energies Article On the Possible Introduction of Mini Gas Turbine Cycles Onboard Ships for Heat and Power Generation Dario Barsi , Matteo Luzzi *, Francesca Satta and Pietro Zunino Department of Mechanical, Energy, Management and Transportation Engineering, University of Genoa, 16145 Genoa, Italy; [email protected] (D.B.); [email protected] (F.S.); [email protected] (P.Z.) * Correspondence: [email protected] Abstract: The recent coming in force of MARPOL 2020 restrictions on shipping pollutant emissions highlights a growing interest in current times towards cleaner means of transport. One way to achieve more sustainable vessels is represented by updating onboard engines to suit current regulations and needs: Gas Turbines are not a novelty in the field and, despite the few applications in commercial shipping so far, this technology is again under evaluation for different reasons. Indeed, it is still a preferred choice in navy, where swift maneuvering is a key factor; it is employed by fast ferries and hydrofoils for its high power/weight ratio; it has been recently applied to LNG carriers to burn boil-off gas in a more efficient way and several studies in literature suggest its possible introduction on large Cruise Ships. Since there seems to be a lack of research concerning small size units, the present work attempts to evaluate the possible usages of Mini Gas Turbine Cycles in the range of 1 to 10 MW of electric output for heat and power generation onboard commercial vessels dedicated to passenger transport. For this purpose, a statistical analysis on existing operating vessels up to 2020 was made, to eplore main engine sizes; a literature review was carried out to find representative onboard heat demands. Once the main vessel electrical and thermal requirements were evaluated, Mini Cogenerative plants based on Gas Turbines were designed within the identified boundaries Citation: Barsi, D.; Luzzi, M.; and compared with state-of-the-art Marine Diesel Engines and Gas Turbines on estimated global Satta, F.; Zunino, P. On the Possible Introduction of Mini Gas Turbine performance, dimensions and weights. Cycles Onboard Ships for Heat and Power Generation. Energies 2021, 14, Keywords: COGES; mini gas turbines; turbo-generators and steam turbines; marine propulsion 568. https://doi.org/10.3390/en plant; cruise ships; statistical analysis 14030568 Academic Editor: Andrzej Teodorczyk 1. Introduction Received: 11 December 2020 A worldwide attention is focusing nowadays on transport systems, looking for more Accepted: 18 January 2021 energy efficient and sustainable means of transportation [1]: in the field of navigation, Published: 22 January 2021 January 2020 saw MARPOL Annex VI [2] convention coming in force, with the aim to significantly reduce pollutant emissions of Nitrogen and Sulphur oxides (NO and SO ) Publisher’s Note: MDPI stays neu- X X onboard ships [2,3]. Such goals can be achieved by installing more efficient power plants: tral with regard to jurisdictional clai- ms in published maps and institutio- among the possible solutions, gas turbines represent a promising choice, as these machines nal affiliations. need to run on clean fuels, such as Sulphur-free liquified natural gas (LNG), and allow for better combustion temperatures control-compared to current internal combustion engines (ICE)—to reduce NOX pollutants. Already tested for commercial applications in the early 2000s [4], gas turbine onboard plants have shown performances comparable Copyright: © 2021 by the authors. Li- to current marine engines [4–8], reaching 50% electrical efficiency with a combined cycle censee MDPI, Basel, Switzerland. for electric-drive propulsion. Applications of gas turbines (GT) as prime movers can be This article is an open access article found mainly in navy [5,7,9] and LNG carriers [8,10–12]: the former ones are justified by distributed under the terms and con- the need for swift maneuvering, thanks to the limited transient times associated to GTs ditions of the Creative Commons At- [5,7]; the latter rely on the possible usage of boil-off gas (BOG) in the main engine, instead tribution (CC BY) license (https:// of re-compressing it, along with some savings in onboard space-allowing for more cargo creativecommons.org/licenses/by/ capacity [8,10,11]. Multiple studies in the field suggest that the introduction of such plants 4.0/). Energies 2021, 14, 568. https://doi.org/10.3390/en14030568 https://www.mdpi.com/journal/energies Energies 2021, 14, 568 2 of 12 on ferries and large cruise ships [9,11,13,14] would allow for competitive efficiencies with space savings-depending on several design choises: a thoughtful selection in Heat Recovery Steam Generator (HRSG) type and layout could improve space [5], engine comparisons could be made to find the best combination of components and reduce weight [15], while vessel emissions are mainly subjected to fuel type and engine efficiency [16], to avoid introducing further components for exhaust gas treatment. However, while there are examples of studies concerning the feasibility of gas and steam turbines (ST) as main or auxiliary engines [13–15] and possible improvements to waste heat recovery through such machines [17,18] on large ships with high energy and power requirements, there seems to be a lack of similar work on smaller size GT units. Usually, compact turbomachines are mainly considered for ICE turbocharging, with niche applications for power generation [19]. We call Mini Gas Turbine Cycles [20] the GT-based energy generation systems, which are suited for ratings in the order of magnitude of 1 or a few electric MW (MWel). Thus, the present paper attempts to define the possible applications, advantages and limits of Mini Gas Turbine Cycles for heat and power generation onboard commercial and passenger ships. To assess this topic, a review of currently operating worldwide vessels is made, in order to define a suitable engine size; a literature review concerning energy and heat requirements onboard cruise ships is performed, with the aim to investigate the magnitudes of onboard heat and electric demands; finally, different plant architectures for Combined Gas and Steam Cycles are compared within the boundaries defined by the previous analysis, to determine the best options in terms of performance, size and weight. A comparison with current naval prime movers - both Diesel engines and marine Gas Turbines at state of the art - is made, to show the possible advantages of the proposed solutions. 2. Analysis Methodology and Modeling 2.1. Worldwide Ships Statistics With the aim to perform statistics on a representative sample of actual operating ships, data from SeaWeb Database [21] were extracted on worldwide vessels registered from 1980 to 2020 for Cruise, Ro-Pax and Passenger ship types. Restrictions on total installed power and passenger number were chosen, to select main engines of relatively small size, but still with non negligible heat requirements. Database characteristics and filter limits are summarized in Table1: here, Gross Tonnage (GT) indicate a conventional measure of ship size, being related to the internal volume of vessels. The acquired data were checked, in order to remove all non-operating vessels, missing information or mistakes. Table 1. Database characteristics. Vessel Type Cruise Passenger Ro-Pax Trip description leisure transport, short distances leisure, transport Cargo people people, vehicles, goods people, vehicles, goods Vessel Nr. 111 464 917 Installed Nominal Power (MW) <30 <30 <30 Pax Nr. ≥30 ≥30 ≥30 GT range 100 to 75,000 100 to 15,000 100 to 70,000 Figure1 describes the two variables chosen for statistical analysis: total installed main engine power and an average engine size, defined respectively as the sum of the nominal power of all engines onboard and the arithmetic average based on the number of main engines. Statistical analysis was performed by the program R, described in [22]; significance level was set to p < 0.05. Since none of the data arrays was normally distributed (Lilliefors; p < 0.001), non parametric Pairwise Wilcoxon test was performed for comparison between ship types. Passenger vessels were found to be significantly different to both Cruise and Ro-Pax (Wicoxon; p < 0.001), while no significative differences were found between Cruise and Ro-Pax vessels. From these findings, it was chosen to focus only on Cruise and Ro-Pax ships, in the following steps. The graphical outliers in Figure1 were further checked, to avoid data inconsistency: the Cruise ones represent vessels from a Energies 2021, 14, 568 3 of 12 specific company, Marella Cruises (Luton, UK), which have slightly larger engines installed compared to the rest of the database; Ro-Pax graphical outliers represent the fleet of MOL Ferry Co. (Tokyo, JP), a japanese ferry company which delivers high-speed travel through the islands of Japan, thus needing power to reach speeds up to 35 knots; finally, Passenger vessels graphical outliers were also checked, but no mistaken or missing data were found. For this last ship type, it was concluded that the scattering of the distribution is related to how different design solutions can be chosen for varying transport needs. (a) (b) Figure 1. Boxplots of (a) Total nominal power installed (Wicoxon; p < 0.001 for Pax vs. Cruise and Pax vs. Ro-Pax) and (b) Average engine size (Wicoxon; p < 0.001 for Pax vs. Cruise and Pax vs. Ro-Pax). *** for p < 0.001 vs Cruise and vs Ro-Pax. Statistics are summarized in Table2: medians were chosen as representative values for both variables, being more robust for non-normal distributions; a confidence interval with 95% accuracy was also defined, based on the distribution quantiles calculated in the program R [22]. Table 2. Database statistics for (a) Total installed power and (b) Average engine size. Quartiles Q1 and Q3 correspond to the lower and upper boundaries of the boxplot in Figure1;Q 2 is the median of the distribution and LCV and UCV are the lower and upper extremes of its 95% confidence interval.