Power Brayton

While there are complexities associated with use of either multiple parallel Brayton units or a single, higher power Brayton unit, the overall component capabilities appear to support either option. Key development for the single Brayton option are the turbine and alternator scale-up, while for the multiple parallel system with spares, development of reliable and large flow area valves appears to the greatest additional challenge. 

In apphcation to electric utihty power generation, MHD is combined with steam (qv) power generation, as shown in Figure 2. The MHD generator is used as a topping unit to the steam bottoming plant. From a thermodynamic point of view, the system is a combined cycle. The MHD generator operates in a Brayton cycle, similar to a gas turbine the steam plant operates in a conventional Rankine cycle (11). 

Fig. 9. Brayton cycle, where A = compressor inlet, B = combustor inlet, C = power turbine inlet, and D = exhaust (a) thermodynamic relationships and...

The Combined (Brayton-Rankine) Cycle The 1990s has seen the rebirth of the combined cycle, the combination of gas turbine technologies with the steam turbine. This has been a major shift for the utility industry, which was heavily steam-tnrbine-oriented with the use of the gas turbine for peaking power. In this combined cycle, the hot gases from the turbine exhaust are used in a heat recoveiy steam generator or in some cases in a snpplementaiy fired boiler to produce superheated steam. 

A comparison of the effect of the various cycles on the overall thermal efficiency is shown in Fig. 29-40. The most effective cycle is the Brayton-Ranidne (combined) cycle. This cycle has tremendous potential in power plants and in the process industries where steam turbines are in use in many areas. The initial cost of the combined cycle is between 800- 1200 per kW while that of a simple cycle is about 300- 600 per kW. Repowering of existing steam plants by adding gas turbines can improve tne over plant efficiency of an existing steam turbine plant by as much as 3 to 4 percentage points. 

The most effective cycle is the Brayton-Rankine cycle. This cycle has tremendous potential in power plants and in the process industries where steam turbines are in use in many areas. The initial cost of this system is high however, in most cases where steam turbines are being used this initial cost can be greatly reduced. 

The work required to drive the turbine eompressor is reduced by lowering the compressor inlet temperature thus increasing the output work of the turbine. Figure 2-35 is a schematic of the evaporative gas turbine and its effect on the Brayton cycle. The volumetric flow of most turbines is constant and therefore by increasing the mass flow, power increases in an inverse proportion to the temperature of the inlet air. The psychometric chart shown shows that the cooling is limited especially in high humid conditions. It is a very low cost option and can be installed very easily. This technique does not however increase the efficiency of the turbine. The turbine inlet temperature is lowered by about 18 °F (10 °C), if the outside temperature is around 90 °F (32 °C). The cost of an evaporative cooling system runs around 50/kw. 

The Joule-Brayton (JB) constant pressure closed cycle is the basis of the cyclic gas turbine power plant, with steady flow of air (or gas) through a compressor, heater, turbine, cooler within a closed circuit (Fig. 1.4). The turbine drives the compressor and a generator delivering the electrical power, heat is supplied at a constant pressure and is also rejected at constant pressure. The temperature-entropy diagram for this cycle is also... 

El-Masri, M.A. (1987a). Exergy analysis of combined cycles Part 1 Air-cooled Brayton-cycic gas turbines. ASME J. Engng Power Gas Turbines 109. 228-23. i. 

On-site combined heat and power (CHP) which has existed for years, includes turbines, reciprocating engines and steam turbines. Gas turbines in the 500-kW to 250-MW produce electricity and heat using a thermodynamic cycle known as the Brayton cycle. They produce about 40,000-MW of the total CHP in the United States. The electric efficiency for units of less... 

Figure 9-13. Regenerative Brayton Cycle Fuel Cell Power System... 

The results of the performance calculations are summarized in Table 9-24. The efficiency of the overall power system, work output divided by the lower heating value (LHV) of the CH4 fuel, is increased from 57% for the fuel cell alone to 82% for the overall system with a 30 F difference in the recuperative exchanger and to 76% for an 80 F difference. This regenerative Brayton cycle heat rejection and heat-fuel recovery arrangement is perhaps the simplest approach to heat recovery. It makes minimal demands on fuel cell heat removal and gas turbine arrangements, has minimal number of system components, and makes the most of the inherent high efficiency of the fuel cell. 

Figure 9-14. Combined Brayton-Rankine Cycle Fuel Cell Power Generation System... 

Another advantage is that the IGCC system generates electricity by both combustion (Brayton cycle) and steam (Rankine cycle) turbines. The inclusion of the Brayton topping cycle improves efficiency compared to a conventional power plant s Rankine cycle-only generating wstem. Typically about two-thirds of the power generated comes from the Brayton cycle and one-third from the Rankine cycle. 

The thermodynamic power cycles most commonly used today are the vapor Rankine cycle and the gas Brayton cycle (see Chapter 4). Both are characterized by two isobaric and two isentropic processes. The vapor... 

An engine operates on the open Brayton cycle and has a compression ratio of 8. Air, at a mass flow rate of 0.1 kg/sec, enters the engine at 27°C and 100 kPa. The amount of heat addition is IMJ/kg. Determine the efficiency, compressor power input, turbine power output, back-work ratio, and net power of the cycle. Show the cycle on a T-s... 

An engine operates on the closed Brayton cycle (Fig 4.8) and has a compression ratio of 8. Helium enters the engine at 47°C and 200 kPa. The mass flow rate of helium is 1.2 kg/sec and the amount of heat addition is 1 MJ/kg. Determine the highest temperature of the cycle, the turbine power produced, the compressor power required, the back-work ratio, the rate of heat added, and the cycle efficiency. 

The maximum and minimum temperatures and pressures of a 40 MW turbine shaft output power ideal air Brayton power plant are 1200 K (Ta), 0.38 MPa (P3), 290 K (TO, and 0.095 MPa (Pi), respectively. Determine the temperature at the exit of the compressor Tj), the temperature at the exit of the turbine (P4), the compressor work, the turbine work, the heat added, the mass rate of flow of air, the back-work ratio (the ratio of compressor work to the turbine work), and the thermal efficiency of the cycle. 

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