A Combined Diesel-Engine Gas-Turbine System for Distributed Power Generation

Transcription

1 A Combined ieel-engine Ga-urbine for itributed Power Generation Mohamed M. El-Awad, Mohamed A. Siraj Abtract hi paper preent a combined dieel-engine gaturbine ytem that enable ditributed power generation plant to attain high thermal efficiencie while enjoying the operational advantage of dieel engine and ga turbine. he configuration of the ytem preented here improve upon previouly propoed configuration through the appropriate ue of regeneration and intercooling. Analye of the ytem' performance were performed uing an Excel-baed thermodynamic model. he energy analyi tudied the effect of the dieel-engine' compreion ratio, the ga-turbine' inlet temperature, and the compreor' preure ratio on the ytem' performance. hi analyi determined the preure ratio that permit the ytem to be deigned with fewer compreor and turbine. Keyword Combined ieel-brayton cycle; Energy and exergy analyi; Small-cale power generation I. INROUCION UE to their excellent thermal efficiency, reliability and high availability, dieel engine have become the mot preferred prime mover in ditributed generation a well a other medium and medium-large application []. ieel engine power plant can be et up quickly, normally in le than twelve month, to generate hundred of megawatt of energy [2]. Another advantage of dieel power plant i that they can run on heavy dieel fuel, a low-grade relatively inexpenive product of oil refinerie. ieel engine are alo uited to burn natural ga and bio-fuel either in their pure form or mixed with petroleum fuel [3]. epite of the high thermal efficiency of dieel engine, their exhaut gae, cooling water and lubrication coil carry out with them a ignificant portion of the energy input in the combution chamber. herefore, combined ytem, uch a the Organic Rankine cycle (ORC) [] and the ieel-kalina cycle [], have been developed for uing the wate energy from the engine to produce additional power. However, both the Organic Rankine cycle that employ organic fluid and the ieel-kalina cycle that ue a waterammonia olution require boiler, condener, etc. herefore, thee combined ytem need ubtantial initial cot and alo limit the plant' flexibility. Moreover, thee ytem do not utilie all the work potential in the exhaut ga. While the exergy loe in the cooling water and lubricant oil are purely thermal, the exergy lo in the exhaut ga i both thermal and Manucript received June 8, 202. Mohamed M. El-Awad i with the Mechanical Engineering epartment, Faculty of Engineering, Univerity of Khartoum, P.O. Box 32 Khartoum, Sudan (phone: ; mmelawad09@ gmail.com). Mohamed A. Siraj i with the Mechanical Engineering epartment, Faculty of Engineering, Univerity of Khartoum, P.O. Box 32 Khartoum, Sudan ( maairaj@yahoo.com). mechanical becaue of it higher-than-atmopheric temperature and preure. A concept that make ue of both the mechanical and thermal exergie in the exhaut ga i the compound cycle engine (CCE) []. he CCE utilie the energy of the dieel-engine' exhaut for producing extra expanion work, part of which i ued in compreing the air prior to the engine' intake manifold. he CCE received interet in military aviation application becaue it combine the lightweight preure rie capability of a ga turbine with the high efficiency of a dieel engine. An extenion of the CCE concept which i more relevant to tationary power generation i the combined ieel-brayton (CB) cycle [,8]. he CB cycle alo take advantage of the energy content of the dieel-engine' exhaut ga but add a complete ga-turbine with it compreor and combution chamber. hi arrangement, which enable the dieel engine and ga turbine to be run independently, give the ytem great flexibility during operation. While the CB cycle enable mall-cale power generation and indutrial cogeneration ytem to attain high thermal efficiencie, the fuel flexibilitie of dieel engine and ga turbine enable them to be fuelled with heavy-dieel fuel, natural ga, or renewable bio-fuel. However, unlike the combined Brayton- Rankine cycle, which received coniderable attention becaue of it uitability for large-cale power generation, publihed tudie on CB cycle are rather carce. he preent paper decribe a combined dieel-engine gaturbine (CG) ytem that improve upon previou propoal through the appropriate ue of regeneration and intercooling. For the analyi of the ytem' performance, an Excel-baed computer model ha been developed. he model enable optimum value of the compreor' preure ratio to be determined for given value of the dieel-engine' compreion ratio (CR) and ga-turbine' inlet temperature (I). Analyi of the ytem' performance with different value of CR and I determined the combination of compreor preure ratio that permit the ytem to be deigned with fewer compreor and turbine while maintaining a near-optimum performance. II. HE COMBINE IESEL-ENGINE GAS-URBINE SYSEM Fig. how a chematic diagram of the combined ytem that conit of a compound dieel engine (compreor C, intercooler IC, dieel engine E, and turbine ) and an intercooled regenerative ga turbine (compreor C2, intercooler IC2, compreor C3, regenerator RG, combution chamber CC, and turbine 2). Before dicharged to the atmophere, the exhaut gae from the two turbine are mixed and paed through the regenerator to reheat the high- 28

2 preure air going to the combution chamber of the ga turbine. Fig. he combined dieel-engine ga-turbine ytem with intercooling and regeneration Compared to the configuration of the combined ytem propoed by Mukul and Agarwal [], the preent configuration offer greater deign and operational flexibilitie by introducing eparate compreor and turbine for the ieel and Brayton cycle. hi arrangement alo allow the two component of the combined ytem to be optimied almot independently ince the only common part i the regenerator. Although intercooling caue an exergy lo, it ha certain advantage in the implementation of the ytem. For the ga turbine, intercooling enhance the effect of regeneration and improve the cycle' thermal efficiency. For the dieel engine, cooling the compreed air before the engine' intake improve it volumetric efficiency and, therefore, reduce it ize and intallation cot. In the cycle propoed by Mukul and Agarwal [], the dieel-engine' intake air i heated after compreion, rather than cooled, which reduce the engine' volumetric efficiency. With two eparate turbine, the preent configuration alo avoid the lo of exergy that i caued by mixing the hot gae coming from the ga-turbine' combution chamber with the cooler dieel-engine' exhaut. Compared to the configuration of propoed by Krihna and Renald [8], the preent configuration improve the thermal efficiency by precompreing the air of the dieel engine and by introducing intercooling and regeneration in the ga turbine cycle. However, compared to the two ytem propoed previouly the preent ytem conit of more component, which i bound to increae it initial cot a well a it overall loe. herefore, the main objective of the following analyi i to explore the poibility of replacing the two low-preure compreor by a ingle compreor and/or replacing the two turbine by a ingle turbine. III. HE HERMOYNAMIC MOEL In the following analyi, the ytem hown in Fig., which ha two different low-preure compreor and two different turbine, i referred to a ytem A. In ytem B, the two lowpreure compreor C and C2 are required to have the ame preure ratio (i.e. PR C =PR C2 ) o that they can be combined in a ingle large compreor. Similarly, in ytem C the two turbine and 2 are required to have the ame preure ratio (i.e. P 0 =P ) o that they can be replaced by a ingle large turbine. ha a ingle low-preure compreor a well a a ingle turbine. Fig. 2 how the - diagram for the four configuration of the combined ytem. A the figure how, ytem A and B, repectively, are different from ytem C and in that the two turbine and 2 have different compreion ratio. A and C, repectively, are different from ytem B and in that the low preure compreor C and C2 have different preure ratio. In the four cycle hown in Fig. 2, proce -2 i an ientropic compreion proce in compreor C, where ambient air taken at, P i delivered at 2, P 2. he compreed air pae through an ideal contant preure intercooler (proce 2-3) after which it temperature i brought down to 3 =. he cooled air goe to the intake of the dieel engine where it i compreed ientropically to tate. Heat i then added to it in proce - followed by the expanion toke (proce -). In ytem A and B, the ga dicharged from the dieel engine i expanded in turbine (Proce -) before dicharged at ambient preure. In ytem C and, the dieel-engine' exhaut i mixed with the ga-turbine' product of combution before the mixture i expanded in the common turbine in proce -3. In all cycle, the ga-turbine' intake air that i compreed in compreor C2 to point 8 alo goe through an ideal intercooler that bring it temperature 9 =. he compreed air then pae through the high-preure compreor C3 (proce 9-0), the regenerator (proce 0-), and the combution chamber (proce -2). In ytem A and B, the heated ga i then expanded in the econd turbine 2 (Proce 2-3) before dicharged at ambient preure and mixed with the exhaut ga of turbine. he mixed ga i ent at to the regenerator to preheat the air going the combution chamber and leave at tate. In ytem C and, the exhaut ga from the ingle turbine at 3 i ent to the regenerator before dicharged at tate to the atmophere. he performance of the dieel engine depend on it compreion ratio (CR) and cut-off ratio (COR) where, CR = v 3 / v and COR = v / v. 29

3 , ,9 (a) A (b) B , ,9 (c) C (d) Fig. 2 he - diagram of four arrangement of the combined ytem Referring to ytem A and B, the relation that determine the ytem' net work(w NE ) and thermal efficiency (η) are: ( h2 ) ( h8 ) ( h ) W C m h W C 2 m2 h W C 3 m2 0 h9 W m ( h h ) W 2 m2 ( h2 h3 ) W E m h h u u3 Q E m ( h h ) Q G m2 ( h2 h ) NE = () = (2) = (3) = () = () [( ) ( )] = () = () = (8) ( W + W + W ) ( W + W W ) W = + (9) E ( Q + Q ) E G 2 C C2 C3 WNE η = (0) A lightly different et of relation apply to the cycle of ytem C and. With the aumed idealiation, viz., ideal compreion and expanion, perfect intercooling, and negligible preure loe, the performance of the combined ytem, in term of it thermal efficiency and net work-output, i affected by the following factor:. he compreion ratio and cut-off ratio of the dieel engine (CR and COR) 2. he preure ratio of the three compreor (PR C, PR C2 and PR C3 ) and two turbine (PR, PR 2 ) 3. he effectivene of the regenerator (ε). he ma flow rate of the air upplied to the dieel engine and the to the ga turbine (m and m 2 ). IV. ANALYSIS OF HE SYSEM'S PERFORMANCE he following analyi of the combined ytem tudie the effect of the key parameter on the ytem' performance o a to determine the value of thee parameter that optimie it thermal efficiency and net work-output. hi wa achieved by varying the dieel-engine' compreion ratio (CR) and the ga turbine inlet temperature (I). he preure ratio of the three compreor were not treated a independent parameter. Intead, Excel' Solver wa ued to determine the preure ratio that maximie the cycle' thermal efficiency at each combination of I and CR. Finally, the tudy compare the performance of the elected ytem configuration in different mode of operation. he analye were carried out baed on the following aumption: - Ambient temperature and the preure are 300 K and 0. MPa, repectively. - he dieel engine' cut-off ratio i 2. - he regenerator effectivene i 80%. 220

4 - Pot-combution product of the dieel engine and ga turbine are treated a air. In the preent model, the thermodynamic propertie were determined by linking REFPROP [9] to Excel. Performance of ytem A with optimied preure ratio he ma flow rate in both the dieel engine and the ga turbine were taken a kg, i.e. m 2 = m =. he thermal efficiency and net work were calculated at value of I in the range K and value of CR in the range -30. Excel Solver wa ued to determine the preure ratio of the three compreor that maximie the thermal efficiency at each et of I and CR. Fig. 3.a how the variation of cycle' thermal efficiency and net work with two value of I (00K and 00K) and CR varying from to 30. he figure how that both the net work and thermal efficiency increae with CR. Fig. 3.b, which how the reult with two value of CR (20 and 30) and I varying from 000K to 00K, how that increaing I increae the net work but reduce the thermal efficiency of the ytem. herefore, in order to maintain the ame level of thermal efficiency at a lower I, the dieel-engine' compreion ratio mut alo be reduced. It wa mentioned earlier that the net work and thermal efficiency at each combination of I and CR were calculated at the compreor' preure ratio that maximie the cycle' thermal efficiency. Fig. how the variation of the optimum preure ratio of the three compreor with I at CR=20 and CR=30. From the figure it can be een that the preure ratio for compreor C doe not change ignificantly with both I and CR. he optimum value of the preure ratio of compreor C2 and that of compreor C3 are alo affected lightly with CR, but they both increae teadily a I increae. he higher compreion ratio i alway that of C3. Fig. alo indicate that the two compreor C and C2 have the ame optimum value at a certain value of I. At CR=20, the optimum value of PR C and PR C2 are both equal to 2. at I approximately equal to 00K. For CR=30, the preure ratio PR C and PR C2 become identical at about I=300K. (a) (b) Fig. 3 Variation of efficiency (Eta) and net work (w): (a) with CR, (b) with I (a) (b) Fig. Variation of optimum compreor preure ratio with I: (a) CR=20, (b) CR=30 hee reult indicate that the two compreor C and C2 can be combined in a ingle large compreor that upplie compreed air to both the dieel engine and ga turbine. Similarly, the two turbine and 2 can be replaced by a ingle large turbine if the preure outlet of compreor C3 can be the ame a that of the dieel-engine exhaut. hi i 22

5 the cae of ytem C and hown in Fig. 2 for which the preure at point i made equal to that of the point 0-2. Apart from the economical and operational advantage of having to deal with fewer compreor and turbine in the ytem, thi arrangement might alo reduce the loe and improve the overall efficiency of the ytem. Performance of the modified ytem B, C and with optimied preure ratio Further imulation tudied the performance of the ytem with the contraint PR C = PR C2 and/or P = P 0. able I and able II compare the key cycle parameter for ytem A, which wa optimied without any condition impoed on the preure ratio of the compreor and turbine, with thoe for ytem B, C and, which were optimied with the retriction PR C2 = PR C and/or P =P 0. able I compare thee parameter at I = 00K and CR=20, while able II compare them at I = 00K and CR = 30. he figure on the table how that ytem B maintain the thermal efficiency of ytem A but lightly reduce the cycle' net work. Although ytem C lead to different value of the compreor' preure ratio, the figure how that it produce more work than ytem A, B and with minor reduction in the thermal efficiency. give the lowet thermal efficiency for both value of CR. hee reult favour ytem B and C over ytem. ABLE I KEY PERFORMANCE PARAMEERS OF HE CG SYSEM A I=00K, CR=20 A B C PR C PR C PR C Net work (kj/kg) η (%).... A combined dieel-engine ga-turbine ytem ha been preented and optimied uing an Excel-baed thermodynamic model. he model enabled the preure ratio of the ytem' compreor that maximie thermal efficiency to be determined at each combination of the dieel-engine' compreion ratio and ga-turbine' inlet temperature. Firtlaw analyi of four configuration of the ytem howed that it can be deigned with a ingle large compreor that upplie air to both the dieel engine and ga turbine without undermining it performance. Alternatively, the ytem can be deigned with a ingle turbine that expand the hot gae from the ga-turbine combution chamber and the dieelengine' exhaut. However, in thi cae the tudy how that the ytem' performance with two low-preure compreor wa better than with one compreor. REFERENCES [] M. Kanoglu, S.K. Iık, A. Abuoglu, Performance characteritic of a ieel engine power plant, Energy Converion and Management, Volume, -2, (200), [2] M. Kanoglu, I. incer, Performance aement of cogeneration plant, Energy Converion and Management 0 (2009) 8. [3] A. Bait, he bi-fuel technology. An Introduction. Energy Engineering, April [] V. oz, R. Novella, A. Garcia, I. Sanchez, H ieel engine equipped with a bottoming Rankine cycle a a wate recovery ytem. Part : Study and analyi of the wate heat energy, Applied hermal Engineering 3 (202) [] M. Jonon, J. Yan, Exergy and Pinch Analyi of ieel Engine Bottoming Cycle with Ammonia-Water Mixture a Working Fluid, Int. J. Applied hermodynamic, Vol.3 (No.2), (2000), - [] G.A. Bobula, W.. Wintucky W and J.G. Cator, Compound Cycle Engine Program, NASA echnical Memorandum 8889, Prepared for the Rotary Wing Propulion Specialit Meeting ponored by the American Helicopter Society Williamburg, Virginia, November 2-, 98 [] S. Mukul, R. Agarwal, Energy and Exergy Analyi of Brayton-ieel Cycle, Proceeding of the World Congre on Engineering Vol II, July - 3, 2009, London, U.K. [8] S.S. Krihna, C.J.. Renald, Numerical analyi of a turbo-compounded ieel Brayton combined cycle, Continuum Mechanic, Fluid, Heat, (200), [9] E.W. Lemmon, M.L. Huber, M.O. McLinden, NIS Standard Reference atabae 23, NIS Reference Fluid hermodynamic and ranport Propertie REFPROP Verion 8.0, Uer Guide, National Intitute of Standard and echnology, Phyical and Chemical Propertie iviion, Boulder, Colorado 8030, Lat acceed July 3, 20. ABLE II KEY PERFORMANCE PARAMEERS OF HE CG SYSEM A I=00K, CR=30 A B C PR C PR C PR C Net work (kj/kg) η (%) V. CONCLUSIONS 222