A Combined Diesel-Engine Gas-Turbine System For .

International Conference on Chemical, Biological and Medical Sciences (ICCBMS'2012) August 25-26, 2012 Kuala Lumpur (Malaysia)A Combined Diesel-Engine Gas-TurbineSystem for Distributed Power GenerationMohamed M. El-Awad, Mohamed A. Sirajlubricant oil are purely thermal, the exergy loss in theexhaust gas is both thermal and mechanical because of itshigher-than-atmospheric temperature and pressure. Aconcept that makes use of both the mechanical and thermalexergies in the exhaust gas is the compound cycle engine(CCE) [6]. The CCE utilises the energy of the dieselengine's exhaust for producing extra expansion work, partof which is used in compressing the air prior to the engine'sintake manifolds. The CCE received interest in militaryaviation applications because it combines the lightweightpressure rise capability of a gas turbine with the highefficiency of a diesel engine.An extension of the CCE concept which is more relevantto stationary power generation is the combined DieselBrayton (CDB) cycle [7,8]. The CDB cycle also takesadvantage of the energy content of the diesel-engine'sexhaust gas but adds a complete gas-turbine with itscompressor and combustion chamber. This arrangement,which enables the diesel engine and gas turbine to be runindependently, gives the system great flexibility duringoperation. While the CDB cycle enables small-scale powergeneration and industrial cogeneration systems to attain highthermal efficiencies, the fuel flexibilities of diesel enginesand gas turbine enables them to be fuelled with heavy-dieselfuel, natural gas, or renewable bio-fuels. However, unlikethe combined Brayton-Rankine cycle, which receivedconsiderable attention because of its suitability for largescale power generation, published studies on CDB cyclesare rather scarce.The present paper describes a combined diesel-enginegas-turbine (CDG) system that improves upon previousproposals through the appropriate use of regeneration andintercooling. For the analysis of the system's performance,an Excel-based computer model has been developed. Themodel enables optimum values of the compressors' pressureratios to be determined for given values of the dieselengine's compression ratio (CR) and gas-turbine's inlettemperature (TIT). Analysis of the system's performancewith different values of CR and TIT determined thecombination of compressor pressure ratios that permits thesystem to be designed with fewer compressors and turbineswhile maintaining a near-optimum performance.Abstract— This paper presents a combined diesel-engine gasturbine system that enables distributed power generation plants toattain high thermal efficiencies while enjoying the operationaladvantages of diesel engines and gas turbines. The configuration ofthe system presented here improves upon previously proposedconfigurations through the appropriate use of regeneration andinter-cooling. Analyses of the system's performance wereperformed using an Excel-based thermodynamic model. Theenergy analysis studied the effect of the diesel-engine'scompression ratio, the gas-turbine's inlet temperature, and thecompressors' pressure ratios on the system's performance. Thisanalysis determined the pressure ratios that permit the system to bedesigned with fewer compressors and turbines.Keywords—Combined Diesel-Brayton cycle; Energy and exergyanalysis; Small-scale power generationI. INTRODUCTIONDUE to their excellent thermal efficiency, reliability andhigh availability, diesel engines have become the mostpreferred prime movers in distributed generation aswell as other medium and medium-large applications [1].Diesel engine power plants can be set up quickly, normallyin less than twelve months, to generate hundreds ofmegawatts of energy [2]. Another advantage of diesel powerplants is that they can run on heavy diesel fuel, a low-graderelatively inexpensive product of oil refineries. Dieselengines are also suited to burn natural gas and bio-fuelseither in their pure form or mixed with petroleum fuels [3].Despite of the high thermal efficiency of diesel engines,their exhaust gases, cooling water and lubrication coil carryout with them a significant portion of the energy input inthe combustion chamber. Therefore, combined systems,such as the Organic Rankine cycle (ORC) [4] and theDiesel-Kalina cycle [5], have been developed for using thewaste energy from the engine to produce additional power.However, both the Organic Rankine cycle that employsorganic fluids and the Diesel-Kalina cycle that uses a waterammonia solution require boilers, condensers, etc.Therefore, these combined systems need substantial initialcosts and also limit the plant's flexibility. Moreover, thesesystems do not utilise all the work potential in the exhaustgas. While the exergy losses in the cooling water andII.Manuscript received June 8, 2012.Mohamed M. El-Awad is with the Mechanical Engineering Department,Faculty of Engineering, University of Khartoum, P.O. Box 321 Khartoum,Sudan (phone: 249-911700885; e-mail: mmelawad09@ gmail.com).Mohamed A. Siraj is with the Mechanical Engineering Department,Faculty of Engineering, University of Khartoum, P.O. Box 321 Khartoum,Sudan (e-mail: maasiraj@yahoo.com).THE COMBINED DIESEL-ENGINE GAS-TURBINE SYSTEMFig. 1 shows a schematic diagram of the combined systemthat consists of a compound diesel engine (compressor C1,intercooler IC1, diesel engine DE, and turbine T1) and anintercooled regenerative gas turbine (compressor C2,39

International Conference on Chemical, Biological and Medical Sciences (ICCBMS'2012) August 25-26, 2012 Kuala Lumpur (Malaysia)intercooler IC2, compressor C3, regenerator RG,combustion chamber CC, and turbine T2). Beforedischarged to the atmosphere, the exhaust gases from thetwo turbines are mixed and passed through the regeneratorto reheat the high-pressure air going to the combustionchamber of the gas turbine.replacing the two low-pressure compressors by a singlecompressor and/or replacing the two turbines by a singleturbine.III. THE THERMODYNAMIC MODELIn the following analysis, the system shown in Fig. 1,which has two different low-pressure compressors and twodifferent turbines, is referred to as system A. In system B,the two low-pressure compressors C1 and C2 are required tohave the same pressure ratio (i.e. PR C1 PR C2 ) so that theycan be combined in a single large compressor. Similarly, insystem C the two turbines T1 and T2 are required to havethe same pressure ratios (i.e. P 10 P 6 ) so that they can bereplaced by a single large turbine. System D has a singlelow-pressure compressor as well as a single turbine. Fig. 2shows the T-s diagrams for the four configurations of thecombined system. As the figure shows, systems A and B,respectively, are different from systems C and D in that thetwo turbines T1 and T2 have different compression ratios.Systems A and C, respectively, are different from systems Band D in that the low pressure compressors C1 and C2 havedifferent pressure ratios.In the four cycles shown in Fig. 2, process 1-2 is anisentropic compression process in compressor C1, whereambient air taken at T 1 , P 1 is delivered at T 2 , P 2 . Thecompressed air passes through an ideal constant pressureintercooler (process 2-3) after which its temperature isbrought down to T 3 T 1 . The cooled air goes to the intake ofthe diesel engine where it is compressed isentropically tostate 4. Heat is then added to it in process 4-5 followed bythe expansion stoke (process 5-6). In systems A and B, thegas discharged from the diesel engine is expanded in turbineT1 (Process 6-7) before discharged at ambient pressure.In systems C and D, the diesel-engine's exhaust is mixedwith the gas-turbine's products of combustion before themixture is expanded in the common turbine in process 7-13.In all cycles, the gas-turbine's intake air that is compressedin compressor C2 to point 8 also goes through an ideal intercooler that brings its temperature T 9 T 1 . The compressedair then passes through the high-pressure compressor C3(process 9-10), the regenerator (process 10-11), and thecombustion chamber (process 11-12). In systems A and B,the heated gas is then expanded in the second turbine T2(Process 12-13) before discharged at ambient pressure andmixed with the exhaust gas of turbine 1. The mixed gas issent at T 14 to the regenerator to preheat the air going thecombustion chamber and leaves at state 15. In systems Cand D, the exhaust gas from the single turbine at T 13 is sentto the regenerator before discharged at state 14 to theatmosphere. The performance of the diesel engine dependson its compression ratio (CR) and cut-off ratio (COR) where,CR v3 / v 4 and COR v5 / v 4 .Fig. 1 The combined diesel-engine gas-turbine system withintercooling and regenerationCompared to the configuration of the combined systemproposed by Mukul and Agarwal [7], the presentconfiguration offers greater design and operationalflexibilities by introducing separate compressors andturbines for the Diesel and Brayton cycles. This arrangementalso allows the two components of the combined system tobe optimised almost independently since the only commonpart is the regenerator. Although intercooling causes anexergy loss, it has certain advantages in the implementationof the system. For the gas turbine, intercooling enhances theeffect of regeneration and improves the cycle's thermalefficiency. For the diesel engine, cooling the compressed airbefore the engine's intake improves its volumetric efficiencyand, therefore, reduces its size and installation cost. In thecycle proposed by Mukul and Agarwal [7], the dieselengine's intake air is heated after compression, rather thancooled, which reduces the engine's volumetric efficiency.With two separate turbines, the present configuration alsoavoids the loss of exergy that is caused by mixing the hotgases coming from the gas-turbine's combustion chamberwith the cooler diesel-engine's exhaust.Compared to the configuration of proposed by Krishnaand Renald [8], the present configuration improves thethermal efficiency by precompressing the air of the dieselengine and by introducing intercooling and regeneration inthe gas turbine cycle. However, compared to the twosystems proposed previously the present system consists ofmore components, which is bound to increase its initial costas well as its overall losses. Therefore, the main objective ofthe following analysis is to explore the possibility of40

International Conference on Chemical, Biological and Medical Sciences (ICCBMS'2012) August 25-26, 2012 Kuala Lumpur 5713,9s(a) System As(b) System B5T5T12664131128103912774142,8101(c) System C13113,9141ss(d) System DFig. 2 The T-s diagrams of four arrangements of the combined systemReferring to systems A and B, the relations thatdetermine the system's net work(w NET ) and thermalefficiency (η) are:WC1 m1 (h2 h1 )WC 2 m 2 (h8 h1 )WC 3 m 2 (h10 h9 )1. The compression ratio and cut-off ratio of the dieselengine (CR and COR)2. The pressure ratios of the three compressors (PR C1 ,PR C2 and PR C3 ) and two turbines (PR T1 , PR T2 )3. The effectiveness of the regenerator (ε)4. The mass flow rates of the air supplied to the dieselengine and the to the gas turbine (m 1 and m 2 ).(1)(2)(3)WT 1 m1 (h6 h7 )(4)IV. ANALYSIS OF THE SYSTEM'S PERFORMANCEWT 2 m 2 (h12 h13 )(5)The following analysis of the combined system studiesthe effects of the key parameters on the system'sperformance so as to determine the values of theseparameters that optimise its thermal efficiency and net workoutput. This was achieved by varying the diesel-engine'scompression ratio (CR) and the gas turbine inlet temperature(TIT). The pressure ratios of the three compressors were nottreated as independent parameters. Instead, Excel's Solverwas used to determine the pressure ratios that maximise thecycle's thermal

gas discharged from the diesel engine is expanded in turbine T1 (Process 6-7) before discharged at ambient pressure. In systems C and D, the diesel-engine's exhaust is mixed with the gas-turbine's products of combustion before the mixture is expanded in the common turbine in process 713. - In all cycles, the gas