ChemE 260 The Brayton Power Cycle and Variations Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 9: E & F CB 8: 1 - 3, 8 - 10 May 25, 2005

The Brayton Cycle The ideal gas cycle for gas-turbine engines Gas-turbine engines are used for transportation Warships Abrams tanks Gas turbines are also commonly used for electrical power generation. Other types of gas power cycles are better suited to other applications. The Diesel Cycle is a reasonable approximation of the cycles used in many trucks, trains and ships. The Otto Cycle is a reasonable approximation of the cycles used in gasoline powered automobile engines. The Brayton Cycle is the only gas power cycle we will consider in this course. The Brayton Cycle is an open cycle because fresh working fluid is draw into the cycle and spent working fluid is rejected from the cycle. This almost invariably means the working fluid is air. The Brayton Cycle is an internal combustion engine. This means heat is not added to the cycle in a HEX. Instead, a chemical reaction (combustion) is carried out within the cycle. The energy released by the chemical reaction increases the temperature of the working fluid (air-fuel-combustion products mixture) This complicates the system a great deal because we no longer have a pure working fluid. Baratuci ChemE 260 May 25, 2005

The Air-Standard Brayton Cycle Air-Standard Assumptions Air is the working fluid and it behaves as an ideal gas. The Brayton Cycle is modeled as as a closed cycle. The combustor is replaced by HEX #1. (External Combustion) All processes are internally reversible. Step 1-2: Isobaric heating Step 2-3: Isentropic expansion Step 3-4: Isobaric cooling Step 4-1: Isentropic compression It is much easier to analyze the performance of the Air-Standard Brayton Cycle. We lose some accuracy by assuming the air is an ideal gas. But we can still learn a great deal about how Brayton Cycles work and how different operating parameters effect their efficiency. In an internally reversible Brayton Cycle, the Compressor and Turbine are adiabatic and therefore, isentropic. But, in Ch 7 and Ch 8 we learned how to use isentropic efficiency to take irreversibilities into account. We will use isentropic efficiencies for the compressor and turbine in Brayton Cycles to develop slightly more realistic models of their performance. We will always consider the heat exchangers to be isobaric. Baratuci ChemE 260 May 25, 2005

PV & TS Diagrams PV and TS Diagrams for the internally reversible Brayton Cycle are pretty simple. Baratuci ChemE 260 May 25, 2005

The Cold Air-Standard Assumption The heat capacities of air are constant and always have the values determined at 25oC. Compression Ratio: Thermal efficiency of an internally reversible, cold air-standard Brayton Cycle: The cold air-standard assumption makes analysis of the Brayton Cycle pretty straightforward. CP and CV are both assumed to be constant. Nothing complicated like the Shomate Equation is used in this model. CP and CV are both evaluated at 25oC. Because gas turbines generally do not operate anywhere near 25oC, this assumption introduces some very significant error. Still, the cold air-standard model helps us understand the trends observed in a real gas power cycle without all the tedious calculations. We are also able to understand how the Brayton Cycle can be improved. The derivation of the thermal efficiency of the air-standard Brayton Cycle is in Thermo-CD. It isn’t long or hard. Take the time to understand it and you will have a good grasp of how to analyze Brayton Cycles. The boxed equation is the result. Baratuci ChemE 260 May 25, 2005

Air-Standard Brayton Cycle Efficiency The shaded band represents the range of compression ratios that are commonly used. The graph shows that the corresponding range of thermal efficiency is from about 37% to about 58%. That is not bad when you consider the efficiency of most automobile engines is less than 35%. Baratuci ChemE 260 May 25, 2005

Improvements to the Brayton Cycle Regeneration Use the hot turbine effluent to preheat the feed to the combustor. Reheat Use a 2-stage turbine and reheat the effluent from the HP turbine before putting into the LP turbine. Intercooling Use a 2-stage compressor with an intercooler. Regeneration with Reheat and Intercooling Use all of the techniques listed above to achieve high efficiency. Regeneration and reheat work in the same way that they do in a vapor power cycle. Regeneration can improve the efficiency if the proper compression ratio is used. Reheat reduces the efficiency of the cycle unless it is used with regeneration. Reheat can help keep operating temperatures down. This can reduce equipment costs. A 2-stage compressor costs more and actually lowers the thermal efficiency of the cycle… Unless it is used in combination with regeneration ! When you use regeneration with both a 2-stage turbine with reheat and a 2-stage compressor with intercooling, the thermal efficiency increases substantially ! In fact, if you could use an infinite number of compressor stages with intercoolers… And an infinite number of turbine stages with reheat… You would have a CARNOT cycle and you would get maximum efficiency ! You would also have infinite cost ! Baratuci ChemE 260 May 25, 2005

Regenerative Brayton Cycle The boxed equation applies for an internally reversible air-standard Brayton Cycle with Regeneration. A closed cycle is shown here because this is the way we will analyze Brayton Cycles, even though they are usually open. The hot turbine effluent, stream 5, is used to preheat the compressed air that enters the combustor, stream 3. Regeneration does not effect the power generated by the cycle, but reduces QH and QC and thereby increases the efficiency of the cycle. Baratuci ChemE 260 May 25, 2005

Regenerative Brayton Cycle Efficiency T1 / T4 = 0.30 Regenerative Brayton Cycle T1 / T4 = 0.20 For certain compression ratios, the regeneration cycle is significantly more efficient. The bonus is that the regenerative cycle is more efficient at LOW compression ratios ! The result is that the compressor is less expensive to purchase and to operate. Baratuci ChemE 260 May 25, 2005

Reheat Brayton Cycle Reheat increases QH and QC, but DECREASES the thermal efficiency. It does help avoid very high operating temperatures which drive up the costs of the equipment. Reheat is never used without regeneration in a Brayton Cycle. Baratuci ChemE 260 May 25, 2005

Brayton Cycle with Intercooling Intercooling reduces the power requirement for compression. It also increases QH and QC. But it increases QC by more ! The net result is a slight DECREASE in the thermal efficiency. Intercooling is never used without regeneration. Baratuci ChemE 260 May 25, 2005

Regeneration, Reheat & Intercooling This is the way real gas turbine power cycles are operated. The increase in thermal efficiency is substantial when all the improvements are combined. The intercooler reduces the compressor work The reheater keeps the maximum operating temperature down while increasing QH. The regenerator facilitates the intercooler and reheater while maintaining a high thermal efficiency. This is the top of the line. It is also the most complex system we will analyze in this course. Baratuci ChemE 260 May 25, 2005

Next Class … Vapor-Compression Refrigeration Cycles Cycle corresponds to the vapor power cycle. TS Diagrams, Deviations from internal reversibility Selecting a refrigerant Enhanced Vapor-Compression Refrigeration Cycles Cascade V-C Refrigeration Cycles Two separate refrigeration cycles, Analogous to Binary Vapor Power Cycles One provides cooling to the other Two different refrigerants Can reach very low temperatures Multi-Stage V-C Refrigeration Cycles Similar to Cascade V-C Refrigeration Two cycles use the same refrigerant Instead of exchanging heat between two cycles, the refrigerant streams are mixed. This is more efficient than heat exchange. Baratuci ChemE 260 May 25, 2005