Thermoelectric Design

INTERAMERICAN UNIVERSITY OF BAYAMON PUERTO RICO Thermoelectric Design Thermodynamic 2 Erik T. Rosado Rolando Santiago 5/15/2012

TABLE OF CONTENTS TABLE OF FIGURE... 2 TABLE OF DATA RESULTS... 2 ABSTRACT... 3 OBJECTIVE AND INTRODUCTION... 4 EQUIPMENT DESCRIPTION AND ESPECIFICATION... 10 Steam Turbines Siemens (SST-100), Case 1... 10 Gas Turbine Siemens (SGT-400), Case 2... 14 RESULTS... 18 Case 1 Simple Rankine Cycle... 18 Case 2 Brayton Cycle... 19 CONCLUSION... 20 Bibliography... 21 1

TABLE OF FIGURE Figure 1 SST-100 8MW Simple Rankine Cycle... 10 Figure 2 SST-100 Legend... 12 Figure 3 STG-400 13MW Brayton Cycle... 14 Figure 4 Design Concept SGT-400... 16 Figure 5 Package for Mechanical Drive... 17 Figure 6 Package for Power Generation... 17 TABLE OF DATA RESULTS Table 1 SST-100 Legend... 12 Table 2 Case 1 Simple Rankine Cycle (constant value)... 18 Table 3 Case 1 Simple Rankine Cycle (variable values)... 18 Table 4 Case 2 Brayton Cycle (constant values)... 19 Table 5 Case 2 Brayton Cycles (variable values)... 19 2

ABSTRACT A natural gas fired furnace in a textile plant is used to provide steam at 130 C. At times of high demand, the furnace supplies heat to the steam at a rate of 30 MJ/s. The plant also uses up to 6 MW of electrical power purchased from the local power company. The plant management is considering converting the existing process plant into a cogeneration plant to meet both their process-heat and power requirements. This project consist on the develop of some designs based on gas or steam turbines. All the pertinent measures were taken to come up with the better design, taking onto consideration the cost and complexity of each one of them. 3

OBJECTIVE AND INTRODUCTION Objective: Develop steam or gas cycles to achieve certain goal and solve a customer s need. Display all the correspondent analysis with all the limitations and advance of each system. Calculate the cycle stage by stage, and show the behavior or changes of the steam or gas while it passes by the cycle components. Solve all the unknowns of the cycle Come with an analysis to determine which of your designs is the more accurate for the customer s needs. Introduction: A natural gas fired furnace in a textile plant is used to provide steam at 130 C. At times of high demand, the furnace supplies heat to the steam at a rate of 30 MJ/s. The plant also uses up to 6 MW of electrical power purchased from the local power company. The plant management is considering converting the existing process plant into a cogeneration plant to meet both their process-heat and power requirements. This project consist on the develop of some designs based on gas or steam turbines. All the pertinent measures were taken to come up with the better design, taking onto consideration the cost and complexity of each one of them. 4

Theory RANKINE CYCLE: THE IDEAL CYCLE FOR VAPOR POWER CYCLES Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser, as shown schematically on a T-s diagram in Figure 1. The cycle that results is the Rankine cycle, which is the ideal cycle for vapor power plants. The ideal Rankine cycle does not involve any internal irreversibilities and consists of the following four processes: 1-2 Isentropic compression in a pump 2-3 Constant pressure heat addition in a boiler 3-4 Isentropic expansion in a turbine 4-1 Constant pressure heat rejection in a condenser Figure 1 The Simple Ideal Rankine Cycle Energy Analysis of the Ideal Rankine Cycle All four components associated with the Rankine cycle (the pump, boiler, turbine, and condenser) are steady-flow devices, and thus all four processes that make up the Rankine cycle can be analyzed as steady-flow processes. The kinetic and potential energy changes of the steam are usually small relative to the work and heat transfer terms and are therefore usually neglected. Then the steady-flow energy equation per unit mass of steam reduces to 5

The boiler and the condenser do not involve any work, and the pump and the turbine are assumed to be isentropic. Then the conservation of energy relation for each device can be expressed as follows: Pump Boiler Turbine Condenser The thermal efficiency of the Rankine cycle is determined from Where BRAYTON CYCLE: THE IDEAL CYCLE FOR GAS-TURBINE ENGINES The Brayton cycle was first proposed by George Brayton for use in the reciprocating oil-burning engine that he developed around 1870. Today, it is used for gas turbines only where both the compression and expansion processes take place in rotating machinery. Gas turbines usually operate on an open cycle, as shown in Fig. 2. Fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The highpressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. The resulting hightemperature gases then enter the turbine, where they expand to the atmospheric pressure while producing power. The exhaust gases leaving the turbine are thrown out (not recirculated), causing the cycle to be classified as an open cycle. The open gas-turbine cycle described above can be modeled as a closed cycle, as shown in Fig. 3, by utilizing the air-standard assumptions. Here the compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constantpressure heatrejection process to the ambient air. The ideal cycle that the working fluid undergoes in this closed loop is the Brayton cycle, which is made up of four internally reversible processes: 1-2 Isentropic compression (in a compressor) 2-3 Constant-pressure heat addition 3-4 Isentropic expansion (in a turbine) 4-1 Constant-pressure heat rejection 6

Figure 2 Open-Cycle Gas-Turbine Engine Figure 3 Closed-Cycle Gas-Turbine Engine 7

The T-s and P-v diagrams of an ideal Brayton cycle are shown in Fig. 4. Notice that all four processes of the Brayton cycle are executed in steadyflow devices; thus, they should be analyzed as steady-flow processes. When the changes in kinetic and potential energies are neglected, the energy balance for a steady-flow process can be expressed, on a unit mass basis, as Therefore, heat transfers to and from the working fluid are and Then the thermal efficiency of the ideal Brayton cycle under the cold-airstandard assumptions becomes Processes 1-2 and 3-4 are isentropic, and and. Thus, Substituting these equations into the thermal efficiency relation and simplifying give Where 8

Figure 4 T-s and P-v diagrams for the ideal Brayton Cycle. 9

EQUIPMENT DESCRIPTION AND ESPECIFICATION Steam Turbines Siemens (SST-100), Case 1 Figure 5 SST-100 8MW Simple Rankine Cycle Descriptions The SST-100 (figure1) is a single-casing multi-stage steam turbine, providing geared drive to 1,500 and 1,800 rpm generators and packaged in a base frame-mounted design. Fields of Application Generator and mechanical drive. Cogeneration Industrial power plants for e.g. sugar industry, textile industry, steel works and mines. Back pressure or condensing turbines, where the steam path is adapted to the needs of the individual customer, provide an optimal solution when medium pressure and temperature conditions are available. 10

Technical Data Power output Rotational speed Inlet steam pressure Inlet steam temperature Exhaust pressure Exhaust area up to 8.5 MW up to 7,500 rpm up to 65 bar / 945 psi 480 C / 895 F Back pressure: up to 10 bar (a) / 145 psi; Condensing: up to 1 bar / 14.5 psi 0.22 m² / 2.4 sq.ft. Benefits Cost effective, high reliable solution Fast and early layout planning Short delivery times because of highly predefined design of both turboset and package Short on-site erection: turbine skid and gearbox / oil unit fully assembled in the workshop before shipping to site Compact design - oil system fully integrated into the base frame Proven components: highly standardized, robust blading; reduction gears taken from the existing range of world-class gear manufacturers Increased safety: no leaks towards hot pipes; oil and steam piping separated Easy maintenance with low maintenance costs Design Concept The single body turbine SST-100 is designed for high thermoflexibility, permitting short start-up times and rapid load changes. Single-casing multi-stage steam turbine Highly predefined proven and compact design Workshop assembly Oil system integrated in base frame Separation of oil and steam piping The SST-100 in back pressure application Steam-driven emergency stop valve Double-seated control valves with hydraulic actuator Up to six impulse stages The rotor is made of solid forging or with forged discs shrink-mounted on the turbine shaft and keyed. It is short and rigid, resulting in operation well below first critical speed, using babbitt-lined sliding type bearings to assure a smooth and well-damped operation. Exhaust flange on side, bottom half of the casing The SST-100 in condensing application The condensing turbine design is derived from the back pressure version. However, there are some design differences, for example: 11

Up to seven impulse stages, including condensing stages Exhaust casing is oriented downwards through the base frame or upwards, depending on the application Diaphragm carrier for the last three stages Package Figure 6 SST-100 Legend Table 1 SST-100 Legend 1. Steam turbine 4. Base frame 2. Speed reduction gear 5. Concrete foundation 3. Generator Dimensions Length (L): 8 m / 26 ft. Width (W): 3.7 m / 12.1 ft Height (H): 3.4 m / 11.2 ft Weight: 40 tons (oil-free) 12

Service For SST-100 steam turbines, Siemens feature a major portfolio of service solutions and products to help optimize efficient plant operation. This service can include: Performance Maintenance Performance Enhancement Service Programs Product-Related Services Training and Consulting Note; all information about SST-100 Steam Turbine was obtained from Siemens [1]. 13

Gas Turbine Siemens (SGT-400), Case 2 Figure 7 STG-400 13MW Brayton Cycle Description The SGT-400 (figure 3) industrial gas turbine is a blend of 25 years experience and the latest combustion technologies. It offers a power output of 12.90 MW(e) for and 13.40 MW for mechanical drive configuration. The gas turbine is equipped with a Dry Low Emissions (DLE) combustion system, achieving low NOX levels with gas and liquid fuels and a full dual-fuel capability. In power generation the SGT-400 gas turbine benefits from the turbine s high simple cycle efficiency (nominal: 34.8 %). In cogeneration applications the unit s consistent, cost-effective power and steam-raising capability contribute to outstandingly high plant efficiencies. As a highly efficient unit the SGT-400 used in the following fields of application: Simple cycle applications Combined cycle applications Combined heat and power (CHP) Power generation for the oil and gas industry Onshore power generator for oil field service, refineries etc. Offshore platforms and FPSO vessels For mechanical drive applications the SGT-400 gas turbine is a proven unit for driving compressors and pumps, primarily for the oil and gas industry. It offers outstanding reliability, efficiency and maintainability and is designed to operate on a wide range of gaseous and liquid 14

fuels. Designed for applications where speeds and loads vary, the SGT-400 can operate over a range of different demands: Drive solution for pumping applications including crude oil, other refinery product transmission and water injection Drive solution for centrifugal compressors used in gas injection, pipeline transmission and boosting, gas processing and similar applications Mixed duty applications Technical Data For Power Generation Power output Fuel Frequency 12.90 MW(e) 14.40 MW(e) Natural gas / liquid fuel / dual fuel and other fuels capability on request 50/60 Hz Electrical efficiency 34.8% Heat rate Turbine speed 35.2% 10,355 kj/kwh (9,815 Btu/kWh) 10,084 kj/kwh (9,700 Btu/kWh) 9,500 rpm Compressor pressure ratio 16.8:1 Exhaust gas flow / Temperature NO x Emissions (with DLE, corrected to 15% O 2 dry) For Mechanical Drives Shaft output Fuel 18.9:1 39.4 kg/s, 555 C (86.8 lb/s, 1,031 F) 44.3 kg/s, 546 C (97.7 lb/s, 1,009 F) 15 ppmv Efficiency 36.2% Heat rate Turbine speed 13.40 MW (18,000bhp) 15.00 MW (20,100bhp) Natural gas / liquid fuel / dual fuel and other fuels capability on request 36.6% 9,943 kj/kwh (7,028 Btu/bhph) 9,684 kj/kwh (6,845 Btu/bhph) 9,500 rpm Compressor pressure ratio 16.8:1 Exhaust gas flow / Temperature NO x Emissions (with DLE, corrected to 15% O 2 dry) 18.9:1 39.4 kg/s, 555 C (86.8 lb/s, 1,031 F) 44.3 kg/s, 546 C (97.7 lb/s, 1,009 F) 15 ppmv 15

Benefits Proven design resulting in high availability and security of supply Dual-fuel Dry Low Emissions (DLE) combustion system, meeting the most stringent legislation Twin-shaft arrangement for both power generation and mechanical drive, allowing commonality of parts in mixed duty installations Competitive cost-to-power ratio Compact size Site maintainability Alternate rapid core engine exchange option, minimizes downtime Design Concept Figure 8 Design Concept SGT-400 The twin-shaft configuration of the SGT-400 (figure 4) provides excellent speed and load turndown flexibility. The design is uniquely simple, employing a single gas generator rotor with twin bearings and a single-stage overhung turbine. The free power turbine is a two-stage overhung design. Rotors are contained in heavy duty casings which are horizontally and vertically split, allowing full site maintenance. The gas turbine is equipped with a Dry Low Emissions (DLE) combustion system, achieving low NOX levels with gas and liquid fuels and a full dual-fuel capability. With a power turbine speed of 4,800 to 10,000 rpm, generator or pump drive from the power turbine is usually via a speed-reducing gearbox. Most compressor drive applications can be met without the need for a speed-changing gear. Package The SGT-400 is available as a factory-assembled package (figure 5 & 6). It is easily transported, installed and maintained at site. The package incorporates the gas turbine, gearbox, the generator for power generation and all systems mounted on a single underbase. Turbine controls, generator 16

control panel, motor control center for package motors and variable speed drive for starter motor are normally also package-mounted. The package is very compact, providing a small footprint and a high power-to-weight ratio. Figure 9 Package for Mechanical Drive Figure 10 Package for Power Generation Note: all information about Gas Turbine Siemens SGT-400 was obtained from Siemens [2] 17

RESULTS Case 1 Simple Rankine Cycle Table 2 Case 1 Simple Rankine Cycle (constant value) State Tempera ture ºC Presion (Kpa) Enthalpy ideal (KJ/kg) Enthalpy actual (KJ/kg) Entropy (KJ/kg K) Specific Volume Win (KW) (m^3/kg) Wout (KW) Qin (KW) Qout (KW) Variable 1 1000 762.51 0.001126 6.193 Cp (water) 4.18 2 6500 768.703 3 6500 3295.6 6.6786 4 1000 2826.43 6.6786 6000 5 25 6 30000 7 130 Table 3 Case 1 Simple Rankine Cycle (variable values) Nth Enthalpy actual (KJ/kg) mc (kg/s) mw (kg/s) Qin (KW) Qp (KW) T6 ºC Nth (sys) 10% 3248.683 127.88541 792.76661 323153.27 317945.27 120.94685 2% 15% 3225.2245 85.256943 546.73846 215435.51 209963.51 116.873 3% 20% 3201.766 63.942707 423.72438 161576.64 155972.63 113.06202 4% 25% 3178.3075 51.154166 349.91594 129261.31 123578.11 109.48926 5% 30% 3154.849 42.628472 300.71031 107717.76 101981.76 106.13306 6% 35% 3131.3905 36.53869 265.56343 92329.506 86555.79 102.97432 6% 40% 3107.932 31.971354 239.20327 80788.318 74986.316 99.99609 7% 45% 3084.4735 28.418981 218.70093 71811.838 65987.837 97.183342 8% 50% 3061.015 25.577083 202.29905 64630.654 58789.053 94.522653 9% 55% 3037.5565 23.251894 188.87933 58755.14 52899.139 92.002018 10% 60% 3014.098 21.314236 177.69623 53858.878 47990.878 89.610661 11% 65% 2990.6395 19.674679 168.23361 49715.888 43837.733 87.338885 12% 70% 2967.181 18.269345 160.1228 46164.753 40277.895 85.17794 13% 75% 2943.7225 17.051389 153.09342 43087.103 37192.702 83.119909 14% 80% 2920.264 15.985677 146.94272 40394.159 34493.158 81.15761 15% 85% 2896.8055 15.045343 141.51562 38018.032 32111.208 79.284515 16% 90% 2873.347 14.209491 136.69154 35905.919 29993.918 77.494678 17% 95% 2849.8885 13.461623 132.37526 34016.134 28099.502 75.782667 18% 100% 2826.43 12.788541 128.4906 32315.327 26394.527 74.143515 19% 18

Case 2 Brayton Cycle State Temperat ure K Presion (Kpa) Enthalpy ideal (KJ/kg) Table 4 Case 2 Brayton Cycle (constant values) Enthalpy actual (KJ/kg) Entropy (KJ/kg K) Specific Volume (m^3/kg) Win (KW) Wout (KW) Qin (KW) Qout (KW) Variable 1 298 K 1.4 2 667.2747 Pr 16.8 3 1828 Cp (air) 1.005 4 828 6000 Cp (water) 4.18 5 6 298 7 30000 8 403 Table 5 Case 2 Brayton Cycles (variable values) Nth (C&T) Compresor Turbine Win (KJ/kg) Wout (KJ/kg) Wnet (KJ/kg) ma (kg/s) Wnet (KW) Qin (KW) Qout (KW) T5=T7 mw (kg/s) Nth (sys) 10% 3990.7466 1728 3711.2103 100.5-3610.71-1.661723 6000 3611.8549-2388.145 n/a n/a 166% 15% 2759.831 1678 2474.1402 150.75-2323.39-2.582433 6000 2418.4234-3581.577 n/a n/a 248% 20% 2144.3733 1628 1855.6051 201-1654.605-3.626243 6000 1152.9826-4847.017 n/a n/a 520% 25% 1775.0986 1578 1484.4841 251.25-1233.234-4.865256 6000-258.6656-6258.666 n/a n/a -2320% 30% 1528.9155 1528 1237.0701 301.5-935.5701-6.413202 6000-1927.68-7927.68 n/a n/a -311% 35% 1353.0704 1478 1060.3458 351.75-708.5958-8.467451 6000-4041.55-10041.55 n/a n/a -148% 40% 1221.1866 1428 927.80257 402-525.8026-11.41113 6000-6959.046-12959.05 n/a n/a -86% 45% 1118.6103 1378 824.7134 452.25-372.4634-16.10897 6000-11484.67-17484.67 n/a n/a -52% 50% 1036.5493 1328 742.24206 502.5-239.7421-25.0269 6000-19906.59-25906.59 n/a n/a -30% 55% 969.40847 1278 674.76551 552.75-122.0155-49.17408 6000-42431.55-48431.55 n/a n/a -14% 60% 913.45776 1228 618.53505 603-15.53505-386.2234 6000-354983.7-360983.7 n/a n/a -2% 65% 866.11486 1178 570.95543 653.25 82.29457 72.90882 6000 70480.561 64480.561 367.75308 203.62158 9% 70% 825.53522 1128 530.1729 703.5 173.3271 34.616629 6000 34875.461 28875.461 347.87264 130.19004 17% 75% 790.36621 1078 494.82804 753.75 258.92196 23.173005 6000 24165.319 18165.319 336.36642 107.70895 25% 80% 759.59332 1028 463.90129 804 340.09871 17.641937 6000 18943.007 12943.007 328.7051 96.601966 32% 85% 732.44077 978 436.61298 854.25 417.63702 14.366542 6000 15818.095 9818.0951 323.1618 89.894731 38% 90% 708.30517 928 412.3567 904.5 492.1433 12.191571 6000 13719.093 7719.0932 318.91661 85.356143 44% 95% 686.71016 878 390.65372 954.75 564.09628 10.636482 6000 12200.005 6200.0054 315.5306 82.051935 49% 100% 667.27466 828 371.12103 1005 633.87897 9.4655294 6000 11041.814 5041.8142 312.746 79.520392 54% 19

CONCLUSION In this project we were presented with the situation that a textile plant was considering on converting their current system used to provide steam for their process, onto a cogeneration plant in which their process and power demand could be reached. Some designs were considered to solve the plant situation and after all the analysis we could come up with a solution for the customer. Based on the analysis done we concluded that the Brayton cycle was the most suitable to meet the customer needs. In comparison, the Brayton cycle prove to have a better performance over the Rankine cycle, giving up to 54% of efficiency on the system vs a 19% of efficiency on the Rankine cycle at its best. Also the Brayton cycle proved to acquire those efficiencies with the less power consumed, needing only 11 MW vs over 32 MW on the Rankine cycle. Therefore, the Brayton cycle is the best match for the customers need. 20

Bibliography [1] "Siemens SST-100 Steam Turbine," 2012. [Online]. Available: http://www.energy.siemens.com/mx/en/power-generation/steam-turbines/sst-100.htm. [Accessed 2012]. [2] "Gas Turbine SGT-400," Siemens, 2012. [Online]. Available: http://www.energy.siemens.com/mx/en/power-generation/gas-turbines/sgt- 400.htm#content=Description. [Accessed 2012]. 21