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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 boiler to supply heat on board. A natural extension to the steam boiler is to add a superheater section and a steam turbine. Several global players of the marine industry [161–164] proposed the use of dual-pressure SRC power systems promising efficiency gains of around 12%, when combining the SRC unit with an exhaust PT. This figure is confirmed by a number of research works. For example, Dimopoulos et al. [165] presented a detailed study of a SRC plant by considering the system performance at different engine loads. They showed that the WHRS allows increasing the thermal efficiency by about 5% points, corre- sponding to an increase in performance of 12%. In this case, the heat sources powering the SRC unit are the exhaust gases, scavenge air, and jacket water heat. Simpler configurations were also investigated. Theotokatos and Livanos [166] presented a techno- economic analysis of a single-pressure SRC unit to recover the waste heat from two-stroke and four-stroke engines. They considered loads ranging from 50% to 100%. The WHRS was fed by the scavenge air and exhaust gas heat. On the two-stroke engine, using a SRC enables in- creasing the thermal efficiency of the vessel energy system by 0.8–1.4%. This improvement on the thermal efficiency can be boosted by up to 3% in the case of four-stroke engines. Hou et al. [167] sys- tematically analyzed four WHRS configurations: 1) no WHRS, 2) PT, 3) single-pressure SRC unit, and 4) PT and single-pressure SRC unit. The application of a variable geometry power turbine was also analyzed. The electrical outputs of systems 2, 3 and 4 were found to be 4.3%, 3.5%, and 9.8% of the main engine power, respectively. The aforementioned studies indicate that a single-pressure SRC plant can be nearly as efficient as a dual-pressure one, and that the PT is of key importance for the WHRS efficiency. Moreover, the SRC unit strongly relies on the high-temperature exhaust gas heat to remain a feasible option. Also, it should be pointed out that SRCs are, in practice, shut off when the engine loads are below 50%, which implies that the potential fuel savings with the use of SRCs would be minor considering the current slow steaming operations. Andreasen et al. [52] carried out a comparison of the ORC and dual- pressure SRC processes for WHR on engines using high-sulfur and low- sulfur fuels. The comparison included considerations about the turbine efficiency and indicated that more efficient turbines employing few turbine stages are possible for organic fluids. In the high-sulfur fuel case, the ORC technology is challenged due to requirements for service steam production and a high boiler feed temperature. However, if it is assumed that the turbine efficiency is 10% points larger for the ORC unit than for the SRC unit, the performances are similar. For the low- sulfur fuel case, both WHR technologies produced significantly more power compared to the high-sulfur fuel case. The design power of the SRC unit increased by 18%, while it increased by 33% for the ORC unit using MM as the working fluid. A comparison between an ORC unit using cis-pentane (with 72% turbine design efficiency) and the dual- pressure SRC unit (with 62% turbine design efficiency) suggested higher performance for the ORC unit at all main engine loads. 5.2. Kalina cycle plants The KC plant, with its ammonia-water mixture working fluid, is claimed to be more efficient than the SRC counterpart in various re- search articles. One example is the doctoral thesis by Jonsson [168] where a thermodynamic comparison of KC and SRC layouts for WHR on large marine four-stroke engines was carried out. It was concluded that the KC plant can supply 40–50% more power than a single-pressure SRC unit, and 20–25% more than a dual-pressure one. Bombarda et al. [169] compared a KC system with an ORC unit for WHR on large marine four-stroke engines. They found that the two plants have similar power outputs. However, they pointed out that the KC unit has important drawbacks compared to the ORC counterpart: i) it has a higher complexity, ii) it requires using more bulky heat ex- changers, and iii) it operates at higher working pressures. More re- cently, Larsen et al. [81] compared a dual-pressure SRC plant with a KC and an ORC unit for a large two-stroke ship engine. The thermal power of the exhaust gases, scavenge air and jacket water was exploited. The results suggested that the ORC module can produce 7% more power than the SRC and KC plants. A number of qualitative aspects were also compared, suggesting that the KC system does not provide any sig- nificant advantage over the ORC and SRC plants. For low-temperature applications, Becquin and Freund [170] pre- sented a thermodynamic investigation of a number of ORC and KC plant layouts. The results indicate that the KC technology can convert 30–50% more power if the heat source temperatures are around 80–90 °C. Unlike Bombarda et al. [169], Becquin and Freund [170] did not find any drawbacks with respect to the HEX area. In this regard, it was found that the literature on KC plants shows contradictory results concerning the claimed advantages. For ship applications, the high toxicity of ammonia is an important drawback of the KC technology. It is also worth considering that the KC is a patented technology. 5.3. Other technologies The use of other technologies for WHR on ships is currently under investigation. For instance, a new technology for WHR on ships is the one developed by Climeon AB [171]. The unit consists of an ORC-like cycle where the evaporation and condensation are replaced with des- orption and absorption processes. This allows achieving high conver- sion efficiencies at low cost. The company has newly installed one unit aboard the cruise ship Viking Grace. The heat source of the WHRS is the high temperature jacket cooling water from the LNG main engines. The thermal efficiency of the system is 10% with an inlet temperature of the heat source of 90 ◦C and a seawater temperature of 20 ◦C [171]. Thermoelectric generators have been also recently proposed as a potential technology for marine WHR [39,172]. Nevertheless, only a few theoretical studies have been carried out, and no unit has been installed aboard ships yet. The main reason is the low energy conver- sion efficiencies (typically around 5%) and the high investment cost. Shu et al. [39] suggested using this technology to exploit the tem- perature difference between the exhaust gases and seawater. In an in- novative application, Shu et al. [173] combined the use of thermo- electric generators with ORC plants using the exhaust gases of an internal combustion engine. They claimed that this electric device can be used to cover the electrical demand of the ORC pump, but at the expense of an increase in investment costs. Loupis et al. [174] evaluated a prototype WHRS based on thermoelectric generators for marine power systems. They concluded that such technology can be econom- ically viable if energy conversion efficiencies around 6.4% are achieved. Another option is the so-called trilateral cycle. Here the working fluid is heated in the liquid phase, and starts expanding from saturated liquid conditions [175]. Given the absence of isothermal evaporation, this feature makes this cycle ideal to harvest the heat from the exhaust gases, as they can be cooled by a temperature difference of up to 100 °C. Choi and Kim [73] studied the use of a water trilateral cycle plant fed by the exhaust gas heat of a marine engine. The system was combined with a bottoming ORC unit. Although no trilateral cycle plant is cur- rently in operation, several works have pointed out that this cycle could attain higher thermal efficiencies than the ORC counterpart (i.e., be- tween 35% and 15% more for heat source temperatures of 115 °C and 160 °C, respectively) [176–178]. In order to realize the trilateral cycle, further research needs to be conducted to enhance the isentropic effi- ciency of expanders operating in the two-phase region. 6. Challenges and future R&D areas As shown in Section 3, the ORC technology can be a viable alter- native to recuperate the heat from the jacket water and the exhaust gases. 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