Energy Conversion And Management-mrzljak-poljak-mrakovcic.pdf

Energy Conversion and Management 140 (2017) 307–323

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Energy and exergy analysis of the turbo-generators and steam turbine for the main feed water pump drive on LNG carrier Vedran Mrzljak a,⇑, Igor Poljak b, Tomislav Mrakovcˇic´ a a b

Faculty of Engineering, University of Rijeka, Vukovarska 58, 51000 Rijeka, Croatia Rozˇic´i 4/3, 51221 Kostrena, Croatia

a r t i c l e

i n f o

Article history: Received 2 November 2016 Received in revised form 21 January 2017 Accepted 2 March 2017

Keywords: Turbo-generator Main feed water pump Energy efficiency Exergy efficiency Exergy destruction

a b s t r a c t Nowadays, marine propulsion systems are mainly based on internal combustion diesel engines. Despite this fact, a number of LNG carriers have steam propulsion plants. In such plants, steam turbines are used not only for ship propulsion, but also for electrical power generation and main feed water pump drive. Marine turbo-generators and steam turbine for the main feed water pump drive were investigated on the analyzed LNG carrier with steam propulsion plant. The measurements of various operating parameters were performed and obtained data were used for energy and exergy analysis. All the measurements and calculations were performed during the ship acceleration. The analysis shows that the energy and exergy efficiencies of both analyzed low-power turbines vary between 46% and 62% what is significantly lower in comparison with the high-power steam turbines. The ambient temperature has a low impact on exergy efficiency of analyzed turbines (change in ambient temperature for 10 °C causes less than 1% change in exergy efficiency). The highest exergy efficiencies were achieved at the lowest observed ambient temperature. Also, the highest efficiencies were achieved at 71.5% of maximum developed turbogenerator power while the highest efficiencies of steam turbine for the main feed water pump drive were achieved at maximum turbine developed power. Replacing the existing steam turbine for the main feed water pump drive with an electric motor would increase the turbo-generator energy and exergy efficiencies for at least 1–3% in all analyzed operating points. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, marine steam turbine propulsion plants can be found in a number of LNG carriers [1,2]. Such steam propulsion plant consists of many components [3]. Because of the complexity, in these systems is required to be familiar with all the components [4] and it is necessary to properly operate with them as a whole [5]. Two of all components of such marine steam plant were taken into consideration in this paper: turbo-generators (TG) and steam turbine for main feed water pump drive (MFP). Energy and exergy analysis of the base loaded conventional steam power plants along with their most important components can be found often in the scientific literature. Some of the important researches in this area are presented below. A study conducted in this paper is based on the methods and conclusions from this literature. Adibhatla et al. [6] presented energy and exergy analysis of a super critical thermal power plant under constant and pure sliding ⇑ Corresponding author. E-mail address: [email protected] (V. Mrzljak). http://dx.doi.org/10.1016/j.enconman.2017.03.007 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

pressure operation at three operating loads (100%, 80% and 60%). Obtained energy and exergy efficiencies of high-power steam turbines were 90% or larger in all operating regimes. At constant pressure, the exergy destruction rate in the steam turbines decreases sensibly when the operating load decreases from 100% to 80%, while the exergy destruction rate remains constant for the operating loads of 80% and 60%. For pure sliding pressure operations, the exergy destruction rate in the turbines decreases significantly with the load condition. Main feed water pump in this thermal power plant was powered by low-power steam turbine, but that steam turbine was not investigated. Energetic and exergetic performance analysis of various coalfired thermal power plants in Turkey were conducted by Erdem et al. [7]. Calculation model for each plant was proposed and the mass, energy and exergy balances were established. Exergy efficiencies of the analyzed high-power steam turbines vary between 80% and 98%, while turbines exergy destruction rates reach mostly between 1.5 MW and 10 MW. Yang et al. [8] presented exergy-based evaluation of a coal-fired ultra-supercritical power plant. Exergy efficiencies of high-power steam turbines from the analyzed power plant are in the range

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Nomenclature

Abbreviations BOG Boil Off Gas BP Basic Plant EM Electric Motor HFO Heavy Fuel Oil HP High Pressure LNG Liquefied Natural Gas LP Low Pressure max. maximum MFP main feed water pump temp. temperature TG turbo-generator Latin symbols _ En energy flow, kW _ Ex exergy flow, kW h specific enthalpy, kJ/kg _ m mass flow, kg/s (or kg/h in Eqs. (10) and (18)) p pressure, MPa P power, kW

75–97%, while turbines exergy destruction rates were between 2.5 MW and 10 MW. Steady state simulation and exergy analysis of supercritical coal-fired power plant with CO2 capture was conducted by Olaleye et al. [9]. This paper, among other elements, presents exergy efficiencies of several high-power steam turbines from the analyzed power plant and that efficiencies vary between 87% and 99% while turbines exergy destruction rates were between 0.5 MW and 7 MW. Exergy analysis of a 300 MW lignite thermo-electric power plant was presented in the paper Koroneos et al. [10]. Exergy efficiency of high-power steam turbine in the presented thermoelectric power plant at the observed load conditions was 96%. Taillon et al. [11] proposed new graphs for thermal power plant exergy efficiency determination. These graphs permit to determine the efficiency ranks compared with the normally obtained values for the industrial systems. A lot of useful data and equations for energy and exergy analysis of base loaded conventional steam power plants and all of its components were presented by Ray et al. [12]. Similar important data and equations for energy and exergy analysis are shown in Kaushik et al. [13] for coal-fired thermal power plant and also for gas-fired combined cycle thermal power plant. Aljundi [14] presented an energy and exergy analysis of a steam power plant in Jordan. Exergy efficiency of high-power steam turbine in the observed power plant was 73.5%, while her exergy destruction amounts 20.407 MW in the observed operating regime. Study of the ambient temperature impact on steam generator, main turbine and condenser exergy efficiency is also provided in this paper. It is concluded that exergy efficiency of the main high-power turbine and steam generator decreases (exergy destruction increases) with an increase in the ambient temperature. The same conclusions for high-power steam turbines and steam generator’s exergy efficiency can be found in the papers Ahmadi et al. [15], Ameri et al. [16] and Kopac et al. [17]. Energy and exergy analysis of a steam turbine power plant in the phosphoric acid factory are conducted in the paper Hafdhi et al. [18]. Energy efficiencies of two analyzed turbo-generators and steam turbine are in the range from 74% to 93%, while exergy

s T V_ Q_ X_ heat

specific entropy, kJ/kg
K temperature, K volumetric flow, m3/h heat transfer, kW heat exergy transfer, kW

Greek symbols v upper and lower limit of the calibration range e specific exergy, kJ/kg q density, kg/m3 gI energy efficiency,% gII exergy efficiency,% Subscripts 0 reference conditions of the ambient in inlet out outlet S isentropic D destruction

efficiencies of the same components vary between 60% and 74%. Both energy and exergy efficiencies are calculated at the highest steam system load. Analyzed steam turbine and turbo-generators are low-power steam turbines with maximum power of about 7 MW. In the paper was also presented the variation of exergy efficiency of turbo-generator 1 and turbo-generator 2 during the change in steam mass flow. For turbo-generator 1 the highest exergy efficiency was obtained for the highest allowed steam mass flow while the highest exergy efficiency of turbo-generator 2 was obtained for the 75.5% of the highest allowed steam mass flow. Not only in the base loaded conventional steam power plants, but also in other steam power plants, energy and exergy analysis provide insight into the effectiveness of each plant component. Optimization and the effect of steam turbine outlet quality on the output power of a combined cycle power plant were presented by Ganjehkaviri et al. [19]. Memon et al. [20] considered three methods to conduct an analytic study on a combined cycle power plant: exergoeconomic, thermo-environmental and statistical. Lythcke-Jørgensen et al. [21] conducted an exergy analysis of a combined heat and power plant. Elsafi et al. [22] presented exergy and exergoeconomic analysis of sustainable direct steam generation solar power plant while Gupta et al. [23] conducted exergy analysis of direct steam generation solar–thermal power plant. Exergy evaluation of 330 MW solar-hybrid coal-fired power plant in China was presented by Peng et al. [24]. The solar system is used to heat the feed water at nearly 300 °C with purpose to substitute the steam extraction from a steam turbine. This improvement raised the net electrical power generated by the steam turbine. A thermal and economic comparison study is also established between solar-only and solar-hybrid coal-fired power plants. It is concluded that the hybrid coal-fired power plant is economically beneficial than the solar-only thermal power plant. Energy and exergy analysis is acceptable method for analysis of various plants, not just necessarily steam power plants. Thus, for example, Taner et al. [25] conducted energy-exergy analysis and optimization of a model sugar factory in Turkey while Jokandan et al. [26] presented an exergy analysis of an industrial-scale yogurt production plant. Energy, exergy, economic and

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environmental evaluations of geothermal district heating systems was conducted in the paper Keçebas [27] while Keçebas et al. [28] presented a thermodynamic evaluation of 6.35 MW geothermal power plant located in Denizli, Turkey. In this study energy and exergy analysis of turbo-generators and steam turbine for the main feed water pump drive during the ship acceleration is conducted. Energy and exergy efficiency rates as well as exergy destruction rates for both of the analyzed low-power turbines in the observed operating range are presented. The influence of the ambient temperature on exergy efficiency of both analyzed turbines was performed. Turbo-generators and MFP steam turbine were analyzed at different loads in order to determine working points with the highest energy and exergy efficiencies. Finally, a method for increasing the turbo-generator energy and exergy efficiency was presented. 2. Steam system and analyzed turbines description Analyzed LNG carrier steam propulsion plant consists of two identical turbo-generator units, Fig. 1, designed to cover all ship requirements for electrical power. Steam turbine for each turbogenerator consists of nine Rateau stages [29]. During navigation, turbo-generators mainly operate in parallel for the sake of safe navigation and they equally share the electrical load. Feed water pump driven by steam turbine is traditionally applied on ships with the steam propulsion due to safety, reliability and easy control during load change. The main feed water pump

309

(MFP) is used for increasing the water pressure and pumping it into the steam generators. Steam turbine for an MFP drive on the analyzed LNG carrier, Fig. 1, consists of single Curtis stage while the whole unit has the following characteristics [30]: – Pump maximum capacity: 175 m3/h – Pump delivery height: 818 m – Steam turbine maximum power: 570 kW It is important to emphasize main feed water pump recirculation line, Fig. 1. During the steam system startup, one part of the feedwater is recirculated back into the deaerator. Recirculation was performed in order to protect the main high-pressure pump at low steam system loads. The main feed water pump has several stages and a pump would significantly warm up feedwater at reduced flow causing its evaporation. Feed water evaporation leads to intense cavitation which can damage pump impeller and sealing elements in a short period. Steam turbines with Curtis and Rateau stages, and their complete analysis, can be found in [31,32]. Many details of classic and special designs of marine steam turbines and their auxiliary systems are presented in [33–36]. Main particulars of the analyzed LNG carrier are presented in Table 1. Also, it is important to describe the steam generators, essential components of the analyzed LNG carrier engine room, which provide superheated steam for each appointed steam turbine operation. A steam propulsion plant of the analyzed LNG carrier

Fig. 1. General scheme of steam propulsion plant of the analyzed LNG carrier.

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Table 1 Main particulars of the analyzed LNG carrier. Dead weight tonnage Overall length Max breadth Design draft Steam generators Propulsion turbine Turbo-generators MFP steam turbine

84,812 DWT 288 m 44 m 9.3 m 2  Mitsubishi MB-4E-KS Mitsubishi MS40-2 (max. power 29.420 kW) 2  Shinko RGA 92-2 (max. power 3.850 kW each) Shinko DMG 125-3 (max. power 570 kW)

consists of two identical, mirror-oriented, natural circulation type steam generators with a wide range of partial load change. During simultaneous parallel operation of two or more steam generators, it is necessary to develop a steam quality management and electronic control system [37], which is in this case provided by the steam generators manufacturer. Essential parts of each steam generator are combined burners, similar to those presented in [38], which can burn HFO, diesel fuel or dual fuel combination (HFO/gas or diesel/gas). Burners are located in the upper part of the furnace. Steam generator is composed of a steam drum and a water drum connected with a bank of inclined steam generating tubes. Other water side components include: front screen tubes shielding the superheater elements from direct radiant heat of the furnace, side and roof water wall, front and rear water wall tubes, down-comers, bottom headers, roof and bottom front wall headers, roof and bottom rear wall headers, and front and rear wall riser pipe. Remaining principal components of construction are superheater, an internal desuperheater, the economiser, steam air heater and housing. Design simplicity of marine steam generators gives them a high reliability [39], which is very important in the shipping industry because safe navigation always has the highest priority. Any marine steam propulsion plant has at least two turbo-generators because it cannot be allowed that the ship fully remain without electricity at any moment. In Fig. 1 the steam streams are marked with red1 and black lines (continuous or dashed) while the water (condensate) streams are marked with blue lines (continuous or dashed). 3. Mathematical description of turbo-generator and MFP steam turbine 3.1. Equations for the energy and exergy analysis Energy analysis is based on the first law of thermodynamics, which is related to the conservation of energy [13]. Mass and energy balance equations for a control volume in steady state disregarding potential and kinetic energy can be expressed as [1,18]:

X

_ in ¼ m

Q_  P ¼

X

X

_ out m

_ out
hout  m

ð1Þ X

_ in
hin m

ð2Þ

The total energy of a flow for any fluid stream can be calculated according to the equation:

_ ¼m _
h En

ð3Þ

Energy efficiency may take different forms and different names depending on the type of the system. Usually, energy efficiency can be written as [40]:

Energy output gI ¼ Energy input

ð4Þ

Exergy analysis is based on the second law of thermodynamics [40]. The main exergy balance equation for a control volume in steady state is [22,41,42]:

X_ heat  P ¼

X

_ out
eout  m

X

_ D _ in
ein þ Ex m

ð5Þ

where the net exergy transfer by heat (X_ heat ) at the temperature T equals [43]:

X_ heat ¼

 X T0 _ 1 Q T

ð6Þ

Specific exergy was defined according to [14,44] by following equation:

e ¼ ðh  h0 Þ  T 0
ðs  s0 Þ

ð7Þ

The total exergy of a flow for any fluid stream can be calculated according to [25]:

_ ¼m _
e¼m _
½ðh  h0 Þ  T 0
ðs  s0 Þ Ex

ð8Þ

Exergy efficiency is also called second law efficiency or effectiveness [45]. It can be defined as:

gII ¼

Exergy output Exergy input

ð9Þ

These governing equations including energy and exergy balances are used in TG and MFP steam turbine numerical analysis. 3.2. Turbo-generator Steam mass flow related to the developed turbine power of each TG turbine according to producer specifications [29] is presented in Fig. 2. Accurate turbo-generator power calculation at different loads was necessary for the turbo-generator energy and exergy analysis. The turbine power curve of one turbo-generator was approximated by the third degree polynomial using data from Fig. 2:

_ 3TG þ 6:7683
106
m _ 2TG þ 0:251318 PTG ¼ 4:354
1010
m _ TG  256:863
m

ð10Þ

_ TG in (kg/h) was placed in where P TG was obtained in (kW) when m Eq. (10). Enthalpy and mass flow through one of the analyzed turbogenerators are presented in Fig. 3(a), where h1 is steam enthalpy at the turbine inlet and h2 is steam enthalpy at the turbine outlet. Steam enthalpy at the turbine inlet was calculated from the measured pressure and temperature. Steam enthalpy at the turbine outlet was calculated from the turbine power PTG in (kW) and mea_ TG in (kg/s) according to [43] with the sured steam mass flow m equation:

h2 ¼ h1 

PTG _ TG m

ð11Þ

Fig. 3(b) presents an ideal (isentropic) and real expansion through a TG steam turbine in h-s diagram. The characteristic points for every expansion were marked in accordance with Eqs. (10)–(16) for energy and exergy analysis. The diagram in Fig. 3(b) shows measured and calculated values of the operating point 21 (Table 2). (a) Turbo-generator mass flow balance:

_ TG;1 ¼ m _ TG;2 ¼ m _ TG m

ð12Þ

(b) Turbo-generator energy balance [14,18]: 1

For interpretation of color in Fig. 1, the reader is referred to the web version of this article.

_ TG;1  En _ TG;2 ¼ m _ TG
ðh1  h2 Þ PTG ¼ En

ð13Þ

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Fig. 2. Steam mass flow in relation to turbine power of one turbo-generator [29].

Fig. 3. Turbo-generator: (a) enthalpy and mass flow, (b) isentropic and real expansion (operating point 21 - Table 2).

Table 2 Measurement results for turbo-generators and MFP steam turbine in various operation regimes. OP.*

Propulsion propeller speed (min1)

Steam pressure at the TG inlet (MPa)

Steam temp. at the TG inlet (°C)

Steam mass flow through one TG (kg/h)

Steam pressure at the TG outlet (MPa)

Steam pressure at the MFP turbine inlet (MPa)

Steam temp. at the MFP turbine inlet (°C)

MFP water volume flow (m3/h)

Steam pressure at the MFP turbine outlet (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

25.00 34.33 41.78 53.50 56.65 61.45 62.52 63.55 65.10 66.08 67.68 68.66 69.49 70.37 71.03 73.09 74.59 76.56 78.41 79.46 80.44 81.49 82.88 83.00

6.21 6.20 6.22 6.09 5.97 5.97 5.98 5.97 6.07 6.06 6.06 6.07 6.07 6.07 6.08 6.06 6.02 6.01 6.03 5.87 5.89 5.91 5.80 5.90

491.0 491.0 491.0 495.0 490.5 491.0 490.5 491.0 491.0 493.0 497.0 500.0 502.5 502.5 503.5 504.5 504.0 504.5 504.5 504.5 501.5 495.5 493.0 493.5

4648.83 4685.54 4556.16 4718.74 4000.58 4102.44 4004.09 4156.83 3838.78 3872.22 3754.24 3794.76 3775.38 3778.91 3798.28 3847.58 3951.37 4070.84 4116.48 4400.42 4689.03 4382.91 4428.43 4487.93

0.00541 0.00512 0.00489 0.00511 0.00425 0.00428 0.00432 0.00451 0.00392 0.00396 0.00404 0.00404 0.00397 0.00397 0.00399 0.00408 0.00412 0.00420 0.00422 0.00433 0.00554 0.00550 0.00557 0.00561

6.21 6.20 6.22 6.09 5.97 5.97 5.98 5.97 6.07 6.06 6.06 6.07 6.07 6.07 6.08 6.06 6.02 6.01 6.03 5.87 5.89 5.91 5.80 5.90

485 487 488 502 496 496 497 497 502 502 507 510 511 511 511 512 513 512 512 510 504 501 500 500

74.49 78.87 75.08 88.14 84.48 70.07 69.71 73.59 76.64 79.11 83.23 82.90 84.98 86.88 87.29 94.24 94.22 100.52 107.91 106.01 111.09 110.66 117.04 118.26

0.2715 0.2701 0.2704 0.2673 0.2763 0.2748 0.2723 0.2708 0.2655 0.2582 0.2564 0.2513 0.2417 0.2411 0.2374 0.2414 0.2391 0.2557 0.2425 0.2351 0.2531 0.2391 0.2495 0.2457

OP.* = Operating Point.

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Isentropic (energy) efficiency of the turbo-generator [40,19,46]:

gI;TG ¼

Dh ðh1  h2 Þ ¼ DhS ðh1  h2S Þ

ð14Þ

(c) Turbo-generator exergy balance [6,7]:

_ TG;1  Ex _ TG;2  PTG ¼ m _ D;TG ¼ Ex _ TG
ðe1  e2 Þ  PTG Ex

ð15Þ

Exergy efficiency of the turbo-generator [15,20,47]:

P

P

TG TG gII;TG ¼ _ ¼ _ TG
½h1  h2  T 0
ðs1  s2 Þ mTG
ðe1  e2 Þ m

ð16Þ

The steam entropy at the turbine inlet (s1 ) was calculated from measured steam pressure and temperature at the turbine inlet. The steam entropy at the turbine outlet (s2 ) was calculated from steam enthalpy at the turbine outlet (h2 ) and measured pressure at the turbine outlet. Steam enthalpy at the turbine inlet, steam enthalpy at the end of isentropic expansion and both steam entropies (at the turbine inlet and outlet) were calculated by using NIST REFPROP 8.0 software [48]. Enthalpies shown in the equations from (11)–(16) do not contain steam kinetic energy. T 0 is the referent ambient (engine room) temperature (298.15 K) while referent ambient pressure p0 is 0.1 MPa. 3.3. Steam turbine for main feed water pump drive Energy and volume flow through the steam turbine and through the main feed water pump is presented in Fig. 4(a), where h1 is steam enthalpy at the turbine inlet and h2 is steam enthalpy at the turbine outlet. Steam enthalpy at the turbine inlet was calculated from the measured pressure and temperature. Steam enthalpy at the turbine outlet was calculated from the turbine _ MFP . required power P MFP and turbine steam mass flow m Fig. 4(b) presents an ideal (isentropic) and real expansion through an MFP steam turbine in h-s diagram. The characteristic expansion points are marked in accordance with the equations for energy and exergy analysis, equations from (17)–(24). The diagram in Fig. 4(b) is drawn by using the measured and calculated values of the operating point 21 (Table 2). Required turbine power is approximated from the main feed water pump volume flow V_ MFP by using third degree polynomial (17). Main feed water pump volume flow in relation to the pump power demand was calculated for medium feed water density q = 937.48 kg/m3 at a temperature of 127 °C, according to producer specifications [30], Fig. 5(a).

PMFP ¼ 1:78582
105
V_ 3MFP  3:08892
103
V_ 2MFP þ 2:002
V_ MFP þ 189:48

ð17Þ

PMFP was obtained in (kW) when V_ MFP in (m3/h) was placed in Eq. (17). Steam mass flow through MFP turbine was approximated with the pump power demand PMFP , Fig. 5(b). Approximation was made according to producer specifications [30], by using third degree polynomial:

_ MFP ¼  3
105
P3MFP þ 3:1326
102
P2MFP  4:396794
PMFP m þ 2386:60

ð18Þ

_ MFP was obtained in (kg/h) when PMFP in (kW) was placed in where m Eq. (18). Steam enthalpy at the MFP turbine outlet, where P MFP in (kW) is _ MFP in (kg/s) is turbine steam mass flow, the turbine power and m was calculated according to [49] by using the equation:

h2 ¼ h1 

PMFP _ MFP m

ð19Þ

(a) MFP steam turbine mass flow balance:

_ MFP;2 ¼ m _ MFP _ MFP;1 ¼ m m

ð20Þ

(b) MFP steam turbine energy balance [14,18]:

_ MFP;1  En _ MFP;2 ¼ m _ MFP
ðh1  h2 Þ PMFP ¼ En

ð21Þ

Isentropic (energy) efficiency of the MFP steam turbine [40,19,46]:

gI;MFP ¼

Dh ðh1  h2 Þ ¼ DhS ðh1  h2S Þ

ð22Þ

(c) MFP steam turbine exergy balance [6,7]:

_ D;MFP ¼ Ex _ MFP;1  Ex _ MFP;2  PMFP ¼ m _ MFP
ðe1  e2 Þ  PMFP Ex

ð23Þ

Exergy efficiency of the MFP steam turbine [15,20,47]:

P

P

MFP MFP gII;MFP ¼ _ ¼ _ MFP
½h1  h2  T 0
ðs1  s2 Þ mMFP
ðe1  e2 Þ m

ð24Þ

The steam entropy at the MFP turbine inlet (s1 ) was calculated from measured steam pressure and temperature at the turbine inlet. The steam entropy at the MFP turbine outlet (s2 ) was calculated from steam enthalpy at the turbine outlet (h2 ) and measured pressure at the turbine outlet. Steam enthalpy at the turbine inlet, steam enthalpy at the end of isentropic expansion and both steam entropies (at the turbine inlet and outlet) were calculated by using NIST REFPROP 8.0 software [48].

Fig. 4. Steam turbine and the main feed water pump: (a) energy and volume flow, (b) turbine isentropic and real expansion (operating point 21 - Table 2).

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313

Fig. 5. MFP turbine: (a) turbine power, (b) turbine steam mass flow [30].

Enthalpies shown in the equations from (19)–(24) do not contain steam kinetic energy. T 0 is the referent ambient (engine room) temperature (298.15 K) while referent ambient pressure p0 is 0.1 MPa.

4. Energy and exergy efficiency of various steam turbines When considering the energy and exergy efficiency of steam turbines, all conclusions and considerations must be divided into two groups regarding the steam turbine power. For high-power steam turbines which are used in land-based power plants, following three important facts are valid: – Energy and exergy efficiencies are very high (approximately 80% or higher) [6,7,9] – Energy and exergy efficiencies have almost identical trends of change [12,18] – Ambient temperature has a little impact on exergy efficiency [14–17] For low-power steam turbines similar to those analyzed in this paper, there are not found exact indicators for energy and exergy efficiency values range, trends of change or indicators for the influence of the ambient temperature on exergy efficiency. Available literature provides only guidelines for energy efficiency values of low-power steam turbines according to their power range. As shown in Fig. 6, it can be expected that the energy efficiency of analyzed steam turbines is much lower when compared to the high-power steam turbines [50]. The same conclusion can be found in [51–53].

5. Measurement results from the analyzed LNG carrier Measurement results of required operating parameters for turbo-generators and MFP steam turbine are presented in Table 2 in relation to the propulsion propeller speed. Turbo-generators are identical and measurements have shown that their operating parameters are also identical (steam pressures, temperatures and mass flows). All the measurement results were obtained from the existing measuring equipment mounted on every individual component of the analyzed propulsion plant. List and specifications of all used measuring equipment are presented in the Appendix A at the end of this paper.

6. Results and discussion Diagram in Fig. 7 shows turbo-generators and MFP turbine power, according to measured values in Table 2. Throughout the whole examined operating range, the highest produced power of both turbo-generators in parallel operation was only 30% of their maximum power (Fig. 7 shows the cumulative power of both turbo-generators). At the beginning of observed operating range, turbo-generators developed power greater than 2000 kW, because during that part of operating range several electrical consumers are in full operation (bow thruster, electric motors of anchor capstan hydraulics and mooring winches). The exact moment when captain shut down mentioned devices (one by one) depends on his evaluation. At the observed propulsion propeller operating range, mentioned devices shutting down starts between 55 min1 and 60 min1 of the propulsion propeller. Electrical consumers’ shutting down is

Fig. 6. Energy efficiency in relation to steam turbine power [50].

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Fig. 7. Turbo-generators and MFP steam turbine power change.

visible as turbo-generators power decrease. The lowest power of turbo-generators in the observed operating range was 1518 kW at the propulsion propeller speed of 67.68 min1. Increase in turbo-generators power at the end of observed operating range is caused by an increase in power of low duty compressor, which compresses the boil off gas (BOG) from the cargo tanks to the steam generators. Steam generators at the analyzed LNG carrier operate in dual fuel regime (HFO/gas). HFO consumption is kept to a minimum and the increase in the steam system load is reflected in the BOG consumption increase. Increase of the propulsion propeller speed is directly proportional to increase of steam system load. From the viewpoint of the produced power it can be concluded that, in the observed operating range, application of only one turbo-generator will be sufficient. The parallel operation of two turbo-generators is justified only from the aspect of safety. An MFP turbine at the beginning of the observed operating range developed more power than necessary. The reason for this fact is relatively small feed water mass flow through the system, so the main feed water pump must recycle a certain amount of feed water into the deaerator, Fig. 1, in order to prevent cavitation. After the recirculation period, the power of MFP turbine decreases (at propulsion propeller speed of 61.45 min1) and then slightly increases with an increase in steam system load. Notable increase in MFP turbine developed power is noticed at propulsion propeller speed of 73.09 min1 due to considerable increase in feed water volume flow (Table 2). The highest achieved power of the MFP turbine during a measurement period was 73% of its maximum power (413 kW) and it can be seen at the highest propulsion propeller speed, Fig. 7. Adibhatla and Kaushik [6] in their analysis concluded that exergy efficiency of high-power steam turbine has a declining trend with decreasing unit load. The same conclusion for highpower steam turbines can be derived from the papers Erdem et al. [7] and Yang et al. [8]. It is therefore interesting to investigate whether the same conclusions are valid also for low-power steam turbines. Fig. 8, according to Table 2, presents energy and exergy efficiency change of TG steam turbine in the observed steam system operating range. Both efficiencies are highly dependent on the generator electric load. Energy and exergy efficiency reduces when the load decreases (propulsion propeller speeds from 56.65 min1 to 70.37 min1). By increasing the load, both efficiencies increase. For the analyzed TG steam turbine it is important to point out that for energy efficiency is valid the same trends of change and dependence on the electrical load as for the exergy efficiency. It can be

assumed that the same conclusions will be valid for the energy efficiency of high-power steam turbines during the load change, when the energy efficiency is calculated according to Eqs. (14) and (22). In the whole investigated operating range, energy and exergy efficiencies of the TG steam turbine have an almost identical trend of change, what is also valid for high-power steam turbines. In the examined operating range, both efficiencies vary between 54% and 61%, what are to be the expected values for the low-power steam turbines (according to Fig. 6). From the viewpoint of energy and exergy efficiencies, usage of only one turbo-generator instead of two in the parallel operation would be advisable in the analyzed operating range. Only one operational turbo-generator would be more loaded and it would achieve higher efficiencies. On the other side, such arrangement could lead to the potential danger in the ship electricity supply network if any of unexpected problems occur. That is the main reason why at least two turbo-generators are always in parallel operation during the ship navigation. Especially in the case of turbo-generators, energy and exergy efficiencies can be greatly reduced due to intense corrosion on turbine blades [54] or due to the increased vibrations of the turbine rotor [55–57]. For these reasons, it is necessary to perform constant maintenance and control of steam turbines driving electric generators [58,59]. The exergy destruction rate of one TG steam turbine is presented in Fig. 9 for all operating points during observed steam system operating range, according to Table 2. Graph in Fig. 9 shows that the exergy destruction rate of one TG steam turbine varies between 621.5 kW and 679 kW. During the increase in propulsion propeller speed, the exergy destruction rate of TG turbine does not show a continuous trend of change. Therefore, it can be concluded that the increase in steam system load has no major impact on the TG turbine exergy destruction rate. From the diagrams shown in the papers Ahmadi et al. [15] and Ameri et al. [16] can be concluded that high-power steam turbines exergy destruction rate is proportional to steam turbine load. Higher load results in higher exergy destruction rate and vice versa. A direct comparison of Figs. 9 and 7 show that the same conclusion is valid also for low-power steam turbine (TG steam turbine). Steam turbine load is the most influential parameter on which depend the change of energy and exergy efficiencies and also the change of exergy destruction rate regardless of maximum steam turbine power. In order to explain the most influential changes in the exergy destruction rate during observed TG operating period, it is necessary to compare results presented in Fig. 9 with the results

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Fig. 8. TG steam turbine energy and exergy efficiency change.

Fig. 9. Exergy destruction rate of one TG steam turbine in observed operating range.

presented in Table 2. It will not be considered the TG developed power, but steam mass flow through one TG in kg/h, according to Table 2 and Fig. 2. A significant decrease in TG exergy destruction rate (from 669.5 kW to 629 kW) is caused by a decrease in steam mass flow from 4718.74 kg/h to 4000.58 kg/h (propulsion propeller speeds 53.50 min1 and 56.65 min1). Notable increase in the TG exergy destruction rate is visible at propulsion propeller speeds 78.41 min1 and 79.46 min1 where steam mass flow increases from 4116.48 kg/h to 4400.42 kg/h. Even a higher increase in steam mass flow, in comparison to a previous case, occurs between propulsion propeller speeds of 79.46 min1 and 80.44 min1 (from 4400.42 kg/h to 4689.03 kg/ h). But, in this situation, exergy destruction rate decreases (from 679 kW to 658 kW). The reason of such decrease in the exergy destruction rate lies in the decrease of steam temperature at the TG inlet (from 504.5 °C to 501.5 °C). Such a decrease in temperature significantly increases the specific exergy at the TG outlet (from 53.49 kJ/kg to 83.71 kJ/kg). According to Eq. (15), a significant increase in specific exergy at the TG outlet along with an increase in TG developed power, results in a decrease of the TG exergy destruction rate. Decrease in TG exergy destruction rate between propulsion propeller speeds of 80.44 min1 and 81.49 min1 is caused by a decrease in steam mass flow (from 4689.03 kg/h to 4382.91 kg/h)

along with further decrease of steam temperature at the TG inlet (from 501.5 °C to 495.5 °C). It can be concluded that the change in the exergy destruction rate of low-power steam turbines is mostly influenced by the change of steam turbine load (steam mass flow rate) or in some situations with the change of steam operating parameters (mainly with steam inlet temperature). The main conclusion derived from TG steam turbine energy and exergy efficiency analysis is also valid for any steam turbine, including MFP turbine. MFP steam turbine energy and exergy efficiencies do not show major changes across the whole range of propulsion propeller speeds and they primarily depend on the MFP turbine load, Fig. 10. Change in MFP turbine load can be considered as a change of feed water volume flow, according to Table 2. After the recirculation period, energy efficiency of MFP steam turbine decreases from 49.36% to 48%, while its exergy efficiency decreases from 60.25% to 59% (propulsion propeller speeds 56.65 min1 and 61.45 min1). From propulsion propeller speed of 61.45 min1 to 71.03 min1 developed power of MFP turbine increases slightly with an increase in feed water volume flow. Due to increase of steam temperature at the MFP turbine inlet (Table 2) in this range of propulsion propeller speeds, energy and exergy efficiencies slightly decrease. The decrease of both efficiencies is small (approximately

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Fig. 10. MFP steam turbine energy and exergy efficiency change.

1% from the beginning to the end of observed operating range). Increase of steam temperature at the MFP turbine inlet causes an intense increase in steam enthalpy, what is the main reason for efficiencies reduction, Fig. 10. Slight increase in MFP turbine power has a smaller influence on exergy efficiency in comparison with the increase in steam enthalpy at the turbine inlet, Eq. (24). At propulsion propeller speed of 73.09 min1 due to considerable increase in feed water volume flow, both efficiencies of MFP steam turbine start to increase. The increase in MFP turbine efficiencies lasts until the highest observed propulsion propeller speed at which the highest values of both efficiencies were achieved. Exergy efficiency of MFP turbine varies between 58% and 62% at all running regimes what indicates that the exergy efficiency is within the same range as for the turbo-generator. Energy efficiency of MFP turbine shows that this component has the lowest energy efficiency (from 46.8% to 51%) within the whole steam propulsion plant at all running regimes, Fig. 10. In a favour of MFP steam turbine low energy efficiency contributes the fact that single Curtis stage has always lower energy efficiency when compared with conventional impulse steam turbine stage. From the standpoint of MFP turbine energy efficiency, the shipowner proposal that the main feed water pump, in such plants, should be powered by the electric motor instead of the steam turbine is justified. The main feed water pump drive by electric motor will increase both turbo-generators efficiencies, because the turbogenerators will be additionally loaded with a new electric consumer. The exergy destruction rate of MFP steam turbine is presented during observed steam system operating range, according to Table 2. Fig. 11 shows that the exergy destruction rate of MFP turbine amounts between 222.7 kW and 264.5 kW. In comparison with TG turbine, the exergy destruction rate of MFP turbine is much lower because MFP turbine develops approximately three times less power than each TG turbine, during the whole observed operating range. According to Table 2 and Fig. 5, change in MFP turbine load can be considered as a change of feed water volume flow. In the propulsion propeller speed operating range from 25.00 min1 to 74.59 min1, change in MFP turbine load is the most influenced parameter of which depend the change in MFP turbine exergy destruction rate, Fig. 11. The only intense jump in the exergy destruction rate in this operating range can be noticed at propulsion propeller speed of 53.50 min1, caused by an increase in feed water volume flow (from 75.08 m3/h at 41.78 min1 to 88.14 m3/h at 53.50 min1). After propulsion propeller speed of 74.59 min1 until the end of complete observed operating area (until 83.00 min1), intense

impact on MFP turbine exergy destruction rate has a steam temperature at the turbine inlet. As can be seen in Table 2, steam temperature at the turbine inlet constantly decreases (from 513 °C to 500 °C) in this operating range. Decrease in MFP turbine exergy destruction rate in this operating range (which occurs at propulsion propeller speeds 76.56 min1, 80.44 min1 and 82.88 min1) has the same explanation as a decrease in the exergy destruction rate of turbo-generator between propulsion propeller speeds of 79.46 min1 and 80.44 min1, Fig. 9. Despite an increase in feed water volume flow (compared to the previous observed operating point), steam temperature at the MFP turbine inlet decreases what significantly increases the specific exergy at the MFP turbine outlet. According to Eq. (23), a significant increase in specific exergy at the MFP turbine outlet along with an increase in MFP turbine developed power, results in a decrease of the MFP turbine exergy destruction rate. In the energy and exergy analysis of land-based steam power plants, some authors investigate the influence of the ambient temperature on exergy efficiency rate of steam plant components. The ambient temperature change has no influence on the energy efficiency of any steam plant component [13]. Ahmadi et al. [15], Ameri et al. [16] and Aljundi [14] investigate the influence of the ambient temperature on high-power steam turbine exergy efficiency. They concluded that the ambient temperature has low impact on exergy efficiency of high-power steam turbines. The ambient temperature increase causes decrease in exergy efficiency of all analyzed high-power steam turbines and the same conclusion is valid for other steam plant components analyzed in mentioned literature. Usually, an increase in the ambient temperature of 10 °C causes a decrease in high-power steam turbine exergy efficiency for about 1% or less. Kopac et al. [17] obtained the same conclusion for the highpower steam turbine exergy efficiency in correlation to the ambient temperature. In contrast to the aforementioned, this paper also describes the influence of the ambient temperature on steam condenser exergy efficiency. An increase in the ambient temperature has a significant impact on steam condenser exergy efficiency. Steam condenser exergy efficiency firstly decreases with a slight increase in the ambient temperature. During the further increase in the ambient temperature, exergy efficiency of steam condenser increases significantly. It is realistic to expect that the influence of ambient temperature on the exergy efficiency of the TG steam turbines and MFP steam turbine will also have low impact. Ambient temperature of marine propulsion steam plants usually has a greater influence when compared to the land-based plants, depending greatly on the geographic area in which ship operates. Likewise, ship steam

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Fig. 11. Exergy destruction rate of steam turbine for MFP drive in observed operating range.

propulsion plants are more frequently affected by different ambient temperatures than land-based steam plants. Trend of exergy efficiency rate for low-power steam turbines (TG steam turbines and MFP turbine) must be the same as for high-power steam turbines. The reason for this conclusion is presented in Eqs. (16) and (24) for steam turbine exergy efficiency calculation. The ambient temperature (T0) is operating parameter which is located in the equation’s denominator. As all other steam operating parameters in those equations remain the same as before, increase in ambient temperature will reduce the exergy efficiencies. The increase in the denominator occurs because steam entropy at each turbine outlet (s2) is always higher than steam entropy at the turbine inlet (s1), Figs. 3 and 4. The only remaining question is will the ambient temperature have a strong or weak impact on the analyzed low-power steam turbines exergy efficiency. TG steam turbine exergy efficiency in relation to the ambient temperature is presented in Fig. 12, according to Table 2. The ambient temperature was varied from 10 °C to 40 °C, what is expected range of ship engine room temperatures. The highest exergy efficiency values of the TG turbine were obtained at the lowest engine room temperature (10 °C) and their values was from 56% up to a maximum of 62% at the highest observed load. With a temperature increase of 10 °C TG turbine exergy efficiency was reduced from

0.8% to 1% at all analyzed loads. At the highest observed engine room temperature (40 °C) exergy efficiency was from 53% up to 60%. The ambient temperature was also varied from 10 °C to 40 °C in order to obtain the exergy efficiency change for MFP steam turbine, Fig. 13, according to Table 2. MFP steam turbine exergy efficiency show trend that is similar to TG turbine. The highest exergy efficiency values were obtained for the engine room temperature of 10 °C (from 59% up to 63%) and the lowest exergy efficiency values were obtained for the engine room temperature of 40 °C (from 56.7% up to 60.5%). A temperature increase of 10 °C causes the reduction of exergy efficiency from 0.8% to 1% also in the steam turbine for MFP drive. In order to determine the energy and exergy efficiency maximum for turbo-generator and MFP steam turbine, additional analysis was performed for all operating points presented in Table 2 and results for operating points 13 and 21 are presented for each turbine. At each operating point of both turbines, steam pressure and temperature at the turbine inlet and steam pressure at the turbine outlet remain identical to the measured data. By changing the steam mass flow, change in developed power of both turbines occurs. Steam enthalpies at the turbine outlets (h2) that are calculated according to Eqs. (11) and (19) have been calculated again as well as outlet steam entropies (s2).

Fig. 12. TG steam turbine exergy efficiency in relation to the ambient temperature.

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Fig. 13. MFP steam turbine exergy efficiency in relation to the ambient temperature.

Fig. 14. TG steam turbine efficiencies for operating point 13.

Fig. 14 presents change in the cumulative power for both turbogenerators in relation to energy and exergy efficiency for operating point 13 (Table 2). Turbo-generator energy and exergy efficiencies increases until cumulative turbo-generators power reach 5500 kW. The highest efficiencies were achieved at 71.5% of maximum developed turbo-generator power. This fact applies not only at this operating point, but also in all other analyzed operating points. Increase in turbo-generators energy efficiency during the steam mass flow increase occurs because until cumulative turbogenerators power reach 5500 kW the increase in developed power has a greater intensity than the increase in steam mass flow, Fig. 2. According to Eq. (11) this fact resulted with the decrease in enthalpy at each turbo-generator outlet (h2) what increases numerator in Eq. (14). All the other elements of Eq. (14) remain the same as before. After the turbo-generator energy efficiency reaches a maximum value, the opposite occurrence happens. From cumulative turbo-generators power of 5500 kW onwards, increase in developed power has a lower intensity than the increase in steam mass flow, Fig. 2. This leads to the increase in enthalpy at each turbo-generator outlet and energy efficiency decreases. The change in turbo-generator exergy efficiency during the increase in steam mass flow is also caused by differences in developed power and steam mass flow. Energy and exergy efficiency trend for turbo-generator, during increase in steam mass flow, will be the same according to Eq. (16). Highest energy efficiency of turbo-generator in this operating point is 66.5%, while the maximum exergy efficiency is 66.79% at a standard ambient temperature of 25 °C.

With the variation of steam mass flow, the ambient temperature was also varied in the range from 10 °C to 40 °C. As concluded before, an increase in the ambient temperature causes a decrease in exergy efficiency and vice versa for any steam turbine. During the change in the ambient temperature along with a simultaneous variation of steam mass flow, turbo-generators exergy efficiency trend remains the same as for the ambient temperature of 25 °C, Fig. 14. Maximum exergy efficiency of 67.92% is achieved at a lowest ambient temperature of 10 °C. It is interesting to note that the highest turbo-generator exergy efficiency at ambient temperature of 40 °C was 65.69%, therefore it would be 1.10% lower than the maximum exergy efficiency at ambient temperature of 25 °C or 2.23% lower than the maximum exergy efficiency at ambient temperature of 10 °C. Also, ambient temperature had no significant impact on turbo-generator exergy efficiency in analysis with a variation of steam mass flow. It is important to note the range of achieved turbo-generators efficiencies, according to measured operating parameters (BP) during ship exploitation. In operating point 13, Fig. 14, energy efficiency of each turbo-generator amounts only 53.82% (grey dot) while the exergy efficiency at the same operating point amounts 54.77% (black dot). It can be concluded that these efficiencies are much lower than calculated maximum values. From the aspect of energy efficiency, MFP steam turbine is detected as one of the poorest components in the steam propulsion system what is confirmed also by this analysis. Introducing an electric motor instead of MFP steam turbine at the analyzed LNG carrier would bring at least two important improvements:

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– The MFP steam turbine has energy efficiency between 46.8% and 51%, while its exergy efficiency is in the range between 57.5% and 62%. Both energy and exergy efficiency of electric motor amounts between 80% and 87%, according to [60]. According to [45], energy and exergy efficiency of electric motor amounts around 90%. Thus, the electric motor has considerably higher energy and exergy efficiency, which will improve the overall energy-exergy efficiency of the whole propulsion plant. – As shown in Fig. 8, energy and exergy efficiencies of turbogenerator increases with the load increase. Electric motor power consumption directly increases the load of turbogenerators. If it is assumed that electric motor would have maximum power of 570 kW and that his power has changed between 200 kW and 570 kW during operation (power change of MFP steam turbine), increase in turbo-generator energy and exergy efficiency would be between 1% and 4% (EM in Fig. 14). The analyzed system would require inclusion of additional electrical consumers if maximum turbo-generator efficiencies will be set as a target (but only until the turbogenerators cumulative power of 5500 kW). In the presented analysis, MFP turbine uses between 3250 kg/h and 3650 kg/h of superheated steam during all observed propulsion propeller speeds. Both turbo-generators will require between 750 kg/h and 2000 kg/h more superheated steam to develop 200 kW to 570 kW more power, Fig. 2. The rest of the steam with higher pressure and temperature will be sent to the deaerator to ensure its smooth operation, Fig. 1. It is an open question whether the remaining steam quantity would be sufficient for the smooth deaerator operation. If not, steam generators will have to produce more superheated steam, which will increase the fuel consumption and propulsion plant costs, at least in some steam plant operating regimes. The real answer to this question can offer only a detailed analysis of the entire steam system before and after the proposed change. The conclusions derived from Fig. 14 are valid in any analyzed turbo-generator operating point (Table 2). In the operating point 21, Fig. 15, the highest turbo-generator energy efficiency is 68.62% and the highest exergy efficiency amounts 69.31% at a standard ambient temperature of 25 °C while at the ambient temperature of 10 °C maximum exergy efficiency grow up to the value of 70.4%. According to measured operating parameters during the ship exploitation (BP), turbo-generator energy efficiency is 59.94% (grey dot) while the exergy efficiency amounts 60.92% (black dot) in the operating point 21. The introduction of electric motor instead of the MFP steam turbine, with the same assumptions as before,

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would cause an increase in both turbo-generator efficiencies between 1% and 3% (EM in Fig. 15). Variations of steam mass flow were made also for the MFP steam turbine and energy and exergy efficiency rates were investigated in the same two operating points as for the turbo-generators. In contrast to the turbo-generators, the highest MFP steam turbine efficiencies appear during the highest load of 570 kW for all operating points. Both efficiencies of the MFP steam turbine are increasing continuously with the driving power. Increase in MFP steam turbine energy efficiency during the steam mass flow increase for the whole observed operating range occurs because the increase in developed power has a greater intensity than the increase in steam mass flow, Fig. 5. According to Eq. (19) this resulted with the decrease in enthalpy at the MFP turbine outlet (h2) what increases numerator in Eq. (22). All the other elements of Eq. (22) remain the same as before. Change in MFP turbine exergy efficiency during the increase in steam mass flow is also caused by differences in developed power and steam mass flow. The increase in developed power has a greater intensity than the increase in steam mass flow for the whole observed operating range, Fig. 5, what results with continuous increase in MFP turbine exergy efficiency. During the change in the ambient temperature along with a simultaneous variation of steam mass flow, MFP steam turbine exergy efficiency trend remains the same as for the ambient temperature of 25 °C, Fig. 16. Highest energy efficiency of MFP steam turbine in operating point 13 (Table 2), is 57.34%, while the highest exergy efficiency for the same operating point amounts 66.99% at a standard ambient temperature of 25 °C, Fig. 16. By reducing the ambient temperature to 10 °C, maximum exergy efficiency increases to 68.12%, while the ambient temperature increase to 40 °C lowers maximum exergy efficiency to 65.89%. According to the measured data from the ship exploitation (BP), energy efficiency of MFP steam turbine in operating point 13 is 46.75% (grey dot) while the exergy efficiency for the same operating point amounts 57.92% (black dot) giving significantly lower values when compared to those at maximum power. The required power of the MFP steam turbine was changed according to the required feed water mass flow in the steam propulsion system. Therefore, it was not possible to maintain the MFP steam turbine at constant maximum power and maximum efficiencies. In the operating point 21 (Table 2), the highest energy efficiency of MFP steam turbine is 58.85%, while the highest exergy efficiency amounts 68.27% at a standard ambient temperature of 25 °C, Fig. 17. Both maximum efficiencies were calculated at the highest MFP steam turbine power. With an increase of the ambient

Fig. 15. TG steam turbine efficiencies for operating point 21.

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Fig. 16. MFP steam turbine efficiencies for operating point 13.

Fig. 17. MFP steam turbine efficiencies for operating point 21.

temperature to 40 °C maximum exergy efficiency decreases to 67.19%, while decrease of the ambient temperature to 10 °C causes that maximum exergy efficiency increases to 69.37%. According to the measured data from the ship exploitation (BP), energy efficiency of MFP steam turbine in operating point 21, Fig. 17, is 50.20% (grey dot) while the exergy efficiency amounts 60.99% (black dot). The same variations of the steam mass flow and influence of those variations on the exergy efficiency for two turbogenerators (STGI and STGII) performed Hafdhi et al. [18]. The obtained trends are comparable with the results obtained for turbo-generator and MFP steam turbine. Analyzed turbo-generators can be compared with STGII from mentioned analysis. Obtained trends of exergy efficiency rates are the same. With the increase in steam mass flow, exergy efficiency increases to maximum value. Further increase in steam mass flow causes exergy efficiency to decrease. Hafdhi et al. [18] concluded that maximum exergy efficiency amounts 75.5% and is achieved at 89% of maximum steam mass flow for STGII. In this paper, maximum exergy efficiency of the analyzed turbogenerator amounts 66.79% (operating point 13) and 69.31% (operating point 21) at the ambient temperature 25 °C. Both maximum exergy efficiencies were achieved at 71.5% of maximum developed turbo-generator power. The MFP steam turbine is comparable with STGI. Again, trends in exergy efficiency are the same for STGI and MFP steam turbine analyzed in this paper. With the increase in steam mass flow, exergy efficiency increases continuously and reaches maximum value at the highest steam flow. Hafdhi et al. [18] maximum exergy

efficiency of 52% for STGI obtained at the highest steam mass flow. MFP steam turbine maximum exergy efficiencies were obtained at maximum developed power of 570 kW and they were 66.99% (operating point 13) and 68.27% (operating point 21) at the ambient temperature 25 °C. Similar research on energy efficiency rate and ambient temperature influence on exergy efficiency of low-power steam turbines authors didn’t find in available literature. Analysis of these two parameters in relation to low-power steam turbine load was performed in this paper. 7. Final remarks Not only the turbo-generators and MFP steam turbine, but also the other components of steam propulsion plant must have a series of measurement and control devices [61,62]. Ship engine room electronic management and control system gives the possibility of monitoring and control of each individual component. During energy and exergy efficiency investigation of any steam system or some of its components, some uncertainties should be considered [63–65]. In the presented analysis, they were not considered in detail, but it was assumed that uncertainties would not change the essential conclusions. Efficiencies optimization for turbo-generators and MFP steam turbine can be performed in many different ways. Nowadays, optimization of steam turbines and their processes is usually conducted through the application of artificial neural networks [66,67], genetic algorithms [44,68,69] or multi-objective optimization [42,70].

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The recommendations in this paper can be helpful to the crew of the analyzed LNG carrier in choosing a more favourable energy solution which is, according to SEEMP (Ship Energy Efficiency Management Plan) convention [71,72], the obligation for the crew and ship-owner. 8. Conclusions Although at the present time ship propulsion systems are mainly based on internal combustion diesel engines, steam propulsion plants are still a dominant occurrence on a number of LNG carriers. Based on the measurement data from the analyzed LNG carrier, energy and exergy analysis was performed in this paper for two components of the steam propulsion plant: turbo-generators and steam turbine for main feed water pump drive. Results of presented mathematical equations show the advantages and disadvantages of these components throughout the whole investigated propulsion operating range. For low-power steam turbines, which were taken into consideration in this research, the following conclusions are valid: – Energy and exergy efficiencies are quite low (between 46.8% and 62%), what is significantly lower when compared to highpower steam turbines – Energy and exergy efficiencies have almost identical trends of change – Ambient temperature has low impact on exergy efficiency (change in the ambient temperature for 10 °C causes less than 1% change in exergy efficiency) By varying the steam mass flow for turbo-generators and MFP steam turbine, a complete insight was obtained in a range of energy and exergy efficiencies. Highest energy and exergy efficiencies of turbo-generator were achieved at 71.5% of maximum developed power while the MFP steam turbine achieves the highest efficiencies at maximum power. The conclusion about maximum efficiencies for both analyzed turbines is valid in all measured operating points. Energy and exergy efficiencies of the turbo-generator and MFP steam turbine that are calculated from measured exploitation points are much lower than the maximum possible values. Therefore, it would be advisable to perform an energy and exergy optimization of the entire steam propulsion system in order to achieve the optimal efficiencies in different operating regimes. The paper presents one of the possible methods for improving the analyzed steam propulsion plant efficiencies by replacing an existing MFP steam turbine with an electric motor. This modification would have at least two useful benefits. One of them is increase in turbo-generator energy and exergy efficiencies for at least 1–3% in all measured operating points. Substitution of MFP steam turbine with an electric motor, analysis of his impact on the entire steam system as well as profitability calculation of such investment will be the topic for future scientific research.

Appendix A A.1. List and specifications of used measuring equipment TG steam mass flow: ? Yamatake JTD960A - Differential Pressure Transmitter [73] Measuring span: Setting span: Working pressure range:

Accuracy

0.25–14 MPa 100 to 14 MPa 2.0 kPa–14 MPa

Linear output: 0:15%   0:1 þ 0:05


3:5

v



ðv P 3:5 MPaÞ

% ðv < 3:5 MPaÞ

Square-root output: – When output is 50–100%: same as linear output – When output is 7.1–50%: 50 linear output
squareroot output % – When output is less than 7.1%: dropout

Pressure at the TG outlet: ? Yamatake JTD910A - Differential Pressure Transmitter [73] Measuring span: Setting span: Working pressure range:

Accuracy

0.1–2 kPa 1 to 1 kPa 70 to 0.206 MPa

Linear output: ð0:15 þ 0:15
v1Þ% Square-root output: – When output is 50–100%: same as linear output – When output is 7.1–50%: 50 linear output
squareroot output % – When output is less than 7.1%: dropout

MFP water volume flow: ? Promass 80F - Coriolis Mass Flow Measuring System [74] Measuring range: Maximum measured error: Zero point stability: Repeatability: Ambient temperature range: Medium temperature range:

0–180,000 kg/h ±0.15% 9.00 kg/h ±0.05% 20 to +60 °C 50 to +200 °C

Acknowledgment The authors would like to extend their appreciations to the main ship-owner office for conceding measuring equipment and for all help during the exploitation measurements. Prof. Vladimir Medica, Faculty of Engineering, University of Rijeka is gratefully acknowledged for helpful suggestions and discussions. This research did not receive any grant from funding agencies in the public, commercial, or not-for-profit sectors.

Temperature at the TG and MFP turbine inlet: ? Greisinger GTF 601-Pt100 - Immersion probe [75] Measuring range: Response time: Standard: Error ranges:

200 to +600 °C approx. 10 s 1/3 DIN class B ð0:10 þ 0:00167
jTemp: in  CjÞ

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Pressure at the TG and MFP turbine inlet: ? Yamatake JTG980A - Pressure Transmitter [76] Measuring span: Setting span: Working pressure range:

0.7–42 MPa 100 to 42 MPa 2.0 kPa–42 MPa

0:15% ðv P 7 MPaÞ    0:05 þ 0:1
v7 % ðv < 7 MPaÞ

Accuracy

Pressure at the MFP turbine outlet: ? Yamatake JTG940A - Pressure Transmitter [76] Measuring span: Setting span: Working pressure range:

Accuracy

35–3500 kPa 100 to 3500 kPa 2.0 kPa–3500 kPa

0:1% ðv P 0:14 MPaÞ    0:025 þ 0:075
0:14 v %

ðv < 0:14 MPaÞ

Propulsion propeller speed: ? Kyma Shaft Power Meter (KPM-PFS) [77] Accuracy Torque Thrust Revolution Power

Absolute < ±0.5% < ±5.0% < ±0.1% < ±0.5%

Relative < ±0.5% < ±5.0% < ±0.1% < ±0.5%

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