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Energy and Exergy Analysis of Brayton-Diesel Cycle

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Energy and Exergy Analysis of Brayton-Diesel Cycle Proceedings of the World Congress on Engineering 2009 Vol II WCE 2009, July 1 - 3, 2009, London, U.K. Energy and Exergy Analysis of Brayton-Diesel Cycle Sanjay, Mukul Agarwal, Rajay Abstract-- In this work the energy and exergy analysis of a hybrid turbines has improved the efficiency of the simple-cycle operation more than 40%. Thermodynamic cycle gas turbine cycle has been presented. The thermodynamic developments, such as recuperation, mixed air steam characteristic of Brayton-diesel cycle is considered in order to turbines (MAST) are among the possible ways to improve the performance of gas turbine based power plants at establish its importance to future power generation markets. feasible costs. Significant work in the field of advanced gas Mathematical modeling of Brayton-diesel cycle has been done at turbine based cycles has been done by Abdallah, H. and component level. Based on mathematical modeling, a computer code Harvey, S [1], Alabdoadaim, M et.al [2], Arrieta, F. R. P [3], Yadav R [4], Bianchi, M [5] , Chiesa. et al.[6] , has been developed and the configuration has been subjected to Gabbrielli [7], Jonsson [8], Kuchonthara [9], thermodynamic analysis. Results show that, at any turbine inlet Heppenstall[10], Horlock [11], Waldyr [12], Lukas [13], temperature (TIT) the plant specific work initially increases with Poullikkas, A [14], etc. After reviewing recent technical papers by leading researchers Brayton-diesel cycle has been increase of pressure ratio (r ), and but at very high values of r , it p,c p,c identified for detailed study. starts decreasing. For a fixed value of rp,c (more than 10) with the increase in TIT, plant efficiency and specific work both increase. II. NOMENCLATURE The cycle is best suited for applications where power requirement -1. -1 cp = specific heat…………..(kJ-kg K ) ranges between 700-900 kJ/kg. The exergy analysis shows that gt = gas turbine . -1 maximum exergy loss of around 27% occurs in during combustion h = specific enthalpy………(kJ kg ) -1 -1 in the plant. ΔH r = lower heating value……(kJ.kg K ) . -1 Index Terms— Diesel cycle, exergy, exergy loss, gas turbine cycle, m = mass flow rate……….. (kg s ) hybrid. Q = heat added/ removed during process I. INTRODUCTION rp = cycle pressure ratio p = pressure………………..(bar) With the increasing population of the world, the energy T = temperature……………(K) consumption will increase rapidly. While as the fossil fuels are TIT = turbine inlet temperature (K) = combustor exit depleting and one can imagine the future energy crisis unless temperature some alternate cheap energy resources are developed. The W = specific work………(kJ.kg-1) increasing energy demand and depletion of fossil fuel resources inevitably necessitate for the optimum utilization of exhaustible Greek symbols fossil fuel and non-renewable energy resources. In this effort, a hybrid gas turbine based power cycle has been conceived for ε = effectiveness ………..(%) achieving maximum utilisation of thermal energy associated with η = efficiency………………(%) the gas turbine exhaust. γ = ratio of specific heat at constant pressure and constant volume In regard to the simple-cycle gas turbine technology, the υ = specific volume ………(m3kg-1) major driver to enhance the engine performance has been the increase in process conditions (temperature and pressure) through Subscripts advancements in materials and cooling methods. On-going development and near term introduction of advanced gas a = air , ambient b = blade Dr. Sanjay . is with the National Institute of Technology, Jamshedpur c = compressor, coolant, INDIA in the Mech. Engg. Deptt. (phone: +91-657-2373813; fax: +91- 657-2373813; e-mail: [email protected] ). C = compression (Diesel Cycle) Mukul Agarwal, is with the Tata Consultancy Services, INDIA(e-mail: comb = combustor [email protected]). dc = diesel cycle Rajay, is with the Bharat Heavy Electricals Ltd. At Haridwar, INDIA as Senior Engineer (email: [email protected] ) ISBN:978-988-18210-1-0 WCE 2009 Proceedings of the World Congress on Engineering 2009 Vol II WCE 2009, July 1 - 3, 2009, London, U.K. Combustion chamber Fuel Compressor Alternator Turbine Air inlet Exhaust Heat exchanger Diesel engine Alternator Fig. 1. Schematic of a Brayton-Diesel cycle III. BRAYTON-DIESEL- CYCLE CONFIGURATION e = exit E = expansion (Diesel Cycle) The configuration identified for parametric study is built up f = fuel of various types of components. Preheating of the inlet air g = gas of a diesel engine can sufficiently improve its performance. gen = alternator The gas turbine exhaust can be applied in order to increase gt = gas turbine the temperature of the air, which is extracted from the he = heat exchanger compressor and fed into the diesel engine. Subsequently, i = inlet, stage of compressor the engine outlet flow expands through the low-pressure in = inlet stage of the gas turbine as illustrated in Fig. 1. Main j = coolant bleed points components of this cycle are gas turbine, gas-to-gas heat net = difference between two values exchanger and diesel engine. The turbine exhaust is used to p = pressure heat up the air bled from the compressor, which goes into plant = brayton-diesel cycle plant diesel engine for further power generation. Modeling of all z = cooled row stage these components/elements that constitute the cycle configuration has been done. Modeling of these Acronym components are based on mass and energy balance across the control volume boundary of each of these components. C = Compressor Modeling the various elements of a cycle and derived CC = Combustion chamber governing equations is detailed in the following section. GT = Gas turbine A. Modeling of Components Modeling of various elements like working fluid/gas, compressor, combustor, and cooled gas turbine has been ISBN:978-988-18210-1-0 WCE 2009 Proceedings of the World Congress on Engineering 2009 Vol II WCE 2009, July 1 - 3, 2009, London, U.K. done in the earlier work [15,16,17]. Following are a few of the V important equations of the model. 4 Expansion ratio, r = (7) T e V3 Enthalpy of air/gas, h = c (T ) dt (1) ∫ p To V 3 Cut –off ratio, r = (8) Compressor work, Wc = c V 2 mehe + (∑ mc, j hc, j − mc,i hc,i )+ mdc hdc (2) It is seen that rk=re.rc (9) Energy balance of combustor : m .ΔH η = (m h − m h ) (3) γ −1 f r comb g,e g,e a.in a.in comb T ⎡V ⎤ 1 The gas turbine work is the sum of the work done by all rows of 4 = 3 = bladings having open loop air-cooled blades. Process 3-4 ⎢ ⎥ γ −1 (10) T3 ⎣V4 ⎦ re = Σm (h − h ) + Σm (h − h ) W gt g.i g.az g.bz cooled g.i g.i g.e uncooled (4) T2 p2V2 V2 1 where ‘cooled’ and ‘uncooled’ in the equation represent rows of Process 2-3 = = = (11) blade requiring cooling and rows of blade not requiring cooling. T3 p3V3 V3 rc Heat exchanger is used to transfer heat from the heated stream to the colder stream flowing through it. The energy balance γ −1 equation of heat exchanger gives: T1 ⎡V2 ⎤ 1 Process 1-2 = ⎢ ⎥ = γ −1 m ⋅ c ⋅ε T − T = m ⋅ c ⋅ T − T T2 ⎣V1 ⎦ rk h p,h he [( he,h )i ( he,h )e ] c p,c [( he,c )i ( he,c )e ] (5) So where ‘h’ stand for hot stream and ‘c’ stands for colder stream. 1 T3 1 T1 = T2 . γ −1 = . γ −1 (12) B. Diesel Engine Cycle rk rc rk Diesel engine works on diesel cycle comprises of two isentropic, As we know that one isochoric and one isobaric process. For 1 kg of air in the cylinder, the efficiency analysis of the cycle can be made as Thermal efficiency Q m ⋅c (T −T ) T −T given below. η =1− 2 =1− v 4 1 =1− 4 1 The efficiency may be expressed in terms of any two of the following three ratios Q1 m⋅cP (T3 −T2) γ (T3 −T2 ) (13) p=c Now substituting the values from equation ( 7) , (8) , (9) , (10) in (13) V=c γ T 1 1 rc −1 WE η Diesel = 1− . γ −1 . (14) Q1 γ rk rc −1 IV. RESULTS AND DISCUSSIONS WC Q2 For predicting the performance of the Brayton-diesel-cycle, a computer code based on the modeling of various cycle components discussed in the previous section has been developed. s For predicting the performance of the Brayton-diesel-cycle, Fig. 2. T-s representation of ideal Diesel cycle mathematical modeling of various cycle components has been discussed in previous section. Based on this modeling a computer code in C++ language has been developed. V1 Results have been obtained for input data listed in Table I. Compression ratio, rk = (6) and the results obtained have been plotted using graphic V2 package ORIGIN 6.0. Based on results of exergy analysis, a Sankey diagram has been drawn for the cycle. The exergy distribution quantifies the losses in various elements of the cycle. The results (Design Monograms and Sankey diagrams) have been discussed. ISBN:978-988-18210-1-0 WCE 2009 Proceedings of the World Congress on Engineering 2009 Vol II WCE 2009, July 1 - 3, 2009, London, U.K. Table I. Input data for analysis for Brayton-Diesel Cycle 830 Component Parameters 825 Gas property cp= f(T) 820 Enthalpy h= ∫ cp(T) dT 815 Ambient condition Ta=288K 810 Pa=1.013 bar Relative humidity=60% 805 Inlet section ∆ploss=1 percent of entry 800 pressure 795 Compressor Isentropic efficiency (ηc)=86% 790 Mechanical efficiency 785 (ηm)=98% Plant Specific work (kJ/kg)) 780 Combustor ∆ploss=2% of entry pressure ηcc=98% 775 Fuel Natural gas 770 0 1020304050 (LCV)f=42000 kJ/kg Fuel inlet pressure = 110% of Cycle pressure ratio Fig.
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