Estimation of the possibility of Stirling engine applications in LNG carrier power systems

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1 Scientific Journals Maritime University of Szczecin Zeszyty Naukowe Akademia Morska w Szczecinie 2010, 21(93) pp , 21(93) s Estimation of the possibility of Stirling engine applications in LNG carrier power systems Ocena możliwości zastosowania silników Stirlinga w układach energetycznych gazowców LNG Arkadiusz Zmuda West Pomeranian University of Technology, Faculty of Maritime Technology Zachodniopomorski Uniwersytet Technologiczny w Szczecinie, Wydział Techniki Morskiej Katedra Maszyn Cieplnych i Siłowni Okrętowych Szczecin, al. Piastów 41, arkadiusz.zmuda@zut.edu.pl Key words: LNG carriers, propulsion system, waste heat utilization, Stirling engines, marine power plant Abstract The article presents a preliminary estimation of the possibility of using Stirling engines in power and waste heat utilization systems of LNG carriers. Flexibility of applying heat sources, very silent operation and very low exhaust gas emission are to the advantage of applying Stirling engines in marine power plants. Unquestionably, one strong point of Stirling engines is the fact that various heat sources can be used to feed them, including waste heat generated by the main and auxiliary engines and burning boil-off gas (evaporated cargo), which is especially important in the LNG carrier power systems. The discussed issues include gas demand by the main propulsion of LNG carriers together with the amount of boil-off, main propulsion power and electric power demand of LNG carriers of various sizes. Finally, an example system for waste heat utilization and reduction of toxic exhaust gases emission of the employing a Stirling engine is described. Słowa kluczowe: gazowce LNG, układ napędowy, utylizacja ciepła odpadowego, silniki Stirlinga, siłownia okrętowa Abstrakt Artykuł przedstawia wstępną ocenę możliwości zastosowania silników Stirlinga w układach energetycznych i w systemach utylizacji ciepła odpadowego gazowców LNG. Elastyczność w możliwości zastosowania źródeł ciepła, bardzo cicha praca oraz bardzo niska emisja szkodliwych składników spalin stwarza duże możliwości zastosowania silników Stirlinga w elektrowniach okrętowych. Niewątpliwą zaletą silników Stirlinga jest fakt, że do ich zasilania można wykorzystać różnorodne źródła ciepła, w tym ciepło odpadowe generowane przez silniki główne i pomocnicze oraz spalanie odparowanego ładunku, co jest istotne szczególnie w układach energetycznych gazowców LNG. Przedstawiono m.in. zapotrzebowanie na ilość gazu do napędu głównego gazowców LNG na tle ilości odparowanego ładunku, moc napędu głównego i zapotrzebowanie na energię elektryczną dla różnej wielkości gazowców LNG oraz przykład systemu do utylizacji ciepła odpadowego i ograniczenia emisji składników toksycznych spalin wylotowych silników okrętowych z wykorzystaniem silnika Stirlinga. Introduction Increasing fuel costs and more restrictive requirements concerning emission of harmful combustion gas components cause growing interest in waste heat utilization systems on ships. Many interesting solutions of these systems that have appeared recently have a positive influence on the efficiency of maritime propulsion and power systems. One of the promising solutions is an application of Stirling engines in marine power systems and waste heat utilization systems. Such solutions are specifically promising for LNG carriers. On the one hand, boil-off gas (BOG) can be successfully 98 Scientific Journals 21(93)

2 Beale No. (P / pfv0) Estimation of the possibility of Stirling engine applications in LNG carrier power systems used as fuel for Stirling engines, on the other hand, significant amount of large waste energy is typical of large LNG carriers. Stirling engines Flexibility of applying heat sources, very silent operation and very low combustion gas emission are promising features for the use of Stirling engines in marine power stations. Intensified research on these engines at the turn of the 21st century accelerated the development of innovative designs and technologies. It was therefore possible to introduce the developed solutions of Stirling engines: STM with the power of 55 kw; Kockums (United Stirling) V4-95, 25 kw and V kw; SOLO Stirling kw. Various heat sources can be applied for feeding Stirling engines which are external combustion engines, including burning evaporated LNG or exhaust gas of internal combustion engines. Heat to the working fluid of the engine can be delivered either directly from a high-temperature heat source or employing intermediate systems. The latter solution requires application of a system to transmit heat to a Stirling engine heater. Stirling engine effective power, according to William Beale, is determined approximately by [1]: N.015 p f V [W] (1) 0 r s where: p r mean working pressure of one cycle [bar], f working cycle frequency [Hz], V s engine displacement volume [cm 3 ]. The equation (1) is true for a majority of designed and tested Stirling engines, independent of their type, size and power transmission gear, either with or without a crank shaft. The value 0.015, called the Beale number, is not constant and depends primarily on the heater and cooler temperatures as well as the engine construction. The higher heat source temperature, the higher the Beale number is (Fig. 1). In most cases the engines analysed by Beale were operated with the heater temperature of approximately 650ºC and cooler temperature of 65ºC [1]. The research on Stirling engines has been intensified in recent years resulting in an improvement of their efficiency, which translated into increased values of the Beale number up to Equation (1) directly indicates the possibility to regulate the Stirling engine power, a key factor for electrical generator operation. The engine power control through the engine speed change is not recommended due to power receiver parameters. Therefore, practical methods of controlling the engine power are based on changes of the working fluid pressure and displacement volume [2]. Application of a Stirling engine control system through the working fluid pressure change is, unfortunately, difficult in practice and makes the construction of the system complex, whereas a swash plate drive employed by STM Power Inc. in STM engines allows to control the engine power smoothly via the regulation of the inclination angle of the swash plate, which corresponds to the piston stroke regulation [3]. Employing a hybrid system composed of a Stirling engine, electric generator and a set of accumulators increases the efficiency and thermal loading and allows the Stirling engine to operate within a narrow load range, significantly simplifying the construction and reducing the control system cost [3] Heater temperature [K] Fig. 1. Beale number as a function of source temperature [1] Rys. 1. Liczba Beale jako funkcja temperatury [1] Propulsion systems of LNG carriers LNG carriers are divided into the following categories by cargo capacity [4]: small up to m 3, small conventional from to m 3, large conventional from to m 3, Q-flex from to m 3, Q-max more than m 3. Until 2007 LNG tankers were mainly ships with a capacity up to m 3, while at present their cargo capacity ranges from m 3 to m 3. Zeszyty Naukowe 21(93) 99

3 Arkadiusz Zmuda The length of LNG carriers varies from 250 m (small) to 345 m (Q-max); they can develop service speed from knots (small), knots (small conventional) and knots (large conventional, Q-flex and Q-max) [4]. During a voyage of the LNG carrier, its cargo in tanks evaporates, therefore it is necessary to discharge this evaporated gas. The rate of evaporation in an LNG carrier is estimated at 0.15%, and considering the cargo and ballast voyage 0.085% per day [5, 6]. One specific feature of the LNG carrier propulsion system is that boil-off gas can be used for feeding dual fuel medium and low speed internal combustion engines. Yhe gas has to be compressed to [5]: approximately 0.5 MPa for medium speed engines, approximately 25 MPa for low speed engines. Steam turbines were mainly applied in the main propulsion systems of LNG carriers until 2007, while presently the dual fuel combustion engines are more and more popular. Due to the required reliability of LNG carrier power plants, producers offer the following propulsion systems instead of a single propeller driven by one main engine [5]: twin-propeller with two main engines, one-propeller with additional drive composed of shaft generator / electrical motor which is a reserve propulsion system in the case of main engine failure. Variants of the main propulsion systems of the LNG carriers and their electric power plants, offered by MAN-B&W, are presented in figure 2. Fig. 2. The alternative two-stroke propulsion and power generation machinery systems [7]: ME-C ME-C engine with reliquefaction, ME-GI ME-GI engine with gas compressor, FPP fixed pitch propellers, CPP controllable pitch propellers, DG diesel generators, PTO shaft generator system, TES MAN Diesel waste heat recovery system Thermo Efficiency System, HFO heavy fuel oil Rys. 2. Alternatywny napęd dwusuwowy i systemy generatorów mocy [7]: ME-C ME-C silnik z systemem upłynnienia gazu, ME-GI ME-GI silnik ze sprężarką gazową, FPP śruby napędowe o stałym skoku, CPP śruby okrętowe o skoku kontrolowanym, DG generatory diesla, PTO system generatora wału, TES MAN Diesel system odpadów ciepła, wydajności cieplnej, HFO ciężki olej opałowy The propulsion system with two main engines is more reliable. For the design speed of 21 knots it can move a ship at 15 knots in the case of one engine failure. However, building and operational costs are higher. Electric power demand Electric power demand of LNG carriers is greater than other types of merchant vessels. The power of the ship electric plant (excluding an emergency generator) can be preliminarily estimated using equations developed by the Ship Design and Research Centre in Gdańsk. For gas carriers the equation holds [8]: n N N 572 [kw] (2) el where: N n * nominal power of the main engines [kw]. Equation (2) was developed at the end of the seventies, therefore it should be employed with care as the marine power systems have been significantly developed ever since that time. Presently, methods for the determination of electric power demand of merchant ships in various operational conditions are continuously developed, especially in the preliminary design, because methods used to date, e.g. balance or index methods are not precise. For example, in [9] an indirect approach to the problem was proposed for general cargo ships. In [10] a complex approach was proposed towards the determination of energy for ship propulsion, electricity and boiler efficiency using the statistic methods, developing equations for modern oil and product tankers on the basis of similar ships. Similarly, in [11] a method was proposed for preliminary determination of the main propulsion and electric plant power using the data base of specific ship types (container ships, tankers and gas carriers) employing artificial neural networks. Although much research is being done at the moment, this author has not found formulations for LNG carriers. In up-to-date marine power systems, apart from typical independent electric generators, suspended or shaft generators as well as gas or steam turbogenerators including utilization generators are used. Additionally, an emergency generator is also installed, not considered in the ship energy balance [12]. Waste heat utilization The key element of the ship power system is the main propulsion engine (or engines) which generate 100 Scientific Journals 21(93)

4 Index of main propulsion demand with respect to deadweight unit Stosunek mocy napędu głównego do nośności statku Estimation of the possibility of Stirling engine applications in LNG carrier power systems large amount of waste heat depending on load operating characteristics and environmental conditions. The changes have significant influence on the possibility of the waste heat usage [13]. To increase the marine power system efficiency, waste heat generated by combustion engines in the form of exhaust gas, cooling water or supercharging air should be used as much as possible. The heat can be used in different plant installations to obtain usable heat and / or electricity. In [14] the following aspects concerning the utilization of marine power system waste heat are addressed: ship speed is a factor determining the main propulsion power, different types of ships with similar deadweight have various power of the main propulsion, amount of waste heat meets the demand for large merchant ships, whereas it is typically insufficient to obtain usable heat for small ships, observed increase of ship speed calls for greater main propulsion power in relation to deadweight, increase of the energy necessary to handle the cargo for certain ship types is noted. With these premises in [14], an index Z has been developed for up-to-date merchant ships, defined as: Z v Nn vk (3) D where: N n nominal power of ship main propulsion [kw], D deadweight [t], v k contractual speed [kn]. A graphic interpretation of index Z is presented in figure 3. Z [kw/t] 0.8 0, , , ,2 Tankers Zbiornikowce Bulk Masowce carriers Container Kontenerowce / vessels Gazowce LNG carriers LNG v [Mm/h] Contractual Projektowa prędkość speed of statku ships Fig. 3. The change of index of main propulsion demand with respect to deadweight unit at contractual speed of up-to-date merchant ships [14] Rys. 3. Zmiana indeksu mocy napędu głównego, w odniesieniu do nośności statku w projektowanej prędkości statku [14] It can be seen in figure 3 that the greatest waste heat amount is produced by container ship and LNG carrier power systems. For such ships the main propulsion power per deadweight unit increases along with speed increase. Possibility of application of Stirling engines on LNG carriers In author s opinion, of the Stirling engines commercially available the STM developed by STM Power Inc. can be applied in the LNG carrier power system. It is a four-stroke engine with a displacement volume of 480 cm 3 and the swashplate drive. In the original version the power of the engine was 32 kw, and the Beale number was After modifications to increase the mean working fluid pressure and the heater temperature, the engine yields 55 kw (the Beale number is approx ). The manufacturer plans a series of STM engines, which at 1800 rpm will produce the following power [15]: STM kw (displacement volume STM cm 3 ), 160 kw (displacement volume 2120 cm 3 ), STM kw (displacement volume 4000 cm 3 ). STM engines are successfully employed in PowerUnit TM heat and electric power unit developed by STM Power Inc. (presently Stirling Biopower Inc.). The systems produce electric power of 55 kw e and supply heat approx kj/h in the form of hot water (91 kw th ). They are characterized by the following working parameters [15]: very silent work 65 dba in distance of 1 m, NO x emission g/kwh, efficiency 80% total system and 31,5% electric (CHP unit), fuel consumption 13.5 kg/h of natural gas per 52 kw output. The gas demand for the main propulsion of LNG carriers is presented in figure 4 together with the amount of boil-off gas. The following assumptions were made for the calculation [16]: boil-off gas rate in loaded condition 0.15% per day, boil-off gas rate in ballast condition 0.06% per day, density of methane 470 kg/m 3, lower calorific value (LCV) of methane kj/kg, specific energy consumption for dual-fuel twostroke diesel engine 7250 kj/kwh. Zeszyty Naukowe 21(93) 101

5 Power consumption [kw] Mass of methane [kg/h] Arkadiusz Zmuda 9000 Q-max 8000 Q-flex MMLc Power consumption [kw] Mass of methane [kg/h] MMNs Large Size of ship, LNG capacity capasity [m 3 ] 3 ] MMNs MMNt MMBc Fig. 4. The mass of methane needed for main propulsion and mass of boil-off gas of different size LNG carriers: MMLc mass of boil-off gas in loaded condition, MMBc mass of boil-off gas in ballast condition, MMNs mass of methane needed for single- -screw main propulsion, MMNt mass of methane needed for twin-screw main propulsion Rys. 4. Masy metanu potrzebne dla napędu głównego i masa gazu odparowanego dla różnych gazowców LNG: MMLc masa gazu odparowanego w stanie obciążonym, MMBc masa gazu odparowanego w stanie z balastem, MMNs masa metanu potrzebna dla jednej śruby napędu głównego, MMNt masa metanu potrzebna dla podwójnej śruby napędu głównego PPs Large Q-flex Q-max PPs PPt EPs Size of ship, LNG capacity capasity [m 33 ]] Fig. 5. The propulsion SMCR power and electrical power demand for single-screw and twin-screw main propulsion of an different size of LNG carriers: PPs propulsion power for single-screw main propulsion, PPt propulsion power for twin-screw main propulsion, EPs electrical power for single-screw main propulsion, EPt electrical power for twin-screw main propulsion Rys. 5. Moc napędu SMCR i zapotrzebowanie na energię elektryczną dla śruby pojedynczej i podwójnej napędu głównego dla różnych gazowców LNG: PPs moc napędu dla pojedynczej śruby napędu głównego, PPt moc napędu dla podwójnej śruby napędu głównego, EPs moc elektryczna dla pojedynczej śruby napędu głównego, EPT moc elektryczna dla podwójnej śruby napędu głównego EPs EPt 102 Scientific Journals 21(93)

6 Estimation of the possibility of Stirling engine applications in LNG carrier power systems The main propulsion power and electric power demand for LNG carriers of various sizes are presented in figure 5. The electric power demand was calculated using equation (2). With relatively low power of the STM Stirling engines, their applications in power systems depending on LNG carrier size can be the following: LNG carriers with cargo capacity up to m 3 Stirling engines can be applied as the only generator propulsion (Fig. 5); LNG carriers with cargo capacity over m 3 with large electric power demand, Stirling engines can be used jointly with auxiliary combustion engines or waste heat turbines. Estimation of possible applications of Stirling engines in the LNG carrier power system requires a detailed analysis of the ship electric plant loads in various operational conditions, e.g. voyage, staying in the port or on the road, and during un / loading. Determining the type of electric generators, the Stirling engines can also be considered besides auxiliary combustion engines or utilization turbines. Application of various engines should be taken into consideration with such criteria as fuel consumption or emission standards. For instance, increasing the power of employed electric generators used to ensure sufficient electric power supply in all operational conditions is not economical as they would remain underloaded in typical operational conditions, which causes fuel consumption increase [12]. This is why application of additional electric generators with Stirling engines can be considered. An undoubtful advantage of Stirling engines is the fact that various heat sources can be used to feed them, including waste heat generated by main and auxiliary engines and burning evaporated cargo, which is especially important in LNG carrier power systems. It follows from figure 4 that for LNG carriers with cargo capacity exceeding m 3 there is a theoretical surplus of evaporated cargo that can be used to feed electric generators with Stirling engines. The combustion engine waste heat can be used to drive electric generators with Stirling engines on practically all LNG carriers. For instance, considering the large power of engines driving compressors of evaporated cargo (on an LNG carrier with cargo capacity of m 3 the power of one engine is 1600 kw), exhaust gas energy can be used to feed electric generators driven by Stirling engines. An example of the system for waste heat utilization and reduction of toxic exhaust gas emission employing a Stirling engine is presented in figure 6. The heat produced by the cooling system of a Stirling engine driving an electric generator can be transmitted further to the heating installations, e.g. heating fuel, lubricating oil or power plant and crew accommodations. Fig. 6. System for waste heat utilization and reduction of emission of toxic exhaust gases of marine engines [17]: SE electronic programmer, TS turbocompressor, SP auxiliary compressor, TM power turbine, zo by-pass valve, zr control valve Rys. 6. System utylizacji ciepła odpadowego i redukcji emisji toksycznych gazów z silników okrętowych [17]: SE programator elektroniczny, TS turbosprężarka, SP sprężarka pomocnicza, TM turbina napędowa, zo zawór by-pass, zr zawór regulacyjny Zeszyty Naukowe 21(93) 103

7 Arkadiusz Zmuda Conclusions The characteristic features of contemporary Stirling engines as well as power demand of the LNG carriers lead to the following conclusions: 1. It is possible to employ in LNG carriers power systems the electric power and heat generators with Stirling engines, which utilize combustion engine waste heat or heat from burning boil-off gas. 2. Application of Stirling engines on LNG carriers can improve the reliability and increase the number of variants of power system solutions. 3. Stirling engines of the nominal output up to 300 kw can be considered in the future, being a subject of the present research. 4. LNG carriers with a cargo capacity exceeding m 3 have a theoretical surplus of boil-off gas which can be burnt to supply heat to Stirling engines driving electric generators. 5. Power systems of LNG carriers with cargo capacity exceeding m 3 produce significant amounts of waste heat which can be used to feed Stirling engines coupled with electric generators. In the light of these considerations, the solution proposed in [5] referring to the development of a mathematical model of the propulsion system and electrical plant of an LNG carrier allowing for their optimal selection is still valid. In author s opinion the model should comprise Stirling engines for the supply of electric power and heat on these ships. References 1. WALKER G.: Stirling Engines. Clarendon Press, Oxford 1980, ŻMUDZKI S.: Silniki Stirlinga. WNT, Warszawa 1993, ŻMUDZKI S.: Perspektywy zastosowania silników Stirlinga w okrętownictwie i żegludze morskiej. XII Sympozjum Paliw Płynnych i Produktów Smarowych w Gospodarce Morskiej. Centrum Szkolenia EXPLONAFT, Warszawa Szczyrk 1997, Two-stroke Propulsion Trends in LNG Carriers. MAN Diesel A/S, Copenhagen, Denmark CHĄDZYŃSKI W.: Trendy rozwoju układów napędowych gazowców LNG. ZN AM w Szczecinie, Explo-Ship 2006, 10(82), JAMROŻ J.: Odparowany ładunek na zbiornikowcach jako alternatywne paliwo dla głównych silników napędowych. XXII Sympozjum Siłowni Okrętowych SymSO Wydawnictwo Uczelniane Politechniki Szczecińskiej, Szczecin 2001, LNG Carriers with ME-GI Engine and High Pressure Gas Supply System. MAN Diesel A/S, Copenhagen, Denmark MICHALSKI R.: Siłownie okrętowe. Obliczenia wstępne oraz ogólne zasady doboru mechanizmów i urządzeń pomocniczych instalacji siłowni motorowych. Wydawnictwo Uczelniane Politechniki Szczecińskiej, Szczecin 1997, BALCERSKI A., ZIÓŁKOWSKI M.: Metoda określania zapotrzebowania na energię elektryczną w typowych stanach eksploatacji statków towarowych dla potrzeb wstępnego doboru siłowni. ZN WSM w Szczecinie, Explo-Ship 2002, 66, GIERNALCZYK M., GÓRSKI Z.: Metoda określania zapotrzebowania energii do napędu statku, energii elektrycznej i wydajności kotłów dla nowoczesnych zbiornikowców do przewozu ropy naftowej i jej produktów przy wykorzystaniu metod statystycznych. ZN AM w Szczecinie, Explo- -Ship 2006, 10(82), ABRAMOWSKI T., ZMUDA A.: Modelowanie okrętowych układów napędowych z wykorzystaniem sztucznych sieci neuronowych. XXII Sympozjum Siłowni Okrętowych SymSO 2001, Wydawnictwo Uczelniane Politechniki Szczecińskiej, Szczecin 2001, HERDZIK J.: Miara zapewnienia dostaw energii elektrycznej na statkach. ZN AMW w Gdyni, 2005, nr 162 K/2, XXVI Sympozjum Siłowni Okrętowych SymSO 2005, BEHRENDT C.: Problemy wyznaczania i wykorzystania zasobów ciepła odpadowego w okrętowych układach energetycznych. ZN AM w Szczecinie, Explo-Ship 2006, 10(82), BEHRENDT C., ADAMKIEWICZ A., KRAUSE P.: Turboprądnica utylizacyjna na parę nasyconą jako alternatywne źródło energii elektrycznej w systemie odzyskiwania energii wtórnej statku. Wybrane problemy projektowania i eksploatacji siłowni okrętowych, XXVII Sympozjum Siłowni Okrętowych SymSO 2006, Wydawnictwo Uczelniane Politechniki Szczecińskiej, Szczecin 2006, Game Changer STM Stirling Engine. Stirling Biopower Inc., Ann Arbor, USA LNG Carrier Propulsion by ME-GI Engines and / or Reliquefaction. MAN Diesel A/S, Copenhagen, Denmark ŻMUDZKI S., KOPER P.: Analiza energetyczna systemu utylizacji ciepła spalin wylotowych silników okrętowych w silniku Stirlinga. Prace Naukowe Politechniki Szczecińskiej nr 536: Badania i rozwój konstrukcji silnika Stirlinga. Wydawnictwo Uczelniane PS, Szczecin 2000, ABRAMOWSKI T., BORTNOWSKA M.: Analysis of Design Solutions and Operational Features of Natural Gas Carriers. Problemy Eksploatacji, 2008, 2 (69), The scientific work financed from resources planned for research and science in the years as an R&D project No. R Recenzent: dr hab. inż. Andrzej Adamkiewicz, prof. AM Akademia Morska w Szczecinie 104 Scientific Journals 21(93)