Brayton Cycle - Methods To Improve Efficiency

Methods To Improve Efficiency

The efficiency of a Brayton engine can be improved in the following manners:

• Increasing pressure ratio - As Figure 1 above shows, increasing the pressure ratio increases the efficiency of the Brayton cycle. This is analogous to the increase of efficiency seen in the Otto cycle when the compression ratio is increased. However, there are practical limits when it comes to increasing the pressure ratio. First of all, increasing the pressure ratio increases the compressor discharge temperature. This can cause the temperature of the gasses leaving the combustor to exceed the metallurgical limits of the turbine. Also, the diameter of the compressor blades becomes progressively smaller in higher pressure stages of the compressor. Because the gap between the blades and the engine casing increases in size as a percentage of the compressor blade height as the blades get smaller in diameter, a greater percentage of the compressed air can leak back past the blades in higher pressure stages. This causes a drop in compressor efficiency, and is most likely to occur in smaller gas turbines (since blades are inherently smaller to begin with). Finally, as can be seen in Figure 1, the efficiency levels off as pressure ratio increases. Hence, there is little to gain by increasing the pressure ratio further if it is already at a high level.
• Regeneration, wherein the still-warm post-turbine fluid is passed through a heat exchanger to preheat the fluid just entering the combustion chamber. This directly offsets fuel consumption for the same operating conditions, improving efficiency; it also results in less power lost as waste heat. However, at higher pressure ratios, the compressor discharge temperature can exceed the exhaust temperature. Under these conditions, regeneration would be counterproductive. Therefore, regeneration is only an option when the pressure ratio is sufficiently low that the exhaust temperature is higher than the compressor discharge temperature.

This feature is only available if the exhaust heat is not used otherwise, as in cogeneration or combined cycle applications.

• A Brayton engine also forms half of the 'combined cycle' system, which combines with a Rankine engine to further increase overall efficiency. However, although this increases overall efficiency, it does not actually increase the efficiency of the Brayton cycle itself.

• Cogeneration systems make use of the waste heat from Brayton engines, typically for hot water production or space heating.