A review of waste heat recovery from the marine engine with highly efficient bottoming power cycles

Highlights

• Different bottoming power cycles on board ships were compared.

• Off-design performance and technoeconomic evaluation were reviewed.

• Integration of waste heat recovery with emission reduction was discussed.

• Fuel savings ranging from 4% to 15% can be expected with different power cycles.

• There is no simple solution to cover all kinds of marine applications.

Abstract

This paper aims at presenting an extensive review of waste heat recovery (WHR) from the marine engine with highly efficient bottoming power cycles which include the steam Rankine cycle, organic Rankine cycle, Kalina cycle and CO₂-based power cycles. After detailed introductions and comparisons of the bottoming power cycles, the design and selection of system components are reviewed. An in-depth survey of the WHR systems operating under off-design conditions is then conducted, followed by a summary of technoeconomic evaluation. Finally, challenges and opportunities of integrating the WHR systems with other emission reduction technologies are discussed.

According to the literature, trade-offs between working fluid characteristics, cycle configuration, size, cost and WHR potential should be made in designing an optimal bottoming power cycle, and fuel savings ranging from 4% to 15% can be expected. The payback time of installing a bottoming WHR system lies typically in the range from 3 to 8 years, depending on the fuel price, ship type, heat and power demands and component costs. The high-pressure exhaust gas recirculation technology is superior to other NO_x reduction technologies due to its high potential of recovering waste heat from the high-temperature recirculated gas. Ship's operational profiles, engine tuning and slow steaming as well as other emission reduction technologies are recommended to be fully considered for future research on WHR systems.

Introduction

According to the “Third International Maritime Organization (IMO) Greenhouse Gas (GHG) Study” in 2014, CO₂ emissions from ships were responsible for roughly 3% of global CO₂ emissions on average from 2007 to 2012 and will increase by 50%–250% from 2012 to 2050 depending on future social and economic conditions [1,2]. Despite international shipping being omitted from the “Paris Agreement” [3], the IMO developed its own regulation for CO₂ emissions in “MARPOL Annex VI” with the Energy Efficiency Design Index (EEDI), which is used to calculate a vessel's energy efficiency and will gradually be tightened. Compared to ships constructed in 2013, ships built between 2020 and 2024 must be 20% more efficient, and those built in 2025 or later must be 30% more efficient [4].

Fig. 1 presents CO₂ emissions from different types of ships in 2012 [2]. It is clearly shown that CO₂ emissions are dominated by three ship types: container ships, bulk carriers, and oil tankers. The container ships, featured with high design speeds and deadweights, make the largest contribution to the total CO₂ emissions of up to 25.6%, followed by bulk carriers (20.8%) and oil tankers (15.5%). Other types of ships, such as general cargo, chemical tankers, cruise and fishing vessels, account for around 38.1% of the total CO₂ productions.

Many technologies and operating strategies have been proposed and adopted to reduce GHG emissions from ships. Slow steaming, optimization of the hull and propeller, hull coating, air lubrication, and wind assistance are typical approaches for improving the ship propulsion efficiency [5,6]. Changing to low-carbon fuels also shows great potential. The resulting CO₂ emissions with liquefied petroleum gas (LPG) and liquefied natural gas (LNG) are roughly 20% lower than that with the conventional heavy fuel oil (HFO) [7]. For marine engines, especially the low-speed two-stroke diesel engines, the high thermal efficiency (approx. 50%) is close to the highest value according to the Carnot theorem, considering the standard engine design restrictions today [7]. Thus, further optimizing the in-cylinder power cycle could not make a large improvement in the engine efficiency. Since around 50% of the fuel energy is lost to the surroundings, waste heat recovery (WHR) has been proposed and testified as an efficient way to produce more power without additional fuel consumption. Decreased specific emissions of nitrogen oxides (NO_x), sulfur oxides (SO_x), and particulate matter (PM) arising from fuel savings also alleviate constraints from more and more stringent emission legislations.

Fig. 2 shows the heat balance of a typical marine two-stroke engine (MAN 12K98 ME/MC) operating at 100% specified maximum continuous rating (SMCR) [8]. It is observed that the exhaust gas accounts for around half of the total waste heat, followed by scavenge air cooling, jacket water cooling, lubricating oil, and a small amount as radiation. In general, the exhaust temperature after the turbocharger lies between 250 and 300 °C for the two-stroke engine and 300–350 °C for the four-stroke engine, depending on load and ambient conditions. For the marine engine burning with HFO, a risk of sulphuric acid corrosion in the exhaust stream should be avoided in the WHR system design [9]. This limits the potential of WHR from the exhaust gas. The scavenge air featured with a high temperature of up to 200 °C at full load, also contains a promising amount of waste heat. Besides, the scavenge air can be cooled without considering the dew point temperature of sulphuric acid formation. However, the heat availability in the scavenge air is load-dependent, since the scavenge air temperature drops down to 50 °C when the engine operates at 25% load. Thus, WHR from the scavenge air is more attractive for ships operating with high load profiles, such as bulk carriers and tankers [10]. Despite being a low-quality heat source with temperature ranging between 70 and 90 °C, the jacket water heat is large in quantity and continuously available during engine operation. It is therefore possible to recover more energy from the jacket water than that from the scavenge air when the engine operates at loads lower than 50%. Waste heat in the lubricating oil shows less attraction, and the small amount of radiation heat cannot be recovered directly. For detailed heat source conditions at full and part loads, the authors collected data of different marine engines and put them together in the ‘Supplementary material’.

Considering on board heat and power demands, waste heat can be utilized for heating, freshwater production, refrigeration, and electric power generation. Heating requirements for HFO, lubricating oil, and accommodation space are always fulfilled with saturated steam produced in an exhaust economizer. Freshwater for daily life is advisable to be abstracted from seawater with multi-stage flash and multiple effect distillation. Apart from driving refrigeration devices with electricity, large cooling requirements, especially on container ships, can be fulfilled with absorption refrigeration and adsorption refrigeration driven by waste heat [[11], [12], [13], [14]]. To cover part or total electricity demand aboard ships, thermoelectric generator (TEG) [15], turbo compounding [16], and bottoming power cycles [17] are three typical technologies, as compared in Table 1. In general, the waste heat conversion efficiency of TEG is still relatively low (<5%), and the system can only become cost-effective when the lower cost of TEG modules and higher value of figure-of-merit are available. The turbocompounding with a power turbine (PT) utilizing surplus exhaust pressure energy can only operate at engine loads higher than 50%. Despite being large scale and high complexity, bottoming power cycles have attracted extensive attention, and potential fuel savings of up to 15% were reported in case studies conducted by Baldi et al. [18] and Mondejar et al. [10].

Table 2 summarizes recent works reviewing WHR technologies applied on different types of vehicles and engines. Chintala et al. [19] and Lion et al. [20] focused their works on heavy-duty diesel engines, while Zhou et al. [21] and Wang et al. [22] reviewed studies conducted on passenger vehicles. Shi et al. [23], Imran et al. [24], Hatami et al. [25], and Sprouse et al. [26] summarized research on working fluid selection, system architectures and component design for small-scale ORC systems. Regarding WHR on marine applications, three review works are available. Shu et al. [27] and Singh et al. [28] aimed at providing general overviews of different WHR technologies including the TEG, refrigeration, desalination, turbocharging, steam RC, Kalina cycle and ORC, and Mondejar et al. [10] presented a comprehensive review of ORC for maritime applications.

Recent years have witnessed a growing number of works on the steam Rankine cycle (RC), organic Rankine cycle (ORC), Kalina cycle (KC), and CO₂-based power cycles on board ships. However, potential fuel savings from case studies depend largely on the engine type, selected waste heat sources, system configurations, and ship's operational profiles. Thus, it is significant to provide a comparative review of the highly efficient bottoming power cycles from different perspectives. In addition, a detailed survey is still lacking regarding component design, off-design performance, and technoeconomic evaluation. Furthermore, no previous review work has been conducted with respect to challenges and potential of integrating the bottoming power cycles with other emission control technologies. Hence, a comprehensive review is still necessary to gain a deeper understanding of the above key issues.