The Instruction of The New Generation Shipboard ower System Wang Chi-Yin,Chiu Liang-Ning 1.Introduction In the early nineteenth century, the mechanical power propulsion syste m which superseded sails of ship was an epochal breakthrough in the ma ritime history. Since the recent fast development of integrated mechat ronic system, the full electric propulsion system has gained more achi evement of high reliability, flexible operation and low infrared signa l than the conventional mechanic propulsion system. Therefore, most o f the developed countries have devoted to the study on this field. A new breakthrough of the full electric propulsion system is expected to come alone and consequently adopted as the main propulsion measure on board their next generation naval vessels. 2.An application profile of the full electric propulsion system in nav ies There was a long history of the full electric propulsion system aboard naval vessels. Especially during the World-War-II, USN had built hun dreds of electric-propulsion vessels. The main reason was the insuffi cient production of gearbox and other associated equipments. Because of the limited technology then, the systems were very heavy wi th disadvantages of large size, low efficiency, and high coast. After the war, only a few German naval vessels applied diesel-electric prop ulsion system. Most of the vessels of other navies just adopted mecha nic propulsion. Since 1980s, along with the development of electric/e lectronic devices, AC generator and its associated control technology, electric propulsion system has been able to meet the requirements of power, efficiency, and power density for ship propulsion. Some fundam ental changes also happened in the applications of the system. Accord ing to the statistics, since the late 1980s, some naval surface vessel s began to be equipped with electric-mechanic hybrid propulsion system , and low power electric propulsion systems were also adopted aboard B ritish mine-hunting boats, French light antisubmarine frigates, Swedis h mining boats, and the mine hunting vessels co-developed by France, B elgium, and Holland. After the Falkland War, The Type-23 frigate of British RN was the firs t vessel adopting the combined diesel-electric and gas propulsion syst em (CODLAG). During the low-speed cruising with range of 7000 nautica l miles, she was propelled by two GEC 750V/1.3MW DC motors at speed of 17 knots. The ship possessed four diesel generators which could prov ide electricity needed for the two DC propulsion motors and other ship board equipments. Each DC propulsion motor directly drove a fixed pit ch propeller. In 1980, USN signed a contract with the Westing House to develop and I ED (integrated electric drive) propulsion system for a two-shaft destr oyer with displacement of 6500 tons. The designed electric propulsion system contained generators which could provide electricity for other shipboard equipments. In order to completely integrate the propulsio n and electrical system, the propulsion motors were common AC motors w ith silicon-controlled frequency converters. The size and weight of t he whole system was larger than conventional propulsion symtem. Durin g the development process, it was concluded that the system would not be economically reasonable if it was going to meet the performance req uirements. Therefore USN proposed the IPS (integrated power systme) c oncept, namely the integrated full electric propulsion system (IEFP) i nstead. A shipboard electric propulsion system generally contains the followin g equipments: Propellers, electric motors, generators, and engines. T he engines can be diesel engines, steam turbines or gas turbines and t heir associated control/regulating devices. High or medium speed dies el engines are commonly adopted. For high power requirement, steam tu rbines or gas turbines are preferable. The generators can be DC separ ately-exited generator, differential compound generators, AC synchrono us generator, and AC rectifying synchronous generator. The most popul ar one is the AC rectifying synchronous generator (DC-AC generator). For the electric motors, DC separately-excited motor (two-armature and two-commutator ), AC synchronous motor, and non-synchronous motor ar e applicable. Presently the most adopted one is the DC two-armature m otor. Besides, the battery used in submarines is also a kind of elect ric propulsion measure. The technology trend of shipboard electric propulsion systems can be g enerally concluded as the following: a.The AC electric propulsion system (AC generators and motors), supers eding conventional DC and AC-DC electric propulsion equipments (DC gen erators, DC motors, and AC rectifying generator). b.The superconducting electric propulsion system. c.The fuel cell propulsion system. The basic arrangement of the above three systems are described as bell ow: 2.1 AC electric propulsion systems This kind of system possesses advantages of high power limit, high eff iciency and excellent reliability. Based on the type of their electri c propulsion motors, this system can be categorized into synchronous e lectric motor type or non-synchronous electric motor type. In additio n, according to the structure of their electric current converter, the y can be sorted into thyristor frequency converting AC type, electric transistor type, and gate-turn-off thyristor AC type. 2.2 Superconducting electric propulsion system Superconducting electric propulsion system uses superconducting electr ic equipments (superconducting generator and superconducting motors) a s its power unit. In comparison with general electric propulsion syst em, it owns merits of light weight, small size, high efficiency, and l ow noise. Since this system has to be operated under critical temper ature, so a set of complex liquid helium facility is required and cons equently limit its application. Recently, the development of acrogeni c technology miniaturization has offered superconducting electric prop ulsion system good conditions for being applied on naval vessels. 2.3 Fuel cell electric propulsion system This system uses fuel cells as the power source of a ship. Fuel cell is an energy transformation device which can directly transforms chemi cal energy into electric energy. A complete fuel cell system consists of cellˇ¦s body, fuel , oxidants and their reservoir. Its energy tra nsformation measure, same as what the common battery uses, possesses t he advantages of quiet and high efficiency. The configuration of a fu el cell system is similar to that of a diesel generator. Its energy s torage part (reservoirs for fuel and oxidants) is separated from the e nergy transformation part. Therefore, not like the batteries which ne ed charge-discharge cycles, it can continuously work for quite a long time with sufficient fuel and oxidants. In recent year, the research on fuel cell has obtained some important technology breakthrough, whic h has made the application of the shipboard fuel cell system just arou nd the corner. Among the persent full electric propulsion systems, fuel cell propulsi on system has been successfully applied aboard submarines. Meanwhile, developed countries are also aggressively applying this system aboard their surface naval vessels because of its higher feasibility. The a rrangement of fuel cells used in a full electric propulsion system is described as bellow: 3.The arrangement of fuel cells used in a full electric propulsion sys tem Fuel cell power system possesses the merits of high power density, hig h flexibility for operation and arrangement, high modulization, low po llution, quiet, and simpler logistic support requirement. However, th ere are more other factors which need to be taken into account for its application aboard naval vessels. In this chapter, the characteristi cs of the proton exchange membrane fuel cells and molten carbonate fue l cells installed aboard some vessels will be described as the followi ngs which contain useful information for future feasibility analysis o f shipboard fuel cell systems. 3.1 General arrangement of fuel cell No mater what fuel cell system (proton exchange membrane type or molte n carbonate type) is applied, its arrangement aboard can be displayed as figure 2. According to figure 2, the fuel cells are used as the main propulsion power source. This system contains two 5.5MW fuel cel l sets and a 22MW gas turbine. The gas turbine can operate with the f uel cell sets in parallel or series. The maximum output of the whole system is 33MW, which is just lower than the propulsion system of PFG2 class (total 50MW, consisting of two 25MW LM2500 gas turbines) and hi gher than Lafayette class (total 21.2 MW consisting of four 5.3MW dies el engines). The fuel cell system is not suitable to be an emergency power system or used as a power source during maneuvering in ports bec ause that the fuel converter of fuel cell needs to spend more starting time and its operation temperature required is also higher. Besides, the starting procedure of fuel cell consists of many steps which will also take much more time. 3.1.1 Fuel process system The purpose of this system, containing air (or oxygen) supply system,h ydrogen supply system, and fuel converter system, is to provide fuel f or fuel cells. This system is equipped with a fuel converter which ca n convert shipboard fuel oil into hydrogen used for cells. Consuming the shipboard fuel oil can get rid of the equipments and pipes needed for hydrogen storage. For the proton exchange membrane fuel cells, th e low operation temperature will cause incomplete reaction and consequ ently generate carbon monoxide harmful to fuel cell if the methane is directly filled into the anodes. Because the process system can not c onvert methane into pure hydrogen, therefore, it has to be equipped wi th a special catalyzing device convert the methane into the hydrogen n eeded for fuel cells. For the molten carbonate fuel cell, the higher operation temperature (about 650˘J) allows the methane to be directly filled into the anodes where methane can be decomposed into hydrogen w ithout generating carbon monoxide. Therefore the process system does not need another device converting methane into hydrogen. 3.1.2 Fuel cell sets This component, made of several fuel cell stacks, is the part where ce ntral reaction is taking place, so as to provide the propulsion power for the ship. 3.1.3 Fuel cell control system The purpose of this system is to control and accelerate system reactio n in order to reach the highest reaction performance. It is made of t he following subsystems: 3.1.3.1 Air filtering system The system is used to purify the air which is necessary for the cathod es of fuel cells. Since air contains certain amount of salt, so air f ilters are necessary for fuel cell system. Since the air filter of ma rine gas turbine and the diesel engines is capable of reducing the sal t of air to 0.005ppm and 0.05ppm respectively, so the present technolo gy level of air filter is already able to meet the requirement of fuel cell. 3.1.3.2 Air pressurization system This system is used to increase air pressure in order to enhance the r eaction efficiency of fuel cell. 3.1.3.3 Electric control system This system is used to convert the electric energy generated by fuel c ell into the electricity needed by shipboard equipments.Different ship board equipments need different types of electricity, so a converter i s required to convert the DC electric energy into the DC or AC electri city with necessary voltages or other characteristics. 3.1.3.4 Automatic control system The system is to perform the automatic control over the fuel cell oper ation such as the input of hydrogen and oxygen and reaction temperatur e, so as to achieve the highest performance and record associated oper ation data which can be useful during the maintenance tasks afterward. 4.The pros and cons of full electric propulsion system In comparison with the mechanic propulsion system, a full electric pro pulsion system is more economic, and can provide higher combat capabil ity and survivability. Its advantages are described as the followings: 4.1 Better economic benefit The fuel consumption of IEFP system is lesser than a mechanic propulsi on system will spend. According to the associated reports from US, du ring the thirty-year service life, a destroyer can save 16% fuel cost full if it applies electric propulsion system instead of a mechanic pr opulsion system. IFEP system can save fuel just because of the follow ing reasons: a.During the low-speed cruising, the electric propulsion system needs lesser engines to provide the same net power. b.During the low-speed cruising, the electric propulsion system can ma intain the engines running at high power working point. For the mechan ic propulsion vessels,the engine efficiency during low-speed cruising is relatively lower, which will cause higher fuel consumption. c.A ship with IFEP system does not need extra generators needed for th e auxiliary machinery and combat systems. d.While being applied onboard a catamaran, trimaran or other non-conve ntional hull form, the automization of IEFP is easy to be accomplished , reducing the complement and its associated training cost. The flexi bility of arrangement can optimize ship structure and improve the prod ucibility. Accordingly, it lower the displacement and construction co st of the ship. During cruising, IFEP only needs least number of engi nes to be operated, therefore their running hours and maintenance cost can be cut and saved. 4.2 Higher combat capability a.Since the electric propulsion system needs few engines and does not possess the power transmission devices needed for mechanic propulsion system, accordingly more room can be saved for carrying more weapons. b.The electric propulsion system can provide the electricity needed fo r the laser and electromagnetic weapons in the future. c.The electric propulsion system can provide better maneuverability, w hose propeller is driven by electric motor. With quick response to th e speed control signal, the step-less speed control can be achieved wi thin the full speed limit. For the mechanic propulsion system, a mini mum limit of shaft speed is existent, and the longer responding time o f shaft coupling also restricts the response to speed control signal. d.This system can reduce the fuel consumption and improve endurance ab ility. The same amount of fuel can make the ship sail farther than th e ship with mechanic propulsion system. e.Neither diesel engines nor gas turbines can easily run in both norma l and reverse directions. The traditional solution is to use controll able pitch propeller which is an approach consumes much fuel. For the electric propulsion system, the problem about the running in reverse direction can be easily solved by changing the polar and phase of elec tric power through power-electronic facilities. Therefore the maneuve rability of ship can be consequently improved. f.The flexibility of system arrangement can reduce the require displac ement of ship.Since the traditional arrangement with engine, propeller , and transmission shaft system located on the same line is no longer to be adopted in the electric propulsion system, the mechanical connec tion means between associated propulsion equipments can be superseded by power cables. Whereas the engines can be located in any position a board which allows the arrangement to become more flexible and consequ ently reduce the displacement of ship. 4.3 Higher combat survivability a.This system can reduce noise and increase stealth features. Since th e engines of this system can be installed in the positions above water line, so the underwater noise can be lowered. The noise caused by stru cture vibration will also be significantly diminished because no gearb ox is needed any more. In comparison with the mechanic propulsion syst em, the noise of the ship with electric propulsion system can be reduc ed by 150dB in broadband, and more in narrowband. b.The operators of the system can select an optimum combination of eng ines, which will accordingly run in the best efficiency. Therefore, th e low-load running of engines can be avoided as far as possible. c.Since the IFEP system supplies electricity to shipboard loads throug h their duel buses arranged on the portside and starboard side, the po ssibility of system failure is quite low. With the backup circuits of the propulsion system, it is not easy to be damaged and malfunctioned . In comparison with the mechanic propulsion system, the full electric p ropulsion system still possesses the following disadvantages: a.Appling low efficient full electric propulsion system will not be be neficial, If the ship needs to sail at high speed and full power durin g most of her service time. b.The full electric propulsion system is not suitable to aircraft carr ier. The IED(integrated electric drive) system might be a possible opt ion for aircraft carrier, however, conventional mechanic propulsion sy stem is more favorable presently. For a huge vessel like aircraft carr ier, the electric propulsion system can not save more space and weight than a steam power system can. The future aircraft carrier may need m ore electricity to meet the requirements of electromagnetic catapult/r etrieve device, electromagnetic weapons, and various countermeasures. However, the most economic way of obtaining sufficient electricity is to use more powerful steam turbines. c.The IPS (integrated power system), with full dimensional test starte d in this year, is not suitable to submarines. Unlike the permanent ma gnet motors, the induction motors adopted by IPS system is too big and noisy, which consequently make the IPS only applicable to naval surf ace vessels. 5.The critical technology of the full electric propulsion system The IFEP (integrated full electric propulsion) system is the combinati on of technologies such as advanced power electronics, AC speed regula tion, manufacturing of electric equipments, permanent magnetic materia l, computer control, and gas turbines. This development of this cutt ing-edged system needs to be based on the following critical technolog ies: a.High power and high power density technology used for permanent magn et generators and electric motors. b.High power power-electronic technology: Presently, most of the players are trying to increase the power level of the IGBT (insulated gate bipolar transistor) in order to reduce the size and weight of converters. c.Advanced gas turbine technology: U.S. and UK have cooperated in developing the WR-21 intercooled regene rated gas turbine and performed the research on lower power high speed gas turbine generator set. d.Technology of regional power distributi on system and monitoring system. 6.Conclusions The full electric propulsion is a great leap of technology in the deve lopment history of ship power system, which is an inevitable trend tha t can largely increase the survivability and combat capability of nava l surface vessels. Although the overweight and oversized dimensions of the system are sti ll a main issue to be addressed, and there are also many technical pro blems about fuel cells which can not be solved overnight, nevertheless , the realization of the full electric propulsion system onboard naval vessels can be expected soon. 7. Bibliography 1.Wen Wu-Yi, "Technology of Fuel Cell," Chuan Hwa Book Co. LTD., Augus t 2004. 2.Naval Technology-U212˘AU214-Attack Submarine. 3.Donald Hoffman, "US Navy Shipboard Fuel Cell Program," NAVSEA 982,Ju ne 2003. 4.G.Steinfeld,R,Distiliate, "Fuel Processing For Marine Fuel Cell Appl ications," Fuel Cell Energy,Inc., June 2000.