Brayton cycle, efficiency

Inereasing the pressure ratio and the turbine firing temperature inereases the Brayton cycle efficiency. This relationship of overall cycle efficiency is based on certain simplification assumptions such as (1) liia > nif, (2) the gas is caloricaly and thermally perfect, which means that... 

Look at the effects of the radiator on the Brayton cycle efficiency and system mass. 

Kidney-shaped Pb-to-C02 heat exchangers must fit inside of the annulus between the shroud and reactor vessel, and provide sufficient heat exchange performance to realize a significant Brayton cycle efficiency. 

Figure XXIII-7 shows the dependency of the S-CO2 Brayton cycle efficiency upon the core inlet temperature. In the calculations, the heat exchanger tube height is chosen to satisfy the peak cladding temperature constraint of 650°C. It is confirmed that a fuel rod outer diameter of 1.30 cm and an inlet temperature of 438°C maximize the Bra)don cycle efficiency.

FIG. XXIII-7. Supercritical carbon dioxide Brayton cycle efficiency versus Pb core inlet temperature subject to peak cladding temperature constraint of650°C. 

There is a strong incentive to operate as closely as possible to the critical temperature to raise the cycle efficiency. The dependency of the S-CO2 Brayton cycle efficiency upon the turbine inlet temperature is presented in Fig. XXIII-12. 

FIG. XXIII-11. Dependency ofS-C02 Brayton cycle efficiency upon cooler... 

FIG. XXIII-12. Dependency of S-CO2 Brayton cycle efficiency upon turbine inlet temperature. 

The HXs must heat the S-CO2 to a high turbine inlet temperature, to achieve high Brayton cycle efficiency ... 

The gas cooler transfers heat from the HeXe working fluid in the gas reactor system to the heat rejection system. The gas cooler is located in the HeXe flow path between the recuperator discharge and the compressor inlet. The performance of the gas cooler has a significant impact on Brayton cycle efficiency because the HeXe flows from the gas cooler discharge to the compressor inlet and the compressor inlet temperature is a key determinant of cycle efficiency. 

Gas-Cycle Systems. In principle, any permanent gas can be used for the closed gas-cycle refrigeration system however, the prevailing gas that is used is air. In the gas-cycle system operating on the Brayton cycle, all of the heat-transfer operations involve only sensible heat of the gas. Efficiencies are low because of the large volume of gas that must be handled for a relatively small refrigera tion effect. The advantage of air is that it is safe and inexpensive. 

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. 

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... 

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. 

The effectiveness of the regenerative Brayton cycle performance will depend on the efficiency of the fuel cell, compressor, and turbine units the pressure loss of gases flowing through the system the approach temperatures reached in the recuperative exchanger and, most importantly, the cost of the overall 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. 

This expression for thermal efficiency of an ideal Brayton cycle can be simplified if air is assumed to be the working fluid with constant specific heats. Equation (4.3) is reduced to... 

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... 

For actual Brayton cycles, many irreversibilities in various eomponents are present. The T s diagram of an aetual Brayton cycle is shown in Fig. 4.6. The major irreversibilities occur within the turbine and compressor. To account for these irreversibility effects, turbine efficiency and compressor efficiency must be used in computing the actual work produced or consumed. The effect of irreversibilities on the thermal efficiency of a Brayton cycle is illustrated in the following example. 

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. 

Air enters the compressor of an ideal Brayton cycle at 100 kPa and 300 K with a volumetric flow rate of 5m /sec. The compressor pressure ratio is 10. The turbine inlet temperature is 1400 K. Determine (a) the thermal efficiency of the cycle, (b) the back-work ratio, and (c) the net power developed. 

An ideal split-shaft Brayton cycle receives air at 14.7 psia and 70° F. The upper pressure and temperature limits of the cycle are 60 psia and 1500°F, respectively. Find the temperature and pressure of all states of the cycle. Calculate the input compressor work, the output power turbine work, heat supplied in the combustion chamber, and the thermal efficiency of the cycle, based on variable specific heats. 

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