property estimation of these new fluids could reduce the current uncertainty about their practical performance, clarifying their potential prospects for utiliza- tion aboard ships. Another issue hindering the use of environmentally-friendly fluids is their high price. In this regard, the manufacturing cost seems to in- crease with the number of fluorine and carbon atoms that the molecules contain. Bivens and Minor [183] pointed out that the manufacturing costs of the new generation of replacements could be higher than those of HFCs. However, the cost of the fluids currently in use will increase as a consequence of supply shortages [184]. This situation will arguably drive the change towards the new generation of fluids. 7. Conclusions This work presents a detailed literature survey of research works on the use of organic Rankine cycles for waste heat recovery on board ships, and provides an analysis of the potential and the challenges of using this technology in retrofitting current vessels and in new-build- ings. The available waste heat and the recoverable energy were esti- mated for three representative operating vessels. Guidelines on the in- tegration on board and on the selection of the optimal cycle architecture, working fluid, components, and control strategy were provided. The economic feasibility was also estimated, and the con- straints imposed by the integration on board were presented. Finally some alternative WHR technologies were reviewed, and potential R&D areas in this field were enumerated. The analysis in this paper indicates that the jacket cooling water and the exhaust gases of the engine are the most suitable heat sources for ORC systems on board ships. Particularly, waste heat recovery from the exhaust gases from engines using low-sulfur fuels can be very pro- mising, with estimated voyage fuel savings of around 10%, although savings of up to 15% are expected, since more advanced design methods in the short term have the potential to boost the ORC per- formance. ORC units recovering the heat of jacket cooling water would 147PDF Image | organic Rankine cycle power systems for maritime applications
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