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Feasibility Study Into Stirling Engines Application in Ship's Energy Systems

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Feasibility Study Into Stirling Engines Application in Ship's Energy Systems Transactions on the Built Environment vol 42, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 Feasibility study into Stirling engines application in ship's energy systems S. Zmudzki, Faculty of Maritime Technology, Technical University of Szczecin, 71- 065 Szczecin, al Piastow 41, Poland E-mail: szmudzkia@shiptech. tuniv.szczecin.pl Abstract The possibility of utilization of solar energy as well as exhaust gas energy from main and auxiliary marine diesel engines for driving Stirling engines has been analysed. The Stirling engines are used for driving current generators. The solar dish Stirling units of 100 kW total effective power may be installed on a board of different type of ships. The system utilizing the exhaust gas energy has been provided with three-way catalyst which may lead to the reduction of emissions below limits established by appropriate regulations. 1 Introduction The Stirling engine belongs to a group of piston engines with so called external supply of heat energy, flowing then through metal walls of engine heater into working gas circulating in the working space. The heat energy supply may be realized or by means of direct effect of the high temperature energy source or by an intermediate heating system i.e. heat pipe or pool-boiler receiver. Thus, the design provides for the possibility of application of different types of heat energy of sufficiently large power. In particular, the advantages of Stirling engines are set off when using the alternative energy sources, especially waste heat energy, the operating costs for which are relatively lower in comparison with capital costs. In case of ship s energy installations, the solar energy available at appropriate geographical latitudes for ships having very large open deck surface / i.e. tankers, bulk carriers, etc /, and exhaust gas energy from main and auxiliary diesel engines have been taken into consideration. The results of analysis are given below. Transactions on the Built Environment vol 42, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 300 Marine Technology 2 Operational properties of Stirling engines The Stirling engine is a cycle machine the working mechanism of which operates with frequency suiting the demands directly or indirectly coupled power receivers / current generator, pump, compressor, propeller, etc. /. They compose a driving set, providing for the necessary technological effect. The basic charakteristics of contemporary Stirling engines are given in table 1. From data given in table 1 it results that Stirling engines, at present, may produce relatively low effective power, taking into account the actual demands of ship's main driving units. Thus, they may only be used for auxiliary purposes i.e. power generating sets. The produced electrical energy may be afterwards used for suppling various ship's installations. Table 1. Charakteristics of contemporary Stirling engines. Working Effective Type of Type of Producer Effektive Frequency gas mean power - drving engine Country power pressure mass ratio mechanism KW Hz MPa W/kg 1 2 3 4 5 6 7 NSO3M Mitsubishi 3,8 20 6 46 Crank Japan NS3OA Aisin Japan 30 25 15 114 Swash plate V4-275 United 100 44 20 1000 Crank Stirling Sweden STM4-120 Stirling 25 30 12 420 Swash plate Thermal 50 75 600 Motors. Inc. USA SHARP Sunpower 3 115 2,4 75 Free piston USA MOD III Mechanical 110 46 15 300 Crank Technology Inc. USA V160 United 9 25 12,5 30 Crank Stirling- SOLO Sweden- Germany Transactions on the Built Environment vol 42, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 Marine Technology 301 Figure 1: Solar dish / Stirling power system. The thermodynamic processes in the Stirling engine working space are very smooth and ensure safe, noiseless and vibrationless operation as well as provide for favourable moment charakteristies. 3 Stirling engine modes of operation on ship s board 3.1 Solar energy supply The example of solar energy application for driving the Stirling engine is given in figure 1. This is a Stirling unit comprising the spherical solar collector- concentrating solar energy on Stirling engine heater; the Stirling engine itself and the tracking mechanism. The solar power intensity on the horizontal surface q% for a given geographical latitude and whole year may be calculated by means of the following Mihelic' et al. [1] formula: ?„ = 1650 + 880sin(90°+1.80>) (1) Since the solar dish Stirling systems are provided with the tracking system it is necessary to consider the solar radiation intensity in vertical direction q^, thus (2) sinh and sinh - sin cp • sin 8 + cos <p • cos 8 • cos P (3) where: (p - geographical latitude, 5 - declination of the Sun, P - hour angle, h - solar altitude. Transactions on the Built Environment vol 42, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 302 Marine Technology AH [ kWh/m*a ] -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Figure 2: Solar power intensity versus geographical latitude. The example of solar radiation qn data versus geographical latitude is given in % 2. Taking into account ship's sailing routes within ± 30° geographical latitude and lOOkW effective power of auxiliary installation, it is possible, according to Baumuller et al. [2], to obtain this goal using 10 dish / Stirling units operating with a spherical collector of 7,5 m diameter and V-160 Stirling engine or 2 dish / Stirling units with 17 m diameter spherical collector and V4-275 Stirling engine. For many ships of the greatest tonnage (tankers, bulk carries, etc.), it is feasible to utilize the solar radiation for the purpose. The assembling of units to ship's deck should be tight and firm against external forces, due to stormy weather and appropriate shielding provided for decreasing the resistance forces of ship movement. Taking into account cyclic supply of the solar energy / day and nighit period /, the systems should be provided with electric acumulator or/and typical installation of generating sets. Considering the present prices of fuel, this proposition seems to be valuable for the future. 3.2 Exhaust gas energy supply The technical and technological progress of main and auxiliary marine diesel engines resulted in very large increase of their total efficiency and the introduction of the so called turbocompound systems. The systems operate with 450-1350 kW power turbine for engine loads exceeding 50% of nominal power. The temperature of exhaust gases fall within the range of 375-450°C for auxiliary engines and 350-400°C for main engines. At present, the turbocompound system operates with the main engine installations, however, the exhaust gas energy of auxiliary engines is lost. Some theoretical analysis of Benvenuto et al. [3] proved that it is possible to use Stirling engine for the cogeneration plant with the utilization of exhaust gas energy of diesel engines. Transactions on the Built Environment vol 42, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 Marine Technology 303 TURBOCHARGER \ EXHAUST S > GAS /^-L^STIRLING V X = 2 ENGINE 1=1.006 DIESEL ENGINE O O o — CHIMNEY 3-WAY CATALYST Figure 3: Diagram of the compound energy system with Stirling engine and catalyst. However, Minamoto et al. [4] analysing the possibility of practical utilization of the Stirling engine NS30A ( Aisin Seiki Co. Ltd. ) in various systems have proved experimentally that estimated Stirling engine power range may be reached for minimum 1300°C temperature in the area surrounding the engine heater. For temperature of 800°C, the effective power drops by ca. 60 %. The redesigning of the heater by increasing its external surface resulted in small gain of power only. Taking into account the above mentioned consideration, author [5] has elaborated his own design of the system utilizing the exhaust gas energy for driving the Stirling engine and interconnected with catalytic reduction of exhaust gas emissions, down to a level accepted by appropriate national or IMO regulations. The main features of the system are given in fig.3. Recording to the diagram, the exhaust gases from diesel engine, of relatively large enthalpy and excess air number of ( A, = 2.0-=-6.0 ) - according to the engine load, flow into the Stirling engine combustion chamber. In the constant pressure combustion chamber the additional fuel bulk is burnt. The amount and quality of the fuel are so selected as to obtain the final value of combustion gases excess air number of A, = 1 ± 0.006 - as required for operation of the 3-way catalyst. During combustion, the gas temperature exceeds 1300°C any time. A part of heat is supplied to working gas through heater walls and exhaust gases of large enthalpy flow to diesel engine turbocharger and power turbine / in parallel or series connection /. After the expansion, they flow to 3-way catalyst, which provides for the reduction of NO*, CH and CO emissions to the level required by Transactions on the Built Environment vol 42, © 1999 WIT Press, www.witpress.com, ISSN 1743-3509 304 Marine Technology national or IMO regulations. This is possible due to reduction of excess oxygen content in exhaust gases to necessary level of X = 1 ± 0.006. Thus, the solution provides for effective utilization of diesel engines exhaust gas energy as well as safe and cheep reduction of exhaust gas emissions. The system should be provided with an oxygen sensor as a controlling element in the Stirling engine fuel system. The system seems to be sophisticated, however, it provides for high total efficiency and what is the most important, considerable reduction of exhaust gas emissions by means of safe and easy in operation method. Selected types of fuels are necessary to obtain required conditions for operation of the catalyst. Liquid fuels / petrol, kerosene, diesel oil, etc.
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