Mechanical Engineering

A Combined Rankine and Brayton Cycle

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One of the biggest challenge mechanical engineers face is how we can make a system more efficient. Many conventional methods involve making the system larger and bulkier, which is why the most efficient cycle we have, the Rankine Cycle, requires so much material that we have to create entire power plants. Power plants are expensive, immobile, large, but they are the most efficient.

Now a disclaimer: I didn’t learn any of this in class. I was trying to come up with stuff for my Thermodynamics research project (this was ultimately scrapped because it wasn’t undergrad level), so I don’t understand it fully. If you combine a Brayton cycle and a Rankine cycle, you can improve the efficiency drastically. You see, a lot of heat is wasted through a standard Rankine cycle, which is why we add reheat and regeneration, which takes up space. What if instead of adding reheat and regeneration processes, we added a Brayton cycle?

A Brayton cycle begins by drawing in air from the surroundings through a compressor. The air is then put through a combustor to heat it up, and the heated air is used to rotate a turbine, and some of that work from the turbine goes back to power the compressor.

What if we took the heat generated by a Rankine cycle and used that to power a Brayton cycle? Then we’d have to add less fuel to the combustor to heat up the air and we wouldn’t waste as much.

Here’s an extremely simple version of what it would look like.

Steps 1-6 are the processes of a simple Rankine Cycle, and steps 7-10 are the processes of a Brayton cycle. If there’s a heat exchanger connecting the two, then you can see how the heat generated as waste by a Rankine Cycle can instead be used to power a Brayton cycle.

I don’t know what it would look like in real life, but something worthy to note is that the Brayton cycle is what is used for modern gas turbines and air breathing jet engines.

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