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Electronic Theses, Treatises and Dissertations The Graduate School
2013 Ship Weight Reduction and Efficiency Enhancement Through Combined Power Cycles Michael J. Coleman
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COLLEGE OF ENGINEERING
SHIP WEIGHT REDUCTION
AND EFFICIENCY ENHANCEMENT
THROUGH COMBINED POWER CYCLES
By
MICHAEL J. COLEMAN
A Thesis submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Master of Science
Degree Awarded: Spring Semester, 2013
Michael J. Coleman defended this thesis on April 1, 2013. The members of the supervisory committee were:
Juan C. Ordonez Professor Co-Directing Thesis
Alejandro Rivera Professor Co-Directing Thesis
Farrukh S. Alvi Committee Member
Carl A. Moore, Jr. Committee Member
The Graduate School has verified and approved the above-named committee members, and certifies that the thesis has been approved in accordance with university requirements.
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This thesis is dedicated in gratitude to my parents, Norwood Sr. and Alice Coleman. It is dedicated in loving memory to my grandparents Viola Smith, and Charles Frank Coleman.
This thesis is dedicated to the prosperity of my daughter, Onyame Coleman, with respect to my brother, Norwood Coleman, Jr, and with thanks and gratitude to God.
This thesis is also dedicated to all of my family, and friends, who are too numerous to mention here. This thesis is dedicated to the educators at every level who have influenced my life and career.
Special thanks goes to my boss, Ferenc Bogdan, at the Center for Advanced Power Systems (CAPS), who has been very patient during this process, Steinar Dale, CAPS’ director, all of the facilities staff at CAPS, and all of my co-workers at CAPS.
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ACKNOWLEDGEMENTS Partial support for this work from the Office of Naval Research (ONR) and the Naval Engineering Education Center (NEEC) is greatly appreciated. I would also like to acknowledge Alejandro Rivera, Carl Moore, Juan Ordonez and Emanuel Collins, whose support and encouragement has been indispensible, as well as Leon Van Dommelen, and Anter El-Azab, who emphasized the role and importance of mathematics in the to me in the pursuit of scientific concepts.
As always, I wish to acknowledge the love and support provided by Norwood Sr. and Alice Coleman, for their unwavering support at every stage of life, in every way possible.
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TABLE OF CONTENTS List of Tables ...... vii
List of Figures ...... viii
Abstract ...... x
1. MOTIVATION AND LITERATURE REVIEW ...... 1
1.1 Combined Cycles for All-Electric Ship Applications ...... 1
1.2 Variations of Hybrid Electric Ship Configurations ...... 4
1.3 Installed All-Electric Ship System ...... 6
2. ANALYSIS OF A COMBINED CYCLE POWER PLANT ...... 8
2.1 Overview of the Combined Cycle Power Plant ...... 8
2.2 The Gas Turbine Prime Mover ...... 10
2.3 The Steam Power Plant ...... 13
2.3.1 The Heat Recovery Steam Generator (HRSG) ...... 13
2.3.2 The Steam Turbine ...... 17
2.3.3 The Condenser and the Pump ...... 21
2.4 Design Strategy for the Combined Cycle Power Plant ...... 23
2.4.1 Combined Cycle Power Plant Configuration Analysis ...... 23
2.4.2 Roadmap to Combined Gas and Steam Turbine Power Plant Configuration ...... 26
2.5. Summary ...... 30
3. WEIGHT ANALYSIS ...... 31
3.1 Weight Considerations for the Combined Gas and Steam Turbine Power Plant ...... 31
3.2 Turbine Weight ...... 33
3.2.1 Gas Turbine Weight ...... 36
3.2.2 Steam Turbine Weight ...... 38
3.2.3 Electrical Generator ...... 41
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3.3 Heat Exchanger Weight and Heat Transfer Area ...... 41
3.4 Fuel Volume and Weight ...... 46
3.5 Summary ...... 56
4. ANALYSIS RESULTS ...... 57
4.1 Case I – The Effects of Varying HRSG Exhaust Gas Temperature (T5) ...... 59
4.2 Case II – The Effects of Varying Steam Quality (x8) ...... 68
4.3 Case III – The Effects of Varying HRSG Pinch Point ...... 73
4.4 Case IV – The Effects of Gas Turbine Performance ...... 77
4.5 Analysis Summary ...... 80
5. CONCLUSIONS & FUTURE WORK ...... 82
BIBLIOGRAPHY ...... 85
BIOGRAPHICAL SKETCH ...... 87
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LIST OF TABLES
Table 1 – Typical Combined Cycle Model Constants ...... 24 Table 2 – General Electric Gas Turbine Scale Factors Exponents ...... 35 Table 3 – Data for Commercial Gas Turbines ...... 37 Table 4 – Data for Commercial Steam Turbines ...... 40 Table 5 – Weight Distribution of Conventional Turbine-Generators ...... 42 Table 6 – FU Values for the Heat Exchangers in the Power Plant ...... 45 Table 7 – Notional Ship Power Specifications [3] ...... 49 Table 8 – Economical Transit Fuel and Volume Savings ...... 53 Table 9 – Surge to Theater Fuel and Volume Savings ...... 54 Table 10 – Operational Presence Fuel and Volume Savings ...... 54
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LIST OF FIGURES
Figure 1 – Major Components of a Combined Cycle Power Plant...... 10 Figure 2 – Energy Flows Crossing the Gas Turbine’s Boundary ...... 11 Figure 3 – Rankine Cycle TS diagram ...... 14 Figure 4 – Heat Recovery Steam Generator ...... 15 Figure 5 – Potential Pinch Point Visualization ...... 16 Figure 6 – Effects of superheated and high quality live steam ...... 19 Figure 7 – Effects of low quality steam turbine exhaust on HRSG design ...... 20 Figure 8 – Condenser and Pump ...... 22 Figure 9 – Logic Flow for Combined Cycle Power Plant Configuration ...... 27 Figure 10 – Power versus Weight for known gas turbine ...... 38 Figure 11 – Power to Change in Enthalpy Ratio versus Weight for known steam turbines ...... 40 Figure 12 – Heat exchanger notation for logarithmic mean calculations ...... 43 Figure 13 – Dry weight of commercially available HRSGs and condensers ...... 46 Figure 14 – Percent fuel weight and volume reduction with increasing power plant efficiency .. 55 Figure 15 – Fuel weight savings with increasing efficiency...... 55 Figure 16 –HRSG exhaust gas temperature is a qualitative measure recovered power ...... 58 Figure 17 – Efficiency versus T5 in the format used to evaluate other parameters ...... 59
Figure 18 – The effects of changing T5 on the steam turbine power output ...... 61
Figure 19 – The effect of reducing T5 on the location of the pinch point ...... 62 Figure 20 – Gross effects on mechanical component, fuel, and net power plant weight ...... 63
Figure 21 – The effects of varying T5 on the net weight ...... 64 Figure 22 – Mechanical Components Breakdown ...... 65 Figure 23 – The effect of HRSG power variation on required heat exchanger surface areas ...... 66 Figure 24 – System weight for a 1/4 range Economical Transit-type mission ...... 67 Figure 25 – Net weight versus for full, 1/2, 1/3, and 1/4 Economical Transit trip durations ...... 68 Figure 26 – Net weight reduction versus efficiency for several Economical Transit-type trips . 69 Figure 27 – Effects of quality on combined cycle efficiency ...... 70 Figure 28 – Effects of quality on combined steam turbine power ...... 70 Figure 29 – Effects of quality on weight savings ...... 71
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Figure 30 – Effects of quality on weight impact per day for full range mission ...... 72 Figure 31 – Effects of quality on HRSG and condenser weight ...... 72 Figure 32 – Effects of quality on mechanical component weight ...... 73 Figure 33 – Effects of changing the HRSG pinch point ...... 74 Figure 34 – Effects of changing the HRSG pinch point on steam turbine power production ...... 75 Figure 35 – Effects of pinch point variation on net weight savings ...... 76 Figure 36 – Effects of pinch point variation on net weight savings per day ...... 76 Figure 37 – Efficiency response to various gas turbine prime movers ...... 78 Figure 38 – Steam turbine power output for various gas turbine prime mover configurations .... 78 Figure 39 – Net Weight Savings for various gas turbine prime mover configurations ...... 79 Figure 40 – Net Weight Savings per day for various prime mover configurations ...... 80
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ABSTRACT
This work describes a tool for configuring and analyzing the weight of combined cycle power plant, designed for shipboard applications. The effects that varying selected combined cycle parameters have on the weight and the efficiency are presented. The combined cycle configuration is limited to a simple Rankine cycle bottoming plant recovering power from a gas turbine prime mover in order to increase efficiency. Although the Rankine cycle analysis could be used to design a steam turbine cycle whose HRSG absorbs power from the waste heat from any prime mover, the weight analysis provided constricts the use of the tool to gas turbines. Unlike much of the weight analysis performed in contemporary literature, this work includes fuel weight as part of the power plant weight, and the analysis results in net weight savings as compared to simple cycle gas turbines operating alone. The model was developed using heat transfer and thermodynamic analysis, turbine scaling techniques, and data from commercially available hardware to size the major power plant components. The analysis reveals that the point of optimal weight does not always coincide with the point of optimal efficiency.
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CHAPTER 1 MOTIVATION AND LITERATURE REVIEW
As the Navy transitions to an all-electric fleet, analytical tools must be developed to facilitate a rational process for the selection of power plant configurations for specific ship designs and modes of operation. Many hybrid designs are being considered for future Navy power plants. These alternatives have been developed in response to the increasing demand of both commercial companies and world Navies to implement all-electric ship (AES) designs. In this work, a tool has been developed that could be used to analyze a combined cycle power plant that uses a gas turbine prime mover and a steam turbine bottoming cycle. The model will be deployed to examine the effects of varying several operating parameters on the power plant’s weight and efficiency. The results of the analysis are presented in contrast to the operation of a simple cycle gas turbine power plant, and used as a tool for optimizing the combined cycle plant for installation on a frigate-sized navy ship; operating in its least power intensive mode of operation.
Section 1.1 provides a brief review of the literature on the topics of combined cycles for marine applications, and also a look at the role of the all-electric ship for future ocean-going vessels. Section 1.2 is an extension of the literature review. A more detailed discussion of options for hybrid-electric marine power plants is presented. The chapter will conclude by considering the effects of a combined cycle power plant installation on a commercial pleasure cruise ship in section 1.3. The findings of that case study will facilitate a brief discussion about the benefits associated with such implementations in the commercial market, and the presumed benefits for such installations on naval ships.
1.1 Combined Cycles for All-Electric Ship Applications
The papers below were very informative illuminating regarding the current state of the all- electric ship, and its future role for the Navy Fleet. In 2008, McCoy conducted a survey of the expansion of electric ship propulsion options since the early 1980s [1]. He indicated that in response to increased electric sensor and weapons loads that are being planned for installation on future Navy ships. He emphasized the movement away from direct drive architecture that has 1
been the staple for traditional ship propulsion. This model requires a separate set of prime movers that are dedicated to the generation of electrical power for ship service loads. In the integrated, all-electric ship architecture, both propulsion and ship service loads are generated and supplied to a main bus at constant voltage and frequency, with variable frequency drives providing the voltage and frequency required by load. The loads on the all-electric ship are both propulsive and ship service, but they provide great flexibility for Naval architects in the design and layout of future ships that is impossible with the segregated plant design.
In 2011, Lundquist looks towards the future an article published in Naval Forces magazine [2]. Quoting Royal Navy Attaché, Ian Atkins, “Going full electric was/is the same step as going from sail to steam.” Lundquist insists that the US Navy is committed to the all-electric ship is the future, due to a continuous rise in electrical demand onboard. In particular, new weapons systems and technologies, such as rail guns and experimental launchers could replace conventional guns. Moreover, experimental, high-powered radar could also draw from the common electrical bus that the propulsion system accesses for power on the all electric ship.
In that Lundquist’ article, he quoted Captain Doerry, who suggests, both in the article, and in his 2007 paper [3] that new standards and ways of sizing power plants for the all electric ship need to be considered. Doerry’s work was immensely useful in this work for both the determination of the combined cycle configuration, as well as the fuel weight calculations. Doerry also collaborated with Robey, Amy, and Petry in a 1996 article in Naval Engineer’s Journal [4], in which the future of the all-electric ship architecture was discussed.
Holsonback and Kiehne published a paper in 2010 [5] that emphasizes the thermal management challenge that all-electric ships face. They produced a simulation that demonstrates the amount of heat generated onboard a ship while conducting a momentum-reversing maneuver. Elsewhere, Ammonia-water absorption refrigeration plants are held up as a good option for converting waste heat into useful energy on marine vessels [6]. They are projected to save 2-4% of fuel consumption.
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In this work, the combined cycle power generating system, which is presented as a candidate for power production on the all-electric ship can also serve as a primary thermal management tool. Implementation of combined cycle power plants onboard ocean-going vessels can help in the thermal management of all-electric ships, while providing other advantages.
Haglind’s 2008 paper, that was issued in three parts [7] [8] [9], points out that the power density of combined cycle power plants is a primary motivating factor in their selection over what has traditionally been the prime mover of choice on ships, two-stroke diesel, operating on heavy fuel oil. In this work, Haglind emphasizes environmental and human health concerns in part 1. He provides the case study discussed later in this chapter, as well as illustrations showing the dramatic space savings associated with the implementation of combined gas/steam turbine cycles in place of diesel engines. In part 3, Haglind describes the dramatic reduction in environmental impact that replacing gas turbines as prime movers onboard, opposed to diesels has. Moreover, the loss in efficiency associated with switching from diesels to gas turbines is largely offset by the addition of the steam turbine cycle.
Haglind emphasizes the declining economical advantage that operating diesels on heavy fuel oil has over operating cleaner burning gas turbines throughout his work. This increasing emphasis on environmental stewardship is underscored by Nord and Bolland’s work, which describes the emissions and efficiency benefits associated with combined gas and steam turbine power plant installations, as compared to gas turbines operating alone [10]. This study was motivated by the desire to decrease operating costs related to CO2 emissions associated with Norwegian regulations for off-shored oil and gas installations.
Another advantage that diesels have historically held over gas turbines is in partial load performance. Haglind suggests that combined cycles can also help to bridge this gap in his 2011 work [8].
In fact, Young, Little and Newell, inform the community that world navies are not leading, but rather trailing the commercial fleet in implementing combined cycles ship-board as power plants in their 2001 work [11]. Emmanuel-Douglas provides a detailed description of several options
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considered or utilized by the cruise line industry to implement combined cycle power plants [12]. This work is the primary focus of the next section’s discussion.
1.2 Variations of Hybrid Electric Ship Configurations
Ship power plants are required to provide propulsion, ship service, and heating power as needed to the vessel in varying ratios, depending on the ship’s size, type and the prevailing mode of operation [12]. In response to the constantly increasing number of electrical loads onboard contemporary ships, and the desire to increase the ease of power distribution onboard military- type vessels, the contemporary all-electric ship (AES) concepts were presented at ASNE day 1994 [4]. In the AES design all of the power generated onboard is converted into electricity, and made available for either propulsion or ship service use. The transition to electric propulsion essentially serves an alternative power transmission option to a mechanical gear train for converting the high speed, low torque prime mover shaft output to the low speed, high torque shaft response that is required to turn propellers and move large ships [1].
Despite their inherent efficiency disadvantages, the gas turbine has been selected over the diesel engine as the prime mover of the future, primarily because of its high power density [13] [11]. The two primary disadvantages of diesel prime movers are that they consume large amounts of space that could otherwise be used to increase payload capacity, and they are significantly heavier than turbine-powered alternatives. With the selection of gas turbines to power the future all electric (AES) fleet, the massive amounts of waste heat produced by the prime mover must be managed effectively. In order to remove this heat using chillers, massive amounts of cooling infrastructure would be required [5]. This solution would add tonnes of additional equipment, offsetting the gas turbine’s power density benefit over diesel prime movers, while providing no benefit to its efficiency deficit as compared to the diesel.
A more effective option for managing the increased heat load of gas turbine prime movers while enhancing their efficiency is to implement combined cycle power plant technology, which features one or several gas turbine prime movers operating in coordination with a steam bottoming cycle. This alternative is still more power dense than diesel engines, and is capable of producing comparable efficiency [13]. Terrestrial combined cycle power plants that operate in 4
the 100s of megawatts level and are capable of achieving efficiencies of approximately 60%. Ship-board applications are less efficient, because weight restraints necessitate the use of less powerful prime movers, which results in less heat rejection, and subsequently less effective energy recovery [8].
In the past combined cycles power plants have rarely been used for propulsion of ships, but in anticipation of expected legislative action, due to increasing environmental awareness, it is expected that the price of heavy fuel oil (which is used to power diesels at low cost currently) will increase sharply in the coming decades. This projected sea change in world affairs further enhances the prospects of combined cycle systems as viable options for AES designs [13]. Furthermore, emissions reductions associated with switching from diesel cycle prime movers to gas turbine prime movers ranges from 67% for nitrous oxide emissions to 98% for carbon monoxide and hydrocarbon emissions. These emission reductions are partly due to the use of the higher quality fuels burned by gas turbines, and partly due to the different combustion processes [9]. Another advantage that gas and steam turbines hold over diesel engines is an artifact of their higher operating frequencies. Noise and vibrations from turbines are more easily damped. It has also been demonstrated that start-up times for diesel engines and combined cycle power plants are comparable [8].
There are several ship-board designs for combined cycle power plants that are either currently in use, or under consideration for future use in Navy ship installations. The options range from conventional direct mechanical propulsion designs, to hybrid mechanical and electric propulsion options, to fully integrated AES designs [12].
Combined cycle designs that employ conventional mechanically coupled gear trains from turbines to propeller shafts include the combined power and heat generation (COGEN), the combined gas turbine and steam turbine (COGAS), the combined gas and gas turbine or alternatively combined gas or gas turbine (COGAG, or COGOG), and the combined diesel or gas turbine or alternatively combined diesel and gas turbine (CODOG, or CODAG) configurations. Designs that operate on a hybrid mechanical and electric propulsion platform include the combined gas turbine and electric (COGAL), the combined gas electric and gas turbine
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(COGLAG), and the diesel engine with waste heat recovery (DE/WHR) configurations. Fully integrated all electric propulsion ship designs can be achieved using the combined gas turbine electric and steam turbine (COGES) or the combined diesel electric and gas turbine (CODLAG) configurations [12]. In addition to the afore-mentioned possibilities, a combined cycle option consisting of a turbo-diesel whose exhaust can be controlled to drive a gas/steam turbine combined cycle and/or provide heat to an HRSG for steam power operation has been proposed [14].
In this work, a COGES-like power plant design has been selected for analysis. While it is clear that AES designs incur additional losses by converting mechanical energy into electrical energy, and then back into mechanical energy, the dramatic increase in power management flexibility has been determined to outstrip the modest efficiency gains associated a direct mechanical coupling from prime movers power to the propellers. For example, an electric propulsion system integrated with the ship service distribution system offers naval architects considerable flexibility and often the choice of a more affordable ship. Electric drive provides flexibility in planning the placement of ship components in the hull. Decoupling prime movers from drive shafts permits location of prime movers to be optimized for maximizing payload carrying capacity [12] [4]. Advantages of implementing the IPS can also be realized in ship manufacturing [4].
1.3 Installed All-Electric Ship System
The world’s militaries trail industry in the implementation of combined cycle power generation systems. The use of a common power system for both propulsion and ship services is now a standard commercial practice for the cruise market and specialized shipping. Efficient operation of combined cycle power plants is being achieved by minimizing the number of prime movers required to meet the mission’s load requirements, and operating the turbines at or near their optimum efficiency [11].
In the year 2000, Celebrity Cruise Lines introduced the world’s first pleasure cruise ship powered by a combined-cycle power plant: the GTS Millenium. The use of a COGES plant to power Celebrity’s ground-breaking Millennium ship design freed up space for 50 additional passenger cabins. The installation also reduces the amount of ancillary machinery required for 6
operation. Similar results have been demonstrated in studies of 2500 other passenger cruise ships [8].
If this type of space savings were applied to a military vessel, ship designers could add munitions, sensory, or other mission-critical equipment to the ship, without increasing the weight of the overall vessel. Traditional Navy power systems require at least four prime movers (two for propulsion and two for ship service load), to comply with redundancy standards. Under many operating conditions, prime movers are idled for lack of demand. However, by feeding the power generated by online prime movers onto a common bus, from which the propulsion and ship service power needs can be pulled on demand, the number of prime movers can be reduced significantly. For example, it has been suggested that a conventional destroyed, that is typically deployed with seven prime movers (four propulsive, and three ship service) could operate with as few as three prime movers if combined cycle power plant designs were implemented [4].
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CHAPTER 2 ANALYSIS OF A COMBINED CYCLE POWER PLANT
As the attention paid to the environmental impact of burning fossil fuels, and fuel costs continue to increase, the requirement to maximize the use of the energy available from hydrocarbons has become paramount in the design of combustion-type power plants [12] [7] [8] [9] [13] [14]. In response to these trends, and the Navy’s call for more dynamic access to the power generated on its ships, the options for its future power plants have been expanded to include combined cycle, all-electric ship (AES) architectures [14]. The combined cycle power plants using gas turbine prime movers and complimentary steam cycle to augment power production, serves as the model for the tool developed in this work. As with the development of any power plant designed for transportation, efficiency and weight are critical design parameters. The efficiency of combined cycle architectures can be calculated from information provided in this chapter. That information will be used in the next chapter to determine the weight of the power plants designed for a frigate-sized Navy ship.
Section 2.1 presents an overview of the combined cycle. In sections 2.2 and 2.3, a discussion of modeling techniques for plant components is conducted. Section 2.4 includes a detailed discussion of the solution methodology developed for configuring combined cycle power plants, and section 2.5 is used to summarize the developments of this chapter.
2.1 Overview of the Combined Cycle Power Plant
Combined cycle power generating systems, merge two power plant designs that interface at a common heat exchanger, known as the heat recovery steam generator (HRSG). In the HRSG, heat from a prime mover that would otherwise be lost to the surroundings is intercepted and used as the power source for the secondary power plant. The cycle that consumes fuel and adds heat to the HRSG is referred to as the topping cycle. The cycle that scavenges heat energy from the HRSG to power its processes is called the bottoming cycle [15]. The topping cycle is typically a chemical engine, but it could be any power generating system that produces sufficient energy in the form of waste heat to power another cycle. For example, the heat rejected from a large bank
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of fuel cells or solar panels could be used as the power source for an HRSG. Typical bottoming cycle candidates are the steam power plant and the Stirling engine.
Regardless of the choice of topping and bottoming cycles selected, the intent of a combined cycle power plant is to maximize energy extraction from the topping cycle’s energy source. The work generated by the bottoming cycle is complimentary to the power generated by the topping cycle. Therefore, the combined cycle efficiency is always greater than the efficiency of the topping cycle operating alone (assuming that there is no supplementary heating). The efficiency of the combined cycle power system and its relationship to the efficiency of the topping cycle working alone is defined in Eq. (1).
(1) (2)