Difference between Regenerative Rankine Cycle & Brayton Cycle with Regeneration
Regenerative Rankine cycle and Brayton cycle with regeneration are provided for increasing efficiency in steam power plant and gas turbine power plant respectively. This article will reveal the basic difference between the two. As you are all aware, in Rankine cycle, the working fluid is water and it changes its state to form steam and you will find a vapor dome in the TS diagram of Rankine cycle. Steam tables and Mollier charts are used to find the property values in Rankine cycle.
However, in Brayton cycle, the working fluid is air which does not undergo any phase change. We are treating air as an ideal gas and the equation of state of an ideal gas is used to find the property values in Brayton cycle. Regenerative arrangement in both these cases is accomplished by providing heat exchangers which utilize the high temperature of the expanded gases in the turbine exhaust. Brayton cycle can be classified into closed Brayton cycle and open Brayton. Among this, only closed Brayton cycle can be considered as a proper Thermodynamic cycle. In open Brayton cycle, the identity of the working substance is changed after combustion of the fuel, so the basic assumption pertaining to the thermodynamic cycle gets violated as the initial mass of the cycle is not the final mass of the working substance in the cycle same like an internal combustion engine. For this reason, the regeneration arrangement mentioned in this article belongs to a closed Brayton cycle category only. The comparative features of Regenerative Rankine cycle and Brayton cycle with regeneration are as follows.
However, in Brayton cycle, the working fluid is air which does not undergo any phase change. We are treating air as an ideal gas and the equation of state of an ideal gas is used to find the property values in Brayton cycle. Regenerative arrangement in both these cases is accomplished by providing heat exchangers which utilize the high temperature of the expanded gases in the turbine exhaust. Brayton cycle can be classified into closed Brayton cycle and open Brayton. Among this, only closed Brayton cycle can be considered as a proper Thermodynamic cycle. In open Brayton cycle, the identity of the working substance is changed after combustion of the fuel, so the basic assumption pertaining to the thermodynamic cycle gets violated as the initial mass of the cycle is not the final mass of the working substance in the cycle same like an internal combustion engine. For this reason, the regeneration arrangement mentioned in this article belongs to a closed Brayton cycle category only. The comparative features of Regenerative Rankine cycle and Brayton cycle with regeneration are as follows.
Regenerative Rankine Cycle
- In this arrangement, bleeding off part of steam from turbine occurs at certain stages of expansion and it is used for heating the feedwater in a separate heater.
- The mean temperature of heat addition is increased and efficiency will improve. Irreversibility is also minimized and heat rejected to the condenser decreases due to bleeding of steam.
- Specific steam consumption increases.
- Turbine work output decreases due to bleeding of steam.
- An increase in pressure ratio increases the efficiency of regenerative Rankine cycle.
Brayton cycle with regeneration
- High-temperature gases coming out from the expansion turbine is utilized for preheating the cold air coming into the compressor.
- Preheating decreases fuel requirements and thus improves efficiency.
- Here also, the mean temperature of heat addition is increased.
- There is no change in the net work output due to the regenerative arrangement in Brayton cycle as there is no bleeding process here.
- Increase in pressure ratio after certain limit decreases efficiency.
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