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Maritime University of Szczecin

Akademia Morska w Szczecinie

2010, 21(93) pp. 98–104 2010, 21(93) s. 98–104

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

71-065 Szczecin, al. Piastów 41, e-mail: 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 źró-deł ciepła, bardzo cicha praca oraz bardzo niska emisja szkodliwych składników spalin stwarza duże możli-wości 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 genero-wane przez silniki główne i pomocnicze oraz spalanie odparogenero-wanego 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 odpado-wego i ograniczenia emisji składników toksycznych spalin wylotowych silników okrętowych z wykorzysta-niem 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 sys-tems. One of the promising solutions is an applica-tion 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

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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 en-gines in marine power stations. Intensified research on these engines at the turn of the 21st century ac-celerated the development of innovative designs and technologies. It was therefore possible to intro-duce the developed solutions of Stirling engines:  STM 4-120 with the power of 55 kW;

 Kockums (United Stirling) V4-95, 25 kW and V4-275 – 75 kW;

 SOLO Stirling 161 – 11 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 solu-tion requires applicasolu-tion of a system to transmit heat to a Stirling engine heater.

Stirling engine effective power, according to William Beale, is determined approximately by [1]:

[W] 015

.

0 pr f Vs

N    (1)

where: pr – mean working pressure of one cycle

[bar], f – working cycle frequency [Hz], Vs – engine

displacement volume [cm3].

The equation (1) is true for a majority of de-signed 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 construc-tion. 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 inten-sified in recent years resulting in an improvement of their efficiency, which translated into increased values of the Beale number up to 0.02.

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]. Appli-cation 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 4–120 en-gines 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 Stir-ling engine, electric generator and a set of accumu-lators 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].

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 90 000 m3,

• small conventional – from 120 000 to 149 999 m3,

• large conventional – from 150 000 to 180 000 m3,

• Q-flex – from 200 000 to 220 000 m3,

• Q-max – more than 260 000 m3.

Until 2007 LNG tankers were mainly ships with a capacity up to 150 000 m3, while at present their cargo capacity ranges from 150 000 m3 to 220 000 m3. 600 800 1000 1200 Heater temperature [K] B ea le N o. ( P / pf V0 ) 0.020 0.015 0.010 0.005

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The length of LNG carriers varies from 250 m (small) to 345 m (Q-max); they can develop service speed from 14÷18 knots (small), 19÷20 knots (small conventional) and 20÷21 knots (large con-ventional, Q-flex and Q-max) [4].

During a voyage of the LNG carrier, its cargo in tanks evaporates, therefore it is necessary to dis-charge this evaporated gas. The rate of evaporation in an LNG carrier is estimated at 0.15%, and consi-dering the cargo and ballast voyage – 0.085% per day [5, 6].

One specific feature of the LNG carrier propul-sion 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 en-gines,

 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 re-serve propulsion system in the case of main en-gine failure.

Variants of the main propulsion systems of the LNG carriers and their electric power plants, of-fered by MAN-B&W, are presented in figure 2.

Fig. 2. The alternative two-stroke propulsion and power ge-neration 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 propel-lers, 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 generato-rów 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 – sys-tem generatora wału, TES – MAN Diesel syssys-tem 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 en-gine failure. However, building and operational costs are higher.

Electric power demand

Electric power demand of LNG carriers is grea-ter 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 Re-search Centre in Gdańsk. For gas carriers the equa-tion holds [8]: [kW] 572 11968 . 0    

Nel Nn (2)

where:

N

n*

– nominal power of the main

en-gines [kW].

Equation (2) was developed at the end of the se-venties, therefore it should be employed with care as the marine power systems have been significant-ly developed ever since that time.

Presently, methods for the determination of elec-tric power demand of merchant ships in various operational conditions are continuously developed, especially in the preliminary design, because me-thods used to date, e.g. balance or index meme-thods are not precise. For example, in [9] an indirect ap-proach to the problem was proposed for general cargo ships. In [10] a complex approach was pro-posed 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 simi-lar ships. Simisimi-larly, in [11] a method was proposed for preliminary determination of the main propul-sion 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 mo-ment, 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 turbo-generators including utilization turbo-generators are used. Additionally, an emergency generator is also in-stalled, 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

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large amount of waste heat depending on load ope-rating 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 uti-lization of marine power system waste heat are addressed:

 ship speed is a factor determining the main pro-pulsion 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 in-sufficient to obtain usable heat for small ships,  observed increase of ship speed calls for greater

main propulsion power in relation to dead-weight,

 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:

 

 

D v N v Z n k (3)

where: Nn – nominal power of ship main propulsion

[kW], D – deadweight [t], vk – contractual speed

[kn].

A graphic interpretation of index Z is presented in figure 3.

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 in-creases 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 4–120 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 cm3 and the

swash-plate drive. In the original version the power of the engine was 32 kW, and the Beale number was 0.0185. After modifications to increase the mean working fluid pressure and the heater temperature, the engine yields 55 kW (the Beale number is ap-prox. 0.0208). The manufacturer plans a series of STM engines, which at 1800 rpm will produce the following power [15]: • STM 4-260 – 80 kW (displacement volume 1040 cm3), • STM 4-530 – 160 kW (displacement volume 2120 cm3), • STM 4-1000 – 300 kW (displacement volume 4000 cm3).

STM 4-120 engines are successfully employed in PowerUnitTM heat and electric power unit

deve-loped by STM Power Inc. (presently Stirling Bio-power Inc.). The systems produce electric Bio-power of 55 kWe and supply heat – approx. 327 000 kJ/h in

the form of hot water (91 kWth). They are

characte-rized by the following working parameters [15]: • very silent work – 65 dBA in distance of 1 m, • NOx emission – 0.227 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 assump-tions 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/m3,

• lower calorific value (LCV) of methane – 50 000 kJ/kg,

• specific energy consumption for dual-fuel two-stroke diesel engine – 7250 kJ/kWh.

St os un ek mo cy n ap ęd u głów ne go d o no śn oś ci s ta tk u

Projektowa prędkość statku 14 12 10 16 18 20 22 0,2 24 0,4 0,6 0,8 Masowce Zbiornikowce Gazowce LNG Kontenerowce Z [kW/t] v [Mm/h]

Contractual speed of ships Container / vessels LNG carriers Tankers Bulk carriers In de x of m ai n pr op ul si on d ema nd w it h re sp ec t t o de ad w ei gh t u ni t 10 12 14 16 18 20 22 24 v [Mm/h] 0.8 0.6 0.4 0.2 Z [kW/t]

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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 50000 100000 150000 200000 250000 300000 M as s of m et h an e [k g/ h ]

Size of ship, LNG capacity [m3]

MMNt MMNs MMLc Small Small Conventional Large Conventional Q-flex Q-max MMBc MMNs

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

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 0 50000 100000 150000 200000 250000 300000 P ow er c on su m p ti on [ k W]

Size of ship, LNG capacity [m3]

EPs PPt PPs Small Small Conventional Large Conventional Q-flex Q-max EPt PPs EPs

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 propul-sion, EPs – electrical power for single-screw main propulpropul-sion, 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 róż-nych 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

Size of ship, LNG capasity [m3] Size of ship, LNG capasity [m3]

P owe r co ns um pti on [ kW] M ass o f m eth an e [k g/ h] Small Small Conventional Large Conventional MMNs Q-flex Q-max MMLc MMNs MMNt MMBc Small Small Conventional Large Conventional PPs EPs EPt EPs PPt PPs Q-max Q-flex

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The main propulsion power and electric power demand for LNG carriers of various sizes are pre-sented 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 de-pending on LNG carrier size can be the following: • LNG carriers with cargo capacity up to 30 000

m3 – Stirling engines can be applied as the only

generator propulsion (Fig. 5);

• LNG carriers with cargo capacity over 30 000 m3 with large electric power demand, Stirling

engines can be used jointly with auxiliary com-bustion 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 tur-bines. 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 genera-tors used to ensure sufficient electric power supply in all operational conditions is not economical as they would remain underloaded in typical opera-tional 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 car-go, which is especially important in LNG carrier power systems. It follows from figure 4 that for LNG carriers with cargo capacity exceeding 180 000 m3 there is a theoretical surplus of

evapo-rated cargo that can be used to feed electric genera-tors with Stirling engines. The combustion engine waste heat can be used to drive electric generators with Stirling engines on practically all LNG car-riers. For instance, considering the large power of engines driving compressors of evaporated cargo (on an LNG carrier with cargo capacity of 210 000 m3 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 sys-tem 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 regula-cyjny

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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 po-wer systems the electric popo-wer 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 180 000 m3 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 120 000 m3 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.

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10. GIERNALCZYK M.,GÓRSKI Z.: Metoda określania

zapotrze-bowania energii do napędu statku, energii elektrycznej i wydajności kotłów dla nowoczesnych zbiornikowców do przewozu ropy naftowej i jej produktów przy wykorzysta-niu metod statystycznych. ZN AM w Szczecinie, Explo- -Ship 2006, 10(82), 183–192.

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układów napędowych z wykorzystaniem sztucznych sieci neuronowych. XXII Sympozjum Siłowni Okrętowych SymSO 2001, Wydawnictwo Uczelniane Politechniki Szczecińskiej, Szczecin 2001, 5–12.

12. HERDZIK J.: Miara zapewnienia dostaw energii elektrycznej na statkach. ZN AMW w Gdyni, 2005, nr 162 K/2, XXVI Sympozjum Siłowni Okrętowych SymSO 2005, 99–108. 13. BEHRENDT C.: Problemy wyznaczania i wykorzystania

zasobów ciepła odpadowego w okrętowych układach ener-getycznych. ZN AM w Szczecinie, Explo-Ship 2006, 10(82), 31–40.

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Turboprądni-ca utylizacyjna na parę nasyconą jako alternatywne źródło energii elektrycznej w systemie odzyskiwania energii wtórnej statku. Wybrane problemy projektowania i eksplo-atacji siłowni okrętowych, XXVII Sympozjum Siłowni Okrętowych SymSO 2006, Wydawnictwo Uczelniane Poli-techniki Szczecińskiej, Szczecin 2006, 19–30.

15. Game Changer – STM Stirling Engine. Stirling Biopower Inc., Ann Arbor, USA 2003–2007.

16. LNG Carrier Propulsion by ME-GI Engines and / or Reli-quefaction. MAN Diesel A/S, Copenhagen, Denmark 2004. 17. ŻMUDZKI S.,KOPER P.: Analiza energetyczna systemu

uty-lizacji ciepła spalin wylotowych silników okrętowych w silniku Stirlinga. Prace Naukowe Politechniki Szczeciń-skiej nr 536: „Badania i rozwój konstrukcji silnika Stir-linga”. Wydawnictwo Uczelniane PS, Szczecin 2000, 83– 96.

18. ABRAMOWSKI T.,BORTNOWSKA M.: Analysis of Design

So-lutions and Operational Features of Natural Gas Carriers. Problemy Eksploatacji, 2008, 2 (69), 139–148.

The scientific work financed from resources planned for research and science in the years 2007–2009 as an R&D project No. R10 003 02.

Recenzent: dr hab. inż. Andrzej Adamkiewicz, prof. AM Akademia Morska w Szczecinie

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Poza tym można podnieść poziom dopuszczalnych spłat długu określonych indywidualnym wskaź- nikiem zadłużenia za pomocą leasingu zwrotnego (sell-buy back czy krzyżowe

Koszt kapitału jednostek gospodarczych to jeden z najważniejszych elementów zarządzania finansami przedsiębiorstw. Oddziałuje on istotnie na wartość podmiotu

Rozwój infrastruktury komunikacyjnej stanowi dla władz regionalnych, jak również dla ośrodków i obszarów metropolitalnych istotne zadanie, gdyż dobra sieć transportowa